Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 212017
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
317 PM CDT Thu Aug 21 2014

.SHORT TERM:  (Through Late Tonight)
Issued at 307 PM CDT Thu Aug 21 2014

Isolated thunderstorms are possible late this afternoon and early
tonight over the eastern Ozarks.

Kanofsky

.LONG TERM:  (Friday through Next Thursday)
Issued at 307 PM CDT Thu Aug 21 2014

Overall thinking remains similar to previous forecasts. An upper
level ridge is expected to remain in place through early next
week, bringing dangerous levels of heat and humidity to the
region. H85 temperatures of 20-24 deg C and H9 temperatures of
24-28 deg C support daytime highs in the mid to upper 90s through
early next week. Dew points are forecast to be in the 60s and 70s.
Heat index values will likely exceed 100 deg F each afternoon
across most of the LSX CWA, and some locations will likely reach
heat index values of 105-107 F. There remains a question about
whether dew points will mix out over the Ozarks on one or more
days during this stretch of hot weather, resulting in lower heat
index values. Since the Heat Advisory was issued primarily for
duration (4 days of heat index values exceeding 100 deg F), we
will wait to see how afternoon dew points respond and will hold
off on expanding the Heat Advisory to the southwest. The Heat
Advisory will be expanded eastward to include two more Illinois
counties after collaboration with neighboring offices. No changes
were made to the Excessive Heat Warning.

Latest models continue to depict significant differences regarding
how/if/when the upper ridge breaks down next week. These
differences stem from how the models handle the interaction
between a shortwave moving down the Pacific Northwest coast and a
closed low over MT/WY. ECMWF/GEM both have less phasing and nearly
develop a cutoff low over NV/UT while the GFS has greater phasing
between these features and is much more progressive with the
overall pattern. Generally followed the recommended CR
initialization for the extended forecast periods pending improved
model agreement. Although there may be some isolated
thunderstorms (especially over the Ozarks), there appears to be
very little chance for widespread rainfall until the middle of
next week.

Kanofsky

&&

.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-Fayette IL-Greene IL-Jersey IL-Macoupin IL-
     Marion IL-Monroe IL-Montgomery 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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