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Area Forecast Discussion
1133 AM CST Fri Nov 28 2014


Earlier altocumulus deck continues to scrape the northeast CWA, but
a larger area of mid and high clouds has been streaming southeast
into the forecast area. Have sent some updates to increase the
cloud cover for today to reflect this particular cloud deck.
Have also made some adjustments to the hourly temperature trends
for this morning as they were a couple degrees too warm, but think
the overall highs are still OK for now as southerly winds have
been picking up quite a bit. 


.SHORT TERM...(Today)

Band of mid-level cloudiness associated with an approaching 850mb 
warm front currently blankets central and southeast Illinois: 
however, these clouds are expected to shift slowly northeastward 
later this morning.  Forecast soundings show skies clearing from 
southwest to northeast across the area by midday, with partly to 
mostly sunny skies expected across the board this afternoon.  As
surface high pressure shifts east of the region, strong S/SE winds
will develop.  Due to the increasing sunshine and southerly flow,
temperatures will rebound quite nicely from very cold overnight
lows in the single digits and teens.  Have gone a couple of
degrees above guidance numbers, resulting in afternoon highs in
the upper 30s and lower 40s.


.LONG TERM...(Tonight through Thursday)

Continued strong southerly flow will bring even warmer air into the 
region on Saturday: however, warming trend may be tempered by 
increasing cloudiness.  Both the NAM and GFS show a fetch of 
low-level moisture developing from the western Gulf of Mexico into 
Illinois on Saturday, with surface dew points increasing from the 
lower 30s early in the day to well into the 40s by late afternoon.  
Forecast soundings and model RH profiles show significant moistening 
within the surface-850mb layer, suggesting the development of a low 
overcast.  Despite the cloudy skies, strong WAA will still push high 
temperatures into the lower to middle 50s.

Strong cold front will approach from the northwest on Sunday, with 
excellent model agreement concerning FROPA during the afternoon.  
Ahead of the front, the depth of the shallow surface-based moist 
layer increases just enough to support a low chance PoP for light 
rain showers, mainly east of the I-55 corridor during the 
afternoon.  High temperatures will be achieved by around midday, 
ranging from the upper 40s northwest of the Illinois River to the 
lower 60s along/south of I-70.  After that, gusty northwesterly 
winds will usher much colder air into the region, resulting in temps 
dropping into the 30s and 40s by mid to late afternoon across all 
but the far SE KILX CWA.  

Models continue to show minor differences with their QPF fields 
Sunday night, with the GEM being the most aggressive with developing 
light precip north of the front into the colder air mass overnight.  
With no clear upper support seen for post-frontal precip that far 
north, have rejected this solution in favor of the much further south 
GFS.  As a result, will carry low chance PoPs along/southeast of a 
Champaign to Taylorville line Sunday evening, then only along/south 
of I-70 after midnight.  As the cold air mass sinks southward, precip 
type may become an issue late Sunday night into Monday morning.  
Forecast soundings continue to show a shallow moist layer, with no 
ice crystals present.  With surface temperatures dropping to near or 
just below freezing, light freezing rain/drizzle may occur 
along/south of I-70.  Will need to keep an eye on this situation 
over the next couple of model runs, as some parts of southeast 
Illinois could experience a light glaze of ice and some potentially 
slick driving conditions for the Monday morning commute.  Elsewhere 
around central Illinois, dry but very cold weather will be on tap 
with highs mainly in the 20s.

After a brief shot of very cold air on Monday, temperatures will 
moderate as a zonal upper flow pattern establishes itself across the 
CONUS next week.  This will help temps recover into the 30s on
Tuesday, then well into the 30s and perhaps the lower 40s by
Wednesday.  Big question in the latter half of the extended
continues to be how models handle closed upper low over the
central Pacific.  00z Nov 28 models continue to be in poor
agreement, although a consensus of a slower eastward ejection
seems to be developing.  Prior runs of the GFS had shown the low
weakening, then shearing eastward across the central U.S. by
Wednesday, while the ECMWF/GEM kept it further west.  Latest GFS
has slowed, but still features a lead short-wave that produces
light precip across Illinois Wednesday night into Thursday.
Meanwhile, the ECMWF shows no lead wave, but instead high pressure
across the area at that time.  It delays the arrival of the
weakening central Pacific wave until Thursday night into Friday.
At this point, will make very few changes to the going forecast
due to model uncertainty.  Will maintain chance PoPs on Thursday,
but these will likely need to be pushed back to Thursday night or
even Friday if current trends continue.


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

Gusty southerly winds to around 20-25 knots will continue for a
few more hours, before the gusts start to subside with sunset.
Have added mention of LLWS at all TAF sites, as southwesterly jet
of 40-45 knots starts to move in mid to late evening, persisting
much of the night. 

Have kept most of the TAF sites VFR for now. Main concern will be
near KSPI, which will be melting some snow today. Most of the
models are showing a good inversion around 2000 feet developing
tonight, which would trap this additional moisture in place. Have
added some MVFR ceilings here for later tonight.




LONG TERM...Barnes

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