FXUS63 KLSX 250826

Area Forecast Discussion
National Weather Service Saint Louis MO
326 AM CDT Sun Aug 25 2019

.SHORT TERM... (Through Late Tonight)
Issued at 300 AM CDT Sun Aug 25 2019

A deep upper level low is expected to push across the US/Canadian
border over the next few days, driving a noticeable change in the
sensible weather in the mid-Mississippi Valley. These changes are
underway this morning thanks to an embedded shortwave currently
over the central Plains downstream of the parent low. This
approaching shortwave has resulted in increasingly southerly flow
through all but the lowest levels of the atmosphere, ushering
relatively deep moisture into the region. This warm, moist flow in
the low levels has resulted in isolated to scattered showers
across the area this morning.

The low level return flow will persist through tonight in
response to the passage the shortwave this evening and tonight.
The combination of WAA and DCVA will support precip chances
through tonight. Primarily expecting a continuation of isolated
to scattered showers initially this morning owing to limited
instability. However, by mid to late morning, instability will
increase, supporting a chance for thunderstorms through
tonight. Winds in the lowest portions of the will remain generally
easterly, which coupled with fairly extensive cloud cover, will
keep temperatures relatively cool.


.LONG TERM... (Monday through Saturday)
Issued at 300 AM CDT Sun Aug 25 2019

On Monday, a surface cyclone will gradually move through the Upper
Great Lakes in response to the progression of the previously
mentioned upper level low. As the surface cyclone develops
eastward, it will drag a cold front through the Midwest, reaching
the bi-state area on Monday evening. Ahead of the front,
continued low level return flow will support a slight chance of
precip through the day before becoming likely as the front
traverses the region on Monday evening into Tuesday morning.

As the front approaches on Monday afternoon, mixed layer
instability is progged to reach nearly 2000 j/kg while deep layer
shear increases up to 40 kts. This combination of CAPE and shear
poses a slight risk for severe storms in central and NE MO on
Monday afternoon and evening. The threat for severe weather will
decrease Monday night as we lose instability, but shear will
remain sufficient to support a marginal risk for severe storms as
the front moves across the area through Tuesday morning.

Behind the cold front, cool and dry air will filter into the
region as a surface high will builds in. This will keep a lid on
precip chances through mid week. The high will then build east of
the Mississippi on Friday, turning low level flow more southerly,
allowing warmer more moist air to build into the region. Forecast
guidance showing some consensus with another cold front moving
through the area on Friday night or Saturday, though
unsurprisingly, differences remain on frontal timing. Given this
uncertainty, just have low end pops going for the time being.



.AVIATION... (For the 06z TAFs through 06z Sunday Night)
Issued at 1028 PM CDT Sat Aug 24 2019

Overall, only minor changes to the previous forecast. With
increasing low level moisture convergence, isod to sct SHRA are
should develop over the next few hours. These should remain east
and southeast of KUIN/KCOU. Chances for KCOU increase late in the
period, but have focused on near term for now.

SPECIFICS FOR KSTL/KSUS/KCPS: Have added light showers early in
the TAF period, expecting isod to sct SHRA to develop in and
around the terminals. These should move NE somewhat quickly.
Another round of SHRA is expected Sun afternoon, but with better
coverage still expected to be south of terminals, have kept as VC
mention for now. MVFR cigs are expected to move into the region
late Sun afternoon and persist through the period, likely lowering
to low MVFR late in the KSTL TAF period.





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