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Area forecast discussion...updated aviation 
National Weather Service St Louis MO
609 PM CST Tue Mar 3 2015

Short term: (late tonight through Wednesday evening)
issued at 405 PM CST Tue Mar 3 2015

12z guidance depicts no major changes in expected weather trends
across the area. The next round of cold air is just now entering
the extreme northwest counties of our cwa, and will attempt to
undercut the bands of rain that continue to develop near the Kansas/OK
border and advected east-northeast into the mid-Mississippi Valley.

The cold air will become deep enough to begin a transition of the
rain to sleet and perhaps freezing rain as the night progresses,
but as the cold air becomes deep enough to create a mix from
central MO into central Illinois it appears that the precipitation will
be winding down. However, it should be a different story a bit
further south late tonight and into Wednesday morning, where the
precipitation will be more sustained as the cold air reaches
critical depths. Overall, the general trend overnight across the
southeast half of the County Warning Area should be for the rain to first become
mixed with sleet and freezing rain as the night progresses. The
continued southward progression of the cold air will then change
this icy mix over to all snow near the I-44 corridor early
Wednesday, with the precip in our far southern counties changing
over to all snow no later than midday, and then winding down
Wednesday evening.

The precipitation late tonight and into Wednesday has a lot going
for it. This mornings upper air data indicated 10c 850mb dewpoints
already into SW MO, and this moisture will increase into
Wednesday as the SW low level flow just above the cold air
continues the moisture transport. Lift generated by already strong
isentropic ascent will be increasing as baroclinicity strengthens
to our south, with the southwest flow aloft ejecting several
shortwaves into the mid Mississippi Valley enhancing the deep
lift further. Believe the intensity of the lift is showing up in
most of the synoptic models as they all have some indication of
the upper level jet over the upper Great Lakes backbuilding into
our area with time.

Quantitative precipitation forecast and thermal profiles suggest heavy wintry precipitation is
likely from Arkansas into southern MO and southern Illinois. Snowfall
generated from these parameters indicate 4 to 6 inches of snow/sleet can
be expected over our far southern counties, and because of this
snowfall along with the light glazing produced by the freezing
rain will be going out with a Winter Storm Warning for late
tonight into Wednesday evening for areas along and south of a
Centerville...Sparta line. Just north of the warning area will be
a Winter Weather Advisory from the Eastern Ozarks into southern
Illinois, for lesser snowfalls of 2 to 4 inches along with a
possible glaze of ice.

It should be noted that there will be an extremely tight gradient
on the northern edges of the snowfall...roughtly from near
Jefferson City...through St. around Vandalia Illinois. If the
system shifts slightly north the advisory may need to be expanded
into these areas...this evenings and overnight shifts will be
monitoring upstream radar trends to fine tune this tight snowfall gradient.

Ah Spring...


Long term: (thursday through next tuesday)
issued at 320 PM CST Tue Mar 3 2015

As upper level long wave trof pushes east, the Arctic air that
enters the region overnight tonight will continue to build and
strengthen over the mid Mississippi Valley. Although the high will
mean few clouds and dry weather, temperatures of this airmass will
be extremely cold for early March. Currently, we are looking for
Thursday morning lows somewhere in the 5 to 12 above range, with
highs only rebounding into the 20s during the afternoon even with
what should be strong sunshine.

Unseasonably cold temperatures will linger on Thursday night, but
return flow should mean the beginnings of a welcomed warmup by Friday.

A split flow regime will initially be in place at the start of the
extended range period featuring a broad longwave trof centered
across eastern noam and a quasi Rex block along the West Coast. The
Block will break down by early next week and the overall strength of
the trof weakens, however we largely maintain northwest flow aloft
into Monday. There will be several notable disturbances embedded
within the flow aloft which will traverse the region, bringing a
cold front through the area on Saturday afternoon/night and then
again on Sunday night. Present indications are that cooling
associated with these fronts will be weak and short-lived. In
general the weekend will be characterized by near average
temperatures and benign/tranquil weather conditions. Next week will
feature a pronounced pattern change, rising heights aloft and above
average temperatures for much of the week.



Aviation: (for the 00z tafs through 00z Wednesday evening)
issued at 546 PM CST Tue Mar 3 2015

MVFR ceilings will last through much of the night at all taf
sites as cold front pushes through and stratus deck remains. KUIN
and kcou will likely be VFR by 10-13z Wednesday, then VFR ceilings
reach kstl taf sites by around 16z. Will have to watch radar and
surface reports carefully this evening as there is some chance
that precip aloft now seen on regional radars may reach the surface at
kstl area taf sites. For now, it appears a dry layer at 6k to 8k
feet may keep this from happening so have left tafs dry. Northwest
winds behind the front will become gusty and last into Wednesday.

Specifics for kstl: MVFR ceilings expected until 16z. Cold front
should reach taf site by around 06z, and north winds behind the
cold front will increase to around 14 knots and become gusty.
Have kept taf dry for now, but will have to watch surface reports
to see if precip aloft now seen on radar begins reaching the
ground this evening. Expect dry air aloft to keep most precip just
to the south of kstl on Wednesday, but it is a close call.



Lsx watches/warnings/advisories:
MO...Winter Weather Advisory from midnight tonight to 6 PM CST
Wednesday for Crawford MO-Jefferson MO-Washington MO.

Winter Storm Warning from 3 am Wednesday to midnight CST
Wednesday night for Iron MO-Madison MO-Reynolds MO-St.
Francois MO-Ste. Genevieve MO.

Illinois...Winter Weather Advisory from midnight tonight to 6 PM CST
Wednesday for Clinton Illinois-Marion Illinois-Monroe IL-St. Clair Illinois-
Washington Illinois.

Winter Storm Warning from 3 am Wednesday to midnight CST
Wednesday night for Randolph Illinois.



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