Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 240531
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1131 PM CST Sun Nov 23 2014

.UPDATE:
Issued at 938 PM CST Sun Nov 23 2014

Updated the PoPs for the overnight. Sped up the timing slightly and
increased coverage to the south over the next several hrs to account
for the precip lifting out of SW MO. Limited thunder potential for a
just a few hrs this evng across the ern Ozarks. Timing on the front
still looks good with winds increasing significantly after FROPA. Not
sold on accumulating snow late tonight into early tomorrow morning due
to lack of cold enough air being advected into the area to cool the
bndry layer enough to switch over to all snow...let alone cold enough
for accums. Think that by the time cold enough air makes it into the
area the precip will be moving out. Plus...2 inch soil temps are in
the 40s across NE MO and W cntrl IL. Have not pulled the accums but
wouldn`t be surprised if there wasn`t any either.

2%

&&

.SHORT TERM: (Through Monday)
Issued at 346 PM CST Sun Nov 23 2014

Main concern for tonight will be will there be any snow accumulation
over northwestern parts of the area as rain changes to snow behind a
sharp cold front. Most of the CWA has seen rain today and the
current band which is being forced by strong moisture convergence
and strong ascent ahead of ejecting shortwave trough will move
northeast of the CWA this evening. This will cause a lull in the
precipitation this evening before a second area currently over the
Plains moves into the area later this evening and overnight. This
area is being frontogenetically forced ahead of a second trough
currently over the northern Plains. Attendant cold front that
currently extends across central Iowa into far northwest Missouri
into eastern Kansas will push eastward this evening, reaching St.
Louis by midnight and east of the CWA by 3 am. Forecast soundings
suggest that the rain will mix with and then changeover to snow
after midnight before the frontogenetical band lifts out between
midnight and 6 am. Current indications are that snowfall amounts
will be less than 1 inch along and north of a Shelbina to Quincy
line given the relatively warm soil temperatures (Novelty Missouri
Mesonet 2 inch soil temperature is currently 50 degrees) and the
relative short amount of time that the accumulating snow will
fall. It will be windy late tonight into tomorrow as decent
gradient winds set up behind the cold front. However it does not
appear that we will hit advisory criteria at this time.

Do not expect much clearing tomorrow as forecast soundings show some
minimal CAPE in the lowest levels tomorrow afternoon suggesting some
isolated snow or rain showers will develop in the low level cyclonic
flow. Highs tomorrow are only a few degrees warmer than lows tonight.

Britt

.LONG TERM:  (Monday Night through Next Sunday)
Issued at 346 PM CST Sun Nov 23 2014

(Monday night through Wednesday)

Next notable trough that will move through the area will be on
Wednesday which the ECMWF and GFS both show light QPF across the
CWA. Have increased PoPs a bit across the area, with the highest
amounts over the northern CWA where current track of the surface
low will be. Temperatures will stay below normal during the period.

(Thursday through next Sunday)

Overall confidence in forecast during this period is not high as the
both operational GFS and ECMWF and their ensembles have had continuity
issues the past few days with the timing and strength of the troughs
that will move across the area later this week. Latest runs are showing
quasi-zonal flow during the period with shortwave troughs moving quickly
across the area. While the GFS is developing some QPF over the region
on Saturday, this is different than yesterday`s run, so will discount
for now. Will go dry during this period given the inherent difficulties
in timing precipitation in zonal flow.

Britt

&&

.AVIATION:  (For the 06z TAFs through 06z Monday Night)
Issued at 1111 PM CST Sun Nov 23 2014

A cold front had already moved through KCOU and KUIN but had not
yet reached the St. Louis metro area TAF sites as of 05z. Expect a
marked wind shift at KSUS/KCPS when fropa occurs between 06-08z.
An area of rain was also moving northeastwards towards the TAF
sites and had already reached KCOU at TAF issuance. Rain will
persist for several more hours then taper off. The rain could
briefly mix with snow at KCOU and KUIN just as the precipitation
is ending, but upstream METARs show that the post-frontal air
mass is warmer than previously anticipated, thus accumulating
snowfall looks less likely than it did during the previous TAF
cycle. Instability snow showers are possible during the afternoon.
Winds should start to taper off once the pressure gradient relaxes
by the late afternoon or early evening.

Specifics for KSTL: Expect a marked wind shift between 06-07z when
a cold front moves through the terminal. Rain will persist for
several more hours then taper off. Instability snow showers are
possible during the afternoon. This TAF is optimistic in that it
depicts a return to VFR cigs by late afternoon/early evening.

Kanofsky

&&

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

$$

WFO LSX





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