Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 242050

Area Forecast Discussion
250 PM CST Tue Feb 24 2015

.SHORT TERM...(Tonight)

Strong 991 mb low pressure near James Bay Canada has its cold front
extending sw through central WI/IA. Low clouds 3-5k ft have spread
southeast across the IL river and approaching SPI, DEC and Rantoul
and will spread into areas from I-72 south the rest of the afternoon
and early evening. Brunt of light snow showers pass north and ne of
central IL but could get some flurries along and ne of I-74 late
this afternoon and early evening. Low clouds decrease behind cold
front in nw IA and expect the clearing to be progressively moving se
across IL river valley during mid evening and into southeast IL
early overnight. Breezy wsw winds 15-25 mph and gusts of 25 to 35
mph to diminish to around 10 mph or less by late evening and
overnight as winds become more northerly behind the cold front.
Not quite as cold as pas few nights with lows in the low to mid
teens by dawn.


.LONG TERM...(Wednesday through Tuesday)

Active weather pattern shaping up during this period with our
first challenge coming along late Wednesday into early Thursday
regarding snowfall potential across a larger area of the central
IL, followed by a larger storm system for the weekend.

Weather system that will affect our area late Wednesday into
early Thursday showing up on the water vapor loop over western
Canada. Models bring the shortwave a bit further north than what
was advertised yesterday at this time. Latest NAM-WRF brings in
the 700 mb frontogenesis by evening across the northwest by 00z
and over the remainder of the forecast area by 06z Thursday. That
band of lift moves out of the area by 12z, however, models are now
advertising another band of 700 mb frontogenesis pushing from
northwest to southeast Thursday morning, before pulling off to our
east by afternoon. As a result, will continue with high chance POPs
Thursday morning across our southwest counties, with a decrease in
POPs CWA-wide for the afternoon as the lift and better moisture
slide off to our southeast. Accumulations of an inch or two will
be possible mainly along and west of Champaign to Taylorville line
with locally higher amounts possible along and west of the Illinois

Colder weather will return in the wake of the midweek system with
afternoon temperatures by Thursday and Friday ranging from the
teens to lower 20s. Overnight lows, with a fresh snow cover,
especially across the west and southwest, will drop to below zero
again Friday and Saturday mornings. Finally, by Saturday, the upper
pattern transitions to a more southwest flow but it will take until
Sunday before we see any significant increase in temperatures. The
Arctic high that visits the area on Friday will slip off to our
east on Saturday with the better warm advection setting up well
to our west ahead of the stronger storm system slated to bring
periodic chances for precipitation starting Saturday night with
basically daily chances or rain or snow in the forecast for the
remainder of this forecast period.

Medium range models agree with the pattern change but the smaller
details with respect to individual shortwaves ejecting out of the
longwave trof are being handled differently with each model. The
GFS is quite a bit stronger with the surface ridge axis extending
west into our area Saturday evening, as a result, is slower in
getting the snow to move in much before midnight, while the
ECMWF is weaker which allows the snow to begin by late afternoon
over our far western counties. Had slight chances going in the far
west by late afternoon Saturday with the highest POPs over most of
the area Saturday night into Sunday and see no reason to change
that at this point. Most of the models now showing a good portion
of the forecast area will see several inches of snow with the
first round of precip Saturday night into Sunday with the precip
gradually shifting southeast in the Ohio Valley by Monday morning,
before the next piece of energy ejects out of the trof to our east
late Monday into Tuesday bringing another round snow and rain.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)

Breezy sw winds of 15-20 kts with gusts in the 20s will occur this
afternoon ahead of a cold front dropping se from the northern
plains into central IL this evening. Some high end MVFR ceilings
are possible over mainly northern airports along I-74 from mid
afternoon into early evening along with chance of flurries. Light
snow showers should stay north of central IL this afternoon and
early evening. Lower clouds scatter out behind cold front during
mid/late evening and then high clouds increase overnight with mid
level clouds arriving during mid/late Wed morning ahead of next
clipper low pressure system dropping se into eastern Nebraska by
18Z/noon Wed. Winds will veer west this evening and diminish to
between 5-10 kts by mid evening and then nw overnight. Winds veer
ene 5-9 kts Wed morning.




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