Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 272338
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
538 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 00z TAFs through 00z Wednesday Evening)
Issued at 532 PM CST Tue Jan 27 2015

Main concern for the overnight hours will be on stratus in
Illinois. Dissipation has occurred over the past several hours on
the western flank mainly due to diurnal effects. Will have to
watch this area of stratus overnight as it is possible MVFR
ceilings advect back southwestward into KUIN and the St. Louis
metro TAF sites. RAP 925-950 hPa winds...albeit quite light...do
suggest this may be possible as initially northerly winds in the
aforementioned layer veer to the east and then eventually
southeast overnight tonight. Otherwise...quiet and tranquil
conditions are expected as sfc ridge of high pressure dominates
the area with winds staying light overnight. By
tomorrow/Wednesday...pressure gradient strengthens with winds
becoming more southerly at the surface with wind gusts up to
around 25 knots.


Specifics for KSTL:

Main concern for the overnight hours will be on stratus deck
currently residing just east of the terminal. Dissipation has
occurred over the past several hours on the western flank mainly
due to diurnal effects...but will have to watch this area of
stratus overnight as it is possible MVFR ceilings advect back
southwestward into Lambert Field. RAP 925-950 hPa winds...albeit quite
light...do suggest this may be possible as initially northerly
winds in the aforementioned layer veer to the east and then
eventually southeast overnight tonight. Otherwise...quiet and
tranquil conditions are expected as sfc ridge of high pressure
dominates the area with winds staying light overnight. By
tomorrow/Wednesday...pressure gradient strengthens with winds
becoming more southerly at the surface with wind gusts up to
around 25 knots.

Gosselin

&&

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

$$

WFO LSX





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