Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 171823

Area Forecast Discussion
National Weather Service Lincoln IL
1223 PM CST Sat Feb 17 2018

Issued at 1223 PM CST Sat Feb 17 2018

Initial batch of snow that was impacting the southeast CWA has
pushed off to the east. Next batch with the upper wave has been
spreading over the northern and central CWA. Surface observations
and dual-pol data from our radar indicate a transition to rain
mixing with snow pellets or graupel occurring toward Springfield
and Jacksonville. Will need to watch the far northern CWA with the
snow band, as visibilities to the west have been down to 1/4 mile
with extensive accidents due to the rapid accumulation. Forecast
was updated to go with around an inch accumulation along and north
of I-74.

UPDATE Issued at 940 AM CST Sat Feb 17 2018

Updated the forecast to increase the snow chances over the next
few hours, plus to lower the forecast highs a few degrees.

Precipitation over the southern third of Illinois has spread
further north faster than originally projected. Latest surface
reports indicate some light accumulations near the southern edge
of Christian County, with some flurries as far north as
Springfield and Jacksonville. Visibilities along and south of I-70
have been below a mile at times, and up to an inch of slushy
accumulation is possible in that area. However, a transition to
rain is anticipated toward midday.

While this particular area of precipitation is moving east, some
light snow is moving across eastern Iowa, associated with a
shortwave in that area. Temperatures should be warm enough for
most of this to be rain by the time it reaches here, though it may
still start as snow across the northern CWA. Short-range models
start to bring this into west central Illinois toward midday,
with a steady eastward march through the afternoon.


.SHORT TERM...(Today and tonight)
ISSUED AT 345 AM CST Sat Feb 17 2018

09Z/3 am surface map shows 1028 mb high pressure over PA/NY and
ridging westward into east central IL. Mid/high clouds were
increasing overnight especially from I-72 north and se of I-70
while light winds prevailed. Temps ranged from 12F at Lacon to
around 30F along highway 50 over Clay, Richland and Lawrence

Weak isentropic lift today over IL ahead of weak 1011 mb surface
low pressure over north central Nebraska, along with a short wave
over western Nebraska, and moving east across northern IL during
this afternoon will likely bring a period of light precipitation
to central IL today, especially later this morning and during the
afternoon. Thermal profiles appear cold enough for light
precipitation to begin as a period of light snow before changing
over to light rain during midday. Also ILX and SGF 00Z soundings
were quite dry from surface to 500 mb and this will tend to delay
start of light precipitation, with virga initially part of this
morning. Skies to cloud up during the morning with light winds
early this morning becoming sw 10-15 mph and gusts up to 20 mph
this afternoon. Highs today in the upper 30s and lower 40s, except
mid 40s wsw of Springfield. A few light showers to linger ne of
Champaign early this evening, otherwise decreasing clouds from the
west during this evening with mostly clear skies overnight. Some
patchy fog could develop later tonight over southeast IL as winds
become light again overnight. Lows tonight in the lower 20s north
of Peoria and upper 20s in southeast IL.

.LONG TERM...(Sunday through Friday)
ISSUED AT 345 AM CST Sat Feb 17 2018

1026 mb high pressure over the northern Rockies settles into the
Ohio river valley by dawn Sunday and moves east off the mid
Atlantic coast by sunset Sunday. This will bring a mostly sunny
and breezy/milder day on Sunday with south winds 10-20 mph and
gusts of 20-30 mph. Highs Sunday range from 48-55F, mildest sw of
Springfield and coolest ne of I-74.

A very unsettled weather pattern takes shape from Sunday night
through Wed across IL, as deep upper level trof digs over the
Rockies and strengthening upper level ridge off the southeast
coast. This puts IL in a sw upper level flow which will bring
ample opportunities for precipitation, and possibly moderate to
heavy early in the work week. The 1st system has low pressure
ejecting out of the northern rockies into southern MN by sunset
Sunday and near Lake Superior by midnight Sunday night, while
a stronger surface low emerges into KS and tracks ne to nw IL/se
IA and sw WI by sunset Monday. Will see rain showers spread ne
across central IL during Sunday night into Monday with isolated
thunderstorms possible late Sunday night or Monday. Lows Sunday
night in the low to mid 40s. Unseasonably mild air over area on
Monday with highs in the 60s, with areas from Springfield sw near
70F. Record high of Springfield on Mon/Feb 19 is 71F set just a
year ago in 2017, and may get close to this record.

A subtropical plume of moisture with widespread showers and some
thunderstorms sets up ahead of a cold front moving into central
IL during Tuesday and approaching the Ohio river valley by dawn
Wed. GFS is further south with its heavy qpf with 2-4 inches in
southeast IL while Ecmwf model is further north this its band of
2-3 inch qpf. Stayed close to model consensus for next work week,
and will issue an ESF product to address the heavy rain threat
especially for southeast IL early next work week. Highs Tue
contrast from mid 40s by Galesburg to near 70F by Lawrenceville.
Cold air arrives behind cold front during Tue night into Wed
changing the rain to mixed precipitation with sleet, freezing rain
and snow possible over IL river valley Tue night and into central
part of CWA overnight Tue night into Wed morning. Highs Wed range
from mid 30s nw of IL river, to mid to upper 40s in southeast IL.

Drier conditions should return Wed night across central IL with
just slight chance of light pcpn in southeast IL where frontal
boundary over TN and eastern OH river valley. Another wx system
spreads precipitation ne across area overnight Thu night through
Friday night, with mixed pcpn overnight Thu night and overnight
Fri night, and mostly rain on Friday and Fri evening.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1120 AM CST Sat Feb 17 2018

Steadier snow has spread as far northeast as KCMI at 17Z, and
some reports of flurries as far north as KBMI. Will be seeing a
period of lower visibilities with this snow, as well as
precipitation that will be pushing from west to east through the
afternoon. Ceilings have fallen to IFR levels with the snow from
KSPI to near KCMI, and will generally stay down until an upper
wave passes mid to late afternoon. Further north, KPIA and KBMI
have recently gone to MVFR conditions, and will see some periods
of ceilings below 1,000 feet as well. Conditions generally should
improve late afternoon with a return to VFR.




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