Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 261157
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
557 AM CST Fri Dec 26 2014

.SHORT TERM:  (Through Late This Afternoon)
Issued at 347 AM CST Fri Dec 26 2014

With a cold front located upstream and poised to move through the
area tomorrow, it appears that the unseasonably warm December
weather will be coming to an end during the upcoming weekend.
However, the warm weather of Christmas Day will certainly linger
into today, with highs in the low to mid 50s. These readings will
be some 10-15 degrees above average, and will make it feel more
like a typical day in late November or early March. Clouds should
start increasing this afternoon due to increasing moisture and
lift ahead of a low pressure system.

Kanofsky

.LONG TERM:  (Tonight through Thursday)
Issued at 347 AM CST Fri Dec 26 2014

(Tonight through Sunday)

Primary challenge heading into the weekend is pinning down PoP
trends.  Increasing southerly low level flow that commences today
and intensifies tonight will certainly be transporting decent low
level moisture into the area, with forecast soundings from both the
NAM and GFS indicating saturation occurring in the lowest 5-10kft by
Saturday morning.  However, mid level moisture is much more scant
and lift well ahead of the approaching front doesn`t appear to be
all that intense and/or focused, so am wondering if the precip
threat based on 00z MOS PoPs and some of QPF output is misleading.

In general, expect a increasing threat of rain overnight from west
to east as the moisture and lift slowly increases across the
region.  Have gone a bit higher with PoPs in our NW counties than in
current forecast, as model consensus indicates an increase of mid
level moisture in this area along with somewhat better lift tied to
approaching front.  Have also upped late night PoPs in our far south
with some fairly strong 85h moisture advection indicated in this
area by 12z.

The precip that does form overnight will wind down from NW to SE on
Saturday and Saturday evening, but with upper trof lagging cold
front by a considerable distance am hesitant to end the precip too
quickly.  With the low level cold air surging south, it`s possible
that there could be some ptype issues if the precip can linger long
enough by the time the freezing level drops to critical levels.
Right now it looks like the Schmocker Rule will be in effect with
most of the precip shutting down before AMS cools enough to support
snow, but due to the lag of the upper trof have added a bit of a
rain/snow mix just as the precip comes to an end.

Temperatures tonight and into early Saturday will certainly be on
the mild side due to the fairly strong and persistent southerly
winds, but the mercury will be taking a tumble during the day on
Saturday and into Saturday evening as the cold front pushes across
the area. Have attempted to reflect this thinking with tonight`s
lows and Saturday`s highs derived from a 3 hourly, non-diurnal
temp trend in the gridded data set.

This colder air will mean a return of more "typical" late December
temperatures by Sunday, with highs primarily in the 30s along with a
fair amount of sunshine.

(Monday-Thursday)

Medium range solutions in fairly good agreement in developing a W-E
trof from S Canada into the western U.S. in the early part of next
week.  The trof is then forecast to sweep across the central
CONUS by midweek while energy in the base of the trof breaks off and
forms a cut-off low over CA.  Passage of the trof in our area should
usher in some arctic air in the Tuesday-Wednesday time frame, but
temps should moderate a bit by Thursday as the trof and coldest air
works into the eastern U.S.

Much of the medium range looks dry with limited dynamics and
moisture progged across area.  However, this could be changing by
the end of the medium range period (next Friday) as the upper low
over CA opens up and is ejected eastward.

Truett

&&

.AVIATION:  (For the 12z TAFs through 12z Saturday Morning)
Issued at 534 AM CST Fri Dec 26 2014

Specifics for KUIN, KCOU: VFR conditions are expected for the
first 0-6 hours of the TAF period before increasing moisture leads
to lowering cigs after 18z. Rain will spread across MO/IL after
27/00z, and the additional moisture should cause ceilings to fall
to IFR overnight. Just beyond the end of the valid TAF period, a
cold front will move through the area and cause a wind shift.

Specifics for KSTL, KSUS, KCPS: VFR conditions are expected for
the first 0-6 hours of the TAF period before increasing moisture
leads to lowering cigs (likely MVFR) after 18-21z. Rain will
spread eastward towards the terminals after 27/03z. IFR cigs are
possible after 27/09z depending on the extent of rain coverage.
Expect a wind shift around 27/18z when a cold front passes through
the area. BUFKIT profiles depict marginal LLWS conditions between
00z and 09z.

Kanofsky

&&

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

$$

WFO LSX





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