Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 230212

Area Forecast Discussion
National Weather Service Lincoln IL
912 PM CDT Thu Mar 22 2018

Issued at 911 PM CDT Thu Mar 22 2018

A few sprinkles are occurring in our southwest counties, which
are associated with a preliminary band of isentropic lift.
Moisture profiles indicate dry air below the cloud bearing level,
which has limited the precip to mainly non-measurable rain. High-
res guidance are in good agreement that the band of forcing will
peak this evening and then wane off after midnight into Friday
morning. Cloud cover will undergo a similar trend, but should
remain across the southwest half of our counties through the
balance of the night. Those clouds will help keep low temps
slightly warmer. Our NE counties will see thin clouds or mostly
clear conditions, allowing for better diurnal cooling. Still
seeing rain progressing into the area from SW to NE tomorrow
afternoon, into tomorrow night. Precip type concerns remain the
primary issue for Friday night into Saturday, with potential for
measurable snows across our far N-NE counties. Will examine the
00z guidance as it rolls in, to see if the southward trend to the
snowband that showed up in the 12z models continues.


.SHORT TERM...(This evening through Friday)
ISSUED AT 240 PM CDT Thu Mar 22 2018

Extremely moist system progged to develop over the Missouri Valley
late tonight and move east into Illinois Friday. Although upper
dynamics is somewhat limited low level isentropic lift (295K-300K)
is very impressive as Gulf moisture lifts over nearly stationary
boundary anchored over Missouri. Friday will see the advection of
precip northeast into the southwest half of the forecast area.
Highs should be only slightly below normal due to the cloud cover,
but as precip begins evaporative cooling should allow temps to

.LONG TERM...(Friday night through Thursday)
ISSUED AT 240 PM CDT Thu Mar 22 2018

Big challenge this package is precip type and any potential
accumulations friday Night into Saturday. Models are consistent in
producing strong isentropic lift and mixing ratios as high as 8.0
g/kg advecting northeast into Illinois. Much of the forecast area
should have a prolonged period of rain which will be occasionally
moderate. Thunder may also be a possibility. Rain and easterly
flow should keep temps well below normal.

For those areas north of I-74 there remains an above average
uncertainty in precip type and any accumulations. Several
different ptype techniques produce slightly different scenarios. A
difference of a degree or two at the surface or even 2-4kft aloft
will make a big difference. Throw in the increased mid-March
insolation and warm ground temperatures combined with impressive
snowfall rates and you have a large range in potential solutions.

Having said that the most likely scenario suggest pcpn beginning
as rain Friday evening and then mixing with or changing to snow
well after midnight and continuing into Saturday. The best chance
of significant snowfall will likely be across northeast
Stark...northeast Marshall...and extreme northeast Woodford
Counties where a period of moderate snow will be possible during
the early morning Saturday. Forecast accumulations may reach to
around 6 inches. However, with warm surface temps the snow may
slowly melt from underneath which may limit the amount of snow on
the ground at any one time. Will also need to keep an eye on
northern Vermilion as some techniques they should also see amounts
approaching warning criteria. Given the uncertainty, will retain
headlines as a watch at this time.

The next system will move into the region Monday Night with more
dynamical support. Precipitation amounts may reach a one to one
and a half inches with this system as well as Gulf moisture is
tapped. This system will be much warmer and precip type is not an
issue. GFS forecasted surface-based CAPE of 300-500 J/kg Tuesday
suggests some thunder will likely accompany the cold front
associated with this system.


.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 700 PM CDT Thu Mar 22 2018

VFR clouds will initially dominate the skies in the short term as
a wave of isentropic lift affects central Illinois tonight. Radar
returns should remain virga over night, as dry air in the sub-
cloud layer evaporates the precipitation. Kept rainfall out of the
TAFs until late tomorrow afternoon when lower levels finally start
to saturate with the next wave of forcing for precipitation. Some
steady light rain could begin at SPI before the end of this TAF
period, after 22z or so. However, better chances of rain will hold
off until after 00z/7pm tomorrow. Winds generally stay under 10
kts through Friday morning, with variable directions until winds
become east-southeast tomorrow morning and increase to 10-12kt.


Winter Storm Watch from Friday evening through Saturday
afternoon for ILZ028-030-031.



LONG TERM...Barker
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