Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 241201

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
601 AM CST Tue Jan 24 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 411 AM CST Tue Jan 24 2017

The north-south surface ridge was centered through far eastern MO at
09z. Stratus was trapped within the ridge and persisted along and
north of a line from the Missouri River in central MO trailing
southeast of St. Louis to Sparta.

The upper low currently over WY and southwestward extending upper
trof will move into the Plains today, with the associated surface
low currently in SE CO tracking northeastward into NE KS by early
evening. In response, the surface high will quickly move to the east
this morning with surface pressure falls across the area through the
day, and winds increasing and exhibiting a strongly veering profile
from southeasterly at the surface to southwesterly above the PBL.
The stratus should begin a gradual north and northeastward clearing
trend this morning as the wind fields evolve, while high and some
mid clouds spread into the area in their wake. There is a chance for
a few showers in far northeast MO/west central IL this afternoon as
weak forcing with the upper system begins to spread into the area as
well as lift associated with the developing southwesterly LLJ. The
decrease in clouds today and good WAA regime should allow for yet
another day of above average temperatures, and warmer than yesterday.

The upper low/trof will then progress into the Mid/Upper MS Valley
tonight with the surface low continuing to track northeast and into
eastern IA by 12Z Wednesday. This is a bit slower than previous
forecasts, and the trailing cold front will move into the western
CWA overnight and be centered thru far eastern MO at daybreak. Large
heights falls aloft and good large scale forcing will spread across
the northern half of the CWA tonight. Normally this would suggest a
pretty good chance of precipitation, however limited moisture would
seem to preclude this possibility, and the forecast currently
has only some low pops across parts of northeast MO/west central IL.


.LONG TERM...  (Wednesday through Monday)
Issued at 411 AM CST Tue Jan 24 2017

Wed thru Wed night

Closed upper low is fcst to track from near OMA to the Grt Lks from
Wed mrng to Thu mrng. The assoc SFC low is expected to move from ern
IA to the ern Grt Lks. The attendant cold front should be bisecting
the CWA by 12Z Wed extending roughly from W cntrl IL to near or just
W of the STL metro area and on S/SSW into the ern Ozarks. The bndry
should clear all of the FA by 18Z. There should be two areas of
precip drng this prd. The more significant area of precip assoc
directly with the SFC low and upper level energy will mostly remain
N of the FA...but could scrape the nthrn CWA mainly Wed PM. It
appears there will be a secondary cold front assoc with the passage
of the short wave trough. An elongated region of vorticity will
slide thru the FA about 6-12 hrs after the first FROPA and
represents the best chance for precip. 850 temps steadily fall below
freezing from NW to SE thru the day meaning p-type will be
determined by SFC temps by aftn. Another area of precip should dvlp
along the cold front thru the day but the bndry is fcst to be E/SE
of the FA before that happens with the models indicating the bulk of
the precip remaining across AR and NE into the OH Vly. Limited
moisture precludes higher PoPs and QPF. This system should also be a
decent wind producer with W winds of 15-25 mph gusting 30-35 mph
peaking Wed aftn with the highest winds along and N of I-70.

Wed may have a non-diurnal trend or at least not a typical diurnal
trend with FROPA occurring in the AM hrs. Temps in the NW sections of
the FA may continue to fall thru the mrng before rebounding slightly
drng the aftn while the SErn CWA may have a late mrng or very early
aftn high and then either level off or fall a few degrees in the

Thu thru Mon

The overall upper pattern will undergo a change at the beginning of
the prd...Thu into Fri. The mid week short wave will lift NE into
Quebec where it will become absorbed by a multi-lobed low near
Hudson Bay. upper level ridge that is fcst to build
into the W Coast of N. A. thru the week will continue to and push into NW Canada. Addtnl short waves are
expected to top the ridge and drop down the back side of the trough
effectively carving out a significant long wave trough across the
ern CONUS from late week thru the wknd. The combination of this
activity will lock the CWA in N/NW flow thru the prd. The N/NW flow
will mean colder temps but there is no Arctic air in
temps are expected to return to seasonal levels which means highs in
the mid 30s N to lower 40s S with lows in the 20s. Temps should be
rather stagnant with not much day to day variability due to
reinforcing shots of colder air about every day or so with each
successive short wave. As is common with a N/NW flow pattern...there
are differences wrt timing and strength of the individual short
waves and it is common for the models to not get a good handle on
these differences much more than a day or so ahead of time. Guidance
currently indicates that a short wave timed for Sun should be the
strongest in the series and therefore represents the best chance for
precip across the FA. But don`t be surprised if addtnl PoPs are
added as we get closer to the end of the week and into the wknd.
There will also be surges of low level cloudiness with each short
wave which could also produce flurries and/or sprinkles even if PoPs
are not needed.  But it is too early to include in the fcst due to
the above mentioned differences to timing and strength of the short
waves. There should also be a persistent NW/WNW flow thru Sun night
due to an expansive stnry SFC ridge locked in across the intermtn W.

Upper ridging finally tries to build into the region late Sun night
and Mon. This allows a lobe of the wrn SFC ridge to break off and
pass S of the FA Mon causing winds to become Sthrly thru the day.
The center of this lobe is fcst to move into the SErn US thru early
week with the ridge axis extending NW to the main area of high
pressure...keeping the Gulf of MX closed for business. The upper
level ridging appears to be short lived with another potent short
wave following quickly behind.



.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 542 AM CST Tue Jan 24 2017

A large area of stratus covered all of IL and much of northern
and central MO early this morning. MVFR flight conditions were
reported at most of the terminals, except KCOU where IFR has
persisted. The general trend expected is that the stratus will
gradually move northward today as southeasterly winds increase.
The southeast winds will get gusty by this afternoon with the
gusts continuing periodically into the early evening. LLWS
conditions will develop this evening thanks to a stout 40-50kt
south to southwest LLJ. The LLJ will also help generate another
round of stratus bringing a return of MVFR flight conditions to
KUIN this evening, with VFR continuing elsewhere. A cold front
will progress across the region overnight into Wednesday morning
bringing a southwest wind shift. There is a chance for a few
showers this afternoon and evening in northeast MO and west
central IL, but the probability is too low to mention at KUIN.


MVFR flight conditions due to stratus will persist until late
morning, then the stratus will lift northward and VFR conditions
should dominate the remainder of the period. Southeast surface
winds will increase by afternoon and become gusty until mid-
evening. Begining around mid-evening, LLWS conditions will
develop thanks to a stout 40-50kt south to southwest LLJ. The LLWS
conditions will cease during the predawn hours on Wednesday,
followed by passage of a cold front on Wednesday morning.





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