Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 011000
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
400 AM CST Sun Mar 1 2015

.SHORT TERM: (Through Tuesday Night)
Issued at 400 AM CST Sun Mar 1 2015

The busy stretch of wx thru mid-week continues.  This major winter
storm is winding down slowly across our region with another spat or
two of winter wx set for Monday night and again on Tuesday night and
perhaps into Wednesday.

Precipitation across our area early this morning can be divided into
two main regions by primary mode of forcing.  A band of snow
stretching across much of northern MO is being driven by
frontogenesis with enough moisture thru the column.  The swath of
pcpn mainly to the S and E of STL Metro is being driven by WAA and
ranges from snow around I-70 and I-64 in IL to sleet and freezing
rain for areas mainly south of Farmington, MO.

Moisture thru the column will hold steady thru much of the morning
before drier air begins to filter in from the north.
Frontogenetical forcing across the north will weaken and exit by
mid-morning but a new area of frontogenesis is anticipated to move
in along the I-70 corridor this afternoon from west to east.  But by
this time, moisture thru the column will be in decline and so the
band of pcpn that will likely result will be something that is
weaker than what we see across northern MO now.  If the model
estimates of moisture are wrong and/or the narrow nature of the
frontogenesis is stronger, we may end up getting a narrow band of
snow that adds 1-2" more than what we have forecast.  Probably a
feature that will need to be nowcast more aggressively once it
finally appears late this morning-midday.

The WAA-generated pcpn is expected to weaken and/or exit our CWA by
late morning.

Add to all of this some dry-slotting near and just south of I-70
this morning with weak lo-level convergence resulting in patchy
freezing drizzle.

Pcpn-types should remain all snow for areas north of I-70 for the
duration of this event today.  Sleet is anticipated to mix in with
the pcpn that falls as far north as the I-70 corridor during the day
today, while freezing rain should be confined to areas along and
south of a Crawford County MO to Washington County IL line, but even
then freezing rain should be done by midday as temps rise just above
freezing.

Putting it all together, additional snow accums of 1-2" appear
likely thru late this afternoon for areas near and north of I-70 in
MO and near and north of I-64 in IL.  Additional snow and sleet
accums of less than an inch for elsewhere.  Light glazing of ice is
likely for much of southeast MO and southern IL for this morning.
Plan on keeping headlines the same for the vast majority of the
counties, but will tack on Marion IL and Washington IL to the
warning with what has already fallen and over an inch of new snow
still expected.

After a break in the action Sunday night and Monday, strong WAA will
generate a new round of pcpn Monday night as a surface warm front
passes thru and resulting in rising overnight temps.  The pcpn-types
should overwhelmingly be liquid, with some sleet possible in far
northern MO and central IL.  Unfortunately, it looks like there will
remain enough cold air in place at the surface early in the event
before the surface temps have had a chance to rise too much to
result in some light freezing rain for much of the area.  It should
be short-lived, with temps by 12z/Tue expected to be above freezing
for all locations.

Rounds of showers with some thunderstorms are still on tap for
Tuesday with an 18-hour period or so of mild temps, as they rise
well thru the 40s and into the 50s for some areas.  A strong cold
front will move thru late Tuesday afternoon and evening and drop
temps rapidly back below freezing.  But much of the pcpn will be
done by then with minimal impact from snow expected.

Large model discrepancies are in place for Wednesday but the
consensus now is to suppress pcpn to our south.  The NAM is a big
outlier here, forecasting another round of heavy snow.  Given its
track record on Day 3, will forego using this for the forecast with
this package, but it is in the realm of possibilities with all
models showing varying degrees of a northern surge of pcpn on Wed.
Something to keep an eye on.

TES

.LONG TERM: (Wednesday through Saturday)
Issued at 400 AM CST Sun Mar 1 2015

(Wednesday through Saturday night)

Colder conditions are expected for Wednesday as a strong surface
ridge builds southeastward into MO from the northern Plains.  Will
keep a chance of precipitation across southeast MO and southwest IL
on Wednesday as a positively tilted upper level trough with embedded
shortwaves remains west of the forecast area.  The GFS model shifts
the precipitation southeast of our forecast area by Wednesday, while
the ECMWF still has light qpf across southern portions of MO and IL,
south of STL.  Precipitation type should be all snow with 850 mb
temperatures falling well below zero degrees C across the southern
portion of the forecast area on Wednesday.  Little diurnal rise in
temperatures is expected on Wednesday due to low level cold air
advection.  Low temperatures Wednesday night will be unseasonably
cold with the surface high dropping into our forecast area, and
850 mb temperatures of only -13 to -18 degrees C across the forecast
area.  Warmer temperatures can be expected for Thursday night and
Friday as the surface ridge moves east of our area and the surface
wind becomes southerly.  A cold front will move southeastward
through our area late Friday night and Saturday.  Precipitation is
not expected with this front due to very limited moisture and only
weak upper level forcing.  Only slightly cooler air will filter into
the region behind the front Saturday and Saturday night.

GKS

&&

.AVIATION:  (For the 06z TAFs through 06z Sunday Night)
Issued at 1140 PM CST Sat Feb 28 2015

Widespread snow should become more scattered or banded late
tonight into early Sunday morning. It appears that the snow should
first taper off in COU, then eventually taper off in UIN and the
St Louis metro area towards morning. The visibilities should
continue to improve through the morning with the snow ending by
18Z Sunday. The low cigs around 1000-2000 feet in height should
scatter out Sunday evening as drier air filters southeastward
into the region. Light surface winds should become nwly Sunday
afternoon as a surface ridge builds southeastward into our area.

Specifics for KSTL: Snow should begin to taper off towards morning
and become less continuous with gradually improving visibilities.
The snow should end by 18Z Sunday. The 1000-2000 feet ceilings
should scatter out Sunday evening. The light surface wind will
become nwly Sunday afternoon.

GKS

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...WINTER STORM WARNING until Noon CST today FOR Crawford MO-
     Washington MO.

     WINTER WEATHER ADVISORY until Noon CST today FOR Iron MO-Madison
     MO-Reynolds MO-St. Francois MO-Ste. Genevieve MO.

     WINTER STORM WARNING until 6 PM CST this evening FOR Franklin MO-
     Gasconade MO-Jefferson MO-Lincoln MO-Osage MO-St. Charles
     MO-St. Louis City MO-St. Louis MO-Warren MO.

     WINTER WEATHER ADVISORY until 6 PM CST this evening FOR Audrain
     MO-Boone MO-Callaway MO-Cole MO-Knox MO-Lewis MO-Marion MO-
     Moniteau MO-Monroe MO-Montgomery MO-Pike MO-Ralls MO-Shelby
     MO.

IL...WINTER WEATHER ADVISORY until Noon CST today FOR Randolph IL.

     WINTER STORM WARNING until 6 PM CST this evening FOR Bond IL-
     Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-
     Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL-
     St. Clair IL-Washington IL.

     WINTER WEATHER ADVISORY until 6 PM CST this evening FOR Adams IL-
     Brown IL-Pike IL.

&&

$$

WFO LSX




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