Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 232241
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
541 PM CDT Tue May 23 2017

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 309 PM CDT Tue May 23 2017

Showers and isolated thunderstorms are currently moving east across
the area along and behind a cold front.  This initial wave of showers
will shift east into south central and southwest Illinois as well as
southeast Missouri.  This is where the RAP is showing low level
moisture convergence becoming concentrated from early evening until
after midnight underneath mid-level ascent ahead of a shortwave
trough coming around the south side of the the upper low.  The
shortwave trough will move off to the southeast by early tomorrow
before the main upper low sinks south into Missouri during the day
tomorrow.  This will bring scattered showers and thunderstorms to
the area under the cold pool of the upper low during the day.

Temperatures will be cooler tonight and tomorrow behind the front.
Highs will be much below normal tomorrow underneath the upper low
with 850mb temperatures only around +5C resulting in highs only in
the mid 60s.

Britt

.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 309 PM CDT Tue May 23 2017

The upper level flow will undergo significant transition during the
period from Wednesday night through early next week. A deep trof
will be centered over eastern portions of the MS Valley with an
upper low in the vicinity of southeast MO at the beginning of this
time frame. Cyclonic flow will be present across the region
attendant with the departing cyclone and residual/lingering
diurnally driven showers will be possible near and east of the MS
River on Wednesday evening. Low level cyclonic flow will diminish
overnight as the upper trof progresses eastward into the OH/TN
Valley Region. Thursday looks to be the tranquil and pick-day of the
week. The upper level trof will continue to depart through the
eastern U.S. with rising heights aloft in the wake of the trof and
ahead of a deepening long wave trof in the western U.S.. Weak high
pressure will dominate at the surface with low-level WAA getting
underway above the PBL.

An active and stormy weather pattern will take hold late Thursday
night through Saturday night. Continued progression of the upper
air pattern and evolution of the upstream broad longwave trof will
result in active west-southwest flow. A migratory short wave
within this flow will move across the area late Thursday night
through early Friday morning. An increasing southwesterly LLJ will
result in pronounced low level WAA and moisture return, and
elevated instability due to these advective processes and steep
mid level lapse rates. The primary focus for showes and storms on
Thursday night will be across central and northeast MO expanding
to the remainder of the CWA on Friday morning. A cold front will
then settle into the region late Friday becoming quasi-stationary
late Friday night into early Saturday. The environment during this
time frame remains quite supportive of deep convection with weak-
moderate instability due to a large expanse of steep mid level
lapse rates atop a warm moist low level air mass within a
persistent low level WAA regime.

There are some model differences with the evolution of the front
Saturday into Saturday night with the new ECMWF a bit more
progressive, and faster moving the cold front out of the area by
daybreak Saturday. Alternatively the consistent GFS and GEFS mean
is suggesting the potential for a very volatile period from
Saturday afternoon into Saturday Night. In this scenario a
deepening surface low would translate northeastward along the cold
front which would advance southeast on Saturday night, and the
warm sector will be quite favorable for organized severe weather
characterized by strong instability and strong deep layer shear.

Cooler weather would dominate in the wake of the front Sunday
through the Memorial Day and thru the early part of next week.
Several disturbances in the northwest flow aloft will bring
reinforcing surges of cooler air, as well as the potential for a
few showers and thunderstorms. The greatest threat of any
additional precipitation should be across northeast MO into west
central IL.

Glass

&&

.AVIATION...  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 517 PM CDT Tue May 23 2017

Upper level low over IA will drop southeastward through eastern
MO on Wednesday. It appears that much of the scattered shower
activity, now mainly in the St Louis metro area will dissipate
this evening with the loss of daytime heating, although could not
rule out isolated light showers through the overnight hours.
Scattered showers and a few storms should redevelop over the area
Wednesday afternoon. For now will likely just include VCSH in the
St Louis metro area tafs early this evening, then include VCSH in
all the tafs Wednesday afternoon. Should have mainly VFR cigs
through the period, but possibly dropping into the MVFR catagory
early Wednesday morning with plenty of low level moisture across
the area. Northwesterly surface winds will continue through the
period with surface troffing across the OH and TN Valley regions
and surface ridging over the Plains.

SPECIFICS FOR KSTL: Upper level low over IA will drop southeastward
through eastern MO on Wednesday. It appears that much of the
scattered shower activity, now mainly in the St Louis metro area
will dissipate this evening with the loss of daytime heating,
although could not rule out isolated light showers through the
overnight hours. Scattered showers and a few storms should
redevelop over the area Wednesday afternoon. For now will likely
just include VCSH in the STL taf early this evening, then include
VCSH again Wednesday afternoon. Should have mainly VFR cigs
through the period, but possibly dropping into the MVFR catagory
early Wednesday morning with plenty of low level moisture across
the area. Northwesterly surface winds will continue through the
period with surface troffing across the OH and TN Valley regions
and surface ridging over the Plains.

GKS

&&

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

$$

WFO LSX



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