Tuesday, May 28, 2013

SPC Convective Outlooks: What does Slight, Moderate, High Risk mean?

Hi, I thought I would post about the slight, moderate, and high terms I use when I’m talking about severe weather events.  So, if you've been a bit confused by the terms; I hope this will help you out.   

Often when severe weather is anticipated, meteorologists will sometimes assign a level of risk to an area for the likelihood of severe weather.

The risk may be Slight, Moderate, or rarely High. Meteorologists will refer to an area having a certain "risk" of severe weather.

When severe weather is likely, most weather outlets will minion the Storm Prediction Center (SPC), located in Norman, Oklahoma.  The SPC is part of the National Weather Service (NWS) and the National Centers for Environmental Prediction (NCEP).

I've been inside the SPC many times (if you ever get the chance I encourage you to visit). The building is shared with several weather centers including the National Severe Storms Laboratory, The National Severe Storms Forecast Center, and the University of Oklahoma School of Meteorology.

The Severe Storms Forecast Center is responsible for tornado watches and other thunderstorm watches. (Warnings come from each NWS office)

Some of the products the SPC is responsible for are:

Forecasting for fire weather (conditions favorable for wildfires) in the contiguous U.S. which are Day 1, 2, and 3–8 fire weather outlooks.

Issuing mesoscale discussions, are issued to give information on a region that is becoming a severe weather threat and states whether a watch is likely and details thereof, as well as situations of isolated severe weather when watches are not necessary.

General severe weather outlooks.

Convective outlooks, which give detail on the risk of severe weather. Since this post is about convective outlooks; I will go into more detail on this Convective outlooks are issued for Day 1, Day 2, Day 3, and Day 4–8, and detail the risk of severe thunderstorms and tornadoes during the given forecast period, although tornado, hail, and wind details are only available for Day 1. Days 2 and 3, as well as 4–8 used a probabilistic scale, determining the probability for a severe weather event in percent.

Risk categories:

Three risk categories (SLGT, MDT, and HIGH) stand for the coverage and intensity of organized severe weather such as supercells, squall lines, multicell thunderstorm complexes, and of course tornadoes.

what is not included in the three risk categories.

Garden variety (pulse type) thunderstorms, which can contain brief severe updrafts.

Isolated (random) severe storms with marginal intensities or short durations of severe weather.



A SLGT risk is issued frequently during the severe weather season and implies well-organized severe thunderstorms are expected, but in small numbers and/or low coverage.  Typically, SLGT Risk areas produce scattered severe weather, including scattered wind damage or severe hail and possibly some isolated tornadoes.

     Use this for day 1  


This isn't issued as frequently as a Slight Risk area. Its reserved for days when substantial severe storm coverage, or an enhanced chance for a significant severe storm event.  A MDT Risk implies a greater concentration of severe thunderstorms, and in most situations, greater magnitude of severe weather. When a MDT risk is issued the forecaster has a much greater confidence that a widespread severe event will occur (as compared to a Slight Risk.) Typical Moderate Risk days include multiple tornadic supercells with very large hail, intense squall lines with widespread damaging winds, or in some cases, land-falling tropical storm systems.

   Use this for day 2      


A HIGH risk area suggests a major severe weather outbreak is expected, with the expectation of either a concentration of strong tornadoes or an enhanced likelihood of a long-lived wind event (derecho) with the potential of higher end wind gusts (80+ mph) and structural damage. The High Risk category is reserved for the most extreme events with the least forecast uncertainty, and is generally only used a few times each year.

   Use for high risk day 3    

For all outlooks, the probability values represent the chance of severe weather occurring within 25 miles of any point, which is about the size of a major metropolitan area.

As an example, if you are in a 15% probability for tornadoes, this means you have a 15% chance of a tornado occurring within 25 miles of your location.

Other things to know about the convective outlook:

The unlabeled (brown) thunderstorm line on the outlook graphic depicts, to the right of the line, a 10% or higher probability of thunderstorms during the valid period.

Sometimes, a blue hatched area will be overlaid with the severe probabilities. Blue hatching means a 10% or greater probability for significant severe events within 25 miles of a location in the area.

"Significant" is defined as:

Tornadoes rated EF2 or greater,

Thunderstorm wind gusts of hurricane force or higher--65 kt or 74 mph;

Hail 2 inches or larger in diameter. 

On days when the forecast severe-storm threat is not organized enough or is too conditional for a SLGT risk, but marginal threat still exists, a SEE TEXT label will be placed on the graphic where a 5% severe probability exists.

The Day 4-8 outlook is a single graphic and short narrative covering well-organized severe-weather potential 4-8 days out. There is no general thunder forecast, significant-severe area or SEE TEXT because of the long time frame and uncertainties involved. Each line represents a 30% probability area for a day, or the same as an "enhanced slight risk" on day-1. If the threat looks lower than 30%, "POTENTIAL TOO LOW" will appear. If the threat might exist, but is too unclear to draw a specific area, "PREDICTABILITY TOO LOW" is the label. The lack of a line does not mean that a severe threat is absent--just that the threat is too low, disorganized or uncertain for a 30% area that far out in time.

Well that is it... I hope this sheds some light on this subject.....


Thursday, May 23, 2013

2013 hurricane outlook

I meant to post this a few weeks ago, but do the active weather and other personal concerns, it took a backseat. Anyway without no farther postponement, here is my tropical cyclone activity outlook for 2103.

Thoughts On The 2013 Atlantic Hurricane Season.

We are a little more than a week before the official start to the 2013 Hurricane season. Here is my thoughts on this year's hurricane season.

Here's the list of names for the 2013 hurricane season. If my personally is any indication, I hope we don't make it to R ;)


As my last blog post noted, we are seeing a similar pattern setup that was predominate in the 1950's. These same ocean oscillations and Sea Surface Temperatures (SST's) will have a major effect on Atlantic Basin tropical cyclone activity in 2013.

The last few years have seen major storms impact the Northeast and Mid Atlantic, Irene, Lee, and Sandy are most likely the first to come to mind. So let's take a look at this year's setup.

Part one can be found here.

Part two can be found here.  


The Pacific Decadal Oscillation (PDO):

The PDO is in a negative (cool phase) which means cool water temperatures off the Pacific Northwest coast. The affect this also has is it helps the Atlantic warm.

The North Atlantic Oscillation (NAO):

This oscillation is in a negative cool phase. The NAO has a strong relationship to temperature, precipitation, and weather patterns along the Eastern United States. The traditional definition of the NAO is the difference of normalized sea level pressure anomaly between Iceland and the subtropical eastern North Atlantic.

During a negative NAO unusually high geopotential height occurs across the high latitude North Atlantic. This is typically found by looking at the 500 mb level and is referred to as blocking. This blocking ridge across the high latitude North Atlantic causes both upstream and downstream troughs across Eastern North America. Which help steer systems on the East Coast.

Recent studies at the NC State Climate Office indicate an increased potential for wintry weather in NC due to the position and availability of cold air, and a more favorable upper level pattern conducive to coastal storm tracks.  And as we saw with hurricane Sandy, blocking in the North Atlantic can have dire consequences for the Mid Atlantic and Northeast.

Atlantic Multidecadal Oscillation (AMO):

The current state of the AMO is positive (warm phase) which means warm water temperatures off the East Coast. When the AMO is in a positive phase there is melting of sea ice in the Arctic. The recent news of the Greenland ice sheet and sea ice melting is the result of the AMO being in a warm phase.

When the AMO is in a positive phase Greenland blocking is more likely, which means cold air pretty much locks in place, in the East; during last winter the AMO being in a positive phase, in conjunction with the NAO being in a negative phase, was the reason New England saw so many nor'easters, with many seeing near record to record breaking snow amounts.

Sea Surface Temperature  (SST):

 I've outlined an area in the Atlantic basin where temps are above average for this time of year.... The sea surface temps off the west coast of Africa over into the Caribbean / Gulf and just off the East Coast, are above normal for this time of year.

   sst atlantic  

Sea surface temperature in the equatorial Pacific Ocean are also important to Atlantic Basin hurricanes.  El Niño is characterized by unusually warm temperatures. La Niña by unusually cool temperatures in the equatorial Pacific. Anomalies. So far, it's been in a neutral phase. and with slight variations this should continue.

All of these ocean temperatures and ocean currents have an important influence on weather. Warmer temperatures will add more moisture to the air. Changes in ocean temperatures will change the wind patterns aloft. Changing the wind pattern will change how moisture is distributed. El Nino and La Nina are examples of how changing ocean temperatures can influence the weather pattern. 

The PDO and AMO help anchor the jet streams which in turn amplify or de-amplify temperatures and moisture distribution. For example, A warm AMO means less arctic ice and with a cold PDO more Atlantic hurricanes.

One more thing I want to mention before I start concluding:

The Tropical North Atlantic Index (TNA), which measures the anomaly of the average monthly sea surface temperatures of the tropical Atlantic Ocean, is in a positive phase and the pattern of the index is eerily similar to that of the active seasons of 2004, 2005. The setup also resembles 2007.

2004 had 15 named storms.

2005 was the most active Atlantic hurricane season in history. They had so many that they had to use the Greek alphabet; when more than 21 storms are named the additional storms are given names from the Greek alphabet.

2005 saw 30 named and one unnamed tropical cyclones (This unnamed subtropical cyclone was identified during NHC's post-season re-analysis) for a total of 31 tropical cyclones.

Tropical Storm Zeta became the final storm of the season when it formed on December 30, six hours short of tying the record of Hurricane Alice of 1954 as the latest-forming named storm in a season (retying back to my post on "Are We Returning To The Weather Patterns Of Past Decades").

2007 was another active Atlantic hurricane season that produced 17 tropical cyclones.

What does all this mean? 

As most of us are aware, normally Atlantic Basin cyclones form in the area between the West Coast of Africa and the Caribbean, along with the Gulf of Mexico (GOM). Hurricanes thrive over warm water; the warmer the better. Tropical cyclones are fueled by warm water evaporating into the air; as I said above, water temperatures 80 degrees (F) or greater will enhance tropical development. So with the area off the west African coast warmer than average, one can conclude that it will help intensify tropical waves moving off the West Coast of Africa.

The NAO has been principally on the negative side during the Winter and into Spring 2013, it looks to say this way for the foreseeable future. Research has shown that negative NAO patterns tend to weaken trade winds. One thing the trades winds do is help mix water along the surface of the ocean. Because the trades are weaker it will help support the warmer water over the tropical Atlantic.  Also with the NAO being negative we will see more in the way of Greenland blocking. And as I alluded to above, blocking helps keep tropical systems closer to the US East Coast. And as was the case with Sandy, blocking can help force tropical cyclones into the East Coast.

As I said above, temperatures in the Pacific have influences on the Atlantic. One of these is wind shear. (if you've followed my weather page or read this blog) you most likely know wind shear is simply a change of wind speed and direction with height. Wind shear can kill a tropical system in its tracks.  With the current neutral to weak La Nina in the Pacific, we should see the less wind shear and warmer SST in the tropical Atlantic condition continue.

What does this year's Atlantic season look like?

Based on the factors I listed above, statistical analysis, along with a few others. Also based on what I posted in part one and part two of the "Are We Returning To The Weather Patterns Of Past Decades" This season will be quite active with 12-18 named systems, with 8-10 becoming hurricanes, four or five (perhaps more) of which will make landfall somewhere in the U.S. Also, I feel 2-4 of these will strike or impact the Northeast and northern Mid Atlantic states. Of the 12-18 named storms 3-4 will be major. Once we get toward the end of July and onward I think the season will become quite active.

last year the SST off the west African coast was cool, which reduced the number of Cape Verde Hurricanes (an Atlantic hurricane that develops near the Cape Verde islands, off the west coast of Africa). So many of the tropical cyclones we saw formed in the GOM or the Caribbean. With cooler SST  not looking to be an issue this season, we should see a return of Cape Verde Hurricanes.

Well that's it... hope you enjoyed reading it......

My next blog post will be on meteorological terms and phases.