Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 022150
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
350 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 18z TAFs through 18z Tuesday Afternoon)
Issued at 1212 PM CST Mon Mar 2 2015

Mid and high clouds to prevail today with winds from the east.
Next weather system to approach region tonight with cigs gradually
lowering. With onset of precipitation will see cigs lower to IFR.
Precip to start off as light freezing rain then transition to rain
between 11z-13z Tuesday as warmer air continues to advect north
into region as winds veer and pickup from the south.

Specifics for KSTL:
Mid and high clouds to prevail today with winds from the east.
Next weather system to approach region tonight with cigs gradually
lowering. With onset of precipitation around 07z Tuesday will see cigs
lower to IFR. Precip to start off as light freezing rain then
transition to rain around 11z Tuesday as warmer air continues to
advect north into region as winds veer and pickup from the south.
Cold front to move through metro area around 22z Tuesday as winds
veer to the west. Not sure on coverage of precipitation with
frontal boundary, so just added vcnty shower mention after 22z Tuesday.

Byrd

&&

.PRELIMINARY POINT TEMPS/POPS:
Saint Louis     29  50  20  23 /  60  70  50  40
Quincy          27  41  13  19 /  70  70  10  10
Columbia        32  48  18  24 /  50  50  20  30
Jefferson City  32  50  20  24 /  50  50  20  40
Salem           27  45  23  26 /  70  90  60  50
Farmington      31  49  23  25 /  70  80  70  60

&&

.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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