HIGH RISK ISSUED: "Numerous Strong Tornadoes LIKELY"

The Storm Prediction Center has issued a very rare HIGH RISK of severe storms and tornadoes for parts of the Middle MS River Valley for late today/tonight.

The High Risk Area (Hot Pink) includes communities like Paducah - KY, Jonesboro - AR and Memphis - TN. Further, a Moderate Risk (Red) has been posted for areas like Louisville - KY, Evansville - IN, Jackson - TN and Little Rock, AR. Outside of the Moderate, an Enhanced Risk (Orange) has been issued from much of the Midwest including Chicago and Cleveland, also south to St. Louis, Nashville, then southwest into parts of the Southern Plains including the Dallas Metroplex.

From the Storm Prediction Center (SPC): "A tornado outbreak is expected today and tonight from parts of the lower Mississippi Valley into the Mid-South and lower Ohio Valley.
Numerous tornadoes, along with multiple EF3+ tornadoes, appear likely. In addition, tornadoes, significant severe wind gusts, and large hail to very large hail will be 
possible across a broad area from north Texas northeastward to the southern Great Lakes".

The strong wording is due to the magnitude of this storm system which looks to bring a dangerous combination of ingredients together. These ingredients include an unseasaonably high amount of instability, as measured in CAPE (convective available potential energy) in the 2-4k range (see above left) and also a robust windfield including low level "helicity" (shown above right) with 0-3km values running in the 400-600 range across the High, Moderate and Enhanced Risk areas.

The combination of these two parameters (instability and low level helicity) help to produce the STP or Significant Tornado Parameter (above) which peak late this afternoon (above left) in the 8-10 range which it top of the scale. These values remain quite high throughout the evening hours as the storms progress east and south (above right).

The High-Res 3k NAM (below animation) shows the progression of storm activity for the remainder of the day and through the overnight.

A broken squall line is expected to gradually grow in intensity reaching St. Louis and Little Rock by late afternoon (3-4pm CT) and then to near Indianapolis, Paducah and Memphis by mid evening (7-9 pm CT) before reaching near Columbus and Louisville late evening and into the overnight (10pm - 2am ET).

Unfortunately, the risk for strong tornadoes looks to exist over a very wide area with SPC posting a risk for "significant tornadoes" which are tornadoes of EF3+ strength in the hatched red area below. This area include areas as far north as Ft. Wayne - IN and all the way south to Shreveport - LA. It also includes areas as far west as St. Louis - MO and as far east as Columbus - OH.

So what do I think?

I think we will have our hands full. The combination of ingredients coming together don't happen that often and are the strongest signals we've seen for tornado activity so far this season. Unfortunately, the storms capable of producing tornadoes will persist well into the overnight hours and could create a very dangerous "night time tornado environment". In addition, these storms will bring damaging to destructive straight line winds (in excess of 70 mph for many) and very large hail (baseball and softball size in some cases)!

Following the outbreak of severe weather tonight, flooding and flash flooding will likely become a major issue as we head into Thursday and potentially into the weekend as heavy storms look to continue to impact the same region for several days.