Area Forecast Discussion
Issued by NWS St. Louis, MO

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000
FXUS63 KLSX 221047
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
547 AM CDT Wed Apr 22 2015

.SHORT TERM: (Through This Evening)
Issued at 237 PM CDT Tue Apr 21 2015

Expansive closed H500 low encompassing the ern half of the CONUS
will continue to spin across sthrn Ontario...Quebec and New England
thru Thursday. This will keep temps below normal thru the end of the
week. The cold front that pushed thru overnight is well south of the
FA area along the AR/MO and KY/TN borders to start the prd. The
bndry is on the leading edge of a cold air mass. 850 temps have
trended slightly cooler in the past 24 hrs with the 0 line now fcst
to be south of I70 with the -2 isotherm down to STL. Temps will
likely not make it out of the mid/upper 50s today in most
locales...which is about 10 degrees below normal. NW winds will once
again kick up today as the sfc ridge continues to build into the CWA
but should not be as strong as the past two days. By tonight the sfc
ridge axis should be aligned along the MO Rvr. Winds will become
lght/vrb overnight across the wrn portion of the CWA with light
nthrly flow across the ern FA...which when combined with a dry
airmass and clear skies should allow temps to drop into the 30s.
Given this set up, frost seems like a pretty good bet, esp for areas
along and north of I70. The frost potential would be higher across
the ern Ozarks if not for the threat of some mid clouds to approach
cntrl/SE MO well after midnight which would obviously hinder the
cooling and limit the frost potential. Based on the potential for a
bit of wind across IL and the threat of clouds across cntrl and SE
MO...have decided in coordination with surrounding offices to hold
off issuing a Frost ADVY. High pressure will remain in control Thu
and Thu night keeping cool and dry conditions in place.

2%

.LONG TERM:  (Tonight through Tuesday)
Issued at 359 AM CDT Wed Apr 22 2015

Mid lvl heights begin to rise Thu night as weak ridging
approaches in advance of an upper lvl system. The next short wave
is fcst to impact the CWA Fri and Sat. Thinking that the upper lvl
ridging should hold the precip at bay until Fri mrng. The ECMWF
has trended towards the GFS wrt the sfc low track with both models
tracking it ESE across cntrl/sthrn MO. All indications are that
the sfc warm front will be somewhere near I70 Fri night into Sat
mrng. That places the best chance for precip along and north of
I70 Fri night into Sat mrng...though it appears that precip may
linger across IL...esp south of I70 into the aftn. The track of
the sfc low will ultimately dictate high temps on Sat...but there
should be a decent temp gradient from N-S on Sat as the sthrn CWA
should be in the warm sector.

High pressure builds out of Canada in the wake of the Sat system
producing a cool albeit dry day on Sun before the next chance of
rain arrives early next week. Solutions vary wrt this system. The
ECMWF has a more progressive closed low that is in the process of
opening as it passes while the GFS has a pair of closed lows that
eventual merge and produce an intense sfc system that is
significantly further north than the ECMWF. Therefore chance PoPs
seem appropriate until a better consensus is achieved.

The cool temp trend will continue through the wknd and into next
week with no persistent prd of above normal temps in sight.

2%

&&

.AVIATION:  (For the 12z TAFs through 12z Thursday Morning)
Issued at 525 AM CDT Wed Apr 22 2015

The cold front has pushed south of the taf sites early this
morning. Mid level clouds around 10000 feet will advect south and
east of the St Louis metro area later this morning. Surface ridge
over the northern Plains will build southeastward into MO today
and tonight. Northwesterly surface winds will strengthen and become
gusty later this morning. The surface wind will diminish this
evening and become light as the surface ridge axis moves over our
area.

Specifics for KSTL: The mid level clouds will shift east and south
of STL later this morning. The northwest wind will strengthen and
become gusty later this morning. The wind will diminish and become
light this evening. Little if any clouds are expected from late
this morning through the end of the taf forecast period.

GKS

&&

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

$$

WFO LSX





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