Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 260259
AFDILX

Area Forecast Discussion
NATIONAL WEATHER SERVICE LINCOLN IL
859 PM CST Wed Feb 25 2015

.UPDATE...
ISSUED AT 859 PM CST WED FEB 25 2015

Light snow continues across the area and is slowly moving south.
The system will continue to weaken as it moves south and east
overnight. Current forecast has a good handle on trends, so no
update required at this time. Winter weather advisory will
continue as is.

&&

.SHORT TERM...(Tonight)
ISSUED AT 256 PM CST WED FEB 25 2015

Alberta clipper low was centered in extreme SE IA at 19z.
Isentropic lift to the N/NE of this feature was producing a wide
swath of snow across IA and into NW IL.  Within the broad snow band
which had widespread 1-2 mile visibilities, bands of heavier
snowfall rates with visibilities down to 1/4 mile in parts of
central/eastern IA, were associated with significant frontogenetical
forcing in the 700-800 MB layer.  As the entire system shifts
southeast this evening and overnight, snow will overspread the CWA
from north to south.  The first flakes have been observed in
Galesburg at 130 PM, so the lift is efficient at overcoming the
initial dry low levels.  Vertical temperature profile looks to support
dendritic growth mainly north of a Canton to Bloomington line this
evening (especially before stronger frontogenesis weakens later this
evening), so have higher snowfall rates (14:1 SLR) through 06z in
this area.  25/12z guidance and more recent hi-res model runs have
shown a slight northeast shift to the track of the surface and mid
level features.  As a result, have expanded winter weather advisory
east to include Marshall, Woodford, and McLean counties.  Advisory
runs through 15z Thu northwest of a Jacksonville to Bloomington line
for 2 to 4 inch totals with isolated higher amounts in the far NW
towards Knox Co.  The weakening trend expected overnight should allow
for a gradual decrease of snowfall amounts to the southeast, with up
to an inch or two down to the I-72 corridor, and then under a half
inch south of I-70, by daybreak.

&&

.LONG TERM...(Thursday through Wednesday)
ISSUED AT 256 PM CST WED FEB 25 2015

The clipper system will dissipate Thursday morning as it departs to
the east. However, a secondary shortwave over southern Saskatchewan
will quickly follow on Thursday, helping to keep light snow going
during the day. Moisture will be very limited on Thursday, so any
additional accumulation should remain less than a half inch. Snow
will come to an end from west to east during the afternoon.

A reinforcing shot of Arctic air will arrive Thursday night as a
1041mb surface high progresses southward into Illinois. There will
be enough wind through the night to push wind chill readings below
-15F in central IL north of I-70, so we may need a wind chill
advisory for late Thur night into Friday morning. Low temps will
likely drop below guidance numbers with a fresh snow cover in much
of the area. Likewise, highs on Friday will struggle to climb into
the teens. The Arctic high will slide east Friday night, keeping
very cold conditions in place through Sat morning. Winds will
shift to the ESE behind the ridge axis on Saturday morning, which
may be enough to put wind chills down around -15F in our northern
counties.

As the high departs to the east on Saturday, a shift in the upper
pattern will begin to develop. The deep persistent trough in the
eastern CONUS will lift to the northeast and a trough will develop
along the western states. That will allow upper level ridging to
progress toward the Mississippi River Valley. The rising heights
associated with that ridge will signal a warming trend, and a
northward progression of a baroclinic zone into Illinois. A
southwest flow aloft along that zone will help to keep the frontal
region nearly stationary across southern Illinois for several days.
The models are advertising several periods of precipitation to
develop during that time, as shortwaves move northeast along that
baroclinic zone.

The first push of significant precip looks to arrive Saturday night
and linger into Sunday night. The precip should start out as all
snow and remain snow until Sunday afternoon, when some rain possibly
develops south of I-70. There appears to be enough frontogenetical
forcing and deep enough moisture to support the accumulation of
several inches of snow in our forecast area, especially north of
I-70 where precip should remain all snow until the system departs
Sunday night. Any change to rain south of I-70 would diminish snow
totals there. Still, slippery travel conditions appear likely from
Sat night into Sunday night.

A lull in the precip is forecast for late Sunday night and Monday,
as dry surface high pressure passes across Illinois.

The next wave of precip is projected for Monday night and Tuesday,
as low pressure progresses from northern Texas and up the Ohio River
Valley. A much stronger surge of warm air is indicated ahead of the
low pressure center, especially aloft in the 900-800mb layer. The
precip should start out as snow Monday night ahead of the low center.
Model consensus has the low crossing southern IL on Tuesday, which
could cause a transition from snow to rain across at least the
southern half of our forecast area. Depending on how cold the
surface temperatures remain Tuesday morning, we could see a period
of freezing rain somewhere north of I-72 during that transition.
NW counties could remain all snow, and see several inches of
accumulation, while southeast areas could see between a half inch
and one inch of rainfall. Due to model differences with warm air
and the track of the low, we will just mention rain or snow in
most areas on Tuesday for now.

Another push of cold air is forecast behind that system for
Wednesday and Wed night, but no precipitation should occur during
that time.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
ISSUED AT 600 PM CST WED FEB 25 2015

Snow is moving into the area and has begun to effect PIA. Will
start in BMI around 01z, and then around 04z at SPI/DEC/BMI.
Conditions will drop to IFR at PIA and BMI first and then at the
other sites when it starts, but improve during the overnight hours
as the system weakens. IFR conditions could last longer at PIA and
BMI, but as the system weakens and drops south, they too will
improve to MVFR conditions. MVFR conditions are expected at all
sites the remainder of the TAF period, based cigs only. Could be
some very light snow or flurries during the remainder of the
morning and into the afternoon, so keeping that in all TAFs. Winds
will be east to northeast and then become northerly tomorrow and
increase in speeds.


&&

.ILX WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY until 9 AM CST Thursday FOR ILZ027>031-
036>038-040-041-047-049-050.

&&

$$

UPDATE...Auten
SHORT TERM...25
LONG TERM...Shimon
AVIATION...Auten






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