Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 251758
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1158 AM CST Sun Jan 25 2015

.SHORT TERM: (Through Tonight)
Issued at 322 AM CST Sun Jan 25 2015

Satellite data at 08z indicates the heart of a strong clipper system
over west-central IA and is chugging southeastward towards our
region at a good rate.  It is also close to forming a closed
circulation as well.  Increasing and solid areas of pcpn exist
around this heart of the system over IA with an arm of patches of
pcpn extending southward thru our region currently.  All pcpn is
currently rain over our region with surface temps in the 40s.  An
initial cold front extends thru northwest MO and southeast KS but
the CAA associated with it is weak.  A stronger shot of cold air is
pushing thru the Mid-MO river valley in eastern NE and western IA.

There is finally excellent model consensus on the structure and
track of this system, with the GFS the big winner on depicting a
closed circulation and farther SW track first with the rest getting
on board:  the closed upper level system will track to close to the
STL metro area by midday and then into western KY by nightfall.  Two
shots of pcpn will be the main players here, with the first out
ahead of the system from STL metro to southwest MO currently
associated with strong vorticity and will shift a bit to the
southeast thru midday.  The second main area of pcpn will be the
deformation zone itself currently in IA, and this will be dragged
bodily southward as the system digs.  The deformation zone is
expected to impact the entire forecast area at some point, entering
the northern CWA in northern MO-central IL by late morning, and then
not exiting areas to the south and east of STL metro until evening.

Thanks to sufficient cold air existing aloft, pcpn-types will
largely be driven by surface temps.  However, the more significant
cold air push is expected to be delayed until shortly after the
arrival of the deformation zone pcpn and will also be accompanied by
strong gusty NW winds.  So pcpn-types look to be all rain at all
locations thru 18z, with a gradual mix in of snow for northeast MO
and west-central IL during early afternoon and making a run towards
I-70 by nightfall.  This trend to then continue into the evening for
STL metro and areas to the south and east.  All-in-all, the
appearance of the snow will probably not amount to much more than an
hour or two, with a bit more for areas around UIN and into central
IL.  Any snow accums will likely be confined to the UIN area and
areas heading into central IL but just a dusting is expected.

The push of the cold air will need to be watched, however, with an
earlier arrival and/or deeper/slower system will lead to increased
snow amounts.

TES

.LONG TERM: (Monday through Saturday)
Issued at 322 AM CST Sun Jan 25 2015

NW flow aloft will continue to prevail thru Tuesday night with quiet
wx expected.  Temps will recover to above seasonable levels in MO
and close to seasonable levels in IL.

A shortwave disturbance is still on track for Wednesday night and
have maintained mentionable PoPs for rain.  If the surface low
reflection can drop more to the south, will need to boost them
significantly.  Slight chances were also maintained for Saturday,
but that is looking less of a possibility due to the northern
influences dominating more--keeping a strong southern storm isolated
well to the southwest.

Mild temps areawide can be expected Wednesday, and perhaps into
Thursday depending on the frontal timing, but it looks like a cooler
period is then in store for next weekend.  The EC and GFS have split
on how to handle an Arctic boundary, with the GFS backing way off.
So for now, a return to seasonable temps for next weekend the best
bet until a better consensus is achieved.

TES

&&

.AVIATION:  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1142 AM CST Sun Jan 25 2015

Low pressure centered just to the east-southeast of St. Louis will
continue moving east-southeast through tonight. Light rain falling
across much of the area will continue into the afternoon, changing
to light snow before ending early this evening. MVFR conditions
south of the I-70 corridor are expected to fall to IFR this
afternoon. IFR flight conditions are expected to continue to
prevail at least into the early evening, with improving ceilings
and visibilities as the precipitation comes to and end and the low
moves further east. Some guidance suggests that the low clouds
will scatter out overnight, however am siding with more pessimistic
models at this time as there is a lot of MVFR and even IFR up
across the Dakotas, Iowa, Minnesota, and Wisconsin. One other
feature of note, wind will be increasing substantially later this
afternoon as the low pulls away. Expect north-northwest wind
sustained between 15 to 20 kts with gusts potentially up to 30 kts.

Specifics for KSTL:

Expect IFR ceilings and at times IFR visibilities in light
rain/drizzle this afternoon. Wind will increase to the northwest
later sustained between 15 and 20 kts with gusts between 25 and
30 kts. Still expecting a little snow late this afternoon or early
this evening. Confidence in the exact timing of the snow is not
especially high though. Did not want to change the current 23Z
transition time for the 18Z TAF because of this, but an hour on
either side of 23Z still seems like the most likely. Snow should
end quickly this evening with ceilings between 1000-2000 FT
persisting through Monday morning.

Carney

&&

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

$$

WFO LSX





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