Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 252144
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
344 PM CST Sun Jan 25 2015

.SHORT TERM:  (Through Late Tonight)
Issued at 323 PM CST Sun Jan 25 2015

Precip over the CWA should be winding down over the next few hours
as upper level impulse works its way SE into the Ohio Valley.
Deformation zone that was so well defined this morning over IA and
n IL certainly falling apart as the afternoon progresses, and have
used a combination of regional radar and HRRR for PoP/weather
trends heading into the early evening. It would appear that the NW
edge of the precip (now all snow) will be near a KCOU-KILX line
by 00z, with the precip dropping southeast and exiting
SE sections of the CWA no later than 06z.  Surface obs trends this
afternoon indicate that rain begins mixing with snow @ 36-37 and
is all snow at 34 or colder, and will use this for ptypes for the
rest of the afternoon and into the early evening. Given warm
ground and the light intensity of the precip once it goes to all
snow, accumulations should continue to be very minor.

Other question for tonight is how much clearing will occur as the
storm exits and a narrow ridge of high pressure works into the
region. Over the last few hours have seen a wedge of clearing
wrapping around the surface low from S WI into NE IA. Some of this
clearing may reach our area, but there is also a lot of additional
low cloud to our north that will advect our way as well, so for
now have only gone with partial clearing during the late evening
and overnight hours.

Truett

.LONG TERM:  (Monday through Next Sunday)
Issued at 323 PM CST Sun Jan 25 2015

(Monday-Wednesday)

Overall, going forecast heading into midweek still looks pretty
good, so this package will include generally minor adjustments and
tweaks to these trends.

Fast moving shortwave in northwest flow will zip into the region on
Monday, with WAA in advance of this feature rapidly shutting down
the CAA occuring overnight in the wake of today`s clipper.  12z
synoptic runs are suggesting a fairly strong N-S warm front will be
located near or just west of STL by late Monday afternoon and have
attempted to reflect this in max temp forecast, allowing highs to
rebound into the middle 50s over mid MO while keeping highs in the
30s over our S IL counties. This temp trend also seems to be
supported by the model RH progs which suggest a fairly thick mid
deck sweeping from W to E across the area in the zone of WAA during
the day, with the most clouds over our eastern counties during the
afternoon.  Some 12z guidance is even trying to kick out some
light QPF near and east of the Mississippi River in the zone of
strongest WAA. Have continued dry forecast for now, but will need
to keep an eye on this as there has been some very subtle suggestions
of this for several days.

The cold air advection behind Monday`s shortwave will try to dip
into the area Monday night, but with the longwave trof working into
the east coast during this time the main thrust of this cooler air
will be east of our area.  Model consensus of thermal profiles over
the region support highs in the 40s over most of the CWA by Tuesday
as weak surface ridge drops into the Mississippi Valley.

Warmup still appears to be on schedule for Wednesday, and main
question is how much of the warming will be able to translate to the
surface.  Forecast soundings are suggesting a large inversion, with
low level mixing up to 900-950mb.  Surface/low level winds...while
strong...are going to primarily be southerly rather than the
southwest direction that normally gives us our biggest warmups.
Also some doubt to how much cloud cover there will be, as GFS
suggests some mid clouds working into the area during the afternoon
while NAM suggests lots of sunshine.  Due to the above uncertainty
will keep highs about where they are now...ranging from the mid 40s
in our far eastern counties to near 60 in the far western tip of our
CWA.

(Thursday-Sunday)

Shortwave that works across the region Wednesday night and early
Thursday appears to be the first of several chunks of energy that
will be deepening long wave trof over the eastern U.S. heading into
the weekend, while cutoff low forms over the southwestern U.S.
Medium range solutions are in very broad agreement in developing
this split flow pattern, and in general this should mean temps
trending back towards more wintry levels after the mid-week warmup.
Will continue some slight chance PoPs on Wednesday night/Thursday
for eastern parts of the FA in advance of aforementioned shortwave,
with slight chance PoPs also continuing for parts of the FA on
Saturday and Saturday night as there is agreement in the medium
range models that WAA, a bit of shortwave energy, and a frontal
boundary will be passing through the area at that time.

Guidance is in reasonable agreement with their thermal profiles
through Saturday, but much like yesterday the operational ECMWF
continues to be deepest with the upper trof and much colder than the
GFS by the end of the period (it`s intesting to note that last
nights ECMWF-based MOS had over a 40 degree difference between the
warm and cold members for Sunday`s highs (58 vs 16).  For now, will
lean towards MOS averages for temp trends on Sunday...which will be
in the 30s.  This seems to be a reasonable compromise at this point,
with additional adjustments occurring as specific trends become less
muddled.

Truett

&&

.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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