Tuesday, November 29, 2011

2011 - 2012 Winter Outlook.

Hi it's Rebecca, I've been meaning to post this for the last 3 weeks. But things and events interfered. Anyway, I thought I would take the time to put it together today. This outlook is based on several key factors: the ENSO (El Nino/Southern Oscillation),the Pacific Decadal Oscillation (PDO), the North Atlantic Oscillation (NAO), the Arctic Oscillation (AO), past years that had a similar pattern, and Snow Cover in Alaska, Canada, and Siberia. I even looked at solar activity.

Before I get into the specifics of the outlook, I want to spend a little time explaining what an outlook is. First a winter outlook is not a forecast. A forecast deals with specific weather events such as storms.  With a forecast you look at data, in order to figure out precipitation type, severity, and timing of a storm. Where as an outlook talks about how the winter pattern will set-up, it attempts to show where the major storm tracks will set-up and how the winter will trend. It will not tell you how many Blizzards, Nor'easters, or ice storms there will be, or when they will occur.

The winter pattern starts with a strengthening jet as it makes it's migration south. Something we're just starting to see. How the jet acts and sets up effects how the troughs will act and move during the winter. Most agree that the winter of 2011-2012 will see the return of a moderate La Nina. The long range models are supporting this idea, more or less. If this is the case, then the key to our upcoming winter will be the return of some strong high pressure blocking over northeastern Canada. The good news is the Euro long-range does not show much if any high pressure setting up over NE Canada. The Bad news is Last year's Euro long range showed some blocking but nowhere near as strong as what actually setup . Last year was a La Nina and we all know how that turned out. Now if the Euro is right we won't have a block setup this would mean just to our north in Ontario and southern Quebec would see tons of snow; while we had a warmer than normal winter with more or less normal snowfall. Now back to the outlook.

For those who don't want to read the science, you can skip to the summary at the end of the post. However, if you do so you will miss a lot of interesting information and how I came to my conclusion.

The Factors:

Factor #1: ENSO:
Probably the most talked-about index. You always hear the words El Nino and La Nina thrown around. The positive phase, or El Nino, is when the winds at the equator are pushed west to east. These keep sea surface temperatures fairly warm. A negative, or La Nina phase, is when the winds reverse. This allows for waters to be "dragged" from east to west, thus pulling to the surface, or upwelling, cooler waters just under the surface around South America.  A weak to moderate  La Nina winter will be the case for  Winter 2011 - 2012. Historically, La Nina favors a warmer than usual fall. As we have seen so far this year supports this idea. Last year’s La Nina November brought much less snowfall than average. The lakes stayed on the warmer side heading into December. Weak to moderate La Nina patterns tend to give more snow to New England, New York State, and the interior mid-Atlantic. La Nina is a little more transient, usually producing a ridge of high pressure across the West which drives the jet stream north, and then it dives down across the Midwest. Up here in the Northeast our weather will depends on how the storms track along the Jet and how the trough axis sets up. How much snow we get really depends on the exact track of individual storms. A difference of just 60 to 100 miles can make a huge difference in snowfall amounts. However, this is just the first factor.

Factor #2 the PDO:
To put it briefly, PDO is another sea temperature phenomena. It has two phases, a warm phase (positive) and a cool phase (negative).  The other oscillations have the same phases. The warm and cool phases in the PDO last a lot longer than El Nino and La Nina, typically lasting 10 to 40 years - with a few shorter intervals occasionally in between. The cool PDO phases have been linked to cooler temperatures, and vice versa. I feel we are at the beginning of a longer cool PDO phase. The PDO focuses on the northern portion of the Pacific Ocean, towards Alaska and the Aleutian Islands. It accounts for Northern Pacific Ocean temperatures.  Since 2008 the PDO has flipped back into a negative cold phase. When the PDO is in a negative phase the Jet tracks north of Alaska, which sets up a trough axis that lines up from Great Lakes down in to Alabama. This kind of track sends a lot of storms into the Great Lakes and Midwest.  When the PDO is in a cold phase the Great Lakes and Ohio Valley into New England tend to see below normal temperatures. The PDO was in this same phase for the  winters from the 1950’s, 60’s and 70’s. I will get more into past years in a little bit. The main thing to remember is: on average, we see more snow during a negative phase than during a positive phase. That's it for the second factor.

Factor #3 the NAO:
There will be a couple of wildcards this winter. The NAO will be one of them. This oscillation is quite different than the ENSO and PDO. Because it's not solely driven by ocean temperatures; which makes it extremely hard to predict.

The top image shows what happens when there is no block in the East, allowing storms to track generally West to East across the United States. This is call a flat pattern

The bottom image shows how a negitave NAO effects the storm track. When the NAO is negative, it sets up a block of the jet stream, making it more amplified, and storms are forced up the coast as potential blockbuster storms. This is also known as the "Greenland block" since high pressure blocking sets up near Greenland. When the NAO is negative the overall pattern is more unsettled weather , frequent and sustained cold snaps, and above normal snowfall.

The NAO this past month has been positive much of the time, and combined with a strong ridging and -PNA out West, it has resulted in warmer weather. In the near future, I see the NAO returning to negative, and will alternate from positive to negative this winter, but I think the primary phase will be negative for the most part, especially the first two thirds of the Winter.

Factor #4 the AO:
This is the other wildcard. The AO takes place over the North Pole.  This flow from the Arctic can and often does work hand in hand with the NAO. The Arctic Oscillation (AO) has a significant influence on winter weather in the U.S. Especially, the northern and eastern U.S.  By this I mean the AO plays a big part in how much cold air will get pulled into systems in the Lower U.S. The AO was at a  record low for the 2010 -2011 winter season.  Because of that many places saw well above average in snowfall. When the AO is negative storms will take a more northern track. The AO has been on the positive side for a little while. However, it's starting to show signs of a tilt to the negative side.  I think the AO will be negative for the first half of this winter (Mid Dec- end of Jan) then starting to turn positive sometime in February. in my opinion, the AO is the hardest oscillation to predict; because it changes so often.  The past two winters have produced times of very negative values. This resulted in  very snowy winters.

Factor #5 snow cover:
One of the things I do in trying to figure out what an upcoming winter will be like; is look at the extent of snow and ice in the Arctic. I start looking at the snow cover in Siberia in October. In November,  I look at the snow cover in both Alaska and Northern Canada.

The images below shows the current snow cover in Alaska, Canada and Siberia along with a comparison to this time last year.

From the images above I can see three very important things:

1) The snow pack in Canada is creating a cold air pocket, you can call it the  "icebox effect" This will cause colder than average temperatures in the Northern half of the US during the typically colder Winter months. The snow pack in Siberia is quite extensive right now.  Snow cover is important to factor into a winter outlook, because it establishes a cross Polar flow. By this I mean the Arctic cold over Siberia will be able to flow over the North Pole/Arctic region into Central and North Central Canada... and right now we don't have that. If there is no cold Arctic air in Canada, which is the case right now, it can't get cold no matter how strong the cold looks to be in long range guidance. So the build-up of serious deep cold Arctic air in Canada is a huge deal.
2) As I've been saying in my weather page, once winter decides to make an appearance Winter will come in with a vengeance once the pattern changes, and cold air in Canada can make it to the lower 48 and Northeast US.

3) From the look of things, I feel the Northern Hemisphere will be under the influence of a strong polar vortex (PV). this will only make things worse, it will help amplify the northern and Southern branches of the Jet.  Combine that with a -PDO, -NAO, -AO. now add in the ridging out West and the expected trough (remember 2011-2012 will have a --NAO and -AO) .... and look out. storms will be following one after another.

Factor #6 Solar
The study “Solar forcing of winter climate variability in the Northern Hemisphere”, led by a team of scientists in the United Kingdom, examined how weather patterns changed when the amount of solar radiation reaching the Earth rises and falls. The authors reached the conclusion of when solar activity is low; it produces colder winters in the United States and northern Europe and milder winters in Canada and southern Europe.

Link to the solar study

"The Sun has recently been in a quiet phase of its regular 11-year cycle, which coincided with three years in which the UK, along with other places in northern Europe and parts of the US, experienced cold conditions unusual in the recent record. But unusually warm weather was felt both further south, around the Mediterranean Sea, and further north in Canada and Greenland." - UK MET OFFICE

The solar minimum of 2008 is said to have played a huge role in the negative NAO that dominated the previous 3 winters. I've read that low solar activity leads to high latitude blocking. Solar activity has started to increase of late. But since the cycle runs for 11 years; I don't think the increase will be much of a contributing factor this year. So it's influence on the negative NAO will be negligible.


A Nor'easter is a storm characterized by a central low pressure area that deepens dramatically as it moves northward along the East Coast.  The heaviest snow in a Nor'easter falls to the north and west of the track of the surface low. Nor'easters can be classified into one of two categories, Miller A and Miller B

Miller A 's are what I call a classic Nor'easter, they develop primarily on the Gulf Coast or East Coast along an old cold front and deepen as they move north.  The "Superstorm of March 1993" is considered to have been a "Miller Type-A" storm.

Miller B's are a little different, they normally dump snow in the Midwest/Ohio Valley . As it approaches the Appalachian Mountains they lose their power and transfer it to a developing low on the coast.  


Above are images of Nor'easter's the top one is a Miller A and the bottom one is a Miller B.

If there are coastal storms, the Miller B pattern set-up will be more common than in a Miller A scenario in this type of pattern.

Past Winters: 
Besides all of the above data, I looked at past winters. I am a firm believer in patterns. The ability to recognize these patterns is what makes a good forecaster. I looked at several winters over the last 60-65 years. I paid special attention to  second and third year La Nina's since 1950. I also looked at years that were neutral. when all was said and done, I came up with this  list of winters for analog years.




The above list comprises the years that strongly matched this year's set-up. There were differences of course. both last year's and 55-56 were fairly strong La Nina's; whereas this year shouldn't be as strong. As for 50- 51 that year didn't have strong blocking. I think this year will see blocking more like last years, though not quite as strong.

Storm Tracks:

These will be the dominant storm tracks for the winter of 2011-2012

These storms tracking to the west would lead to rain, mix, and ice and slop storms along the coast. Places inland would see more in the way of snow. what you saw would depend on the exact setup and location of where you live. I expect a fair share of these this winter.

The red tracks are the coastals and the purple tracks are clippers.

The blue line shows the average axis of the trough for 2011-2012
The purple lines show how the jet will set-up the main storm track

Here is the temperature outlook for the United States for the winter of 2011-2012

The Purple is well below normal
Dark blue is below normal
Lite blue  slightly below normal
Orange is normal
Yellow is above normal
Red is well above normal

Above is the Northeast snowfall outlook for 2011-2012

Purple well above average
Indigo above average
Green slightly above average
Dark blue average
Lite blue below average


Now the good stuff. What  the heck does this mean?

Now that were officially going into winter; we have to start thinking  pattern change. I think we will see a pattern change by the middle of December. for the last five or six weeks we've had above normal temperatures. (Even with the Halloween and Thanksgiving snowstorms thrown in) However, out in the western US and Canada along with Alaska its been the opposite. So, the weather out there has not been as nice and they've seen colder temps. Over the last month and a half, the AO and NAO as be positive to neutral. The result our relatively nice weather. Well all of this is about to change, starting next week we will see the pattern began to change. By Hanukkah we will be in a much colder pattern. This will be especially true for the Great Lakes.

What do I think:

As I've outlined above, Storms will enter the Pacific Northwest move into the Plains and Midwest then head into the Northeast. It will be difficult to get a lot of Nor' Easters this year; the pattern doesn't support them as much as it did last year. For the snow lovers, the Halloween Nor' Easter is not a sign of things to come. But, that doesn't mean there won't be some. The pattern will have the storms phasing more west than we saw last year. this is why I feel 2011-2012 will be a snowy winter for the Great Lakes. But those in eastern NYS and New England shouldn't laugh. Because we will see quite a few Clippers these will make the northeast quite cold for the winter of 2011-2012.  

What to expect from the winter of 2011-2012 across the U.S.

The West:
Because of the way the storm track will setup. The Pacific northwest looks to see normal to above normal precipitation. whereas California and the Southeast will see normal precipitation.

The states in the Rockies:
If you like snow skiing Stay in the Northeast, The ridge that will develop over them, which will keep  places like Aspen Colorado from seeing as much snow as normal.

The Northern Plains:
Like last year, the northern Plains will see an extremely cold winter. I’m expecting yet another very cold winter for the Northern Plains Because I think there will be a higher than average number of Alberta Clipper's this year they should see as much snow as last year.

The Southern Plains:
They will see quite a few ice storms this winter. Parts of Texas could also see more severe weather outbreaks than normal.

The Mid West and Ohio Valley:
The storm track for the most part will be to the south and southeast. So they will see a cold and snowy winter.  

The Southeast and Gulf Coast:
If you like the sun, this will be the place to be. So they will be warmer than average. However, this could come with a price. they run the risk for more severe weather than normal.

The Northeast:

The pattern looks to be classic for a cold and snowy 2011-2012 winter. With the PDO,  AO, and the NAO being negative. we will see a deeper and more easterly jet this year. this will lead to a higher than average number of cold spells. There will be quite a few lake cutters. Also, with the larger than average number of Clippers and the greater threat of Miller B's, will keep the Great Lakes busy.  Because of the warm summer and warm fall the Lakes are very warm this year. So lake effect snows will be above normal to well above normal this winter.  Much of the Northeast will be affected by the same storm systems. Most of NYS and New England will see more snow than avearge. The exception being extreme southeast NYS and areas closer to the coast. This is because  most of the storms will track more to the west this winter. So places like the  Philadelphia and New York City metropolitan areas will see less snow and more of a mix or sloppy mess. I don't think we will see an early spring.  Traditionally, La Nina years see a cold and snowy March and first part of April.

Here is my call for snowfall amounts for some places in the Northeast.

The Tughill east of Lake Ontario will see 200-425 inches of snow this year.
Syracuse looks to see  170-180 inches.

Boston Massachusetts 46-50 inches.

Along the Mohawk Valley  100-130 inches (The higher amounts Just to the south and east of Utica.)

Albany 75-85 inches.

New York City right around 26 inches.

Buffalo 125-135 inches

Here is a link you can also checkout.

NOAA 2011-2012 winter outlook.

Well that's about it......remember this is an outlook not an forecast. I tried to be as accurate as I could. But when all is said and done, an outlook is no more than a educated  guess. I looking forward to seeing how right I am. However, we will have to wait until next spring to see.


Friday, November 11, 2011

Weather Models part 2

Hi It's Rebecca again,In part two, I will show five GFS model plots at most of the mandatory levels. As I go along, I will explain what you're seeing on the chart and explain what the information it's giving you is used for. I will also go into a little about Forecasting wintertime precipitation. This part will build off my post How to Read a Skew-T-log-p .  This lesson is geared toward an intermediate level and assumes you know what most of the terms used stand for. If I had to stop and explain every term this post would go on and on. However, at the same time I will try not to make it overly complicated. That said, let's dive into the wonderful world of the GFS model.

Below is the GFS surface plot.

Above is the GFS showing Saturday's model forecast. If you look at the model you will see writing on the top and bottom of the image.

To make it easier to read I've retyped them below.

11/09/11 12UTC 066HR FCST VALD Sat 1112/11 06UTC NCEP/NWS/NOAA

This is the top line. it tells you the date the model was produced, The model run, how far it's looking in the future; in this case 66 hours, the date and time in the future it is showing, and who developed and maintain the forecast model; in this case it's the National Centers for Environmental Prediction which is part of the National Weather Service.

111112/0600V066 GFS MSLP.06-HR PCPN (IN). 1000-500 THICK

The first set of numbers is just restating the date, UTC time, and where it's looking at in the future. Then it tells you which model it is; in this case it's the GFS. The next part shows the Mean Sea Level Pressure and 6 hour precipitation, and the important field of 1000-500 mb thickness. One final thing before I move on to thickness - you will notice the colored bar on the left hand side. This shows the rainfall accumulations the model thinks will fall. This is only a rough idea and you should always view it with caution.


When you look at the model you will see the thickness lines running across the image. These represent the physical thickness of the atmosphere between the 1000hPa level and the 500hPa level. This is normally around 5500m, but varies by up to 200m on either side of this value.

Because colder air is denser than warmer air, it's also thinner, because of this the thickness values of cold air will be lower than those of warm air. Being able to see this will give you a a general idea how warm or cold the atmosphere is over a certain point.

You will notice that thicknesses rise from south to north, reflecting the warmer air near the equator compared to the air near the poles. If you look you will see the 540 (5400m) line is shaded blue. This is to make it easier to find. One more thing I should mention is upper troughs; these are areas where colder (lower thickness) air advances to the north. The areas where warmer (higher thickness) air proceeds south are called upper ridges.
By and large, when an upper trough is moving through, there is a greater chance for weather that is more unstable. Whereas upper ridges have weather is usually more settled.

Why is the 540 line important:

The 540 line is in reference to a 5,400 geopotential meter thickness between 1000 and 500 mb. Thickness is a principal function of the air temperature and a secondary function of the moisture content present in the air. Moisture and temperature and moisture are combined together to produce the virtual temperature. The average virtual temperature from 1000 to 500 mb determines the thickness displayed for model analysis. If the temperature of the air is warmed or if moisture is added to the air; it will raise the virtual temperature; which will increase the 1000 to 500 mb thickness. When the thickness gets low enough, snow has the chance to reach the surface. Because of this, the 540 thickness is generally called the snow line. When the thicknesses is 540 or lower it indicates there is a 50% chance that snow will fall at elevations below 1000 feet. However, this is a very lose rule, There are times when lower than 540 thickness will generate rain; and visa versa , when a higher than 540 thickness can produce snow/ice pellets. So never use the thickness value as the sole means of determining precipitation type.

Some of the things that must be taken into consideration:

1 Elevation
2 Warm or cold biasing of 540 thickness
3 Evaporative cooling potential
4 Temperature in the planetary boundary layer  (PBL)

The PBL is the lowest layer of the troposphere where wind is influenced by friction. The thickness of the PBL is not constant. During nighttime and the winter season the PBL has a tendency to be lower in thickness. Then during the daytime and Summer it most likely to have a higher thickness.

The first, elevation is easy to understand. This is because the higher the elevation the closer it is to mid-level colder air. Also the higher the elevation the lower the pressure.

540 thickness can have a warm or cold bias. Generally, temperatures decrease with height in the troposphere. However, differential advection (two or more air masses increasing or decreasing of advection with height) can cause layers in the atmosphere to be warmer or colder than they normally would be. You may have heard terms like Vorticity advection or warm air advection affecting a forecast.

Warm air advection occurs between 800 and 650 mb, which causes the temperature in that layer to increase and thus results in the thickness increasing above 540. If the temperature between the surface and 650 millibars continues to stay below freezing, the precipitation will fall as snow. Whether the 700 mb temperature is -20 C or -5 C, the precipitation will fall as snow if the temperatures are below freezing at all levels aloft in the troposphere. Cold thickness bias is when a shallow layer of warm air in PBL melts the snow before it reaches the ground.

As you can see the temperatures in the PBL are very important in determining the precipitation type. If you have a shallow layer of cold air at the surface you can get freezing rain or sleet even if the thickness is well above 540. This is because deep layers of the troposphere will have more influence in determining the thickness than shallow layers.

Evaporative cooling causes the thickness value to decrease. If precipitation falls into a deep layer of dry air, expect thicknesses to lower.

850mb plot:

The 850mb plot: is good for seeing the locations of highs/lows along with isobars, winds, and temperatures. If you remember, In part one I said the 850mb level corresponds to roughly 5,000 feet in the atmosphere. This height can a lot more useful for determining precipitation types at the ground. The temperature readings at 850mb are high enough above ground level that it's not influence to a great degree by topography as a surface plot. Therefore it is very useful in determining where warm or cold air advection might affect a forecast.

To get a fairly good estimate at the surface for a particular day's high temperature use this method  with the the 850mb temperature chart:

1) find the temperature at 850 mb  for the area in question
2) add the following to the 850mb temperature depending on the time of year: spring/fall(+12 C); summer (+15 C); winter (+9 C); this will give you an estimate of surface temperature.
3) to convert to Fahrenheit: C x 1.8 +32 = F

For example, if the 850 mb temp is 8 C in summer, the afternoon temperature could reach 8+15=23C= just a tad under 73 and a half degrees F. In the winter, 9+9=18C= just over 62 and a half degrees F. This formula works fine in the eastern half to the US and it also assumes little to no cloud cover or precipitation is expected. However, out west the altitude is a lot higher so it doesn't work there.

If you look at the map you will find a line that says 0C on it; this line will give you a good idea of where it will rain, or where it will snow. This tends to be more accurate than using the 5400m line on surface plots. Temperatures 0 C or lower at 850mb mean that precipitation will likely fall as snow (but not always).

The wind barbs show wind direction (remember wind direction is named from which direction the wind is coming FROM)  and speed. The barbs can give you a good clue where there the atmosphere will be unstable. This can come in very handy when you're trying to figure out the severe weather potential. The thing to look for is southern warm moist air northward.
The height field works very similar to the sea level pressure field. Lows and highs can found and compared to sea level locations. Strength of winds are again related to the packing of the height contours.

700mb plot:

The 700mb Heights, Relative Humidity, and Omega Panel is an important panel used by meteorologists. The black contours are called isohypse at the 700mb level, and are contoured every 3dm (the isohypse are in decameters, so 300dm is actually 3000m). The height of the 700mb level is how far you would have to go up in the atmosphere before the pressure drops to 700mb. You will notice that there are colors on the map. The green shading indicates the amount of relative humidity in the atmosphere at 700mb. (NOT the relative humidity at the surface). The green shading represents areas with a 700mb RH greater than 70% as indicated by the legend below the panel, with dark green areas displaying RH greater than 90%. White areas on the map indicate regions where the RH is between 30% and 70%.

The 700mb height lines are similar to the isobars shown on the surface map, they can help you determine where pressure systems are located, and the relative speed of winds(tightly packed height lines indicate areas of strong winds). 700mb height field .... The height field works very similar to the sea level pressure field. Lows and highs can be found and compared to sea level locations. Strength of winds are again related to the packing of the height contours.

The 700mb wind barbs work the same as the 850mb barbs At the 700mb they can be a good indicator of divergence or convergence; in the middle and upper levels of the troposphere.

700mb “omega” is also contoured here. Areas of negative omega indicate upward vertical motion in the atmosphere and areas were precipitation could very well occur. while areas of positive omega show where there is downward motion and areas which normally will be dry.

500mb plot:

The 500mb plot is very important. This level of the atmosphere is often referred to as the steering level. Because most major weather systems follow the winds at this level. Again, the wind vectors will give a clue as to how fast and from which direction the storm system will track. This information will allow you to approximate the storms time of arrival.

You will notice the color shaded areas on the map. These are the vorticity values at 500mb. These eddies in the atmosphere can affect weather in many ways. They can strengthen an existing low pressure center, or cause one to form. They often produce lighter precipitation as they pass over an area. During the winter months,when passing overhead they can produce light snow showers.

Large storm systems will often run parallel to the height lines. So you can use them to give yourself an idea how a system will move.

300mb plot:

The 300 mb plot investigates the upper portion of the troposphere where most of the weather making occurrences happen. This level is often referred to as the jet stream level. Jet streams are simply rivers of fast-moving winds aloft in the atmosphere. Jet streams form along the upper-air boundaries of large masses of warm and cold air. They are created by strong temperature contrasts in the lower and middle tropopause and reflect areas of potential storm development. The temperature contrasts control the speed of the jets. The larger the temperature difference, the tighter the pressure gradient and the stronger the wind speed. There are several jet streams influencing weather in all areas of the globe. I will discuss a few of them, The most commonly referred-to jet streams are the polar front jet. The polar jet typically occurs at latitudes between 50°and 60°, both north and south of the equator. Then there is the subtropical jet located around the 20° to 30° latitudes. A jet stream can theoretically occur anywhere in the upper atmosphere when wind speeds exceed 50 knots. Wind speeds in jet streams average about 110 mph, although they can reach as high as 300 mph. Another example is the tropical easterly jet, which occurs between India and the equator during July and August. Low-pressure systems form along these boundaries and are often near the jet streams. The jets, highs, and lows are nothing more than waves in the ocean of air over our heads; they are always interacting with each other which causes storms to amplify and strengthen. One thing to remember is storms don't create the jet stream. However, a strong jet stream can provide enough influence that it helps create a very powerful storm. The jets move south in the wintertime. As I said above, Jet stream winds blow along boundaries between warm and cold air. As cold air advances farther south in the winter it pushes warm-cold boundaries, and jet streams, south with it. When warm air moves north, the boundaries and jet streams are pushed back to the north.

The 300mb chart is useful for the prediction of temperature. The jet stream divides colder air to the north from warmer air to the south. The transition between temperatures on each side of the jet is very abrupt. When a trough builds over a region it often indicates cooler temperatures due to cloudier weather and northerly winds. A ridge builds by low level warm air advection between the 1000mb and 700mb sections of the atmosphere and upper level forcing (negative vorticity). Air in a ridge sinks and expands thus creating higher heights. Therefore, temperatures are warmer in a ridge due to the compression of the air that results from the sinking motion that occurs within a ridge.

Windspeed: a color-coded map indicates the wind velocities at the 300mb level. The jet stream longwave pattern usually shows up clearly here, as the regions of higher wind speeds indicate the location of the jet stream. Height Lines: indicate the height (in meters) of the 300mb level. Again, the more tightly packed the height lines, the stronger the winds.

Another phenomenon associated with the jet stream are jet streaks. these are detached, high-speed units of the jet stream surrounded by slower winds. Jet streaks are often big weather makers as they move over an area. They can aid in the development of severe weather as air is exhausted from the top of the troposphere or enhance the development of a surface low as they move overhead.

300 mb wind vectors at the 300mb level work the same as vectors at the lower levels.

Winter Forecasting:

Forecasting wintertime precipitation is a huge challenge. Above I said, the 850mb is better at deterring who will see snow than using the 540 line. However, don't think that the 850mb level is the only way or the best way to forecast wintertime precipitation. The method of choice that's best to use when forecasting cold season precipitation is the profile sounding method. If you go to the Penn State e-Wall you will see a link on the left hand side for forecast atmospheric soundings. If you want to make an accurate winter forecast viewing atmospheric soundings for precipitation events is a must. The reason for this is that you want to follow the temperature profile of the atmosphere. I covered how to do this In my post on reading a SKEW-T, there are several atmospheric temperature profiles that correlate to the different types of winter precipitation that may fall. I won't rehash SKEW-T soundings. However, I will show three different types of atmospheric snow soundings.

For sleet to fall, you would expect a layer of air from 500mb up to the 700mb to be below freezing. Then you would like to see a layer between 700mb and 850mb to be slightly above freezing (so it will melt the snowflakes). The layer of air below the 850mb level should then be below freezing all the way to the surface. Use the 850-700mb thickness to help determine if sleet may be a possibility when you are forecasting cold season precipitation.

To forecast freezing rain, you want to see a large layer in the atmosphere from just above the surface to about 850mb above freezing. This layer is sufficient to melt the snowflakes to rain drops. However, to get freezing rain, you want your surface temperature below freezing.

To forecast rain during the cold season, you would expect to see the same setup for freezing rain as explained above, but with a surface temperature that is above freezing.

For snow to fall, you would expect the whole layer from 500mb to the surface below freezing (As I said in past blog post, the surface temperature itself doesn't have to be below freezing for snow to fall. However, if the surface temperature is above freezing you want to see a below freezing temperature profile to at least 950mb to get snow to fall at the surface

OK lets get to our snowstorm. Elevation will pay a huge role. The surface temperatures will be too warm to support all snow in places below 500 feet. Other than the surface conditions the upper air looks great to support wintry precipitation.

The yellow shows the freezing line and the blue lines (temperature and dew point) remain below that point except at the surface. The location this sounding covers shows snow and rain. Making accumulating snow hard to come by. there will be times when there will be heavy bands of precipitation. The evaporative cooling will help to cool the surface layer for brief periods and allow snow to accumulate in those areas. Now let's look at a sounding for the same storm; but a different location.

NOW this is an all snow sounding for the whole event since the temp remains below freezing all the way to the surface.

Here is a SKEW-T that shows a classic setup for lake effect snow.

The things that are key are

1) Unidirectional shear intensifies moisture convergence
2) Inversion is above 700 millibars
3) Plenty of PBL moisture (saturated in PBL)
4) Steep lapse rate from surface to 700 millibars
5) Average wind speed below inversion is 30 knots
6) Wind direction results in a large lake fetch

The distance that an airmass travels over a body of water is called fetch.

For a big lake effect snow event off the east end of Lake Ontario; A flow of 270 is key, A 270 flow allows the wind to move over 193 miles of open water. I will be getting into lake effect snow in a future blog post.

Well that's about it. I hope you learned something and enjoyed going through part 2. If you want to learn more, there are two good books that will teach you more about weather models. The books are "Weather map Handbook" and another called "weather forecasting handbook" by Tim Vasquez. You should be able you find them online at Amazon or at a local bookstore.


Thursday, November 3, 2011

Weather Models part 1

Hi It's Rebecca again. Weather Models part 1 will cover the basics of weather models. I will explain what a weather model is and cover some of the models out there. As well as some of their weak and strong points. I've decided to divide the subject of weather models into two sections.

In the previous post;  I went into too much detail on Skew-T's; as a result I fear I may have lost some of you in the details. Therefore, this part will cover only the basics. So that when you hear a certain model name you will have an idea of what it is and how it works.  I've been fielding a few questions from some who want to know a little more about how this stuff works.  So for those adventurous souls, in the second part of this installment, I will go into a little more detail. The section will cover things like 500mb and 700mb Heights, Vorticity, 850mb Temperatures, and 1000mb to 500mb thickness, and maybe a few other things. I will give you the fundamentals on how to read a model to make a forecast. because winter is around the corner (yes it's still Fall in spite of the past nor'easter). I will focus on winter forecasting.  I hope this is acceptable for everyone. It's my hope that if I tackle it this way,  you can read the section you want.
What Are weather Models?
Weather models are computer generated forecasts of how the current or initial state of the atmosphere will change over a period of time. There are many different models, and they all have their uses. Some computer models, such as the Global Forecast System (GFS) run forecasts out to 16 days while other models, such as the Rapid Update Cycle (RUC) will only run for 12 hours.
 How does a weather model work?  
Each computer model will forecast weather for a different amount of time but all models are based on initial weather observations and weather data. This data is then ingested into a powerful supercomputer. The mathematical equation used by the computer involves uncertainty analyses (which is hard to explain in this format) Anyway, computers running these forecasts take in such things as temperature, pressure, humidity, wind speed and direction, etc in an attempt to represent the current weather conditions, then process the data and output it in a certain amount of resolution (detail) attempting to make a prediction of the future state of the atmosphere. Resolution is based on a grid system that  divides up the atmosphere at the mandatory  levels in the vertical, from the surface  to the stratosphere. At each level the horizontal spacing between the points is very regular. The closer the points the higher the resolution ( Supposedly, the higher the resolution the more accurate the model)  Some of you might be asking. Then why not make all the models high resolution? The answer, the computer processing power needed is the fourth power of the increase, Therefore if you want to make the grid half as small you would have to increase processor power by 16 times. So you can see based on current computer technology it becomes impossible at a certain point.
The Models.

Mid and Long Range Models:

NAM (North American Mesoscale) This is one of 4 American models. The NAM runs 84 hours into the future. Updates are produced every six hours at 0z, 6z, 12z and 18z,

Strong points:

It normally has a good handle on temperatures. This model is very useful in forecasting winter storms and severe weather that are expected occur within 48 hours or less.
Weak points:
Normally it shows systems moving slower than they actually are. It also likes to show more precipitation than actually accrues. The model has a hard time handling shortwaves.
GFS (Global Forecast System) Model Forecasts are also produced every 6 hours. (00, 06, 12, & 18). Like the NAM they are available in 6 hour increments. The GFS is a longer range model and goes out to 384 hours. However, the current GFS output from NCEP is 3-hourly up through hour 192, after that it then shifts to 12-hourly. There is also a resolution change at the 192 hour time.
Strong points
It's great for looking at overall patterns and temperatures.
Great for finding the conditions for major winter storms.
There are associated ensemble forecasts that run off the GFS operational run that are used for Quantitative Precipitation amounts (QPF)
Weak points:
It's not very accurate out past 5-6 days. It can have a storm in the 11-15 day range, only to "lose" it for a few days and then "find" it again when the storm is 3 or 4 days out.
GFS can miss the short term forecast in bizarre ways, such as the location a front will be six hours from the initialization time.
It has a northern dry bias
Like the NAM it has a tendency to overdo the precipitation a bit especially when the precipitation is light and it tends to over forecast the amount and extent of precipitation in cold air masses.
It has a very large grid spacing

ECMWF (European/Euro) Some of the data from the Euro is available by subscription only. Many meteorologist call the Euro the King, because it's known for  its accuracy rate in the mid-range. The model runs 240 hours into the future. However, it's between the 72hr and 120hr that it is the most reliable. The Euro It runs every 12 hours, at 0z and 12z, and there are no "off-hour" runs.
Strong points

Handles winter weather events very well.
Like the GFS it has its own ensembles.

Weak points 
Doesn't handle shortwave interaction with storms during winter as well as some of the other models.
Forecasts using this model are a bit more complicated

There are two other models I will briefly mention. The first is the UKMET. It is run 4 times a day; the 00z and 12z runs go out 120 hours; the 06z and 18z runs go out through 72. Most of the time it's used to collaborate with other models during the winter. The second is the GGEM. This is a Canadian model; it runs twice a day out to 144 hours. A major problem with this model is it has a warm and wet bias

Short Range Models:

RUC (Rapid Update Cycle) This model updates every three hours beginning with 0z, 3z, 6z, etc. It takes current surface readings add any changes into the model.  It is a high resolution model that's  intended to forecast looking out for 12 hours; because of this it's very valuable for now casting.  Because of its rapid updates and higher resolution it's great for severe and winter weather forecasting.  
HRRR (High-Resolution Rapid Refresh) The HRRR is a 3-km resolution, hourly updated, cloud-resolving atmospheric model. Its enhanced Rapid Refreshing capability replaced the Rapid Update Cycle (RUC) earlier this year. This model has a much higher resolution than the RUC. The HRRR "looks" farther out into the future than the RUC.

What the heck is Z time
The forecast hours on weather models is  UTC, which is Coordinated Universal Time; this is also called or Zulu time (That's where Z comes from) which is military time, or GMT, Greenwich Mean Time.

0z is midnight at the Greenwich Meridian and 12z is noon.

Here is something to help you convert Eastern time to Z time.
Eastern Standard Time (winter) is 5 hours behind UTC
Ex 12z = 7am
00z= 7pm (Remembering that 00z is actually midnight)

During Eastern Daylight Savings (summer) is 4 hours behind the UTC
Ex 12z = 8am
00z = 8pm

How to read a weather model
Here are the basics. I will  focus on the GFS and NAM.
As I said above the GFS is a long range model, going out to for 16 days. whereas, the NAM is used in the shorter term going out 84 hours. On each one you will see OZ, 6Z, 12Z and 18Z. you will have to check to see what your local time is and then convert it to Z time.... Z time  is the time they run the model, each runs four times a day.
Now you will scroll down the page and click on an hour. My  advice is to click on fine and then  select the 850 MB and 6 hr Precipitation.  A map should then appear.
Now you will see a lot of  graphics on the map. Don't let the graphics scare you.  The graphics will give you all kinds of information. However,  you are most likely looking  for a storm. So look for green. The amount of precipitation is broken down by color.  The colors move from light green to blue to purple. Light green represents light precipitation and Purple represents very heavy precipitation.
If you are looking for a snowstorm, you must look for the O line. It will be a blue line labeled 0. The color blue represents cold air. If any blue line is to your south and you see precipitation it will likely be something in the frozen state. The orange line represents warmer air and means rain.

To give you an idea of what a model looks like here is the 1000-500 mb thickness from todays GFS. I added an arrow that points to the 540 0c line.

Here is a link to the e-wall Penn State site.


Well that's about it.....This should give you an understanding of what a weather model is and what their used for.........I will be back with part 2 Monday or Tuesday.