Friday, April 8, 2011

Why Moderate Risk areas from the SPC are a big deal

Moderate Risk areas issued 2 days out from the date of the severe weather is rare.
It displays the forecaster's confidence that a severe weather event could occur with force.

Supercells Developing in Oklahoma: April 8 2011

Supercells are developing in Oklahoma at this time with a vengeance.
Craven/brooks Significant Severe Parameters, where the threshold for significant severe weather is 20,000 is currently at 100,000. That is 5x the threshold.

April 9-10 Severe Weather Outbreak Series: Short Range Analysis

Low pressure will eject into the Plains with a cold front on its back. An assisting low pressure system in the Upper Midwest will help advect warm air north. That warm air will be very warm and humid. Tight pressure gradient will make for windy conditions ahead of the cold front.
As the low pressure moves east, the assisting low pressure system will be absorbed and the cold front will push into the Midwest and Plains, making for a severe weather situation.

April 10 Significant Severe Weather Outbreak Forecasted (Issued April 8)

There is growing concern over a large and potentially significant severe weather outbreak over the Plains and Midwest.

Table of Contents
-The Forecast
-The Set-Up
-The Initiation
-The Threat


These images are from the Storm Prediction for 2 days out, Sunday. The Storm Prediction Center has been outlining the area for a severe outbreak, potentially significant.
With the SPC forecasting this for at least 3 days, there has been concern aroused, and more with each model run. 
In the Moderate risk area, we can expect somewhat of a tornado outbreak. If one does happen, it may not be significant. The main threat making the moderate risk worthy is the damaging wind risk. There is a looming threat of damaging winds throughout the entire section of the risk area.

A very strong cap will set up in the Midwest, Great Lakes, Plains and South Plains. The cap will be extremely hard to break, thus amounting instability and activity in the atmosphere. Under this strong of a cap, there is always concern. (A cap is something suppressing storms in the atmosphere.)
Temperatures will be skyrocketing, potentially as high as 88 degrees in Chicago. This warm air will be a determining factor in some of the strength of the storms. These temperatures beg to be manipulated for storms. Another issue will be dewpoints.
In the above image we see dewpoint value forecast. These dewpoints are very high and also a big factor in the storm development. With dewpoints this high, large amounts of humidity in the air could be present. 

It can be assumed this outbreak is like a big tower of objects. One big object (like instability) at the bottom falls and the entire tower falls. But one object near the top (such as a 8 degree temperature difference) falls, the entire tower won't fall. Right now, the tower seems a bit wobbly, if you get my drift.

Looking closely at this image above we see the jet stream in its glory. But there is a small section of the jet stream branching out, around the Arkansas/Oklahoma border. When air diverges like that, this means air is rising on such a scale that it disrupts the jet stream. This indicates extreme instability.

The main face of instability are CAPE values, showing instability. Values above 2000 mean the beginning of an unstable environment. 
As we can see, CAPE values are skyrocketing across the Plains into the Midwest. This will be around the time the image above shows the jet stream diverging.

All in all, the set-up is ripe for intense, damaging storms.

In this image, it shows initiation (beginning) of storms at 4:00 pm CDT on Sunday in Oklahoma into extreme southwest Missouri.
With such an environment as mentioned above, this initiation will fire up quickly into a squall line. It is crucial to identify when the beginning time of the storms are to determine how able the environment will be to support storms to what extent.

The threat will definitely be damaging winds, but a tornado threat will also be present.
This first image above is from the GFS. It is the Significant Tornado Parameter at 7:00pm CDT on Sunday. By then, we see the height of the tornado threat in Wisconsin. For other areas, the threat just isn't good enough to provide a concerning risk for tornadoes.
The above image is from the NAM of the same parameter as above. However, this being the height of the threat, it is 6 hours faster than the GFS and noticeably stronger. The main threat here seems more viable, as the agencies in the weather world have been more hyped than what the GFS shows.
Large hail will also be an issue, but damaging wind is hands-down winner in the way of primary threat.

A potentially large and intense outbreak is possible over the Midwest, Great Lakes and South Plains Sunday evening and overnight. We can expect the primary threat to be damaging winds, but Wisconsin will see a good tornado threat.

Another Write-Up to be issued this afternoon

Due to the circumstances, another write-up will be issued by The Weather Centre this after noon, Friday April 8, 2011.