Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 272200
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
400 PM CST Tue Jan 27 2015

.SHORT TERM:  (Through Late Tonight)
Issued at 400 PM CST Tue Jan 27 2015

Surface ridge extending from WI southwest into northern MO will
strengthen and move southeastward into the Ohio Valley region by 12Z
Wednesday.  Persistent low level cloud cover extends across eastern
IA and much of IL this afternoon with some diurnally driven
cloudiness across eastern MO.  Much of the cloud cover over eastern
MO should dissipate early this evening.  The models try to slowly
dissipate and advect eastward the more solid cloud cover across
eastern IA and IL tonight.  While this is possible due to at least
weak subsidence across the area, would not be surprised if this
cloud cover remains trapped under the inversion and actually
retrogrades some westward tonight as the surface/boundary layer wind
veers around to an easterly direction this evening.  If this low
level cloud cover remains through the night the MOS minimum
temperature guidance may be a little too cool across the IL counties
of our forecast area.  Lows tonight across the western portion of
the forecast area will be warmer despite less cloud cover due to
increasing low level warm air advection on the nose of a
southwesterly low level jet.

GKS

.LONG TERM:  (Wednesday through Next Tuesday)
Issued at 400 PM CST Tue Jan 27 2015

(Wednesday-Friday)

12z guidance continues to support thinking that a strong but brief
warmup will occur tomorrow, followed by a cooling trend
that should send temps to below average levels by the end of the
work week.  Have maintained current forecast highs for Wednesday,
which is several degrees warmer than warmest GFS MAV.  In spite of
the very strong inversion that is forecast to limit mixing still
think warmer is the way to go, as guidance hasn`t been robust enough
the past few times we`ve had strong warming.  I`m also uncertain how
much cloudiness there will be; worse case for warming would be that
this afternoon`s low clouds manage to hang around in the morning,
but increasing southerly surface winds should advect them back out
of the area for the afternoon.

Cold front will sweep through the region Wednesday night, with the
resultant cold advection only allowing a minimal daytime rebound
during the day on Thursday.   I have also continued some slight
chance PoPs late Wednesday night and into Thursday morning as
shortwave drops into the mid-Mississippi Valley, but moisture
certainly looks scant at this time.

The chunk of cold air that pushes into the area on Thursday should
continue to exert its influence on Friday, with highs about a
category lower than those on Thursday.

(Saturday-Tuesday)

Over the past few days models have come into much better agreement
for this upcoming weekend, with upper low cutting off over the
southwestern U.S. while fairly active westerlies continue from the
Pacific NW, through the Mississippi Valley, into the east coast.
All medium range guidance now supports the idea that broad
overrunning will develop in the wake of the cold surface ridge on
Saturday, which should lead to at least a chance of precip over much
of the area.  However, a strong shortwave in the westerlies will add
additional large scale lift Saturday night and into Sunday, and have
upped PoPs a bit more during this time.

While precip is looking more promising, still lots of unknowns about
ptype.  850mb temps are genenerally below 0C and certainly on
Saturday evaporative cooling should try to cool the lower levels of
the AMS enough to support snow.  However, forecast soundings are
also suggesting low level temp profile will become isothermal at
around 0C on Saturday night and Sunday, which suggests ptype go be
either liquid or frozen.  I have leaned ptype a bit more towards
snow, but still kept some mention of a mix because of the above
thermal profile.

A large chunk of very cold air will drop into the region behind the
weekend system, and by the start of the new work week we will likely
be reminded that winter is certainly not over.   Have stayed with
initialization guidance for highs on Monday...generally ranging from
the lower 20s to the lower 30s.

Truett

&&

.AVIATION:  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1032 AM CST Tue Jan 27 2015

Low level, MVFR cloudiness across eastern IA and much of IL still
advecting southwestward late this morning. There has also been
some diurnal development of cumulus or stratocumulus clouds
further southwest which were impacting STL and CPS. This diurnal
development will likely continue this afternoon despite some
subsidence across the area, so will need at least a tempo group in
UIN and the St Louis metro area tafs for MVFR cigs at times.
Surface winds will gradually veer around to a nely direction this
afternoon, and then to an ely direction this evening due to surface
ridging strengthening across the Great Lakes and Ohio Valley
regions tonight. As these winds take on a more e-nely component it
may cause the low level cloud cover to advect back into UIN and
the St Louis metro area late this afternoon or this evening. For
now make keep prevailing conditions VFR during this time period,
but will need to keep a close eye on satellite trends. Sely
surface winds will strengthen late tonight and Wednesday morning
as the surface pressure gradient tightens due to developing low
pressure over the northern Plains.

Specifics for KSTL: MVFR clouds have develped over STL and may
continue for much of the afternoon. The more solid cloud cover
east and north of STL may also try to advect back into STL late
this afternoon and this evening. For now may keep cloud conditions
scattered tonight, but would not be surprised if it goes MVFR again
for at least part of the night. The nly surface wind will
gradually weaken and veer around to a nely direction late this
afternoon, and to an ely direction by late evening. The wind will
intensify again Wednesday morning from a sely direction.

GKS

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX





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