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.


1 comment:

  1. Thanks for all this - certainly makes it more understandable! One question, how does all this translate into a forecast of snowfall amounts ?


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