Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 130442
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1142 PM CDT Sat Jul 12 2014

.UPDATE:
Issued at 1007 PM CDT Sat Jul 12 2014

SHOWERS AND THUNDERSTORMS BECOMING MORE NUMEROUS OVER SOUTHERN
IOWA INTO FAR NORTHWEST MISSOURI AND SOUTHEAST NEBRASKA IN
RESPONSE TO UPSTREAM SHORTWAVE AND COOLER TEMPERATURES ACROSS MID
LEVELS. DELTA THETA-E STILL LO0KS IMPRESSIVE WITH RESPECTABLE
VALUES BETWEEN 30 TO 35 k FROM NORTH CENTRAL ILLINOIS THROUGH
CENTRAL AND WESTERN MISSOURI. INTERESTING 500 TEMPS OVER AREA
RANGE FROM -3.0 TO -4.0C OVER CENTRAL AND SOUTHERN MISSOURI RELATING
TO WARM LAYER ALOFT THUS HARD TO DEVELOP CONVECTION. BANDS OF
CONVECTION FROM SOUTHERN IOWA THROUGH SOUTHEAST NEBRASKA WILL MOVE
ACROSS NORTHEAST MISSOURI BETWEEN 06 TO 09 UTC. PRECIPITATION WILL
GRADUALLY WORK ITS WAY SOUTHWARD INTO CENTRAL AND EASTERN
MISSOURI. THUS RAISED POPS FOR NORTHERN PART OF CWA FOR AFTER
MIDNIGHT. SLIGHTLY ADJUSTED TEMPERATURES FOR EASTERN AND SOUTHWEST
PARTS OF CWA.

PRZYBYLINSKI

&&

.SHORT TERM:  (Through Late Tonight)
Issued at 301 PM CDT Sat Jul 12 2014

Still expect scattered thunderstorms to develop along a cold
front over Iowa later this afternoon and early this evening as a
shortwave trough currently over the Dakotas drops southeastward
over the Midwest. These storms should become more widespread by
mid-late evening along the cold front as both the GFS and the LSX
local-WRF is showing strong 925-850mb moisture convergence
developing ahead of the front. This idea is supported by the 12Z
NSSL WRF which shows showers and thunderstorms thunderstorms
becoming widespread along the cold front and moving south to near
the Missouri/Iowa border by 04Z. I expect that these storms will
eventually move into the northern CWA by midnight, so will
maintain likely PoPS over northeast Missouri and west central
Illinois with chance PoPS down to just north of I-70 between
06-12Z.

Britt

.LONG TERM:  (Sunday through Next Saturday)
Issued at 301 PM CDT Sat Jul 12 2014

(Sunday through Tuesday)

LSX Local-WRF and GFS is still showing some moisture convergence into
Sunday morning that will begin shifting southward ahead of the
surface cold front.  This will move in sync with shortwave trough,
so will maintain chance PoPS into the afternoon.  Do have some
concern about the potential of a few severe thunderstorms developing
during the afternoon hours on Sunday afternoon and early evening
ahead of the cold front as MLCAPEs will climb into the 1500-2500
J/kg range with deep layer shear around 30kts which may support some
organized multicells/possible supercells capable of producing
primarily damaging winds.

This cold front will move south of the area on Sunday night followed
by a secondary cold front on Monday which will bring another chance
of thunderstorms as the atmosphere will become unstable ahead of it
again on Monday afternoon.  Temperatures will be much cooler behind
it on Tuesday as 850 mb temperatures will only be around 8C.

(Wednesday through next Saturday)

Pattern will undergo a change in the extended part of the forecast
with the deep eastern trough lifting north into eastern Canada by
late in the week.  Both the GFS and ECMWF show a surface high moving
through the area under northwesterly flow aloft on Wednesday and
Thursday, so will keep with the current dry forecast.  850mb temps
are expected to be in the 8-12C range supporting surface high
temperatures in the middle to upper 70s.  By Friday and Saturday,
the GFS and ECMWF show more run to run continuity differences with
the pattern shift, namely they have been too quick to move a trough
toward the area next weekend.  Will continue to keep the forecast
mainly dry with a slow warm up toward next weekend as 850mb
temperatures by next Saturday warm into the 16-18C range which
supports highs in the low to mid 80s.

Britt

&&

.AVIATION:  (For the 06z TAFs through 06z Sunday Night)
Issued at 1123 PM CDT Sat Jul 12 2014

Fcst remains focused on precip and clouds assoc with a cold front.
Front currently stretches from nthrn IL SW thru SE IA into NW MO
and on into KS. Earlier convection struggled to maintain intensity
as it moved east and has since all but dssptd. A new band of
SHRA/TSTMs has dvlpd in the past couple of hrs along the cold
front across sthrn IA...nthrn MO and SE NEB. This activity should
push SE thru the night. Models have the precip dssptng between 8Z-
11Z as it moves away from better upper level support...into warmer
mid level temps and during a climatologically unfavorable time for
convection to exist. Have MVFR VSBYs at KUIN and KCOU as this band
passes thru overnight. Not confident at all that it will hold
together long enough to affect the STL metro area terminals so
have a few hrs of VCSH to address the threat. Beyond that...the
front is fcst to be just south of the STL metro sites by early
aftn when convective dvlpmnt should begin. Think that the front
should be far enough south by that time that precip and/or TSTMs
should not be a problem...but due to uncertainties in the exact
placement of the front left the going VCTS in place until there is better
confidence that convection will not be an issue for the metro area.

Specifics for KSTL:

Dssptng band of SHRAs possibly affecting the terminal around 12Z with
addntl TSTMs possible during the aftn. Otherwise expect a VFR fcst with
winds aob 10kts.

2%

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX






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