Area Forecast Discussion
Issued by NWS St. Louis, MO

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402
FXUS63 KLSX 140423
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1123 PM CDT Sun Aug 13 2017

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 321 PM CDT Sun Aug 13 2017

High pressure over the region will continue to influence the weather
over most of the region through tonight. An upper level disturbance
over southeast Kansas will slide across far south central Missouri
and southwestern Illinois this evening. Scattered showers over
extreme south central Missouri will move east slowly over the area
the rest of this afternoon into the evening. Scattered showers will
be possible through this evening in extreme south central
Missouri and southwestern Illinois. A series of impulses will drop
down from the upper Midwest on Monday along a boundary north of
the Iowa and Missouri border with showers and thunderstorms
possible in north central Missouri. Temperatures will remain at
climatological normals through Monday afternoon with light
southeast winds.


Kelly

.LONG TERM...  (Monday Night through Next Sunday)
Issued at 321 PM CDT Sun Aug 13 2017

(Monday Night - Tuesday)

Quasi-zonal flow aloft is expected to continue through Tuesday.
At the surface, a stalled front is forecast to be oriented roughly
west to east across the mid Missouri Valley into the Upper
Midwest. Believe this boundary will be the primary focus for
convection Monday night through Tuesday. A secondary area of
showers and thunderstorms appears likely across the south-central
states associated with a weak, subtle vorticity maxima moving
through the region. This area of precipitation may try and clip
our region, particularly western and southern sections of the area
on Tuesday afternoon. Trended PoPs downward a bit for this
period, particularly for Monday night due to the aforementioned frontal
boundary being draped well to the north of the CWA.

Temperatures will continue to moderate Monday night and Tuesday.
Gone will be the below to well below normal temperatures the area
has experienced so far this month, replaced by near normal
temperatures and humidity levels. Forecast lows on Monday night
should be in the mid 60s to near 70 degrees, with highs Tuesday
afternoon in the mid 80s to near 90 degrees. MOS numbers looked very
reasonable given the synoptic pattern, and stuck close to those
values.


(Tuesday Night - Next Sunday)

A more active weather pattern is expected for the middle of this
week with southwest flow aloft and increasing low-level moisture and
instability. Highest chances for showers and thunderstorms appear to
be in the Wednesday afternoon/night period as a cold front moves
through the bi-state area. Timing of frontal passage (most likely
late Wednesday night) and broad low-level jet/unfocused moisture
convergence however suggest the potential of at least of
decreasing convective coverage and less widespread beneficial
rainfall for southeastern portions of the area.

Forecast uncertainty begins to increase heading into Thursday and
beyond as how far south the front gets is a question mark as well as
the track and timing of a shortwave trough from the northwest.
Believe much if not all of Thursday night through Friday will be
dry as there should be at least a brief period where surface high
pressure and drier/more stable air infiltrate the mid-Mississippi
Valley behind Wednesday night`s cold frontal passage. Chances for
showers and thunderstorms then should increase a bit ahead of the
next upper-level disturbance beginning Saturday.

Temperatures will begin above normal on Wednesday as 850-hPa
temperatures approach +20C ahead of the cold front. Highs in the
upper 80s to low 90s are expected with peak heat index values
mostly in the 95-100F range, but due to some uncertainty with
clouds and precipitation chances, actual highs may be a bit higher
or cooler than currently forecast. Behind the front, forecast
temperatures both day and night should get back down to around
normal for the Thursday through next Sunday period. Some spread
with the strength of the cooler airmass coming in behind the front
however, with the ECMWF suggesting the potential of cooler
conditions than the GFS. For now, leaned toward the warmer GFS
solution given the track of the upper-air impulse and the fairly
weak area of surface high pressure settling into the area behind
the front.


Gosselin

&&

.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 1112 PM CDT Sun Aug 13 2017

Surface ridge extends from MI southwest into southeastern MO. An
upper level disturbance was bringing mainly VFR, low-mid level
cloudiness to central and southern MO along with sprinkles to
southeast MO. Any precipitation should remain south of the taf
sites with cloud ceilings as low as around 4000 feet in COU and
the St Louis metro area late tonight/early Monday morning. Much
of the current cloud cover should shift east of the taf sites
Monday morning. Scattered diurnal cumulus clouds will develop late
Monday morning and afternoon with some mid-high level cloudiness
as well. Light southeast surface winds late tonight will become
southerly on Monday, remaining weak.

SPECIFICS FOR KSTL: Surface ridge extends from MI southwest into
southeastern MO. An upper level disturbance was bringing mainly
VFR, low-mid level cloudiness to central and southern MO along
with sprinkles to southeast MO. Any precipitation should remain
south of the STL area with cloud ceilings as low as around 4000
feet late tonight/early Monday morning. Much of the current cloud
cover should shift east of the STL area Monday morning. Scattered
diurnal cumulus clouds will develop late Monday morning and
afternoon with some mid-high level cloudiness as well. Light
southeast surface winds late tonight will become southerly on
Monday, remaining weak.

GKS

&&

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

$$

WFO LSX



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