Area Forecast Discussion
Issued by NWS St. Louis, MO

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043
FXUS63 KLSX 210001
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
701 PM CDT Sat Jul 20 2019

.SHORT TERM...  (Through Late Sunday Night)
Issued at 328 PM CDT Sat Jul 20 2019

A cold front is forecast to slowly move southward to just south of
the Missouri-Iowa border toward dawn Sunday morning. An area of
showers and thunderstorms is forecast tonight along and behind the
front across Iowa and northern Illinois. CAM guidance suggests this
complex of storms will move southward into northern sections of the
forecast area between 0600 and 1200 UTC. Storms are then expected to
weaken and may eventually dissipate completely toward midday as low-
level moisture convergence wanes and shortwave ridging briefly sets
in behind a departing trough at midlevels of the atmosphere. An
outflow boundary should continue a bit southward, and this boundary
or a differential heating zone, is expected to help storms refire by
mid afternoon. Exactly where this will take place is a bit
uncertain, but best guess at this time is in the southeastern half
of the CWA. The trailing cold front should also see an uptick in
convection by early evening, at first likely in western Missouri.
This area of storms should move east/southeast across much of the
area on Sunday night. Thunderstorms across the area during the
afternoon and evening hours that are able to amalgamate a cold pool
will be capable of producing damaging wind gusts in a high CAPE/low
shear environment characterized by steep low-level lapse rates and
high DCAPE values.

Temperatures are the other main concern on Sunday with ongoing heat
headlines continuing for parts of the area. The evolution of the
complex of storms late tonight into Sunday morning is of course
paramount, and there is a possibility that parts of the excessive
heat warning may need to be cancelled early if storms last a bit
longer than expected or debris clouds thin out slowly. The outflow
boundary that is expected to push further to the south is also a
complicating factor because this will switch the surface winds from
the south to the northwest or even north and at least temporarily
slow the warmup on Sunday. Given everything outlined above, will
make no new changes to the heat headlines. High temperatures within
the warning are forecast to range from the low to mid 90s, with peak
heat index values in the between 100 and 105 degrees.


Gosselin

.LONG TERM...  (Monday through Next Saturday)
Issued at 328 PM CDT Sat Jul 20 2019

The cold front early Monday morning should be near the southeastern
CWA border, with post-frontal rain and embedded thunderstorms
affecting mainly southeast Missouri and southwest Illinois. This
area of rainfall should exit the area toward midday. Behind the
front, strong low-level cold air advection combined with thick cloud
cover will yield much cooler conditions to start off the work week.
In fact, temperatures may fall for portions of the area through
midday before rebounding back into the mid 70s to near 80 degrees.
This certainly will be a major change compared to today, with it
feeling some 30-35 degrees cooler after factoring in the
humidity across much of the region.

Below normal temperatures and humidity levels are forecast across
the area Monday afternoon through Thursday as a strong area of
Canadian high pressure slowly moves across the Mississippi Valley.
Some moderation in temperatures is likely heading toward the end of
the work week as mid/upper level heights rise and 850-hPa
temperatures climb back above +15C.


Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 630 PM CDT Sat Jul 20 2019

Cold front extending from southern WI southwest through southern
IA to southeast CO will move slowly southeastward with convection
along it which will move southeastward into UIN later tonight.
This convection will likely weaken late tonight. Convection will
redevelop along and just ahead of the cold front and along outflow
boundaries from tonight`s convection which will likely impact COU
and the St Louis metro area Sunday afternoon and evening. MVFR
clouds are likely in UIN on Sunday. South-southwesterly surface
winds tonight will become northwesterly after passage of the cold
front and outflow boundary from MCS currently over south central
IA.

SPECIFICS FOR KSTL: Cold front extending from southern WI southwest
through southern IA to southeast CO will move slowly southeastward
with convection along it which will move southeastward into UIN
later tonight. This convection will likely weaken late tonight.
Convection will redevelop along and just ahead of the cold front
and along outflow boundaries from tonight`s convection which will
likely impact STL Sunday afternoon and evening. South-
southwesterly surface winds tonight will become northwesterly
after passage of the cold front and outflow boundary from MCS
currently over south central IA.

GKS

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Excessive Heat Warning until 8 PM CDT Sunday for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
     Gasconade MO-Jefferson MO-Lincoln MO-Moniteau MO-Montgomery
     MO-Osage MO-Pike MO-Saint Charles MO-Saint Francois MO-
     Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO-
     Warren MO-Washington MO.

     Heat Advisory until 8 PM CDT Sunday for Iron MO-Madison MO-
     Reynolds MO.

     Excessive Heat Warning until 8 PM CDT this evening for Knox MO-
     Lewis MO-Marion MO-Monroe MO-Ralls MO-Shelby MO.

IL...Excessive Heat Warning until 8 PM CDT Sunday for Bond IL-Calhoun
     IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-
     Madison IL-Marion IL-Monroe IL-Montgomery IL-Pike IL-
     Randolph IL-Saint Clair IL-Washington IL.

     Excessive Heat Warning until 8 PM CDT this evening for Adams IL-
     Brown IL.

&&

$$

WFO LSX



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