Area Forecast Discussion
Issued by NWS Central Illinois

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000
FXUS63 KILX 131158
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
558 AM CST Fri Jan 13 2017

.SHORT TERM...(Today and tonight)
ISSUED AT 400 AM CST Fri Jan 13 2017

The surface pressure pattern today will be dominated by cold high
pressure in the Plains building eastward into the Great Lakes, with
a cold front drifting farther away to the southeast of our forecast
area. The high pressure will has already pushed a cold dome of air
into central and southeast Illinois. The eastward movement of the
high will set the stage for persistent low level cold air to be
reinforced by northeast winds into central Illinois, as overrunning
precipitation surges northward today and tonight. The south to
southwest winds in the 900mb to 700mb layer will provide the
moisture and warmer air to drive the extended freezing rain
scenario. Radar returns to our S and SW indicate that some virga is
already working to saturate the lower levels early this morning. We
expect some light freezing rain to reach the Highway 50 corridor by
mid-morning, then advance north to near I-70 by mid-day, and to I-72
this afternoon, and eventually toward I-74 tonight. There continues
to be model differences creating some concern as to icing amounts.
The 00z ECMWF still continues its trend of spotty precip and not as
far north as the NAM/GFS/Canadian models. Do not want to totally
shift gears away from headlines at this point, so have opted to
start a freezing rain advisory for our southern 9 counties along and
S of I-70 at 18z/Noon today, then expand the advisory to the I-72
corridor starting at 00z/6pm this eve. Icing in that area could
reach a tenth of an inch, especially closer to I-70 and north toward
I-72 where surface temps will remain below freezing. Areas farther
south toward highway 50 from Flora to Lawrenceville/Robinson may
climb above freezing for a few hours this afternoon, possibly
limiting icing amounts there.

Due to uncertainty on the northern extent of icing, we kept a winter
storm watch going from Schuyler and Cass counties east through Logan
Co to Champaign/Vermilion counties starting at 00z/6pm this eve. Do
not see warning levels of ice in the eastern portion of that row of
counties, but may eventually need a freezing rain advisory there if
the NAM/GFS/Canadian model group start to verify correctly.

Overall, high temps today will not climb much, with upper 20s from
Rushville to Bloomington and north, 30 to 32 along the I-72
corridor, and low 30s along and south of I-70. Lows tonight will
likewise only drop 3 to 5 degrees below daytime highs, and in some
cases maybe only drop a couple degrees below highs S of I-70.

.LONG TERM...(Saturday through Thursday)
ISSUED AT 400 AM CST Fri Jan 13 2017

A slowing of the low out of the SW continuing to subtly alter
the forecast. Frontal boundary stalling to the south along the Ohio
River Valley providing a focus for precipitation through the
weekend. Weak high pressure limiting the northerly extent of the
precip, with weak easterly flow further eroding the precip.  Deep
trof over the west aloft and weak ridging in the SE resulting in
SWrly flow over the Midwest becoming more westerly, but in enough of
a WAA pattern to result in melting of any ice introduced into the
column...and a continued freezing rain threat. It warrants
mentioning that the latest NAM BufKIT soundings also have a deeper
frozen layer under the melting than prev runs, meaning a threat for
some sleet mixing. The dry air moving in at the surface at least
temporarily suspending the saturation of the low levels and delaying
the precip early in the weekend will also affect the ice
accumulation total.

Models are struggling with how far north the moisture will surge and
making decisions difficult for Central IL.  For now, the GFS and
ECMWF are limiting the northerly extent of the precip on Saturday
while the NAM persists up to I-74 for the morning before settling
southward later in the afternoon. Main low over the SW taking even
longer with this run to move into the region... resulting in the
threat for FZRA through much of the day Sunday until the warmer air
can advect northerly and transition over to mostly rain near
sunset...only for the temps to drop and the FZRA to linger in the
northern half of the state.  In summary...FZRA forecast for most of
the state at different times over the weekend and watches and
advisories are currently staggered accordingly.  This system is very
dynamic and the forecast will continue to shift and fine tune right
into the weekend.  Closely monitor the forecast for changes.

Monday and Monday night and into Tuesday the bulk of the rainfall
moves through with the surface low passing to the NW, but plenty of
WAA ahead of the wave keeps the rainfall liquid. So after a brief
cooler break behind the front midweek...warm air from the
persistent ridging in the SW moves in and pushes the temps in
Central Illinois well above climatological norms through the end
of the week.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 558 AM CST Fri Jan 13 2017

VFR conditions are expected to continue over most of the area
until this evening, when MVFR ceilings develop first at SPI and
DEC after 02z, then expand northward overnight. In addition to the
MVFR conditions, the threat for some light freezing rain will
increase from south to north. Best timing of the onset of the
light freezing rain and possibly sleet will be from 21z-00z at SPI
and DEC. Farther north, have kept a mention of VCSH at CMI, BMI
and eventually PIA later tonight, although confidence that any
precip will shift along or north of I-74 this evening is low at
this time, due to quite a bit of dry air being fed into our area
from a large Canadian high pressure area to our north. Higher
probabilities for the threat for ice will be at DEC and SPI along
with lower cigs, at least through this forecast period. Surface
winds will start out north-northeast and increase to 10-15 kts
this morning with a few gusts around 17 kts. Winds should veer
more into an easterly direction tonight with speeds of 10 kts or
less.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Freezing Rain Advisory from noon today to noon CST Sunday for
ILZ061>063-066>068-071>073.

Winter Storm Watch from this evening through Sunday morning for
ILZ040>048.

Winter Storm Watch from late Saturday night through late Sunday
night for ILZ027>031-036>038.

Freezing Rain Advisory from 3 PM this afternoon to noon CST
Sunday for ILZ049>057.

&&

$$

SHORT TERM...Shimon
LONG TERM...HJS
AVIATION...Shimon



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