Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 232337
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
537 PM CST Mon Jan 23 2017

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 334 PM CST Mon Jan 23 2017

Primary concern tonight will be cloud trends.  Missouri is stuck
between two low pressure systems this afternoon, one over the
Carolinas, and the other moving across the rockies.  We`re still in
low level northwest flow over our area which is keeping low clouds
over the area.  However a weak is moving over western Missouri and
will be drifting into our area this evening...bringing the wind
around to the southeast after the ridge axis passes tonight.  After
lingering most of the morning and early afternoon, the western edge
of the stratus is finally moving east into central Missouri as the
ridge drifts east.  Latest low level RH progs are now indicating
that the clearing line could make it to the Mississippi River as
early as 01-02Z.  Opted to be more pessimistic than that for the
moment and am keeping mostly cloudy conditions over the majority of
the area until 06-07Z since this moisture is trapped under a strong
inversion and decent cold advection will continue to occur until the
ridge axis moves through.  Expect lows to dip into the low to mid
30s across the area.

Should see any remaining stratus dissipate and move east rapidly
Tuesday morning and the wind will increase from the south-southeast
ahead of the next low pressure system.  Kept some slight chance PoPs
in northeast Missouri Tuesday afternoon ahead of the low. Guidance
is all dry in our area, there`s some decent moisture convergence on
a 40-50kt low level jet during the afternoon.  Could squeeze a
shower or two out of the atmosphere with that kind of forcing.
Should be very mild (though windy) tomorrow with temperatures in the
50s.

Carney

.LONG TERM...  (Tuesday Night through Next Monday)
Issued at 334 PM CST Mon Jan 23 2017

Another strong Pacific storm system will approach late Tuesday.  The
models have come into much better agreement on timing, track, and
strength of this system over the past 24hrs, with a timing that will
result in its closest approach Tuesday night, a track that will be
completely north of our region, and a strength that is more
reminiscent of the stronger more closed model runs.  The bottom line
on how it affects our forecast is minimal, with previous forecasts
handling this system well, with low PoPs, mainly across northern MO
and central IL with what should be mostly rain pcpn-types but some
small possibility for snow mixing in as well.  These low PoPs will
continue thru Wednesday for the same areas to the north as a
strongly sheared lobe of vorticity tracks thru nearly coincident
with a surface cold front. Limited moisture will be the main
reason this does not give us a better chance for pcpn.

For Thursday and thru the upcoming weekend, models continue to show
additional weak upper level disturbances sliding down into our
region from the resultant northwest flow aloft as a longwave TROF
carves into the eastern CONUS early in this period and then
persists.  Confidence on pcpn from any one of these individual
disturbances remains low, but this is typical for a northwest flow
scenario and do not expect to get a decent handle on any one of
these until we get within 36hrs or closer of the event, so probably
not until beginning Wednesday.  Don`t be surprised that the vastly
dry late week and weekend forecast has PoPs introduced at some later
date.

Otherwise, temperatures are expected to remain above average until
the middle of the week when the longwave TROF develops, with
temperatures more appropriate for late January for late week
and into next weekend.

Beginning the work week next week, there is some discrepancy on
whether to persist the longwave eastern CONUS TROF or not, with the
CMC most bullish on that and the EC and GFS not as much.  This
equates at the surface to a chilly NW flow persisting on the CMC,
and flow switching around to the SW on the EC and GFS.  A blended
approach is what was done here, with low PoPs persisting and temps
rebounding to average until a better consensus can be attained.

TES

&&

.AVIATION...  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 531 PM CST Mon Jan 23 2017

Not a lot of confidence in this TAF set. MVFR cigs remain in place
and expect them to be slow to clear. Will likely take the ridge
axis to move into the region to push the clouds out, which may be
a couple of hours later than currently forecast. However, did not
want to make drastic changes from the prev TAFs and will continue
to monitor and update as needed.

Tilly

&&

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

$$

WFO LSX



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