Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 280856

Area Forecast Discussion
256 AM CST Sat Feb 28 2015

.SHORT TERM...(Today through Monday)

First of two significant winter weather systems poised to begin
spreading across the area later today. Latest water vapor imagery
showing an upper trough along the Montana/North Dakota border, with
a strong southern stream jet across the southern tier of states.
Upper low over Oregon will be sinking southward into California,
which will help orient the southern stream into a southwest-
northeast configuration in time, with various disturbances riding
along it toward our area.

Latest guidance indicates the morning hours will remain dry, but
rapid development of light snow is expected to our southwest toward
midday as isentropic lift strengthens into the afternoon. The
forecast was updated during the evening to hit the PoP`s harder
during the afternoon, and have kept this basic philosophy with some
minor refinements. Bulk of the snow threat will be from this evening
into Sunday afternoon. However, the models have continued with their
slight southward trend in the 00Z runs, shifting more into the I-72
to I-70 corridor. Will issue a winter weather advisory for most of
the area except the extreme northwest CWA during this period. While
we cannot rule out some areas seeing just over 6 inches, the
duration of it (about 24 hours) would be too long to justify a
winter storm warning. However, the general thought is more of a
broad 4-6 inch total, with lesser amounts across the northwest and
southeast parts of the CWA. There also starts to be a precip type
question on Sunday. Forecast soundings north of I-70 generally
support all snow, but a rain/snow mix is more likely along and south
by late morning as temperatures rise just above freezing. Have also
added a short period of freezing rain toward Flora and Lawrenceville
during the morning before temperatures rise above freezing.

Any lingering precipitation should exit the southeast CWA during the
evening on Sunday, with Monday dry as an area of high pressure moves


.LONG TERM...(Monday night through Friday)

An active split flow pattern to amplify into a large central U.S.
trof by the middle of the week as energy from the northern stream
phases with the southern stream upper wave. Very dynamic subtropical
jet will aid in the strong moisture transport northward into the
Midwest starting late Monday ahead of our next weather maker with
precipitable water values by Tuesday in the 1-1.25 inch range, some
2-3 standard deviations above normal for early March. This will be a
concern for area rivers and streams as the snow melt combined with
0.50 to 1 inch of rainfall will lead to significant rises along many
of our basins by midweek.

Strong warm advection/isentropic ascent will lead to a rapid increase
in clouds late Monday night with precipitation breaking out late
evening or just after midnight. Models have trended a bit slower with
the onset of the precip but all forecast soundings continue to suggest
a period of sleet and freezing rain will develop from southwest to
northeast late in the evening and encompass most of our forecast area
by morning. Ice accumulations Monday night still look as if it will
average a tenth of an inch or less with temperatures slowly rising
overnight. Have most of the area above freezing by 15z/9am Tuesday
morning with rain expected for the remainder of the day with even
some isolated thunder possible in the south as forecast soundings
across southern Illinois indicate some elevated instability during
the afternoon hours as the deepening surface low tracks to our north
dragging a cold front through our area by evening.

The strong surface low will push well to our north but the boundary
will start to slow down as it becomes parallel to the upper flow
late Tuesday night into Wednesday. Highest POPs will shift into
southeast Illinois Tuesday night into Wednesday when models indicate
another wave to push east across the southern Plains and at the same
time an intensifying jet with the right rear quadrant and enhanced
upper level divergence/lift over the Ohio River valley during the
day Wednesday. In response to the upper wave and jet lift, models
indicate a surge northeast with precip again by Wednesday afternoon
which would bring the threat for snow to southeast Illinois into
the late afternoon or early evening hours before the lift departs
Wednesday night. GFS is a bit more progressive with the wave and
does not show as much precip building back to the north as the
ECMWF. For now will continue to support the more conservative look
to the GFS with just chance POPs over southeast IL during the day
with just slight chances for light snow Wednesday night as the
system pulls away from our area.

After that, colder air moves in for a brief stint as high pressure
settles over the region on Thursday with our coldest overnight lows
Thursday morning when we will see single digits CWA-wide. The cold
air will not hang around very long as the surface high slips off to
our east by Friday as a northern stream shortwave tracks to our
north for the end of the week. This will bring a southerly flow back
into our area with afternoon temperatures by Friday rising back into
the 30s, with 30s and 40s by Saturday before a cold front sweeps
across our area Saturday afternoon. No significant rain or snow is
expected with the late week shortwave.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)

Mid and high clouds will overspread the area overnight. Then in
the morning, beginning at SPI and then spreading east, lower
clouds around 5-6kft will move into the area and effect all the
TAFs. Now looks like the snow will arrive little sooner and begin
to effect the TAFs in the late afternoon. So will be bring snow
into SPI at 21Z and then the other sites after that. Conditions at
the beginning will be IFR but then 3hrs later, conditions will
drop to LIFR for SPI/DEC/CMI. With heaviest snow being along just
south of I-72, PIA and BMI conditions will not be as bad as the
other three. So, will keep IFR conditions at PIA and then LIFR
conditions at BMI, just a little better than at SPI/DEC/CMI.
Models showing a short break in intensity of the snow late
evening, but for now, will not indicate that in the TAFs. Winds
will be light and variable overnight and then become easterly,
then southeasterly tomorrow. Wind speeds will remain less than 10kts.


WINTER WEATHER ADVISORY from 6 PM this evening to 6 PM CST
Sunday FOR ILZ029-031-036>038-040>057-061>063-066>068-071>073.



SHORT TERM...Geelhart
AVIATION...Auten is the U.S. government's official web portal to all federal, state and local government web resources and services.