Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 120434
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1134 PM CDT Wed Oct 11 2017

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 328 PM CDT Wed Oct 11 2017

Persistent low level cloudiness was across our forecast area this
afternoon on the backside of a surface low now over the OH Valley
region.  Surface ridging was building eastward into our area from
western MO with clearing across parts of northwest and west central
MO and into a small part of southeast MO aided by daytime heating,
drying, and mixing.  It appears that this clearing trend may end or
at least slow down this evening as the boundary layer cools and the
low level subsidence inversion strengthens.  Could see at least some
breaks in the overcast across parts of northeast, central and
southeast MO tonight.  Any areas which clear out tonight will see
the formation of fog with light surface winds and a very moist
boundary layer.  Could not rule out patchy drizzle late tonight
across parts of west central IL, but it appears that most of the
drizzle will remain northeast of our forecast area.  This persistent
cloud deck should finally completely erode across our area late
Thursday morning and afternoon as the surface wind becomes
southeasterly on the west side of the surface ridge shifting east of
our area, and as southwesterly low level winds bring warmer air into
our region.  Highs will be warmer on Thursday and close to seasonal
normals.

GKS

.LONG TERM...  (Thursday Night through Next Wednesday)
Issued at 328 PM CDT Wed Oct 11 2017

The synoptic pattern at the start of the long term period will
feature fairly amplified flow aloft, with a strong ridge located
over the East Coast and an upstream trough digging into the western
CONUS.  This will place the Midwest within a southwesterly flow
regime through this weekend, before the trough swings through and
switches the flow to more northwesterly early next week.

Surface high pressure will move off to the east by Friday and
especially into Saturday as a lee surface low deepens across the
Central Plains. This will promote south/southwesterly surface flow
Friday into Saturday.  Temperatures will top out in the low 80s on
Friday, but will be even warmer on Saturday as the 850mb thermal
ridge settles over the area.  While there are some cloud cover
concerns over northeast MO, think enough sunshine will peek out
over eastern MO and Illinois to warm temperatures well into the
upper 80s, if not close to 90 degrees in the warmer locations (St.
Louis metro).  This would put temperatures near record values for
this time of year.

The western trough and an associated cold front will slide into the
region Saturday afternoon and slide east through most of the area by
Sunday morning.  Ahead of it, instability will build across
western/northern MO Saturday afternoon/evening.  Plenty of deep-
layer shear coupled with 1000-2000 J/kg of MLCAPE should be enough
to support a severe weather risk over portions of northern MO
Saturday afternoon/early evening (barely clipping the far northwest
LSX CWA).  The front will rapidly advance southeastward Saturday
night, but will encounter a weakening thermodynamic environment.
However, the ascent associated with the right entrance region of a
130-knot jet streak coupled with the strong forcing along the front
may help compensate a bit for the weakening instability, keeping the
threat for strong convection going through at least the first part
of the overnight hours (into central/eastern MO and western IL).
This is certainly a system we will continue to monitor in the coming
days.

Behind the front, another shot of cooler and drier air will follow
for next week.  Highs will dip back into the 60s/low 70s with dry
conditions expected as a surface ridge builds into the Midwest.

KD

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1121 PM CDT Wed Oct 11 2017

MVFR conditions for SUS, CPS, and STL through the overnight hours,
while COU and UIN will have lowering ceilings with IFR conditions
until tomorrow late morning. Period looks dry with the exception
of some stray light drizzle. Southerly winds tomorrow afternoon
bring dry air that will help erode the cloud deck. VFR conditions
should return to all terminals by mid afternoon.

SPECIFICS FOR KSTL:

STL should remain at MVFR conditions overnight. Light drizzle
could affect the terminal. VFR conditions should return by mid
afternoon tomorrow as dry air from southerly winds will help to
clear the cloud deck.

Walsh

&&

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

$$

WFO LSX



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