Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 270810
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
310 AM CDT Tue Jul 27 2021

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 307 AM CDT Tue Jul 27 2021

Surface high pressure has continued to expand and shift east of
the region under the influence of a broad upper ridge dominating
the western CONUS. The ridge will continue to build eastward
through at least midweek, keeping precipitation chances near zero
while the surface high drifts into the lower Great Lakes. With the
high pushing further to our east, we`ll see weak low level winds
shift to the ESE today and the south on Wednesday. This will allow
heat and humidity to build back into the region, potentially
reaching dangerous levels by Wednesday. That said, it does look
like afternoon dewpoints may mix out, especially in the Ozarks,
which would limit maximum heat index readings. Nonetheless, the
current forecast does have the heat index right around 105 for a
large portion of the forecast area so a heat advisory has been
issued for those areas.

BSH

.LONG TERM...  (Wednesday Night through Monday)
Issued at 307 AM CDT Tue Jul 27 2021

The upper ridge is expected to remain in place through the day on
Thursday before it begins to break down in response to a shortwave
dropping through the NW flow its northern flank. This will cause the
surface high to shift even further east before a cold front moves
into the area. It still appears as though the front will not arrive
until after 00z on Friday, so dangerous heat is again expected on
Thursday. In fact, another day of weak return flow will push
temperatures a bit higher than what we`re expecting on Wednesday
while dewpoints pool on the warm side of the approaching front. This
should result in even higher heat index readings than what we`re
expecting on Wednesday, with much of the area pushing into the 105-
110 range. While this is the most likely scenario, there is still a
chance that the front moves into our area earlier than expected.
Should that occur, the maximum heat index would be a bit less than
expected, especially in our northern counties. Nevertheless, it
appears quite likely that a heat advisory will be needed on
Thursday. However, given the above uncertainty, and to allow us to
highlight the hottest areas for Wednesday, we`ve chosen to forego
the heat advisory on Thursday for the time being.

Guidance is in good agreement that the pattern will remain bit more
active after Thursday thanks to the above front stalling in the
vicinity of the Missouri River Valley a deepening upper trough over
northeastern North America. A series of embedded shortwaves are
expected to reinforce the front, resulting in at least a few rounds
of precipitation somewhere in the region. The primary source of
uncertainty at this time is where exactly the front will set up
and when the shortwaves will move through the region. Given this
uncertainty, we`ve got a long period of 10-50% PoPs to end the
forecast. The actual period of showers and storms will be quite a
bit more limited and there should be plenty of dry time in between
rounds of precipitation. Temperatures should also be notably
cooler during this period.

BSH

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1026 PM CDT Mon Jul 26 2021

No major updates. VFR flight conditions will persist with light
and variable winds. Even though patchy river fog is expected to
develop once again, confidence is low that it will affect KJEF,
KSUS or KCPS, so left out mention at this time.

Byrd

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory from noon to 10 PM CDT Wednesday for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Franklin MO-Gasconade MO-
     Jefferson MO-Knox MO-Lewis MO-Lincoln MO-Marion MO-Moniteau
     MO-Monroe MO-Montgomery MO-Osage MO-Pike MO-Ralls MO-Saint
     Charles MO-Saint Louis City MO-Saint Louis MO-Sainte
     Genevieve MO-Shelby MO-Warren MO.

IL...Heat Advisory from noon to 10 PM CDT Wednesday for Adams IL-
     Brown IL-Calhoun IL-Clinton IL-Greene IL-Jersey IL-Madison
     IL-Monroe IL-Pike IL-Randolph IL-Saint Clair-Washington IL.

&&

$$

WFO LSX


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