Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 212057

Area Forecast Discussion
257 PM CST Sun Dec 21 2014

.SHORT TERM...(Tonight)

Low pressure currently over southern Alberta will track into eastern
South Dakota by 12z Mon.  Ahead of the system, a tightening pressure
gradient will produce southeasterly winds of 10 to 15 mph across
central Illinois tonight.  Due to these sustained winds and a partly
to mostly cloudy sky, overnight low temperatures will be a few
degrees warmer than last night and several degrees warmer than
either the MAV or MET guidance.  Lows will once again be coolest
across the SE KILX CWA where skies will initially be mostly clear
and warmest across the W/NW where clouds will hold firm.  Readings
will range from the upper 20s near the Indiana border to the middle
30s along/west of I-55.  Any precipitation associated with the
approaching system will remain west of the Mississippi River until
later Monday morning, as forecast soundings remain dry through 12z.


.LONG TERM...(Monday through Sunday)

Beginning to look like a good news/bad news scenario setting up with
the storm system expected to impact central and southeast Illinois
from Monday into Christmas Eve. If recent model trends hold, the
good news will be that significant weather related travel impacts
are not expected on Christmas Eve in the area. The bad news, for
some, would be that a "White Christmas" is looking less likely

We are still looking at active weather the next few days as a
complex, slow moving, storm system impacts the area. A 160+ kt
upper-level jet crashing into the Pacific NW & northern Rockies will
continue to amplify troffing over the middle of the nation, a trof
that will be with us for a few days.

A closed upper low is expected to develop over the northern
Plains/upper Midwest Monday, nearly vertically stacked with an
associated surface low. The WAA/isentropic lift ahead of these lows
will be accompanied by a periods of rainfall from Monday into Monday
night. Then, another vigorous piece of upper energy will round the
base of the mean trof Tuesday into Wednesday, accompanied by the
development of another surface low. This second low will become the
dominant low, with the first low expected to dissipate. A dry slot
is progged to wrap around the initial upper low, no doubt aided by
the secondary wave intercepting much of the deeper moisture. This
dry slot may result in much of Tuesday/Tuesday night ending up dry
across the northwest 1/2 of the forecast area, certainly drier than
it looked yesterday. The southeast 1/2 of the forecast area should
see rain chances persisting Tuesday and Tuesday night, as this area
will be closer to the track of the secondary wave. The remaining
precipitation will change over to light snow on Christmas Eve, but
model agreement is strengthening and this suggests that snowfall
will not be a significant impact across the forecast area.

Mostly dry conditions are expected by Christmas Day as the system
heads northeast into Canada. Another, much weaker, system is progged
to impact the area by Friday. However, guidance suggests that this
system will have minimal moisture to work with until it moves east
of the forecast area. This is because there will not have been much
time for the atmosphere to recover from the cold/dry advection in
the wake of the midweek system. So, only have a slight chance of
rain and/or snow for Friday into Friday night. While temperatures
will be above normal for much of this week, they will trend below
normal behind Friday`s system.


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

MVFR ceilings prevail at the KILX terminals late this morning:
however, the cloud cover is beginning to erode across east-central
Illinois. 1730z visible satellite imagery shows clearing line from
KPRG to just north of K1H2 and lifting slowly northward. Based on
timing tools and latest HRRR forecast, skies will partially clear
at KCMI between 20z and 21z. Further west, forecast soundings
suggest a continued MVFR overcast through the afternoon before
ceilings gradually improve to VFR this evening in advance of an
approaching storm system. IFR ceilings will develop/spread across
the area between 13z and 16z Mon as rain showers arrive from the
southwest. Winds will be from the S/SE at 10-5kt through the
entire period.




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