Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 190424

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1124 PM CDT Sun Aug 18 2019

Issued at 853 PM CDT Sun Aug 18 2019

Latest guidance is showing a considerably weaker low level jet
overnight compared to the last several nights. With this lack of
focused forcing, I believe any widespread convection will struggle
to break through the cap in western MO, limiting both storm
chances and cloud coverage over the mid-Mississippi Valley. These
clearer skies coupled with light winds may result in some patchy
fog in the early morning hours. In fact, there are several
location over IL in which the dewpoint depression is already less
than 5 degrees, and it`s in areas toward IL in which I think fog
is most likely.



.SHORT TERM...  (Through Late Monday Night)
Issued at 325 PM CDT Sun Aug 18 2019

Focus through Mon night will be temps on Mon.

Ongoing convection is expected to continue to move east/southeast
out of the CWA over the next few hours. The threat for storms
should linger a bit longer across southern portions of the CWA,
but going forecast may hold on to PoPs too long. With lingering
outflow boundaries, have kept low PoPs over much of the area
tonight. With the remnant MCV pushing east this evening,
subsidence should prevent precip from forming for much of tonight.
However, as the nose of the LLJ edges into the area late tonight
into Mon morning, "sunrise surprise" storms will be possible and
have increased PoPs into Mon morning to account for these.

With the upper ridge building into the western portions of the
CWA, best chance for storms would be across eastern portions of
the CWA during the afternoon hours.

Focus then turns to temps and dewpoints. Models prog 20-22C 850mb
temps building into the region on Mon. This should allow temps to
reach lower 90s across the southern half of the CWA. Model solns
also suggest dewpoints reaching the middle to perhaps upper 70s
across much of the area. Believe these warm dewpoints are too
high, but lower to perhaps mid 70s are expected. This results in
heat index values around the century mark and perhaps around 105
degrees for portions of central MO. With questions regarding
convective debris and outflow boundaries and given the borderline
levels, have held off issuing a heat headline for now.


.LONG TERM...  (Tuesday through Next Sunday)
Issued at 325 PM CDT Sun Aug 18 2019

Upper ridge builds further west into the area on Tues with 850 mb
temps in the 22-26C range. This should result in the warmest day
of the week with temps in the lower to mid 90s across the region.
Would expect warmer dewpoints as well as a warm front lifts into
the region. The NAM would suggest a decaying MCS dropping into
northern portions of the CWA Tues morning. If this occurs,
redevelopment should occur during the afternoon hours along
whatever outflow boundaries exist. This results in a fair amount
of uncertainty in temps for Tues. Have therefore held off issuing
a headline for now. If confidence increases, would expect a
headline would be needed for at least portions of the CWA on Tues.
The best chances overall appear to be the southern half of the
CWA where less impacts from convection are expected.

Models still suggest a cold front pushing south of the region mid
week with cooler temps expected as a surface ridge builds into the



.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 1114 PM CDT Sun Aug 18 2019

Precip is looking rather unlikely through the overnight hours so
attention turns to fog potential. Given clearer skies than
previously expected, dewpoint depressions have dropped, currently
sitting between 0 and 10 degrees. With additional cooling
expected in the early morning, coupled with weak and variable
winds, there seems to be a reasonable chance for patchy fog across
the area. This is especially likely in the typically foggy areas
like river valleys and east of the Mississippi where current
dewpoints depressions are near 0. Guidance agrees with this
assessment, painting patchy fog across those areas. All that said,
I`m not confident the patchy fog will actually form over a
terminal, and I`m not sure how low the visbys will drop. For now,
have stuck close to guidance with just 6SM fog at all terminals
save COU through just after sunrise.

Beyond that, the primary focus will be for shower and storm
chances along the cold front expected to sag south through the day
tomorrow. Forcing along the front will be fairly weak, so I don`t
anticipate much more than initially isolated storms across our
north, turning scattered in our south in the afternoon. Generally
VFR conditions will prevail, though if a terminal is hit by one
of the storms, conditions will obviously drop.



Saint Louis     75  92  78  97 /   0  40  20  20
Quincy          70  87  74  92 /   0  20   5  30
Columbia        70  90  74  94 /  10  20   5  20
Jefferson City  70  92  74  96 /  20  20   5  20
Salem           69  89  73  94 /   0  30  20  20
Farmington      68  91  73  95 /   0  30  20  20




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