Area Forecast Discussion
Issued by NWS Paducah, KY

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FXUS63 KPAH 211740
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
1240 PM CDT Wed Oct 21 2020

.UPDATE...
Issued at 1232 PM CDT Wed Oct 21 2020

For aviation section only.

&&

.SHORT TERM...(Today through Friday night)
Issued at 401 AM CDT Wed Oct 21 2020

With respect to the dense fog advisory for southeast Missouri,
some mixing appears to have occurred near the previous col along
the frontal boundary. There is still some fog evident in the
southeast MO valley areas, so may keep dense fog advisory in place
until close to expiration time, should conditions warrant.

With respect to the Tuesday evening and overnight convection
north of the WFO PAH forecast area, there will be some translation
of showers and a few thunderstorms further southeast through mid-
morning, until the upper level channeled vorticity and lift move
east of the area. May be too pessimistic with PoPs along the
northern and northwest borders of the WFO PAH CWA (mainly
southeast missouri and southern Illinois), but tried to find a
collaborative solution with neighboring NWS offices.

With the upper level forcing expected to shift the east today, it
may take decent insolation and some mechanical mixing to dislodge
cloud over southern Illinois and parts of the southeast Missouri
Foothills today. Given the cloud cover, suppressed temperatures
into the 60s north of a Mount Carmel Illinois to Perryville
Missouri line, with 70s expected south of this line.

With warm air advection and ridging at middle and upper levels by
this evening, the surface frontal boundary will move north of WFO
PAH forecast area this evening. Under the increased thicknesses
and with the ridge, temperatures will rise into the 80s Thursday
and Friday.

On Thursday evening, a minor shortwave along the leading edge of
the broad northern U.S trough will move across the area, sharpened
by the flow of colder air moving south from Canada. Enough
moisture and instability will be in place to generate scattered
showers and a few thunderstorms late Friday and Friday night.

The NAM-WRF, RAP, and ECMWF model family output was blended into
the forecast package, given the reasonable solutions for
convection and frontal passage Friday night.

.LONG TERM...(Saturday through Tuesday)
Issued at 401 AM CDT Wed Oct 21 2020

To start out the weekend, plenty of deep residual moisture will be
in place post frontal, meaning mostly cloudy skies for Saturday.
Even though there is not much in the way of colder air moving in
behind Friday`s weak FROPA, clouds will definitely help to keep
temperatures down into the upper 50s to lower 60s. The flow aloft
will be southwesterly and while there are no major disturbances
noted as of yet, we could see the threat for some minor
precipitation developing at some point, if any weak shortwaves push
through. No substantial signals now to latch onto, so Saturday is
rain free for now.

This deep moisture will remain in place Saturday night into Sunday
night, as a large upper trough develops across the western part of
the CONUS and southwesterly flow continues across our region. Mid to
upper level flow will eventually increase across the area, but the
main jet dynamics will be positioned just to our north. A surface
low develops in the lee of the Rockies on Saturday night and
migrates southeast into the southern Plains by 12Z Sunday.

During the day on Sunday, a boundary develops to our west and it
slowly shifts east with time. A series of waves will likely develop
along this boundary during this time frame, allowing for multiple
rounds of rainfall across our area. Therefore, rain chances will
definitely increase Sunday through Tuesday. Starting on Sunday, the
best chances will be in our northwestern half, closer to influences
of the upper jet and location of the boundary. As surface winds
become southerly on Sunday and low level flow strengthens, we should
see temperatures rebound well into the 70s for highs in most
locations, despite the mainly cloudy skies. Upper 60s will be more
likely in the northwestern sections however, due to more clouds and
possible rain.

Better, more widespread rain chances should arrive Sunday night
through Monday night, as the boundary slowly pushes through. This is
the time frame where we will likely experience multiple rounds of
precipitation. Still a lot of differences in QPF between the
operational GFS/ECMWF and their related ensembles. Way too many
idiosyncrasies to work out yet to be able to nail down where and
when the best rain chances will be this far out, so we keep PoPs
still in the chance category for now, but definitely foresee that
changing with time. The same goes for actual rainfall amounts,
although PWs have jumped a bit from yesterday, running 1.5 to 1.7 at
their highest Sunday night into Monday. Will continue to mention the
threat for heavy rainfall. Given instability parameters, thunder
seems to be a good bet. Not everything is lined up yet to support
any severe threat just yet, but it will be watched closely.

The main western trough will begin to undergo a split by on Tuesday,
to where a closed low develops somewhere over the four corners
region. As this occurs, we will continue to be in unsettled
southwesterly flow with enough moisture to warrant continued rain
chances. Much cooler air will filter in behind that front/boundary
that moves through on Monday, resulting in highs on Tuesday only in
the mid to upper 60s, maybe low 60s east.

&&

.AVIATION...
Issued at 1236 PM CDT Wed Oct 21 2020

Will be a fairly sharp cut off between IFR cigs along a frontal
boundary this afternoon up along the I-64 corridor in srn IL
(KMVN) and sw IN, to general sct VFR bases over much of the rest
of the forecast area. The sfc front is expected to lift northward
by this evening, allowing for VFR conditions to prevail area wide
for much of the night. Patchy late night IFR fog is possible in
climatologically favored locations.

&&

.PAH WATCHES/WARNINGS/ADVISORIES...
IL...None.
MO...None.
IN...None.
KY...None.
&&

$$

UPDATE...GM
SHORT TERM...Smith
LONG TERM...CW
AVIATION...GM


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