© 2019 Terry Swails

SEVERE WEATHER THREAT WEDNESDAY?

May 8, 2019

Wednesday has the potential to be the first severe weather day of the 2019 season for parts of the central Midwest. The culprit is a strong spring storm that will gradually push northeast during the day. Tuesday's models continue to track the system further NW which puts about the southern half of my region in a potentially vulnerable location.

Wednesday starts with elevated showers and storms lifting north during the morning. Some heavy downpours are likely but nothing severe is anticipated with this wing of precipitation as it lifts north during the morning. The scope and speed of movement of this rain band will determine how far north a warm front will advance during the day. 

 

Simulated radar at 7:00am

After that wave of rain departs, a warm front will push northward into southern Iowa as low pressure heads west of Des Moines. Much of the area near and south of HWY 30 is shown getting into the warm sector with temperatures here reaching 60 to 70 degrees with dew points into the 60s. If that amount of heating is realized bulk shear is projected to exceed 50 knots with CAPE up to 1,000 j/kg. Any storms that can get rooted to the surface near the warm front and triple point would have a tornado potential, especially those that remain discrete. LCL's (cloud bases would also be low which would contribute to the tornado threat). The Storm Prediction Center did push its slight risk northward Tuesday to account for the latest trends.

 

SPC in its overnight discussion amended its outlook saying: As a convectively enhanced circulation shifts northeast across the mid Missouri Valley today, a weak surface low is expected to do likewise and is progged to reach Iowa during the afternoon. With favorably strong deep-layer flow, and some possible backing of the low-level winds east of the low, the kinematic environment would appear favorable for severe potential. That said, the warm sector thermodynamics remain a question, with an evolving/well defined MCS currently over Kansas/Oklahoma likely to negatively influence potential for heating/destabilization across a large portion of the region. At this time, will maintain the broad slight risk area across the region, though acknowledging the uncertainty. Adjustments to later outlooks can be expected, once the ongoing scenario further evolves, and hopefully helps to clarify the degree of risk across the area. 

 

Simulated radar at 7:00pm

I did look at the CIPS analog forecast which looks at similar events from the past to assist with conditional forecasts for the current set-up. The top 15 analogs show a 55-65% chance of at least one severe weather event over SE Iowa or WC Illinois.

The chances of at least 1 tornado within 124 km of a point came me in at 23% near the Quad Cities. Not great but far from negligable.

Moisture is going to be plentiful and heavy rainfall rates are anticipated where thunderstorms develop. Amounts of an inch or more per hour are possible which could produce the threat of some minor flash flooding considering the saturated soils that exist. Depending on how much rain ends up falling and where, some rises are also possible on rivers and streams. These are wild cards that need to be watched.

 

To the point, this is a very low confidence forecast that is highly dependent on mesoscale features that are impossible to see at the time of this post. We'll know much more about the threat (or lack of one) by late morning. It is worth keeping an eye on, especially from I-80 south. Roll weather...TS

 

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