Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 250538

Area Forecast Discussion
1138 PM CST Tue Feb 24 2015


Skies are clearing this evening and winds have become lighter.
Front is slowly moving through the area but will continue to do so
the remainder of the night. Current forecast looks on track, but
will be making some minor adjustments to overnight lows, clouds
and winds. Update will be sent, but changes small.


.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 06Z TAFS through 06Z Wednesday night)

VFR conditions will continue the rest of the night and through
most of the day tomorrow. Scattered high clouds will advect into
the area and then thicker mid clouds around 10kft will move into
the area during the morning hours. During the afternoon, lower
clouds will push into the area in advance of the next system.
These clouds will be around 4kft. Current thinking is that snow
will not start until after 00z, first at PIA around 01z, BMI
around 02z, then 03z at SPI and DEC, and then 04z at CMI. Once the
snow arrives, cigs will fall, with PIA and CMI down into the IFR
range; and CMI/DEC/SPI only down to lower MVFR. Vis will also
decrease as the snow begins, dropping to at or below 1sm. Winds
will be west to northwest and then become light and variable. Then
east to northeast winds can be expected tomorrow.




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