Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 250505
AFDILX

Area Forecast Discussion
NATIONAL WEATHER SERVICE LINCOLN IL
1105 PM CST Sat Jan 24 2015

.UPDATE...
ISSUED AT 846 PM CST SAT JAN 24 2015

Overnite forecast generally on track. 18z models and HRRR indicate
that low-level thermal profile may be conducive for some sleet to
mix with rain prior to 12z in the far NW. Will add wording for
that potential as far east as Peoria.

Still plenty of uncertainty in low-level temperature forecast for
late tonight and Sunday with most models indicate temperatures
near or possibly just above freezing at the surface and warmer
temps around 925 mb through the morning then rapid cooling behind
the expected cliper system during the afternoon and a changeover.
The trend of the solution envelope has also shifted south over the
past few days which if it continues will bring CAA into the
northern counties faster. Still appears that accumulations of an
1"-3" still possible mainly northeast of I-74 with the best
chances of accumulation occurring on elevated and grassy surfaces.

&&

.SHORT TERM...(Tonight)
ISSUED AT 236 PM CST SAT JAN 24 2015

Abundant sunshine occurring across the forecast area this afternoon,
which has allowed temperatures to warm well into the 40s, with some
50 degree readings about to occur in west central Illinois. The
leading edge of the high clouds from our incoming storm system are
about to cross the Mississippi River, and should become more
widespread over our area toward sunset.

Latest surface map shows the low center in central North Dakota. The
12Z models have trended a bit south again with the track, but are in
general agreement with a position in the Quincy area by sunrise
Sunday, and associated warm front extending east across the northern
CWA. With the CWA largely in the warm sector overnight, have shown
only a couple degrees of temperature fall tonight with the entire
area remaining well above freezing. Have updated the alignment and
timing of the PoP`s to focus on the period after midnight, and also
increase them from the west. Higher resolution models start
spreading precipitation in from the west after about 3 am, which
soundings show as rain, although a bit of sleet is hinted at across
the northwest CWA, with a dry wedge between 925 and 850 mb.

&&

.LONG TERM...(Sunday through Saturday)
ISSUED AT 236 PM CST SAT JAN 24 2015

After several days of significant spread and large north/south
swings in the storm track with Sunday`s clipper system, the models
have finally come into better agreement. The current consensus is
kind of a middle ground of the north/south extremes of the past few
days, although it leans more toward the southern tracks that had
been advertised. A more southern track is preferred considering how
much digging the clipper wave still appears to be doing on water
vapor imagery as it is diving into the northern Rockies/Plains.
Would not be surprised to see the ultimate track to be a little
south of the current model consensus, a track which result in an
associated shift of the main snowfall track.

The surface low center should be near the extreme tip of
west-central Illinois at 12Z Sunday, and will quickly race to the
southern tip of Ohio by 06Z Monday. The bulk of the precipitation
with this system should occur during the daylight hours Sunday as a
quick shot of modest isentropic ascent ahead of the wave, and jet
forcing/DPVA as the upper trof axis, swing through.

Measurable precipitation now appears to be a given areawide with
this system, and have boosted PoPs to Categorical (80%+) on Sunday.
The main question is rain versus snow, and how much snow will
accumulate. Temperatures to start the day across most if not all of
the forecast area will be too warm for snow to reach the ground
thanks to the southerly flow ahead of the clipper. Forecast
soundings suggest the low level thermal profile will steadily become
cool enough for a rain to snow transition from northwest to
southeast during the day as cold air wraps in on the north side of
the surface low. The change over should occur around Galesburg near
sunrise, along a Springfield to Champaign line around midday, and
south of I-70 by shortly after sunset. The current track and speed
of change over to snow supports 1-3 inch accumulations along and
north of the I-74 corridor, with an inch or less south of there. Any
lingering snow will quickly come to an end Sunday evening as the
clipper races south/east of the area.

Another wave, much weaker than the first one, will pass north of the
forecast area on Monday. Have started to back down from the already
low snow chances we had over northern portions of the forecast area
with this wave. The lead clipper is expected to develop into a major
east coast storm after it leaves the area, and Monday`s wave is
essentially shearing out into the much stronger system.

A stronger wave coming off the Pacific is expected to move through
the Midwest by Wednesday night and Thursday. At this point, this
system appears warm enough to be mainly a rain producer. Another
system may arrive to start next weekend, but model agreement in the
details is pretty low at this time.

Temperatures through the next week should be at or above normal. The
coolest readings are likely Monday/Tuesday behind the main clipper,
and again Friday/Saturday behind the midweek system.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
ISSUED AT 1105 PM CST SAT JAN 24 2015

Low-level thermal profiles still a challenge as the 00z models
continue to be on the warm side of the current forecast despite
the low tracking a bit further south. Latest water vapor imagery
and upstream observations suggest that the southward trend in the
forecast envelope may continue. However one impact of the further
south trajectory may be to delay post-system low-level CAA. Cold
air remains bottled up over eastern North Dakota, Minnesota, and
northern Wisconsin and is not yet being funneled into the storm.

Will delay switchover of precip from liquid to frozen and confine
a period of possible sleet to KPIA and KBMI.

Gusty north winds still look likely after storm passage Sunday
afternoon as forecasted gradient tightens.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE...Barker
SHORT TERM...Geelhart
LONG TERM...Bak
AVIATION...Barker





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