Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 070502
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1102 PM CST Tue Dec 6 2016

.UPDATE...
Issued at 825 PM CST Tue Dec 6 2016

The low clouds that plagued the area today have been shunted off
to our east this evening as high pressure builds in from the
northwest. Meanwhile, energy dropping into the Rockies has spread
some high and mid level clouds east into Missouri which should
continue to push east into our area overnight. Upstream dew points
have lowered into the mid and upper teens this evening with the
dry air mass flowing east into central Illinois. Thanks to the
scattered to broken high cloud cover, temperatures will be held
up somewhat overnight with current temperature trends supporting
our overnight lows in the low to mid 20s. Forecast soundings and
time height cross sections suggest the thicker cloud cover should
be across the west and southwest, while more of a scattered cloud
cover will dominate the north and east where our colder overnight
lows will be.

Other than the usual tweaks to the evening temperatures and cloud
cover, the remainder of the forecast is on track for the overnight
hours. As a result, no evening update will be needed at this time.

&&

.SHORT TERM...(This evening through Wednesday)
ISSUED AT 312 PM CST Tue Dec 6 2016

High pressure ridging will build into the area tonight and
Wednesday. Clearing line will make its way across the remainder of
the forecast area this evening in the wake of a cold front pushing
through the Midwest. Plume of mid-level moisture over Oklahoma will
push northeast into the area late tonight and Wednesday.  The
combination of the incoming cloudiness, dry air advecting in with
the ridge, and winds remaining up around 5 kts should reduce the
potential for fog tonight and keep temps mostly in the 20s overnight.

Mostly Cloudy skies and continued cold advection will keep high
temps on Wednesday below normal for early December. Latest models
have backed off on producing any light precip for Wednesday
afternoon and have removed slight chc pops that had been in earlier
forecast packages.

.LONG TERM...(Wednesday night through Tuesday)
ISSUED AT 312 PM CST Tue Dec 6 2016

The models have all come into agreement that the Wednesday night
system will no longer affect central Illinois in terms of
precipitation. The surface ridge axis is now expected to prevail
over what portion of the 500mb shortwave that moves across Illinois
Wed night.

With the 12z model updates, it appears we may end up seeing some
virga develop on Thursday as the upper trough axis progresses across
Illinois. Any precip that reaches the ground should remain to the
north of our forecast area. In any event, clouds will increase on
Thursday as a result of the trough passage.

The bigger story for Wed night and Thursday will be the arrival of a
significantly colder air mass. Steady NW winds later Wed night will
contribute to wind chill readings in the zero to 5F range N of I-74,
as low temps bottom out in the mid teens. Lows for areas south of I-
70 will drop into the low 20s with wind chills around 10F by 12z/6am
Thurs. That cold dry air will remain the main influence on our
weather through Friday night, with daytime highs in the 20s and
overnight lows in the teens. Wind chills Friday morning will range
from around zero towards Galesburg to around 8F near Lawrenceville.

There appears to be some better agreement for the weekend system in
terms of precipitation timing. Differences remain in the surface
pressure pattern. The Canadian and GFS develop a closed low at the
surface Saturday night west of Illinois and progress it across NW
Illinois into the Great Lakes by late Sunday afternoon. The ECMWF
keeps more of an open wave of low pressure with a frontal passage
across Illinois in the same general time frame. The resultant
weather is generally the same, with precipitation developing in
central Illinois Saturday night and continuing through Sunday
afternoon. The precip type looks to start out as snow, then a surge
of warm air on Sunday could progressively change snow to rain as far
north as I-74 Sunday afternoon. Snowfall amounts this far out are
very difficult to pinpoint, but around an inch of snow north of
Lincoln would not be out of the question based on current model
trends. The strength and path of any surface low will play the
major role in how the snowfall scenario plays out.

One major model difference occurs Sunday night as the ECMWF develops
a secondary low to the southeast of IL and moves it northeast into
Ohio by Monday morning. During that time, an additional wave of snow
is spread across central and southern Illinois Sunday night into
Monday morning. For now, we only left slight chance PoPs for snow
during that time frame, due to two opposing dry model solutions.
High pressure should provide dry conditions on Tuesday, but clouds
look to remain in place.

A warming trend is indicated for later this weekend and early next
week, as warm air surges north ahead of any low pressure system. So
high temps should return to the middle to upper 30s Sunday, with
slightly cooler highs on Monday and Tuesday but still all of our area
in the 30s.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1100 PM CST Tue Dec 6 2016

VFR conditions are expected through this forecast period. Little
overall change from previous thinking with some mid and high level
clouds streaming northeast into the forecast area. Forecast
soundings indicating cloud bases AOA 8000 ft on Wednesday before
the mid level clouds shift east and south of the TAF sites by
evening. However, as a secondary cold front moves through the
area, we will have to watch the progress of some stratocumulus
clouds to our northwest which may start to affect our northern
TAF sites just after 06z Wednesday night/Thu morning. Surface
winds will be westerly tonight at 5 to 10 kts and then westerly
winds on Wednesday at 10 to 15 kts will become northwest and
increase to 12 to 17 kts Wednesday evening.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE...Smith
SHORT TERM...Barker
LONG TERM...Shimon
AVIATION...Smith


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