Wednesday, April 13, 2011

Spring weather pattern means more severe weather possible

April showers may bring May flowers, but April severe thunderstorms could bring May insurance claims!

Yet another potent weather system will move through the Mid-South Thursday night and Friday, following previous systems the past two Mondays, particularly on April 4 when 70,000+ MLGW customers lost power due to damaging straight line wind.  While we enjoy fantastic springtime weather during the middle of the week, the atmosphere is commencing a recharge phase with wind shifting back to the south today and humidity beginning to rise over the next 24 hours.  On Thursday, the change will be a little more palpable as highs climb back to 80 and south wind increases.

A strong low pressure system will be the root cause of possible severe weather on Friday as it moves slowly across the Plains into the middle Mississippi Valley during the day. I expect we cold see at least a couple of good chances at thunderstorms before Friday evening, the first associated with a warm frontal passage late Thursday night or early Friday morning and another later in the day with a cold front.  The timing of these systems (the cold front in particular) is still debatable as the model comparison graphics from this morning shown below demonstrate (click each for larger image).

The first, valid at 7am Friday, shows the NAM and GFS models in decent agreement with low pressure over eastern KS and widespread rain and thunderstorms (greens/blues) to the southeast of the low (triggered by the warm front and an upper-level disturbance).  The GFS is a little faster, showing the bulk of the precipitation centered on the MS River, while the NAM indicates it is mostly west of the river.

In the second set of images, shown below and valid at 7pm Friday, the disagreement is more pronounced.  The GFS continues to be the faster model with the front having crossed the river and the heaviest precipitation moving over the TN Valley, east of Memphis.  The NAM on the other hand, indicates that the Memphis area cold still be seeing strong thunderstorms at that time.

Both models (there are others, but these two are primary and shown for example purposes), indicate two widespread areas of thunderstorms on Friday, so I believe I'm safe with a high likelihood of thunderstorms. The details could greatly affect the severity of the episode(s) though (especially the latter event). The later in the day the storms arrive, the more likely they are to be able to tap into very warm air due to heating after the morning round of storms. This could increase the risk of the storms being severe.

Other environmental parameters (from both models) are indicative of a severe weather outbreak that could include the threat of tornadoes, large hail, damaging thunderstorm wind, and flash flooding.  The Storm Prediction Center's risk area for Friday is shown below, along with the probabilities of severe weather within 25 miles of a point.  The probabilities map indicates to me that SPC is leaning towards the GFS model solution at this time, and also that when the risk area is updated tomorrow, it could be upgraded to a Moderate Risk, especially for locations east of the Mississippi River and especially east of the Memphis metro.
Day 3 outlook area from SPC - Slight Risk of severe weather

Probability of severe weather within 25 miles of a point
I'll update the situation tomorrow here on the MWN Blog, then stay with us Friday on Facebook and Twitter as we nowcast the situation as it unfolds.  Now would be a good time to download the MWN App for iPhone or Android, as well as sign up for severe weather alerts for your county by clicking here: Severe Weather Notification

We'll chat again tomorrow afternoon.

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