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Area Forecast Discussion (AFD)

Issued by St. Louis, MO (LSX)

FXUS63 KLSX 230908

Area Forecast Discussion
National Weather Service Saint Louis MO
408 AM CDT Mon Sep 23 2019

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 317 AM CDT Mon Sep 23 2019

A cold front continues to progress to the southeast this morning
with a narrow band of broken showers running along the front. This
activity parallels the southeastern CWA and the front will effectively
clear the area within a couple hours after this update.

Closely behind the front, there are two minor issues to address.
First is the low and mid-level cloud cover on IR imagery, which 
generally resides between 1-2k feet and ranges from scattered to 
broken in nature. This cloud deck quickly erodes to just a few low
level clouds and clears out further behind the front. This brings
us to the second item, which is patchy fog potential in areas that
clouds have cleared, mainly over areas of northern Missouri. This
is where recent rainfall has been more abundant and winds have
lightened just enough to result in visibility reductions. Much of
this will be short-lived up through sunrise and elsewhere there 
should be enough mixing to help mitigate fog development.

The rest of the period looks tranquil. Surface high pressure
sets up overhead with drier air resulting in ample sunshine today.
The high begins to move into the Ohio Valley Tuesday, turning
surface winds more southerly on Tuesday. Despite a few high
clouds, it should be another nice day. Both will feature high
temperatures that are near normal (upper 70s/lower 80s).


.LONG TERM...  (Tuesday Night through Sunday)
Issued at 317 AM CDT Mon Sep 23 2019

The main concerns in the long term period will be the potential
for showers and thunderstorms along a cold front late Tuesday
night and Wednesday. Another front drops south into the region
again late Thursday into Friday, but stalls with decreasing
confidence in any clear solution as we head into the weekend. 

Surface high pressure will continue to depart the area Tuesday
night as a broad upper trough over the Canadian provinces places
us under west-to-east upper flow. A cold front will extend
southwest through the plains as the trough continue eastward over
the northern Great Lakes. This will introduce the next chance for
showers and thunderstorms late Tuesday night, more likely by early
Wednesday morning as the actually front approaches the forecast
area. As of now, I'm not too concerned for severe potential with
much of the supper remain well north over Iowa, Minnesota and
Wisconsin. While convection may get started Tuesday evening to our
northwest, current thinking is that instability wanes with
eastward travel and relies mostly on frontal convergence and 30
knots of shear to maintain storms. 

Frontal timing could be key in introducing additional
thunderstorms to the forecast Wednesday. There are some hints of a
midlevel feature running just ahead of the front through southern
Missouri, which could ignite additional storms Wednesday
afternoon. However, better instability remains just southwest of 
the CWA as the front clears the area. This will be something to
keep an eye on as far as coverage and intensity during the day

Later in the period, another front attempts to drop down from the
north as an upper ridge builds over the easter CONUS. The strength
of this ridge has varied between the CMC/GFS/ECM, along with the
location of the stalling boundary. The strength of this ridge and
placement of the boundary will be key in determining exactly where
the active pattern sets up, but in general, Friday and Saturday
seem to be the better bet for any wet weather. By Sunday,
southwest flow takes over with the ridge building west into early
next week. Much warmer weather is likely to return with high
climbing near 90 or warmer by Monday.



.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 1107 PM CDT Sun Sep 22 2019

The front has pushed into the area, now with little more than
isolated to scattered showers along and in front of the front.
Precip will remain possible at each terminal until the front
pushes to the east, so will keep VCTS for the first few hours of
each TAF. 

As the front arrives, cigs are likely to drop into MVFR and remain
there until clouds associated with the front scatter out a few 
hours later. A handful of upstream obs are even briefly dipping to
IFR before clearing out, though model guidance does not show this
well. Therefore, I will be putting an hour of IFR conditions at 
COU and UIN, with just SCT009 in the STL metro TAFs where 
confidence in IFR cigs is lower.