Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 220513
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1113 PM CST Sun Dec 21 2014

.UPDATE:
Issued at 925 PM CST Sun Dec 21 2014

Cloudy skies have moved into all but the far eastern part of the
CWA at this hour. Latest trends suggests that this area will cloud
over within the hour, so expect the entire area to return to being
cloudy the rest of the night. With clouds and a bit higher winds,
do not expect temperatures to fall more than a few degrees tonight,
so raised going temperatures a bit in a few areas. Expanded the
slight chances for light rain eastward late tonight across eastern
Missouri as low level moisture convergence will increase ahead of
weak shortwave trough that is rounding base of trough currently
over the southern Plains.

Britt

&&

.SHORT TERM:  (Through Late Tonight)
Issued at 334 PM CST Sun Dec 21 2014

Low level moisture will be on the increase late tonight ahead of
next weather system, but rain will not move into central and
northeast Missouri, west central Illinois til after midnight.
Otherwise, cloudy conditions to persist with low temperatures only
in the low 30s over far eastern sections to around 40 over central
Missouri, so do not need to worry about any freezing or frozen
precipitation.

Byrd

.LONG TERM:  (Monday through Next Sunday)
Issued at 334 PM CST Sun Dec 21 2014

(Monday - Wednesday)

All guidance based off of the 21/12z model cycle still giving strong
support to going forecast trends that indicate a rainy Monday is in
the offing for most of the CWA.  Upper trof over the central CONUS
will be deepening over the next 12 hours as several shortwaves work
into the mean trof position, with the strongest of these pushing
into the mid MO valley on Monday morning and closing off into an
upper low over IA by Monday afternoon and evening. As this system
approaches late tonight and early Monday, southerly flow will
increase through the lower troposphere leading to a significant
deepening of the moisture over the region.  Ascent from the
approaching upper level dynamics combined with the low level
isentropic ascent should produce a fairly rapid increase of rainfall
across most of the area during the morning hours, with this rain
continuing over much FA into the afternoon.

Dry slot will be wrapping into the system Monday night as the system
occludes, with model consensus of the translation of the dry slot
suggesting that rainfall will be diminishing from NW to SE across
the CWA on Monday night.  By 12z Tuesday, relatively dry air will be
over NW sections of the CWA, while on the se fringes of the dry
slot deep moisture and baroclinicity will draped from AR into the
lower Ohio Valley. will carry a chance of precip over about the
SE 2/3 of the CWA on Tuesday, with highest PoPs over SE sections
of the FA closer to the aforementioned moisture and baroclinic zone.

This leads into a very tricky forecast heading into midweek, as
longwave trof lingers over the CONUS. This is one of the trickier
patterns of the winter season, as models often struggle with speed
and intensity of shortwaves that are meandering through the trof.
And in this case, forecast concerns are amplified due to the fact
that strong low level baroclinicity and deep moisture...two key
ingredients for strong synoptic scale winter systems...will be
lurking just off to our SE.  Models have already been struggling
with the evolution of this pattern... last nights 00z GFS and off-
cycle 06z NAM runs hinted at low closing off to our south with
accumulating snow for a large part of our FA, but 12z runs are
less bullish on this forecast, and suggest more of an open system
that will produce heavier precip in the warmer air over parts of
the Ohio Valley and areas east.

In this pattern, odds are extremely high that individual solutions
will continue to flip-flop on exact trends, so at this juncture feel
most comfortable leaning on model consensus for the large scale
forecast details.

In general, the weather on Tuesday night and Christmas Eve Day
should become a bit more unsettled as another shortwave enters
the base of the longwave trof and the entire system begins to push
east. This should certainly generate another wave of precip, but
the big question is where.  GFS remains the most robust model in
regards to precip during this time, with measurable QPF over about
the eastern half of the CWA, while all the other solutions focus
this axis from extreme e AR into KY and TN. Forecast soundings
indicate the low level AMS over our SE counties (where the QPF
would be heaviest) will be too warm for any frozen precip Tuesday
night, but may cool off enough to support some snow during the day
on Wednesday as the precip winds down. Thermal profile looks more
favorable for snow over our NW counties, but here the precip
should be quite light.

At this point, accumulating snow in any part of our CWA look
iffy. But once again this is a tricky pattern...so holiday
travelers should continue to monitor the latest forecasts.

(Christmas Day - Sunday)

It now appears that Christmas Day will be fairly tranquil and
relatively mild as upper trof pushes east and ridging temporarily
allows warmer air to build into the central CONUS.

Medium range consensus remains fairly good into Friday, as a lead
shortwave begins the process of developing another trof axis over
the central CONUS.  Have maintained some slight chance PoPs as the
shortwave pushes through the region.

Although medium range models diverge heading into next weekend
with the location and intensity of the upper level trof, lowering
of heights should yield below normal temps.

Truett

&&

.AVIATION:  (For the 06z TAFs through 06z Monday Night)
Issued at 1103 PM CST Sun Dec 21 2014

Focus continues to system to impact the region on Mon. Latest
guidance, while not having a great handle on current conditions,
continues to suggest prev TAFs are on track. Believe conditions
will deteriorate shortly after sunrise Mon. Cigs shud lower to low
end MVFR as well as visbys reduced into MVFR range due to either
RA, BR or both. Latest indications suggest RA may have a more SHRA
component, but during dry periods, while brief, visbys shud still
be reduced due to BR. Have low confidence on visbys on Mon and may
drop to IFR at times. Confidence in IFR at SUS/CPS remains low and
will keep trends going for now. Winds will veer to eventually swly
Mon evening with visbys and cigs improving with this wind shift.

Specifics for KSTL: VFR and dry expected thru the overnight hours
with conditions deteriorating shortly after sunrise. Confidence in
IFR conditions is still too low and will therefore keep sct cloud
deck mentioned for now. Conditions will improve Mon late
afternoon or evening as winds become swly.

Tilly

&&

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

$$

WFO LSX





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