Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 241017

Area Forecast Discussion
National Weather Service Saint Louis MO
517 AM CDT SUN JUL 24 2016

.SHORT TERM... (Through Monday Night)
Issued at 414 AM CDT Sun Jul 24 2016

Today should be the last day of excessively hot and humid
conditions across the CWA. A cold front will approach from the
north and move into the area overnight and tomorrow, bringing a
cooler air mass and the chance for widespread precipitation.

The primary forecast issue for today is determining daytime high
temperatures. There are several factors to consider, including:

0. Warm start. Temperatures were in the mid to upper 70s (low 80s
in the St. Louis metro area) across most of the CWA at 09z. It
won`t take much insolation to reach the 90s.

1. Moist air mass. Dew points have been in the 70s for the last
few days. It is very unusual in this area to reach 100 deg F when
the sfc dew points are in the mid to upper 70s, as is expected
to continue today.

2. Wind direction. The topography of the region is such that a
southwesterly to westerly wind produces slight compressional
warming in our area, but a southerly wind does not. It will be
difficult to reach 100 deg today if the wind remains nearly
southerly and does not have a significant westerly component.
Models are roughly evenly split on the wind direction forecast
for the St. Louis area; some show the winds remaining southerly
through the day and others show southwesterly winds during the
afternoon. Models are in better agreement that southwesterly winds
will be present over parts of central MO during the afternoon

3. Daytime mixing. If dew points near the surface mix out during
the afternoon, then air temperatures in the near-sfc layer of the
atmosphere can become warmer than would otherwise be expected
based on sfc Td considerations alone.

4. Cloud cover and pcpn. Pcpn chcs will increase with the
approach of a cold front from the north. Pcpn might affect high
temperatures today across central and northeast MO as well as
west central IL. Mid and high clouds will also encounter less
subsidence now that the upper ridge has started to break down, and
the presence of moderate filtering from mid/high clouds would
reduce sfc heating.

Although 100 deg F is still within reach today, it no longer
appears to be the most likely outcome, therefore high
temperatures were lowered by 1-2 degrees across parts of the
area. If there were no clouds upstream, no pcpn expected today,
as stout of an upper ridge as was seen a few days ago, and higher
confidence in a sustained westerly component to the winds, then I
would have higher confidence in reaching the century mark across
central and eastern MO. Given the approaching front and the
increasing pcpn chcs tdy, however, this fcst is more likely to
bust by being several degrees too high than by being a degree or
two too cool.

There is also a possibility of a few strong thunderstorms this
aftn and evening. SHRA/TSRA are expected near and ahead of the
front when it moves into an unstable air mass. LLJ interaction
with the bdry will also support a thunderstorm complex tonight.
SHRA/TSRA will on Monday with the cold front in the region, then
taper off on Monday night as the front sinks farther southward.


.LONG TERM... (Tuesday through Saturday)
Issued at 414 AM CDT Sun Jul 24 2016

The sfc high behind the cold front is slightly stronger than
previously fcst, which gives greater confidence that the front
will make more southward progress, likely to at least I-70 and
possibly even farther south. The front is still expected to stall
across the region and eventually lift northward, serving as the
focus for SHRA/TSRA dvlpmt through the end of the week when
several disturbances move across the central CONUS within weakly
NW flow aloft. High temperatures during this time will be in the
mid/upper 80s to near 90 degrees...a welcome relief compared to
the recent heat wave.



.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1142 PM CDT Sat Jul 23 2016

Strong capping across the mid-Missouri Valley should prevent
thunderstorms from initiating. expecting a dry
forecast for the terminals for the rest of tonight and into at
least early on Sunday. Scattered thunderstorms are expected to
develop along the cold front by afternoon and slowly sag
southward. Have added a VCTS group at each terminal to account for
this. In addition...could also see some isolated diurnal type
thunderstorms within the warm sector on Sunday afternoon/very
early evening...particularly across the eastern Ozarks. This
activity will likely stay south of the terminals however. Front
expected to be through KUIN by late Sunday evening and KCOU and
the metro terminals likely after this valid TAF package. Winds
behind the front will veer from the south to the northwest.

Specifics for KSTL:

Dry and VFR for rest of tonight and early Sunday. Cannot totally
rule out an isolated storm affecting terminal on Sunday afternoon
but probability too low to mention in TAF. Did add a VCTS group in
for expectation of scattered showers/storms along the approaching
cold front for Sunday evening. Frontal passage looks to be late
Sunday night toward the end of the forecast period with winds
switching from a southerly direction to the northwest behind the



MO...Excessive Heat Warning until 8 PM CDT this evening 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 this evening 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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