Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 180508
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1108 PM CST Tue Jan 17 2017

.UPDATE...
Issued at 920 PM CST Tue Jan 17 2017

Slowed down the clearing a little, and raised the minimum
temperatures for tonight a tad across southeast MO and southwest
IL. Clearing line is advancing slowly across parts of northwest
and west central MO this evening. The model guidance continues to
show clearing of stratus cloud deck from west to east tonight, but
satellite trends appear to indicate the clearing may be a little
slower than previously forecast. There will likely be some patchy
fog late tonight in areas that clear out with surface winds
becoming light underneath the surface ridge axis. Lows tonight
will be cooler than the previous night, but still about 10 to 15
degrees above normal.

GKS

&&

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 324 PM CST Tue Jan 17 2017

Skies have remained cloudy this afternoon as low level moisture
remains trapped under an inversion.  Latest regional satellite does
show that the clearing line is currently over far eastern Kansas.
RAP low level progs suggest that drier air will work into the area
during the late evening and overnight hours allowing low clouds to
erode.  This will be about the same time a surface ridge will move
into the area causing the winds to turn light allowing some patchy
fog to form, particularly across central Missouri where the clearing
will occur first. This fog will remain over the area into the
morning hours before it burns off by mid morning.

Should see more sun on Wednesday as a shortwave ridge moves across
the area.  More clouds will move into southeast Missouri late in the
day as mid-high level moisture streams northeastward ahead of an
upper low in the Southern High Plains.

Lows tonight will not fall much with the clouds staying in most of
the night.  Went with a compromise of MOS guidance for low tonight.
With sunshine tomorrow and winds turning out of the south, highs
will once again be close to the agreeable MOS highs.

Glass

.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 324 PM CST Tue Jan 17 2017

The models remain in relatively good agreement with the large
scale mass fields this cycle depicting a stout ULJ emanating from
the Pacific and contributing to an energetic lower latitude storm
track.

The upper low currently over the southwestern U.S. will lift
northeast into the central Plains Wednesday afternoon/night. Then
the main upper low takes a more northerly track as a strong
negatively-tilted short wave rotates around it and thru the Lower
and Mid MS Valley region Thursday afternoon/night. The models while
still showing some differences with respect to QPF with this system,
have trended closer and generally all show more backed low and mid
level flow. This results in a faster and more westward shift of the
warm conveyor belt and associated moisture transport, beginning on
Wednesday night and increasing on Thursday. The combination of large
scale ascent and the moisture transport on Thursday will
correspondingly lead to greater coverage and northward/westward
extent of precipitation. Moderately steep mid level lapse rates will
also support a threat of at least isolated thunderstorms.

Friday and Friday night look dry in the wake of this departing first
system. Another upper low and trof will lift north/northeastward
from the southern/central Rockies and through the central U.S.
Friday night and Saturday morning bringing deepening south/southwest
flow aloft. The are some decent height falls and weak large scale
forcing for ascent on Saturday. Moisture however seems pretty
minimal in the wake of the previous system and a bit low for much
in the way of any decent precipitation threat.

The eye-catching system is still on track to impact the region late
Saturday night-Monday as a potent upper low tracks across the
southern Plains and across the lower/mid MS Valley. The guidance has
all shifted south with this system and present indications the main
impact would be for the southern/southeast 2/3rds of the CWA with
a good rain event.

Mild, above average to well above average, temperatures will
prevail thru the forecast period.

Glass

&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1043 PM CST Tue Jan 17 2017

Ridge of high pressure over southwestern MO will move eastward
into the OH and TN Valley regions Wednesday and Wednesday
evening. Stratus cloud deck around 1000-1500 feet in height will
gradually clear out from west to east late tonight and Wednesday
morning. The clearing line was advancing slowly eastward through
western MO this evening. Will scatter out the low level cloud
ceiling in COU shortly after 06Z, and at the rest of the taf sites
around 12Z Wednesday. At least light fog will develop late
tonight/early Wednesday morning, especially where it clears out
well before sunrise. Westerly surface winds will back around to a
south-southwest direction Wednesday morning as the surface ridge
shifts east of our area.

SPECIFICS FOR KSTL: Ridge of high pressure over southwestern MO
will move eastward into the OH and TN Valley regions Wednesday
and Wednesday evening. Stratus cloud deck around 1000-1500 feet in
height will gradually clear out from west to east late tonight and
Wednesday morning. The clearing line was advancing slowly eastward
through western MO this evening. Will scatter out the low level
cloud ceiling in STL around 12Z Wednesday. At least light fog
will develop late tonight/early Wednesday morning. West-northwest
surface wind will back around to a south-southwest direction
Wednesday morning as the surface ridge shifts east of our area.
Low level cloudiness will spread back into STL late Wednesday
night.

GKS

&&

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

$$

WFO LSX



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