Monday, April 6, 2015

Thursday, April 9th Potentially Widespread Severe Weather Outlook

This is the latest outlook (as of Monday, April 6th) for Thursday's potentially widespread severe weather.

Storm Prediction Center
(click all images to enlarge)
The above image shows the Storm Prediction Center outlook for Thursday, April 6th.
The following areas have been outlooked for a 15% chance of severe weather within 25 miles of any given point:

- extreme southern Wisconsin
- extreme southwestern Michigan
- Indiana
- northern and southern Illinois
- extreme eastern Iowa
- eastern Missouri
- western Kentucky
- western Tennessee
- Arkansas
- extreme northwest Mississippi
- northeast Texas
- northern Louisiana
- southeast Oklahoma

The 15% chance of severe weather, put into words, generally suggests that there is a noticeable risk for severe weather in the areas mentioned. Residents in those areas should keep an eye on the forecast as the date approaches, and watch for inclement weather when the event arrives.

The SPC has outlined the following regions for a 30% chance of severe weather within 25 miles of any given point:

- Illinois
- eastern Missouri

The 30% chance of severe weather, put into words, indicates that there is a relatively good chance that severe weather will strike the areas mentioned above. In addition, there is a risk for more significant severe weather. Residents in those areas should continue to keep an eye on the forecast for this date, and closely monitor all watches and warnings when the event date arrives.

Weather Prediction Center
We now turn our attention to the forecasted surface map on the morning of Thursday, April 9th. At this point, we see a rather potent set-up over the nation. A strong low pressure system will be moving northeast into the Midwest, dragging the warm front northward as it does so. The trailing cold front will provide the focal point for thunderstorm formation inside the warm sector.

The NAM model is expecting that we will see over 3000 joules per kilogram of CAPE form by late afternoon over Iowa, Wisconsin, and Illinois. Convective Available Potential Energy, or CAPE, is the parameter that defines the amount of instability in the atmosphere. In short, higher instability means that the air is more buoyant- it is able to rise easier, and thus create thunderstorms. It is uncommon to see CAPE values in excess of 3000 j/kg, particularly this far north in the Midwest, so it is rather noteworthy.

The next thing we want to look at is the wind field in the lower levels of the atmosphere. Here, we have the NAM model's projected 700 millibar wind speeds on the evening of Thursday, April 9th. We can clearly see the storm in question over Iowa and Minnesota, but what's the most eye-catching is the jet streak rounding the base of the trough earlier on, now pushing out into the Chicagoland/Milwaukee metro areas. As this happens, it should increase the wind shear for when storms begin to enter those same areas a few hours later. Summed up, this increases the tornado threat for the Illinois/Wisconsin/Missouri areas, likely why the SPC has a 30% risk over those regions.

For the tornado threat...

The Weather Channel's Dr. Greg Forbes has put out a rather ominous forecast using his TORCON indicator, which predicts the chance of a tornado within 50 miles of any given point.

Severe thunderstorms in south WI, central and south MI, northwest
OH, IN, IL, east IA, MO (except northwest corner), southeast KS,
east and south-central OK, AR, west KY, west TN, northwest MS,
northwest LA, central and northeast TX. TORCON - 4 south WI,
south MI, north IL, north IN; 3 - MO, rest of IL and IN; 2 to 3
rest of area above
A TORCON of 4 has been assigned to southern Wisconsin, southern Michigan, northern Illinois, and northern Indiana. That means that there's roughly a four in ten chance that a tornado will touch down within 50 miles of any given point; that's a pretty high number.

All of the factors that support tornado formation appear to be in place. There will be buoyancy/CAPE (though the abundance of that instability is still in question), there will not be a temperature inversion aloft to prevent storm formation, the cloud bases should be low to the ground, and there will be abundant wind shear and helicity in the atmosphere. Plugging all of this into my own research (described in more depth in a previous post), the environment appears conducive for EF2 to EF3 tornadoes in that north IL/south WI corridor, if supercells can form and produce tornadoes. This is all subject to change, but there's certainly the risk of some tornadic activity for the aforementioned areas.

To summarize:

- There is an increasing threat of widespread severe weather on Thursday, April 9th.
- The most intense severe weather looks to focus on Illinois and Missouri.
- An appreciable tornado threat may exist over southern Wisconsin into northern Illinois.
- Crunching numbers indicates that IF tornadoes do form in the aforementioned areas, they may be of EF2 to EF3 strength.
- Uncertainty still exists with this event.


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