Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 231735

Area Forecast Discussion
1135 AM CST Mon Feb 23 2015


1046 MB Arctic high pressure centered over northern Missouri this
morning, responsible for the bitterly cold air on tap today. Winds
have decreased to under 10 MPH in most locations and wind chills
have "warmed" to above -15, so allowed wind chill advisory to
expire at 9 AM. Still expecting chills below zero through noon
mainly due to the bitterly cold air. Otherwise skies will remain
mostly clear with only some passing cirrus, and highs of 12-16
look on track.


.SHORT TERM...(Today)

08z/2am surface analysis shows 1046mb high centered over northern
Iowa, with CAA continuing to its east across central Illinois.
Current temperatures range from the single digits below zero across
the far N/NW KILX CWA to the teens above zero far southeast around
Lawrenceville where clouds have been slow to depart.  Latest
satellite imagery shows clouds slowly shifting southward, so even
the far SE will become mostly clear over the next couple of hours.
Based on hourly temp falls of 1 to 2 degrees, low temperatures
early this morning will bottom out in the single digits below zero
everywhere except south of the I-70 corridor where readings will
likely remain slightly above zero. Thanks to northerly winds of 5
to 10 mph, corresponding wind-chills will drop into the -15 to -20
range across the board. Therefore the Wind Chill Advisory will
remain in effect through 9am. Despite abundant sunshine,
temperatures will remain well below normal today. Given higher
late February sun angle and light winds/advection, think MET
guidance numbers in the single digits are too cold. Have gone
closer to the warmer MAV numbers, resulting in afternoon highs in
the teens.


.LONG TERM...(Tonight through Sunday)

High pressure will be centered over central Illinois early this
evening, but will then shift into the Ohio River Valley by dawn
Tuesday.  Thanks to southerly return flow developing on the back
side of the departing high, low temperatures will be achieved during
the evening across the western half of the CWA, with steady or
slowly rising readings overnight.  Lows will range from the single
digits below zero along/east of I-57 in closer proximity to the high
to the single digits above zero further west where return flow will
become strongest.  As the high moves further away and a fast-moving
cold front approaches from the northwest, breezy and warmer
conditions will prevail on Tuesday.  Partial sunshine and strong WAA
will boost highs into the lower to middle 30s in most areas.
Further north, may see a few snow flurries along/north of I-74
during the afternoon as the front swings into the area: however,
limited moisture will prevent any measurable precip.

Once front passes, another shot of colder weather is expected for
the middle of the week as persistent upper low remains in place over
southern Canada and the central/eastern CONUS remains under the
influence of northwesterly flow.  Next short-wave trough embedded
within the flow will approach the region late Wednesday, with the
00z Feb 23 suite of models all shifting this feature and its
associated precip further east into Illinois than previously
forecast.  Due to this consistent signal, have raised PoPs Wednesday
night.  Based on the expected track of the 500mb vort max, think
best precip chances will be across the SW CWA.  Will carry low
chance PoPs there, decreasing to just slight chances along/north of
I-74.  A light snow accumulation of less than 1 inch is expected
along/southwest of a Effingham
line.  This wave will quickly exit the region Thursday morning,
followed by cold and dry weather through Friday.

Many questions still remain concerning the potential storm system
this weekend.  As has been advertised for several days, a major
pattern change is expected as upper low over southern Canada shifts
northward and a southwest flow develops across the central CONUS.
This will bring warmer air into Illinois, but it will also bring
unsettled weather.  Models are still in disagreement on how quickly
precip will arrive/depart.  The ECMWF is the most aggressive model,
suggesting a lead southwest wave may spread light precip into parts
of the area as early as Friday night.  Given initial presence of
strong high pressure, think this is too quick.  Have therefore
opted to follow the slower GFS, which delays overrunning precip
until Saturday night.  Since there are still so many discrepancies
among the various models as they try to resolve the coming pattern
shift, have made very few changes to the extended other than
limiting mentionable PoPs to the Saturday night through Sunday
time frame.  This will likely be fine-tuned as solutions become
more consistent with time.  Precip type will also be an issue as
warmer air arrives.  Light snow will initially occur Saturday
night, but temps reaching the upper 30s and lower 40s across the
southern half of the area on Sunday will likely mean a transition
to rain at that time.


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

VFR conditions for the 18z TAF cycle. High pressure centered over
northeast Missouri at midday, will shift southeast into southern
Illinois tonight. This will cause initially light north winds to go
light/variable this afternoon, then light southwest this evening.
The pressure gradient will tighten towards daybreak, as low pressure
tracks north of the Great Lakes. As a result, southwest winds will
increase to 10-15 kt with gusts over 20 kts by mid morning. From
09z-15z Tue wind shear criteria may be approached especially
across the north (PIA/BMI) but this looks marginal due to higher
sfc wind speed by that time, so will monitor for later TAF
forecasts. Just high cirrus to affect the terminals through
tonight, with some VFR mid level clouds on Tue morning ahead of a
sfc/mid level trough.




LONG TERM...Barnes
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