Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 271742

Area Forecast Discussion
1142 AM CST Sat Dec 27 2014


Weak cold front making its way through the state this morning and
has just passed over I-55 according to the depiction on radar and
the wind shift to more westerly in the obs. Cooler temps on the way
and a non-diurnal trend anticipated for locations behind the
front. Some minor precip along the boundary, but majority of radar
returns post-frontal not producing much in the way of measurable precip,
with the vast majority being drizzle. Only a few obs even picking
up on that. Not going to update the pops too much as the better
chances for rain remain in the southeast for the rest of the day.
Some small updates here and there, but nothing large scale. Will
reissue the zones only to refresh the morning wording closer to


.SHORT TERM...(Today)

08z/2am surface analysis shows 1013mb low just north of KMLI, with
frontal boundary extending southwestward into Texas. Airmass ahead
of the front was initially quite dry as evidenced by the 00z KILX
upper air sounding: however, shallow low-level moisture has been on
the increase throughout the night.  Surface dewpoints have reached
the middle 40s across central Illinois, while a plume of 50s
dewpoints is noted further upstream pushing into Arkansas.  LAPS
soundings show this increasing moisture, with most local profiles
now saturated below 700mb. As low pressure tracks further
northeastward into Wisconsin, it will pull the cold front into the
area later this morning, then to the Indiana border by mid to late
afternoon. Continued moisture pooling along/ahead of the front
will result in increasingly deep moisture profiles, particularly
across the E/SE CWA this afternoon. HRRR suggests only scattered
light showers across much of the area this morning, then shows
more widespread precipitation developing along/east of I-57 this
afternoon. Think this solution makes sense as front gradually taps
into increasing amounts of Gulf moisture as it moves further east.
As a result, will carry chance PoPs across the western half of the
CWA throughout the day, with chance PoPs increasing to likely
across the E/SE this afternoon. With cold front passing through
the area, high temperatures will be achieved this morning,
followed by brisk westerly winds and falling temperatures later in
the day. Due to very mild current temps, have generally gone 2-4
degrees above MAV guidance numbers. End result will be highs in
the upper 40s and lower 50s, with temps dropping into the 30s
along/west of I-55 during the afternoon.


.LONG TERM...(Tonight through Friday)

Cold front will slowly push eastward into the Ohio River Valley
tonight, with clouds and showers lingering across the E/SE through
at least the evening hours.  NAM is fastest with the departure of
the precip, while GFS tends to keep it around through the entire
night.  Based on slow forward movement of the front, have trended
toward the slower GFS.  As a result, will maintain low chance PoPs
along/south of I-70 through the overnight hours.  As temperatures
slowly fall into the lower to middle 30s across this area, any
lingering rain showers may mix with or change to light snow late.
Further northwest, skies will clear across the Illinois River
Valley, allowing lows to bottom out in the lower to middle 20s.

Sunshine and seasonably cool temps in the 30s will prevail Sunday
into Monday, before a surge of cold air arrives next week.  Models
have been consistently showing strong 1050mb high building southward
out of western Canada into the central CONUS by Tuesday/Wednesday.
Weak synoptic lift along the leading edge of the cold air may be
enough to trigger a few flurries across the area Monday night, but
the main story will be the colder temps.  High temps will remain in
the 20s Tuesday/Wednesday, while overnight lows dip into the single
digits and teens.  After that, Canadian high pressure will gradually
slide east of the region, allowing a southern stream system to
approach by the end of the week.  Model solutions vary with this
feature, so forecast confidence remains low at this point.  ECMWF
aggressively brings precip northward into south-central Illinois as
early as Thursday night, while the GFS is about 12-18 hours
slower.  Meanwhile, the GEM suggests the system may remain largely
southeast of the area across the Ohio River Valley.  Given the
uncertainties, have opted to go with a dry forecast Thursday night,
and to slow precip onset on Friday.  Have trimmed Friday PoPs to
just slight chance, with higher precip chances arriving Friday night
into Saturday.  Think slower/colder is the way to go, especially
given the Canadian airmass that will initially be in place.  End
result will be a chance of snow or a rain/snow mix mainly Friday
night into Saturday.


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

Conditions behind the front improving to MVFR, and IFR still ahead
of the boundary. DEC in midst of fropa, and CMI to follow. Tempos
in to hold for lower cigs and lesser vis associated with the
front. Winds more westerly/northwesterly, but a more southwesterly
flow aloft. Major concern is the longevity of the lower clouds
behind the boundary. Where as the sfc boundary may race out ahead
the remaining llvl moisture may contribute to keeping the clouds
around a little longer than expected. Models not really picking up
on the current situation. All the weak returns out to the west and
SW also point to abundant moisture. Think the sky will eventually
scatter, though later than prev expected.




LONG TERM...Barnes
AVIATION...HJS is the U.S. government's official web portal to all federal, state and local government web resources and services.