Area Forecast Discussion
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FXUS63 KILX 290908

Area Forecast Discussion
308 AM CST Mon Dec 29 2014

.SHORT TERM...(Today)

08z/2am surface analysis shows strong 1057mb high over northern
British Columbia/Alberta.  Bitterly cold airmass beneath the high
with current temps in the teens and 20s below zero is building
southward into the Northern Plains/Upper Midwest, with the leading
edge of the colder air sinking toward northern Illinois.  Front will
remain north of the KILX CWA today: however, increasing mid/high
clouds in the vicinity of the boundary will be noted across the
north.  End result will be partly to mostly cloudy skies north of
Peoria, with mostly sunny skies further south across the remainder
of the area.  High temperatures will once again be near normal for
this time of year in the middle to upper 30s.


.LONG TERM...(Tonight through Sunday)

Main weather story this week will be the shot of very cold air
expected Tuesday through Thursday.  Arctic cold front will settle
southward into central Illinois tonight, accompanied by clouds and
perhaps a few flurries.  NAM time-height cross-sections show much
shallower moisture profiles and weaker lift than they did on
previous runs: however, think strong CAA will still be enough to
produce scattered flurries overnight.  Any flurries will come to
an end Tuesday morning as high pressure builds into the region and
skies gradually clear.  High temperatures will be much colder than
today, ranging from the lower 20s northwest to the lower 30s south
of I-70.  Heart of the coldest airmass will arrive Tuesday night
into Wednesday.  Thanks to clear skies and diminishing winds, low
temperatures Tuesday night will bottom out in the single digits
and teens, with corresponding wind-chill readings dropping below
zero.  Despite full sunshine, highs on Wednesday will struggle to
reach the lower to middle 20s.  As high pressure sinks south of
the region, winds will back to the southwest and help boost temps
back into the lower 30s by Thursday.

Forecast becomes more complicated later in the week, as a southern
stream system slowly approaches from the southwest.  Model-to-model
as well as run-to-run consistency remains poor, so forecast
confidence beyond Thursday is low.  Latest trend observed with the
00z Dec 29 model suite suggests a deeper cut-off low over the Desert
Southwest and thus a slower northeastward ejection.  Current water
vapor imagery shows a strong jet streak diving southward along the
West Coast, which is progged to carve out a closed upper low over
California within the next 48 hours.  Models are in very good
agreement initially, showing the low meandering slowly eastward to
the Texas/Oklahoma panhandles by Saturday evening: however, they
have very different ideas on how to handle precip ahead of the
system.  The ECMWF shows precip breaking out well in advance of
the low across the southern Plains Thursday, then spreading
northeastward into south-central Illinois and the Ohio River
Valley late Thursday night into Friday.  Given presence of
cold/dry airmass across the region, am very skeptical about this
aggressive push of precip.  As a result, will continue with a dry
forecast Thursday night and will reduce PoPs significantly during
the day Friday.  The GFS seems to have a much more realistic
solution, showing precip remaining south of Illinois until Friday
night into Saturday.  Models are in poor agreement with what
happens to the southwest system over the weekend, with the ECMWF
maintaining a closed 500mb low longer than the GFS, which opens
the wave and allows it to be absorbed into the northern stream
much faster.  If the ECMWF verifies, much of the precip associated
with the low will remain southeast of Illinois across the Ohio
River Valley.  This is a drastic change from its previous runs, so
am looking at this with a great deal of skepticism.  Prefer the
more consistent GFS, which opens the low and brings it northward
into the area on Saturday.  As a result, have introduced chance
PoPs across the board Friday night/Saturday, then have ended
precip chances by Sunday as the system exits into the Great Lakes.
Precip type will be an issue as well, with forecast soundings and
surface temps initially supporting snow or a rain/snow mix.
Atmosphere will likely warm enough to change the precip to mostly
rain on Saturday before colder air returns Saturday night into
Sunday.  Overall this does not look like a major winter storm for
central Illinois, but future model runs will need to be monitored
for any potential changes.


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

High pressure centered across our counties will keep VFR
conditions in general. Some fog is indicated in the HRRR and RAP
models along the ridge axis for CMI and DEC. The RAP goes all the
way to VLIFR with the fog across CMI/DEC/SPI, while the HRRR keeps
the LIFR fog east of CMI. Will add MVFR fog at CMI and DEC as
light fog is developing along that axis already near-by.

A weak cold front extending from SE Nebraska to southern Michigan
will slowly drift to the southeast over the next day. As the cold
front approaches Monday evening, MVFR clouds are expected to
arrive from NW to SE.

The increasing clouds will be triggered by a shortwave moving
east along the front as it arrives in the area. A few flurries
could develop for the northern terminals of PIA and BMI. Models
have trended drier with the last 2 runs, so do not expect any snow
accumulation as the NAM was suggesting previously. Will add -sn at
PIA and BMI Monday evening, and keep the other TAF sites dry.

Winds will remain calm under the surface ridge axis tonight. A
northeast wind will develop by mid-morning on Monday and continue
through the rest of the TAF period. Wind speeds will remain less
than 10kt.




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