Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 130328
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
1028 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 IMPRESSVE 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 00z TAFs through 00z Sunday Evening)
Issued at 639 PM CDT Sat Jul 12 2014

Fcst package will focus on convective trends assoc with a cold
front to push thru the terminals. Convection currently in the
process of dvlpng from NW IL SW into sthrn IA and extreme NW MO.
This activity is expected to continue to increase in coverage this
evng and slowly sink ESE reaching KUIN late this evng. Coverage
and intensity are both fcst to diminish as the precip moves SE.
Due to uncertainties in coverage...have covered threat at KUIN
with a VCTS group. Do not expect this activity to hold together
long enough to reach the I70 corridor...so have left the rest of
the sites dry overnight. The actual cold front is fcst to enter
the nthrn CWA around 12Z and then slowly sink south thru the day.
Left in the MVFR conditions at KUIN early Sunday morning as the
site is closer to better upper level support. TSTMS are expected
to redvlp Sunday aftn along the front...so exact placement of the
front during that time is key. I think the front will be south of
the I70 corridor prior to convective initiation...but it will be
a close call...so either maintained the existing VCTS or introduced
a new group late in the prd to cover the uncertainty. Unless a
terminal is directly impacted with a convective core...then the
fcst should remain VFR. Winds are fcst to remain aob 12kts.

Specifics for KSTL:

VFR fcst with a cold front pushing thru the terminal around noon
tomorrow...with convection dvlpng Sunday aftn mainly south of the
terminals. Due to uncertainty in exact placement of the front
during convective initiation...opted to retain the VCTS group for
the aftn.

2%

&&

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

$$

WFO LSX





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