Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 211727

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1227 PM CDT THU JUL 21 2016

.SHORT TERM...  (Through Late This Afternoon)
Issued at 324 AM CDT Thu Jul 21 2016

This will be the hottest day of the heatwave so far as the RAP
and GFS are showing upper ridge building over the area today.
Forecast soundings are showing mixing up to 800mb with 850mb
temperatures around 22C by this afternoon.  This supports the
warmest MOS temperatures in the mid-upper 90s for highs with
dewpoints staying in the mid to upper 70s today.  This results
in the afternoon heat index values in the 105 to 110+ range.

Low level moisture convergence is farther east this morning over
central Illinois.  Latest runs of the HRRR are suggesting some
development of showers and thunderstorms over this region over the
next few hours.  Isolated thunderstorms will also be possible later
this morning and this afternoon across the eastern Ozarks into
southern Illinois where there will be some weak orographic lift in a
very unstable atmosphere with weak CINH.  The thunderstorm chances
will end by evening with the loss of daytime heating.


.LONG TERM...  (Tonight through Wednesday)
Issued at 324 AM CDT Thu Jul 21 2016

Friday and Saturday are shaping up to be even hotter than today.
850mb temperatures on all models are in the 22-24+ Celsius range,
which mixes down to 95-100+. Operational guidance continues to
print out areas of light QPF each afternoon. While I can`t
completely rule out precip, it seems very unlikely given the
strength of the upper level ridge overhead, and forecast soundings
showing a fairly substantial capping inversion. Lack of a synoptic
boundary to focus updrafts is another strike against widespread
convection. Will keep current slight chance PoPs Friday and
Saturday in the forecast for now.

Guidance is settling on a slower FROPA which keeps us in the heat
and humidity for Sunday.  The boundary looks like it might be into
northern Missouri by 00Z Monday and there may be some scattered
thunderstorms ahead of it Sunday afternoon.  However, dewpoints are
pooling ahead of the front and it looks like we`ll be locked in the
teeth of the low level thermal ridge with 850mb temps of 22+ across
the entire area.  I bumped temps and dewpoints up a bit over
previous forecast which puts heat index values up over 100 for the
entire area.  Will therefore extend the excessive heat
warning through Sunday afternoon until 01Z Monday.

Tonight`s model runs are pretty consistent with last night`s.  Cold
front moves south of I-70 by 12Z and hangs up over southern
Missouri.  Temperatures should fall off a few degrees and drier air
will filter in across the region north of the front.  Convective
debris from storms over southern Missouri will also help to suppress
temps a bit.  It will still be hot, but we should be out of heat
warning territory.  Medium range models seem to agree that the front
will stay put over southern Missouri Tuesday, but ECMWF and GFS
diverge on Wednesday.  GFS pushes the front back to the north
Tuesday night and Wednesday.  ECMWF is not as aggressive with return
flow during the same period and keeps the front draped across the



.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1222 PM CDT Thu Jul 21 2016

Diurnal cu has begun to flair-up across eastern MO and southwest
IL with continued heating and destabilization. There could be a
few isolated showers or thunderstorms develop within the region of
growing cu this afternoon, however warmer air aloft will likely
limit coverage. As such I think VFR flight conditions will
dominate the terminals through the forecast period.

Specifics for KSTL:

VFR flight conditions should prevail through the forecast period.
Isolated showers or thunderstorms are not out of the question this
afternoon but probabilities are low and best chances would be
south of the terminal.



MO...Excessive Heat Warning until 8 PM CDT Sunday for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
     Gasconade MO-Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln
     MO-Madison MO-Marion MO-Moniteau MO-Monroe MO-Montgomery MO-
     Osage MO-Pike MO-Ralls MO-Reynolds MO-Saint Charles MO-
     Saint Francois MO-Saint Louis City MO-Saint Louis MO-Sainte
     Genevieve MO-Shelby MO-Warren MO-Washington MO.

IL...Excessive Heat Warning until 8 PM CDT Sunday for Adams IL-Bond
     IL-Brown 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



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