© 2019 Terry Swails

FALL STORM RAISES SEVERE WEATHER THREAT SATURDAY...

October 13, 2017

Heading into the weekend the weather focus turns to a significant storm that promises to bring heavy rain and severe thunderstorms to parts of the central Midwest. While it's still too early to define the extent of the severe weather, most models are indicating sufficient shear and instability for surface based supercells. Here's the current risk area for Saturday as laid out by the Storm Prediction Center.

The severe threat will exist ahead of a surface low that's projected to track from Kansas Saturday morning to northeast Iowa Saturday evening. Here's the GFS depiction of the 2 time periods.

Saturday morning:

Saturday evening:

Near and just east of the track of the surface low is where the strongest storms are anticipated...roughly where SPC has its outlook. The NWS in the Quad Cities issued this story board late Thursday showing the potential impacts. You will notice there is the mention of possible tornadoes.

Some of the key ingredients regarding the ultimate strength of storms will be instability tied to heating and moisture. Here's what the GFS shows for highs.

Dew points are quite high for mid October pushing 70 in spots.

The warmth and moisture will generate CAPE (convective available energy) Saturday that should reach 1-2,000 j/kg. That's not extreme but combined with the bulk shear that some models show up to 60 kts could prime the pump for strong storms in the afternoon and early evening in my local area.

One thing that is very impressive is the high moisture levels. PWAT's (precipitable water vapor) are off the charts (over 2" near Cedar Rapids). 

 To put that into perspective 2" PWAT's represent moisture levels more that 300% greater than normal.

 

 

That's 4.1 standard deviations above normal.

With such deep moisture and potent dynamics the potential is there for heavy rains to once again fall. The GFS has a band of 1-3" totals near the track of the surface low and triple point.

 

By Sunday the storm is gone but a quick pop of cold air will drop temperatures substantially. Here's Sunday's highs which will be accompanied by strong NW winds.

Needless to say, there's some weather brewing as another fall storm stomps across the central Midwest. Roll weather...TS  

 

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