Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 120443

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1143 PM CDT Fri Aug 11 2017

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 256 PM CDT Fri Aug 11 2017

Isolated showers and thunderstorms will dissipate with the loss of
diurnal heating of the day over southeast Missouri and southwestern
Illinois by early this evening.  High pressure will slide across the
upper Mississippi valley and filter in cooler and drier air into the
region through Saturday afternoon.  Temperatures will be right at or
just below climatological normals with light northeast winds
expected through Saturday afternoon.


.LONG TERM...  (Saturday Night through Next Friday)
Issued at 256 PM CDT Fri Aug 11 2017

The large scale pattern aloft late Saturday will feature a broad mean
trof centerd through the Great Lakes/Ohio Valley with broad
cyclonic flow from the northern Rockies eastward through the Mid
MS Valley. This will maintain surface high pressure as the
dominant weather feature on Saturday night into early Sunday and
below average temps, low humidity and dry conditions.

However from later Sunday through next week there is growing
uncertainty and lessening confidence in the forecast details.
Initially the high pressure begins to slowly retreat and there is
some question on the location and eastward advancement of low
level moisture return in its wake. This is coupled with
variations amongst the models in the location and timing of
migratory short waves in the cyclonic flow impacting the area.
Also the high pressure system will persist across the Great Lakes
where several shortwaves in the northern portion of the cyclonic
flow will maintain its presence and the associated air mass.
However in the mid MS Valley the weakening and gradual eastward
retreat of the surface ridge axis and lessening influence will
allow a boundary to return northward. Complicating the assessment
is each for model cycle the models are different to some degree
from Sunday through Thursday, and they also have little run-to-
run continuity among themselves.

In general, more progressive flow and a pattern shift from broad
cyclonic flow early week to west-southwest progressive flow and rising
heights midweek should signal a moderation and return to near
normal temps. As far as a precipitation threat, it would appear
the main focus Sunday through Monday/Tuesday will be across the
southern 1/4 to 1/2rds of the CWA. The threat then appears to
increase and expand northward Wednesday and beyond as low level
flow becomes south-southwesterly and moisture transport ensues,
the boundary lifts northward, and disturbances aloft traverse the



.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1143 PM CDT Fri Aug 11 2017

VFR conditions and dry wx will prevail at the TAF sites for much
of the valid period. Only item of concern is radiational fog
potential at sheltered locations, mainly SUS and CPS. Fog chances
are starting to look less likely with decently large dewpoint
depressions and dry air continuing to filter in behind the cold
front and have backed off to a TEMPO for a few hours of 5SM in
mist. Otherwise, northerly surface winds will prevail thru the

SPECIFICS FOR KSTL: VFR, dry wx, with northerly surface winds.





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