Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 211742
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1242 PM CDT Thu Aug 21 2014

.SHORT TERM:  (Through Tonight)
Issued at 200 AM CDT Thu Aug 21 2014

Focus continues to be precip chances influencing temps thru the
period.

Mdls are in good agreement regarding synoptic features and mass
fields. However, subtle differences in thermal profiles and
mesoscale features will be important over the next few days. Main
question is centered around ongoing convection over NEB and nrn
KS. Believe the GFS handles this s/w the best tracking it thru
cntl IA today along the sfc boundary. Any subsequent outflow
boundaries will determine how far S convection will propagate this
afternoon and tonight. Believe the area will remain dry this
afternoon due to timing of the s/w as well as a lack of stronger
forcing. A broad LLJ tonight shud be enuf forcing to support TSRA
that may impact nrn portions of the CWA. The most likely timing
will be late tonight at will at least in part depend upon how
convection over IA region plays out today.

As for temps, have trended twd the cooler guidance which has
verified well even in places that did not see TSRA over the past
couple of days. Mdls are slightly cooler with 850mb temps over the
region. For nrn counties, forecast soundings suggest those regions
will not mix to 850mb anyway. Prelim forecast of 90 degrees at UIN
may be too warm due to convective debris. Further S, warmer temps
will be offset by better mixing and therefore, cooler dewpoint
temps.

Overall, the resulting heat index values thru this afternoon are
similar to what was forecast yesterday. Have therefore not made
any changes to the going headlines.

.LONG TERM:  (Friday through Wednesday)
Issued at 309 AM CDT Thu Aug 21 2014

Focus continues to be temps thru the period with ongoing heat
headlines.

Overall, not many changes to the prev forecast. The GEM has come
into better agreement with other mdl solns, but does still break
down the upper ridge quicker. Questions regarding the need to
expand the current advisory remain the same as they did yesterday.
Mdls depict a weaker cap across much of the region, but especially
across nrn portions of the CWA. This area is also closer to the
sfc boundary and may therefore experience more convective debris
into the weekend, if not storms. Have therefore kept low PoPs
going for this area and trended temps cooler. Elsewhere, continued
a trend twd the cooler MAV guidance as mentioned above.

Mdl solns begin to diverge early next week with the ECMWF now
reinforcing the upper ridge, keeping it in place thru the middle
of the week. However, the GFS/GEM suggest a weaker upper ridge
across the cntl U.S. Regardless, all mdl guidance suggests cooler
mid level temps which may allow for afternoon/nocturnal convection
across the region. Have kept PoPs on the low end until mdls come
into better agreement and confidence builds.

Due to questions mentioned, have not made any changes to the
ending of the going headlines. However, headlines may need to be
extended if the upper ridge holds into next week.

&&

.AVIATION:  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1225 PM CDT Thu Aug 21 2014

A very warm, moist and unstable air mass currently resides across
a good portion of Missouri into Illinois. Present indications are
isolated thunderstorms could develop during the afternoon with
the most likely location across southern Missouri into southern
Illinois. This would be south of the terminals, and also given
the anticipated low coverage and random nature, I have no mention
in the TAFS. The only other precipitation threat would be at KUIN
late tonight if activity across Iowa drops south, but again the
probability is too low to mention at this point. Otherwise VFR
conditions will prevail through the forecast period.

Specifics for KSTL:

VFR conditions are expected through the forecast period. A very
warm, moist and unstable air mass currently resides across the
region. Present indications are isolated thunderstorms could
develop during the afternoon south of KSTL.

Glass

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...HEAT ADVISORY until 7 PM CDT Sunday FOR Audrain MO-Boone MO-
     Callaway MO-Cole MO-Franklin MO-Gasconade MO-Jefferson MO-
     Lincoln MO-Madison MO-Moniteau MO-Montgomery MO-Osage MO-
     St. Charles MO-St. Francois MO-Ste. Genevieve MO-Warren MO.

     EXCESSIVE HEAT WARNING until 7 PM CDT Sunday FOR St. Louis City
     MO-St. Louis MO.

IL...HEAT ADVISORY until 7 PM CDT Sunday FOR Bond IL-Calhoun IL-
     Clinton IL-Greene IL-Jersey IL-Macoupin IL-Marion IL-Monroe
     IL-Randolph IL-Washington IL.

     EXCESSIVE HEAT WARNING until 7 PM CDT Sunday FOR Madison IL-St.
     Clair IL.

&&

$$

WFO LSX






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