Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 270420

1120 PM CDT Tue Aug 26 2014

.SHORT TERM:  (Through Late Tonight)
Issued at 324 PM CDT Tue Aug 26 2014

A fairly well-defined convergence zone extends along an axis from
Champaign through St. Louis towards Springfield this afternoon
associated with an old convective boundary. SBCAPE is over 4000
j/kg with little CIN along this corridor, and scattered storms
were noted along portions of the boundary. Short range HRRR and
RAP guidance suggests some of this activity may linger past sunset
and thus have included some low pops within the region. Otherwise the
other threat of showers and thunderstorms tonight will shift
northward along and north of the real east-west frontal boundary
which will be drapped across northern KS and northern MO into
central IL, mainly late tonight.


.LONG TERM:  (Wednesday through Next Tuesday)
Issued at 324 PM CDT Tue Aug 26 2014

The east-west frontal boundary will gradually sink southward
throughout the day Wednesday settling into southern MO and
southern IL by early evening. Very unstable conditions due to
daytime heating and high dew points will persist and maintain a
threat of scattered showers and thunderstorms will the front, and
just to its north. Present indications are that overall the
atmosphere will be slightly cooler in the low-mid levels. This
cooling along with the front and attendant clouds/precipitation
are expected to diminish the magnitude of the recent heat. However
despite slightly cooler temperatures, the pooling of surface dew
points in the 70s will continue to produce afternoon heat index
values of 100-105 degrees along and south of a Columbia to
Litchfield line. I have extended the heat advisories and warnings
in this region, while the excessive heat appears to be over across
northern MO and central IL where cooler conditions have also been
noted today.

The east-west front is expected to retreat back northward on
Thursday as an upper trof migrates out of the Rockies and the
associated surface low lifts northeastward. The highest pops will
remain along and north of the retreating boundary, primarily
northern sections of the CWA Thursday-Thursday night. Heat is a tough
call on Thursday and the current forecast calls for highs in the
upper 80s to lower 90s. There could potentially be some lingering
pockets of afternoon HI values of 100+ but there is too much
uncertainty to extend any heat headlines to this time period.

The heat should definitely come to an end by the weekend. The models
move the upper trof from the Plains on Friday into and through the
MS Valley by late Sunday. A good chance of showers and thunderstorms
will accompany this trof and the attendant frontal system, with
cooler temperatures closer to normal for this time of year.



.AVIATION:  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1054 PM CDT Tue Aug 26 2014

Cluster of storms northeast of the St Louis metro area will
likely slowly dissipate late tonight, although could not rule out
an isolated storm along the outflow boundary moving southwestward
through the St Louis metro. The surface wind will temporarily
switch to a northeast direction and pick up behind the outflow
boundary. A cold front was across northern MO, just south of UIN
this evening. This front will sag slowly southward late tonight
and Wednesday with scattered convection possible along and just
north of this front. Latest HRRR model run keeps UIN dry late
tonight, but there may be at least isolated showers/storms in the
UIN area late tonight into Wednesday morning. Due to uncertainty
in timing and coverage may keep the UIN taf dry for now. With
mostly light surface winds, mainly just high level clouds, and
high surface dew points there will likely be some fog late
tonight/early Wednesday morning mainly in SUS and CPS. The surface
wind will be mainly nely in UIN and COU Wednesday, and becoming
nely Wednesday afternoon in the St Louis metro area after fropa.

Specifics for KSTL: May need to include VCTS in the STL taf around
06-07z late tonight if storms develop along outflow boundary
dropping southwestward through STL. The wind will also briefly
switch to a nely direction and become gusty around 06z behind the
outflow boundary. Should have scattered diurnal cumulus clouds
late Wednesday morning and afternoon as the atmosphere becomes
very unstable along the weakening cold front which will sag
southward through central MO. Will likely be isolated to scattered
thunderstorms Wednesday afternoon, but may leave out of the STL
taf for now. The surface wind will become nely Wednesday
afternoon as the front sags south of STL.



MO...HEAT ADVISORY until 8 PM CDT Wednesday FOR Audrain MO-Boone MO-
     Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Madison MO-Moniteau MO-Montgomery MO-Osage MO-
     Reynolds MO-St. Francois MO-Ste. Genevieve MO-Warren MO-
     Washington MO.

     EXCESSIVE HEAT WARNING until 8 PM CDT Wednesday FOR Jefferson MO-
     Lincoln MO-St. Charles MO-St. Louis City MO-St. Louis MO.

     Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-
     Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL-
     Randolph IL-St. Clair IL-Washington IL.



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