Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 241810

Area Forecast Discussion
110 PM CDT Tue Mar 24 2015

High pressure sliding east as another low develops over the Plains
and moves into the Midwest later this evening. For now, a frontal
boundary not making much progress northward up into the region.
Between the slower approach of the front and the persistent cloud
cover, have pulled down todays highs. Would like to bring them
down further, but sat imagery starting to show some holes here and
there...and any clearing in the afternoon hours may allow for some
warming...but the highs for today will definitely wait until the
end of the day...with temps steady or climbing through the
overnight hours.


.SHORT TERM...(Today)

A band of mid-level warm frontogenesis will advance into central IL
from the west this morning, causing rain showers to eventually
develop through the dry low level airmass. The leading edge of the
rain band will likely encounter areas with surface temps at or just
below freezing, but not sure how far south will freezing rain
affect. 08z obs show only Jacksonville and Lawrenceville above
freezing, while the remainder of our C and SE IL obs at or below
freezing. Will need to include a mention of freezing rain a bit
farther south this morning as a result. Road surface temps on IDOT
sites showing road temps ranging from 34 to as low as 31F. We could
see some slippery road conditions develop with this initial rain
falling on dry pavement. Air temperatures should rise above freezing
across the board by 15z/10am, so any icing would melt quickly.
However, any ice on initially dry roads can be very slippery.

Will follow the HRRR depiction of rain advancing eastward in a band
that matches the current radar trends rather closely. Likely and
categorical PoPs this morning will be confined to the western
counties where low level saturation will occur the soonest.

The band of warm F-gen will gradually shift north this afternoon,
giving the precip a northward push as well. Have kept likely PoPs
across the northern half of our area through the afternoon.

Instability params support elevated thunderstorms today, generally
in the southwest half of our forecast area.

High temps today will be held in check by the cool east-northeast
flow that will persist through the day. Most areas will see the
mid-upper 40s this afternoon.


.LONG TERM...(Tonight through Monday)

Deepening surface low pressure currently at 1006 mb over the OK
panhandle to eject ne into the IL river valley over central IL by
06Z/1 am tonight and then to southern part of Lake MI by 12Z/7 am
Wed and swing a cold front through central and eastern IL overnight.
Showers likely ahead of this front along with chances of
thunderstorms. SPC has marginal risk over much of central IL
(excluding Knox, Stark and Marshall counties) with slight risk of
severe storms this evening across central and sw MO from St Louis
southwest. Lows tonight range from upper 30s to near 40F over IL
river valley to upper 40s and lower 50s in southeast IL.

Central IL to be in between weather systems during the day Wed as low
pressure passes ne of the Great Lakes and another surface low
pressure organizes over the southern plains. Isolated showers and
thunderstorms to be possible later Wed afternoon south of I-72.
Milder highs Wed mostly in the 60s, ranging from around 60F from
Peoria north to upper 60s and lower 70s in southeast IL. 1007 mb low
pressure to track ne into far southern IL by 06Z/1 AM Wed night and
up the Ohio river to near the OH/KY border by dawn Thu. Models have
trended further south with this low pressure track, and SPC has
shifted slight risk of severe storms Wed night south of our CWA with
marginal risk into southeast counties along and south of I-70. Heavy
rains of 1-1.5 inches possible south of I-70 Wed night while rain
chances taper off to 20-30% nw of the IL river. Lows Wed night range
from mid 30s over IL river valley to 40-45F in southeast IL from I-70

Lingered a chance of rain showers in southeast IL Thu morning then
low pressure moves into south-central PA by midday Thu and far enough
way to end rain chances Thu afternoon. Much cooler highs Thu in the
mid to upper 40s central IL and near 50F southeast IL. Even colder
Friday with highs in the upper 30s and lower 40s, coolest from I-74
ne. Models have been consistent in digging strong upper level trof
into the eastern states Thu/Fri bringing in the very cool
temperatures. 1030 mb Canadian high pressure settles into the Great
Lakes region Fri night and Sat with clouds and winds decreasing and
continued cold temps. Lows Fri night in the low to mid 20s. Highs
Sat in the low to mid 40s.

A cutoff upper level low passes north of the Great Lakes Sunday
keeping best chances of precipitation north of central IL, though
did carry small chances of light rain showers Sunday. Milder highs
Sunday and Monday in the mid to upper 50s with west central IL
around 60F.


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

Quasi stationary boundary at the surface just south of Central
Illinois terminals slow to move this afternoon, delaying the onset
of the MVFR/IFR stratus deck in PIA BMI and CMI. SPI BKN012
now...DEC soon to follow suit. Showers moving along SPI to DEC
path as well, with more rain to the west. Afternoon showers and
thunderstorms late remaining possible, though coverage anticipated
to be rather sparse...keeping the mention to VC since the warm
sector of the approaching low is delayed until after sunset.
Tightening pressure gradient however, will result in some rather
gusty winds after sunset/before midnight gusting to 30kts...mainly
out of the SE. IFR through the overnight hours. Clearing in the
morning with the llvl moisture trapped/enhanced may be delayed
considerably and since the operational models are not clearing
until closer to holding off mention of improvement
just yet.




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