© 2019 Terry Swails

COMPLEX SEVERE WEATHER SET-UP...

April 19, 2017

 

For the last few days models have been promoting a severe weather set-up over the central Midwest Wednesday afternoon or evening. At this time the Storm Prediction Center has the enhanced risk area centered on the eastern half of Iowa

The forecast will be complicated by ongoing showers and storms Wednesday morning focused along a nearly stationary front near the Iowa Missouri border. The extent of the convection and outflow boundaries will determine where a warm front lays out later in the day. There is some disagreement in model placement but generally the warm front towards evening will be running close to I-80. It will extend west to a surface low in SW Iowa.

The position of the warm front is critical because it will determine where the best instability for strong storms resides. Just look at the sharp temperature contrast shown in the forecast highs across Iowa. Upper 40's northwest and low 80s southeast.

That sort of set up yields CAPE (convective available potential energy) that looks like this. The highest instability is established near the surface low eastward along the elongated warm-front.

What I will be watching Wednesday is the amount of daytime heating that's realized. If the sun can get out in and near the moistening warm sector it will focus low level ascent near the low and its associated boundaries. With CAPE up to 1500 j/kg and significant shear, organized rotating updrafts are likely where storms develop. They would be able to produce large hail and damaging winds. Near the warm front and low pressure center, backed winds could also produce a tornado threat. That will be better defined later in the morning but as of now the warm front has the best chance of seeing a few tornadoes, especially if discrete surface based cells can anchor or cross over the boundary.

 

The graphic below shows where the best chance of surface based supercells is projected as of Tuesday evening.

Any shift north or south on the system will alter the risk area. I doubt there will be much of a change in Wednesday's data but if there is a shift it will likely be more south than north due to potential outflow boundaries.

 

The GFS really has storms firing between 6 and 10pm over much of my local area just ahead of the triple point.

The EURO is also bullish on the same general area.

Behind the storm winds kick up and temperatures turn much cooler Friday. In fact, readings will hold the 50s in many areas. A change I called for a week ago with the sharply falling and negative SOI I was anticipating. The rest of the weekend looks cool but dry.

In summation, Wednesday will be an interesting day. Severe weather set-ups are always challenging as so many variables come into play. Leave one ingredient out of the recipe and what looks bad on paper ends up timid in reality. That said, I think there will be some tough storms in some part of central or eastern Iowa later Wednesday. Roll weather...TS 

 

 

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