Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 240547

Area Forecast Discussion
1147 PM CST Sun Nov 23 2014


Double low pressure centers have developed this evening, with one
circulation just northeast of Lawrenceville and another just west
of Galesburg. The trough connecting the two lows generally marks
the line of the back edge of steady rains and the onset of drizzle
and fog. Additional rains are approaching from across Missouri and
Iowa associated with increasing jet dynamics and a potential
vorticity anomaly. The return of steadier rains after midnight
from west to east will diminish fog and drizzle conditions as
winds steadily increase from the WSW. Colder air will approach
Illinois late tonight, but there will not be a deep enough layer
of cold air at the surface to change any rain to snow before
12z/6am in our counties. However, the blast of cold air will
quickly advance across IL Monday morning, with west winds
increasing to near wind advisory levels /30 mph sustained winds or
gusts to 45 mph/. Most areas should remain below those numbers, so
no wind advisory has been issued in our forecast area to this

As the cold air changes any rain to snow, snow accums still look
possible generally north of a line from Rushville to Lincoln to
Bloomington, especially north of Peoria where up to an inch of
snow could accumulate. The snow will be blowing sideways as it
falls due to the strong winds, which will make it very difficult
to measure, and any accumulations on the ground will be blown
around as well. Visibility could drop to low levels during the
snows from mid morning to early afternoon, before snowfall rates
diminish during the afternoon.

Updates this evening have focused on weather grids through
tomorrow afternoon, and updated info is already available.


.SHORT TERM...(Tonight)

Deepening 993 mb low pressure between Oklahoma City and Tulsa at mid
afternoon will strengthen further as it ejects northeast across
central IL near or east of I-55 during this evening, and into
northern lower MI overnight at it deepens to 979 mb. Widespread
light to moderate rain showers have spread NNE across central/se IL
early this afternoon. Still no reports of thunder over IL but will
continue to carry isolated thunderstorms over eastern IL into this
evening as deepening low pressure approaches.

Showers will break up later this evening and overnight east of the
IL river while deformation zone of precipitation lingers west of the
IL river. Strong cold front sweeps across central and eastern IL
during overnight, passing through the Wabash river valley early
Monday morning. Light rain to mix with light snow nw of the IL river
late tonight and change to snow early Monday morning as temps fall
into the mid 30s. Temperatures currently in the low to mid 50s and
dewpoints are in the upper 40s and lower 50s. Lows tonight to range
from mid 30s from Peoria and Springfield west, to the mid to upper
40s near the IN/IL border with temps continue to drop Monday
morning. Southeast winds 10-17 mph and gusts of 18-25 mph this
afternoon to subside this evening to around 10 mph or less as low
pressure moves in. But as low deepens and pulls away from central/se
IL overnight, winds will turn westerly behind the cold front and
increase to 15-25 mph and gust 25 to 30 mph later tonight. RUC model
shows even higher wind gusts of 35-40 mph going into southeast IL
behind cold front early Monday morning around sunrise. MET/MAV
guidance shows strongest winds during the day Monday (strongest in
eastern IL) after 15Z/9 am Monday.


.LONG TERM...(Monday through Sunday)

The strong low pressure area will continue to lift northeast through
the Great Lakes area with a very strong pressure gradient behind the
system. In addition, gusty westerly winds and strong cold air
advection (CAA) will move into the area Monday. Expecting rain in
the eastern half of the CWA and a mix of rain and snow in the
western half,in the morning. Then as the CAA occurs, the all snow
line will move across the area from west to east, so that by early
afternoon, most of the CWA will be all snow, with the mix of rain
and snow only in the eastern parts of the CWA. By late afternoon,
the whole area will be cold enough for snow. As the system lifts
northeast through the day, the higher chances of pcpn will move from
southwest to northeast. So, the areas mostly likely to see any
accumulation of snow will be Northwest of I-55 during the morning and
then north of a Galesburg to Bloomington line during the afternoon.
Total snowfall accumulation expected tomorrow will be an inch or
more northwest of a Macomb to Pontiac line; and then lesser amounts
southeast of that line. Highest amounts are expected to be 1.5 to 2
inches north of a Galesburg to Lacon line.

The tight gradient behind this system will also bring very gusty
winds to the CWA tomorrow, beginning tomorrow morning and lasting
through the day. Sustained winds or around 25 mph with gusts between
35 and 40 mph will be possible across the whole area tomorrow. Will
not be issuing a wind advisory as it will be just below criteria.
However, would not be surprised if overnight shift issues one in the
morning. The gusty winds will also create blowing snow, mainly in
the northern parts of the CWA, so visibilities could be reduced at

Pcpn will end by tomorrow night as the low pressure area lifts
northeast into Canada. After this system, dry weather is expected
for Monday night through Tue night as brief high pressure area moves
through the CWA. Then another clipper system is expected to move
through the area and bring another chance of rain and/or snow for
Wednesday. There will be some warm air ahead of this system so the
pcpn type could be a mix before it changes to all snow. Any
accumulations with this system will be confined to the north with a
half inch or less this time. The remainder of the
forecast should be dry with temps below normal, as Canadian high
pressure moves in and dominates the weather through the rest of the
week and into the weekend.

Temps will steady or slowly falling tomorrow due to the strong CAA
expected. Then temps will be cool/cold most of the rest of the
forecast, except for Wed, just ahead of the next clipper system.


.AVIATION...(For the 06Z TAFS through 06Z Monday night)

The pair of surface low centers have merged and lifted to the NE
of our forecast area, with a trailing cold front to the SW across
IL into southeast Missouri. Drizzle, fog and low clouds are
lingering behind the departing low along that trough, with VLIFR
ceilings at BMI, and LIFR ceilings at the remaining TAF sites.
Expect ceilings to improve slightly with the return redevelopment
of showers well behind the low, but remaining LIFR until Monday
afternoon when levels possibly climb all the way to MVFR.

Cold air will filter into central IL late tonight and Monday
morning, causing the rain showers to mix with or change to light
snow from west to east toward 6 am for PIA/SPI, 8 am for BMI, and
10 am for CMI/DEC. As the precipitation becomes all snow, vsbys
could drop below 1 mile at PIA and BMI where snowfall accumulation
could climb up as high as 1 inch. Vsbys should remain 2-4 miles
farther south toward I-72 where snowfall will be lighter. Snowfall
rates will diminish quickly after 18z-19z, with high winds blowing
any snow around and possibly keeping visibility low at times.

Winds have already become SW behind the low. Pressure rises will
climb to 6 mb/3 hours in at least the southeast half of our
forecast area, which would include DEC/CMI. Wind speeds in our
southeast areas could climb as high as 30g40kt between 12z and 15z
this morning, with other areas 25g35kt. Those speeds could
continue into early afternoon, but we should see some decrease in
peak gusts as the afternoon progresses.




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