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000
FXUS63 KLSX 270531
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1131 PM CST Thu Feb 26 2015

.UPDATE:
Issued at 922 PM CST Thu Feb 26 2015

Clouds along with flurries and a few light snow showers across
southeast MO and southwest IL, south and east of STL should
advect southeast of our forecast area by midnight due to
increasing subsidence as strong 1040 mb surface ridge across the
northern Plains builds southeastward into MO. Very cold
temperatures are expected tonight due to continued cold air
advection, very cold 850 mb temperatures of -15 to -20 degrees C,
a mostly clear sky late tonight, and diminishing north-
northwesterly surface winds. New snow cover across northeast MO
and west central IL will enhance radiational cooling. Wind chill
values will drop to around 15 below zero across this area late
tonight.

GKS

&&

.SHORT TERM:  (Through Late Tonight)
Issued at 327 PM CST Thu Feb 26 2015

Cold air continues to filter into the area as a strong surface
ridge noses into the area. The final glimpses of light snow
flurries are coming to an end near the St. Louis metro area, with
maybe another hour or two before the snow completely cuts off.
Regardless of how much longer the flurries continue, further
accumulations through the evening are very unlikely, so have
generally trended PoPs downward through the rest of the evening.
Overnight lows will be rather chilly as the surface high moves
overhead, with temperatures on Friday morning reaching between 0
and 10 degrees along and south of I-70, with temperatures between
0 and -5 across areas north of I-70. With the new snow pack over
far northern Missouri and good radiational cool and little to no
wind, there could be few isolated spots dropping to between -5 and
-10 over far northern Missouri and Illinois. Since the surface
high will be overhead for the night expect winds to drop off to
around 0 to 5 mph, especially over the snow pack where the surface
ridge will be more centered. This will help keep the wind chill
values from plummeting too much. With WC values generally in the
-10 to -15 range in far N Missouri and Illinois have opted to
forgo a wind chill advisory for that area, at least for now. Will
let the evening and midnight shift take one more look at it.

.LONG TERM:  (Friday through Next Thursday)
Issued at 327 PM CST Thu Feb 26 2015

With the cold air solidly in place over the are it will set the
stage for a winter system to affect a large portion of the
forecast area for the weekend and again early next week. Broad
troughing over the northern part of the CONUS will bring mostly
zonal mid level flow over the forecast area. As a more potent area
of low pressure energy digs into the west coast a low level trough will
take form over the interior mountain west and start a flow of warm
air advection into the area. With the warm/moist air
isentropically lifting over the cooler air in place it will
gradually re saturate the atmospheric column and start to
precipitate Saturday afternoon to evening across central Missouri
and probably Saturday night for eastern Missouri and western
Illinois. Since the thermal profile will be very cold, expect this
activity to be all in the form of snow. At least for now it does
not appear that P-type will be in much question, especially for
the Saturday night period. At this time it does not appear that
there will be much in the way of heavy snow, but the prolonged
nature of the warm air advection, as the low stays anchored over
the west coast will provide at least 24, and perhaps 36 hours of
very light snow, which will likely accumulate several inches
through the weekend. Right now the models still favor a track of
the higher snow amounts to be generally along and north of
Interstate 70, and this makes sense since this corridor will have
the better chance to maintain a cool enough thermal profile to
keep everything snow. Farther south, in southern MO and IL there
is a chance that some warmer low level temps could creep into the
thermal profile, which would maybe cause some mixed precip types
on Sunday. Given some lingering model discrepancy the confidence
on specific amounts in a given location are still somewhat low to
medium, however it does appear that areas along and north of I-70
will see several inches of accumulating snow, perhaps getting up
to 4 to 6 inches. As stated above, standard uncertainty caveats
apply, so amounts and locations will likely shift as we approach
the event.

The focus then shifts to early next week as another wave of energy
ejects out of the desert southwest and into the forecast area.
This trough will be a bit more potent than the weekend system, and
will likely draw in more low level warm air and more moisture. At
this point both EC and GFS would hint toward a large portion of
the forecast area, especially areas south of I-70 getting mostly
rain, with a few hours on either side of the rainy period of
wintry mix as the thermal column warms up. Confidence is rising
that there will be at least a decent amount of precipitation over
a widespread area through the early to mid week period, but again,
thermal column uncertainty will preclude any hard amounts and
locations for any specific precip type. By late next week another
strong cold front will sweep through and scour out the moisture,
effectively ending chance for PoPs and ushering another cool down
for the end of next week.

&&

.AVIATION:  (For the 06z TAFs through 06z Friday Night)
Issued at 1121 PM CST Thu Feb 26 2015

For COU/UIN...Sfc ridge builds into the region overnight and
Friday. Nly winds will persist thru the TAF period and become
light and vrb Fri afternoon. Expect CU to develop late
morning/around Noon and become bkn persisting into the afternoon.
CU dissipates in eve leaving ovc cloud deck with bases around 9
kft.

Specifics for STL/SUS/CPS: Sfc ridge builds into the region
overnight and Friday. Nly winds will persist thru the TAF period
and become light and vrb Fri eve. Expect CU to develop late
morning/around Noon, but do not anticipate bkn cigs. CU dissipates in
eve leaving ovc cloud deck with bases around 9 kft.

Tilly

&&

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

$$

WFO LSX