Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 212156
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
356 PM CST Sun Dec 21 2014

.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 18z TAFs through 18z Monday Afternoon)
Issued at 1132 AM CST Sun Dec 21 2014

Southeast flow at the surface is helping to bring in drier air, so
cigs have lifted to low end VFR at taf sites along I-70. KUIN
should see cigs lift to low end VFR by 00z Monday. Otherwise,
winds to persist from the southeast to south through forecast
period. Next weather system to approach region tonight, will see
rain develop over western MO and spread east, moving into KUIN and
KCOU by 14z Monday and metro area by 18z Monday. When rain begins
to move in, will see cigs/vsbys lower to mvfr.

Specifics for KSTL:
Southeast flow at the surface is helping to bring in drier air, so
cigs have lifted to low end VFR in metro area. Otherwise, winds
to persist from the southeast to south through forecast period.
Next weather system to approach region tonight, will see rain
develop over western MO and spread east, moving into metro area by
18z Monday. When rain begins to move in, will see cigs/vsbys lower
to mvfr.

Byrd

&&

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

$$

WFO LSX





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