Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 032318

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
618 PM CDT TUE MAY 3 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 257 PM CDT Tue May 3 2016

An amplified longwave RIDGE was located over western North America
with a TROF over the east, leaving N-NW flow over our region.  Two
disturbances of note within this flow, with the first located just
south of the center of MO, with a stronger one located over far
western Lake Superior.  The first disturbance was interacting with
the diurnal CU cloud field already in place to produce spotty light
showers across southeast MO and in a few areas of STL metro and
southern IL.  Temps were in the 60s, with the coolest readings in
southeast MO where the cloud cover is thickest.

The first upper level disturbance will exit our region by sunset,
and along with the loss of diurnal influences, will precipitate an
end to the light shower activity in southern MO and IL.  Skies will
clear for a time in-between disturbances.

The second upper level disturbance will drop almost due south
tonight and track along the MS river late.  It will be accompanied
by a surface cold front.  If the current radar is any indication,
the coverage of showers should be sufficient, even in the middle of
the night, to justify high-end chance or likely PoPs for IL, with
lesser PoPs into MO where the upper support drops off significantly.

Min temps tonight will range from the lowest values--in the mid to
upper 40s--in southeast MO where the skies will remain clear the
longest, to the low-mid 50s for areas further north where clouds and
a late FROPA will keep the temps from dropping off too much.


.LONG TERM...  (Wednesday through Next Tuesday)
Issued at 257 PM CDT Tue May 3 2016

(Wednesday through Friday)

Forecast is still on track that cold front will move south of the
area by tomorrow morning.  Will keep a slight chance of showers over
the far eastern part of the CWA in close proximity to the upper low
that will be dropping southeastward into the Ohio Valley.  Thursday
and Friday will be dry as the upper low moves away and upper ridge
builds into the area.

Temperatures on Wednesday and Thursday will be below normal as 850mb
temperatures stay around 5C.  Temperatures will climb back above
normal on Friday as low level flow turns out of the west and 850mb
values climb into the 10-15C range.

(Saturday through Tuesday)

GFS/ECMWF show that the upper ridge will move across Missouri and
Illinois before southwesterly flow sets up early next week.  A front
will move south into the area and stall over the weekend which will
act as a focus for scattered shower and thunderstorm development.
Models are showing significant shortwave trough that will move
across the Midwest Monday night/Tuesday that will keep the chance of
thunderstorms going early next week.

Temperatures during the period should be mainly above normal with
850mb values in the 10-15C range.  Warmest day may be Saturday
before the front moves down into the area allowing highs to climb
into the 80s.



.AVIATION...  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 540 PM CDT Tue May 3 2016

High based cumulus clouds along with mid level clouds will
dissipate or shift southeast of the taf sites early this evening
due to the loss of daytime heating and as an upper level
disturbance shifts southeast of the area. Another upper level
disturbance over the Great Lakes region will send a cold front
southeastward through the taf sites late tonight and Wednesday
morning. A band of VFR, low-mid level cloudiness will advect
southeastward through the area along the front. A band of showers
will also drop southeastward through IL late tonight and
Wednesday morning with the western edge possibly impacting UIN
and the St Louis metro area. For now will just include VCSH in the
the tafs as the showers should be brief and light in nature.
Southwest wind will veer around to a northwesterly after fropa and
become relatively strong and gusty Wednesday morning.

Specifics for KSTL: Mid level clouds will dissipate early this
evening with the northwest wind backing around to a southwest
direction. VFR, low-mid level cloudiness will advect into STL
late tonight. A band of light showers may drop southeastward
through STL between 08-12Z Wednesday. For now will continue the
VCSH in the STL TAF during this time frame. The southwest wind
will strengthen late tonight as the surface pressure gradient
tightens, then veer around to a northwest direction early
Wednesday morning after fropa and become gusty. The surface wind
will diminish Wednesday evening as a surface ridge over the
northern Plains builds southeastward into MO.





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