Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 030028
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
628 PM CST Mon Mar 2 2015

.SHORT TERM:  (Through Late Tonight)
Issued at 350 PM CST Mon Mar 2 2015

Interesting forecast for tonight, as return flow develops in the
wake of the retreating surface ridge.  Resultant WAA/lift and
moisture return should mean precipitation threat ramping up as the
evening progresses, generally right over the CWA and in the
residually cold and relatively dry low level AMS.  Soundings over
the northeast half of the CWA have a produced warm wedge aloft (as
much as +5C around 850mb) with low levels remaining below freezing.
With the above in mind, the key to the forecast is how rapidly
surface temperatures will rebound after a brief bit of cooling this
evening.  Low level warm advection should also be attempting to push
surface temps above freezing with time,  but think that this will be
a rather slow process with the existing snow cover and believe that
the evaporative cooling that occurs once the precip starts may
temporarly retard the slow northward push of the warmer air.  In our
far north, evaporative cooling processes are likely to cause a bit
of sleet and perhaps even some snow to occur along with the freezing
rain.

Although QPF will be quite light, am concerned about the below
freezing surface temps and the cold ground causing icing conditions
on untreated pavement.  Based on this reasoning, have opted to issue
a winter weather advisory for about the northeast half of the CWA
(along and north of a Mexico-St Louis-Salem IL line), where the
coldest surface temps and slightly higher QPF will be co-located.
Uncertain as to exactly when the precip will start, but think the
"heaviest" will likely be during the overnight hours so will begin
the advisory at 06z.   Will run it long enough to the morning rush
hour...14z Tuesday.

Obviously, additions and/or deletions to the advisory area may be
needed as precip and temp trends become a bit more clear.

Truett

.LONG TERM:  (Tuesday through Next Monday)
Issued at 350 PM CST Mon Mar 2 2015

(Tuesday-Thursday)

Once the freezing rain threat ends early, primary forecast concern
on Tuesday is PoP trends, as warm advection continues lift and
moisture transport into the mid-Mississippi Valley.  Hard to get a
handle on exact precip trends as lift is rather broad and somewhat
unfocused, but there does seem to be some model consensus that
highest PoPs will become focused over the southeast half of the CWA
heading into the afternoon and early evening, with the strongest
moisture advection and lift along and ahead of approaching cold
front.  Instability looks even more meager than before, so have
removed thunder from forecast.

Winter weather concerns return to the forecast Tuesday night and
Wednesday as next round of cold Canadian air undercuts energetic
southwest flow aloft.  The rain over the southeast half of the CWA
will gradually change over to snow across the southeast half of the
CWA as the low level cloud air deepens, and by Wednesday morning
thermal profiles support all snow across the CWA.

All of the synoptic guidance suggests a potent shortwave ejecting
out of the longwave trof over the southern Rockes and pushing into
the southern Plains during the day on Wednesday.  Resultant WAA over
the top of the deepening cold air to our south is expected to
produce a  band of heavy precip from AR into the lower Ohio River
Valley.  Southern sections of our CWA will be on the northern
fringes of this heavier precip.  The last 2 days the NAM has been
the northern outlier with this band of precip and this trend
continued with the 12z run...so have continued to trend forecast to
the more southward solutions of the GFS, GEM and ECMWF.

Early indications are that snow accumulations of 4-6 inches will be
possible in our far southern areas, but a very slight shift in the
track of the system is going to have a huge impact on exact
amounts.  Coordinated the potential for a winter storm watch, but
since the system is still 2 days away opted to wait another forecast
cycle to see if we can get a better handle on the northern fringes
of the system.  However, snow potential will be highlighted in the
HWO and an SPS.

Current timing of shortwave is such that precip should wind down
rapidly on Wednesday evening across the south. Secondary surge of
colder air in the wake of the shortwave should mean clear, dry and
cold weather heading into Thursday.

(Friday through Monday)

Surface ridge to move to the south allowing southerly winds to
return to the region Thursday night.  After a cold start Friday
morning in the single digits to upper teens, temperatures to
moderate through the extended with highs in the mid 30s to mid 40s
Friday, rising into the 40s and 50s for the rest of the weekend and
into early next week. Dry conditions to prevail through the period.

Truett/Byrd

&&

.AVIATION:  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 537 PM CST Mon Mar 2 2015

Specifics for KUIN: A lifting warm front will spread warmer air
northward atop the colder air in place at the surface, creating
conditions for freezing rain and sleet for a couple of hours
overnight. There may also be some snow mixed in at times. Ceilings
will likely fall to IFR once precipitation begins. As
temperatures warm up above freezing, the rain will no longer
freeze on contact with colder surfaces. Just beyond the end of the
valid TAF period, winds will turn northwesterly behind a cold
front.

Specifics for KCOU: A lifting warm front will bring rain showers
to the area tonight and tomorrow, but it is possible that
precipitation will remain to the north and east of KCOU. If
any precipitation does occur at KCOU overnight, BUFKIT soundings
indicate that it would initially fall as light freezing rain until
temperatures rise far enough to keep the rain from freezing on
contact with cold surfaces. Ceilings will likely fall to IFR due
to the increasingly moisture and nearby rain showers. Just beyond
the end of the TAF period, a cold front will cause a sharp wind
shift from southwesterly to northwesterly.

Specifics for KSTL, KCPS, KSUS: A warm front will lift through the
region tonight. The warm air aloft spreading over the much colder
air at the surface will produce freezing rain and perhaps even a
brief period of sleet for a couple of hours overnight based on
BUFKIT soundings. The increasingly warm air mass spreading into
the region will eliminate the "freezing" part of the freezing rain
during the morning, but rain showers are expected to continue for
most of the day on Tuesday. Ceilings will likely fall to IFR once
precipitation begins. Even after the rain ends, lingering moisture
should be sufficient for continued IFR ceilings and a few hours of
drizzle until a cold front moves through after 04/00z. Initially
southeasterly winds become southerly then southwesterly behind the
warm front on Tuesday, then turn sharply northwesterly behind the
cold front on Tuesday night.

Kanofsky

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...WINTER WEATHER ADVISORY from Midnight tonight to 8 AM CST
     Tuesday FOR Audrain MO-Knox MO-Lewis MO-Lincoln MO-Marion
     MO-Monroe MO-Montgomery MO-Pike MO-Ralls MO-Shelby MO-St.
     Charles MO-St. Louis City MO-St. Louis MO-Warren MO.

IL...WINTER WEATHER ADVISORY from Midnight tonight to 8 AM CST
     Tuesday FOR Adams IL-Bond IL-Brown IL-Calhoun IL-Clinton IL-
     Fayette IL-Greene IL-Jersey IL-Macoupin IL-Madison IL-
     Marion IL-Montgomery IL-Pike IL-St. Clair IL.

&&

$$

WFO LSX






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