Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 202350

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
550 PM CST Wed Feb 20 2019

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 304 PM CST Wed Feb 20 2019

Area remains under SW flow aloft through Thurs with some weak
ridging aloft tomorrow. At the surface, a weak ridge will build into
the area tonight, with the axis lifting through the region Thurs.

Low clouds remain in place this afternoon, other than a few holes in
the clouds that has allowed a brief period of insolation. Expect low
clouds to clear out of the area early this evening. However, model
solns continue to suggest high clouds moving back into the region
this evening and becoming more widespread and thicker late tonight
into Thurs. Believe models also continue to hold on to too much snow
cover today, with this lingering into tonight. Have therefore
trended toward warmer guidance tonight.

As the surface ridge settles over the area tonight, expect winds to
become calm. With dewpoints above freezing, would expect any snow to
continue to melt this evening, adding to the low level moisture.
Currently, the best conditions appear to be across the southern half
or so of the CWA. However, if northern portions can remain free of
clouds longer, some fog may develop here as well. While not a great
setup, have mention of patchy fog for these areas late tonight into
Thurs morning.

For Thursday, believe many of the model 2m temps are again too cold
due to the snow cover mentioned above. MOS is also in good agreement
across the region for Thurs. Regardless, with the ridge axis lifting
north of the area, temps should rebound. Have therefore trended just
below the MOS guidance for highs on Thurs despite not being a great
warming setup.


.LONG TERM...  (Thursday Night through Next Wednesday)
Issued at 304 PM CST Wed Feb 20 2019

Upper level flow will remain out of the southwest from Thursday
night until a potent shortwave pushes through the region on
Saturday. The upper level flow will slide high pressure to our east
on Friday morning, though a lingering baroclinic zone across the mid-
South will keep the high north of the Ohio River Valley. This high
position will keep our surface winds out of the east through the day
on Friday, though winds are expected to quickly veer with height.
South to southwesterly low level flow will begin to advect warm,
moist air into the region on Friday. This lift and moisture will
result in mostly clouds skies across the region and gradually
increasing rain chances across southern MO and IL through the day.
Despite the WAA just above the surface, cloudy skies and easterly
surface winds will conspire to keep high temperatures at or just
below tomorrow`s expected highs.

The southerly low level flow will persist into Saturday ahead of the
previously mentioned shortwave. This impulse is expected to result
in rapid cyclogenesis across the Southern Plains before the low
pushes through NW MO and into the Great Lakes by Saturday evening.
With jet level winds approaching 140 kts and the low level jet
topping 50 kts by Saturday afternoon, this extremely dynamic system
has the potential to result in strong to severe thunderstorms across
the mid-Mississippi Valley.

However, all the moisture streaming in ahead of the low could keep
skies mostly cloudy through the day on Saturday, limiting
instability. Additionally, the previously mentioned baroclinic zone
to our SE will likely limit surface based instability in our region
and serve as a focus for the more significant, surface based storms.
All that said, the current consensus low track should allow at least
some elevated instability to bleed into the region, which coupled
with the impressive dynamics, may give us our first severe weather
event of the year. This thinking is backed up by the CIPS severe
probability guidance, which has the entire CWA within a 15% contour
and a 30% bullseye over southern IL.

Precipitation should quickly clear out of the area on Saturday
evening as a robust dry slot punches into the region, but the
potentially hazardous weather will not end there. The strong
gradient flow will result in very gusty winds beginning Saturday
evening before topping out near 35 kts early Sunday morning. Winds
will then gradually diminish through the day on Sunday, but remain
between 25 and 35 kts through noon. Those gusty post-frontal NW
winds will drop Sunday`s high 10-15 degrees compared to Saturday
despite mostly sunny skies.

Zonal upper level flow will then prevail through the first half of
next week with high pressure dominating the lower levels. Winds will
remain out of the north through Monday which will result in a very
cold start to the week with lows on Monday morning in the teens and
20s. By Monday evening, winds will swing back around to the south
and we`ll briefly warm up to near normal on Tuesday. By Wednesday, a
shortwave is progged to push through the Midwest, pushing another
cold front into the region. With that, we`ll see temps drop back
below normal and yet another chance for precipitation, some of which
may be frozen.



.AVIATION...  (For the 00z TAFs through 00z Thursday Evening)
Issued at 538 PM CST Wed Feb 20 2019

An area of stratocu with cig heights from 2500-3500 feet persists
from across much of western IL through central and eastern MO.
Both KUIN and KCOU are near the back edge and should clear out of
the stratocu by 02z, with high clouds increasing later this
evening and continuing into Thursday. The St. Louis area terminals
should see the stratocu through late evening with flight
conditions varying between high-end MVFR to low-end VFR, with high
clouds then the remainder of the night and into Thursday. There is
some question as to fog potential overnight into Thursday morning
and I have kept with a TEMPO group for 5SM BR give the uncertainty.


Stratocu will continue through late evening with flight conditions
varying between high-end MVFR to low-end VFR. High clouds are
expected after the stratocu exits with the high clouds continuing
on Thursday. I think the probability of fog overnight into
Thursday morning at KSTL is too low to mention at this time.





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