Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 220008
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
708 PM CDT Mon Aug 21 2017

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 344 PM CDT Mon Aug 21 2017

Scattered afternoon thunderstorms are mainly being driven by
afternoon heat and humidity as well as a pair of outflow boundaries
left over from this morning`s storms. Temperatures have rebounded
back into the upper 80s and low 90s in the wake of the eclipse which
has invigorated convection.  Had a few reports of wind gusts to 40-
50 mph, frequent lightning, and brief heavy downpours as the cores of
the storms fall out, but deep layer effective shear is only 25-30kts
at this time, so think widespread severe weather is unlikely even
with the fairly high MLCAPE values of 2500 J/Kg indicated on the SPC
mesoanalysis at this time.  Should see these afternoon storms
diminish this evening as we lose daytime heating.

Attention turns to the cold front over the Northern Plains tonight.
This front will push southeast into northern Missouri and west
central Illinois between 08-11Z.  Showers and thunderstorms look
likely along and ahead of the front after midnight tonight. Guidance
seems to be in good agreement that a line of storms will move into
the area ahead of the front after 06Z and move southeast to near the
I-70 corridor by between 09-12Z.  The line slows as elevated
instability wanes toward morning...with scattered thunderstorms
likely continuing across the southern 1/2 to 2/3 of the forecast
area on Tuesday before the front finally pushes all the way through
the area on Tuesday night.

Carney

.LONG TERM...  (Tuesday Night through Next Monday)
Issued at 344 PM CDT Mon Aug 21 2017

Should have lingering showers and storms southeast of the STL area
Tuesday evening until the cold front shifts southeast of our
forecast area and a surface ridge builds southeastward into the
region from the northern Plains.  Cooler and less humid air will
filter into the area Tuesday night leading to lows about 5 to 10
degrees below normal.  This will begin a period of relatively cool
and dry weather from Wednesday into the weekend as surface ridging
remains locked in place from the Great Lakes region southwest into
eastern MO with any convection likely remaining west and north of
our forecast area.  Although weak northwest flow shortwaves will
move through the region it appears that the low levels will be too
dry to generate any precipitation. The ECMWF model does develop an
upper level trough over the northern Plains on Sunday, but it looks
like any precipitation associated with this feature will stay north
of our forecast area.  Highs will typically be around 80 degrees
with lows in the upper 50s to around 60 degrees during this period.

GKS
&&

.AVIATION...  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 702 PM CDT Mon Aug 21 2017

Scattered storms ahead of front through this evening so have
vicinity ts mention. Then cold front to begin moving thru taf
sites by 05z Tuesday with storms along it. So timing similar to
last set of tafs. Behind cold front will see MVFR cigs for a few
hours, so kept mention in. As for winds, southeast to south winds
to veer to the west then northwest to north behind cold front.

SPECIFICS FOR KSTL:
Scattered storms ahead of front through this evening over portions
of NE and Central MO, but should diminish before reaching metro
area. Then cold front to begin moving thru metro area by 08z
Tuesday with storms along it. So timing similar to last set of
tafs. As for winds, southeast winds to veer to the west then
northwest to north behind cold front.

Byrd
&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$
WFO LSX



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