Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 072052

Area Forecast Discussion
National Weather Service Lincoln IL
252 PM CST Wed Dec 7 2016

.SHORT TERM...(This evening through Thursday)
ISSUED AT 252 PM CST Wed Dec 7 2016

Sprawling ridge of high pressure extending from western Canada to
the Ohio River Valley will dominate the weather across central
Illinois for the next 24 hours.  Mid/high clouds currently
blanketing the area will gradually dissipate and shift further
southeast tonight, resulting in clearing skies overnight.  W/NW
winds around the periphery of the high will continue to bring colder
air into the region, allowing lows to drop into the teens and lower
20s.  Wind-chill values by dawn Thursday will range from around zero
northwest of the Illinois the teens along/south of I-70.
Chilly weather will continue through Thursday, with afternoon highs
remaining in the 20s and wind chills hovering in the single digits
and teens. upper trough currently over the Upper
Midwest will pivot southeastward into central Illinois by Thursday
afternoon.  Model RH progs suggest overcast conditions will spill as
far south as the I-74 corridor, with partly to mostly sunny skies
persisting further south.  May even see a few flurries across the
far N/NE CWA Thursday afternoon, but have left them out of the
forecast for the time best lift/moisture should remain
across northern Illinois.


.LONG TERM...(Thursday night through Wednesday)
ISSUED AT 252 PM CST Wed Dec 7 2016

Quiet and cold weather is expected to start the period, with
widespread single digit Wind Chill values expected into midday
Friday. A quick moving storm system will track toward the area on
Saturday. Strong WAA/isentropic lift ahead of the system should have
light snow starting to break out by late Saturday afternoon into
Saturday night. The initial arrival of the system is pretty well
agreed upon by the model guidance. However, significant disagreement
and run-to-run inconsistency exists Sunday into Monday. The GFS has
remained progressive with the system, suggesting dry weather for the
end of the weekend. However, the operational ECMWF has been
insistent that a wave will develop along the front, spilling
precipitation back into the area Sunday into early Monday. In fact,
the 12Z ECMWF significantly strengthened this secondary wave and
could bring significant precipitation to the area if it verifies.
The strength of the ECMWF system would actually be warm enough for
rain to the east of the storm track. The latest Canadian model has
trended toward what the previous run of the ECMWF had (a secondary
but much weaker wave to end the weekend). This significant
variability between operational models, and within their respective
ensembles, suggests a low confidence forecast for the weekend. Have
not made considerable changes to prior forecast, with the bulk of
the precipitation (light snowfall) occurring Saturday night.

One other system is progged to impact the area during the forecast
period. It is a fairly weak upper-level weak wave that is expected
to arrive sometime between Tuesday and Wednesday. However, there is
at least a 24 hour spread between the slowest and fastest solutions
with this wave. The unfortunately result of this disagreement is an
extended period of low PoPs (for snow), when the eventual period of
expected snowfall will be rather short.

Temperatures will be well blow normal to start the period. Then,
they will trend warmer, with only slightly below normal readings for
the weekend and early next week. Another blast of cold air, colder
than the one currently pushing in, is likely to arrive by the middle
of next week.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1132 AM CST Wed Dec 7 2016

Overcast from earlier this morning has thinned, with mainly just
high/thin clouds noted at the central Illinois terminals. Based on
latest visible satellite loops and upstream obs, it appears mid
clouds with bases between 9000 and 12000ft will return from the
W/SW this afternoon. HRRR continues to suggest MVFR ceilings will
develop across northern Illinois later this afternoon/evening...then
will spread southward into the eastern KILX CWA tonight.
Meanwhile, the NAM keeps any low ceilings well to the N/NE until
Thursday afternoon when a trough of low pressure drops southward
into the area. Since there is no evidence of widespread low clouds
currently forming across northern Illinois/Wisconsin, have sided
with the NAM and decided to wait until the trough arrives Thursday
afternoon to mention MVFR ceilings. It appears only mid/high
clouds will prevail tonight, with lower clouds sinking southward
toward the I-74 terminals by midday Thursday.




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