Area Forecast Discussion
Issued by NWS Central Illinois

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626
FXUS63 KILX 100515
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1115 PM CST Fri Dec 9 2016

.UPDATE...
Issued at 845 PM CST Fri Dec 9 2016

High pressure was centered right over central Illinois this
evening with the ridge axis extending northwest into North
Dakota. Clouds dominating again over the region this evening
with the very persistent low level cloud cover and scattered
flurries continuing mainly over the eastern half of the forecast
area. Further west, widespread mid and high level clouds were
tracking east and will overspread the entire area overnight.
Surface low pressure now over southeast Colorado is forecast to
track towards the Midwest Saturday night into Sunday and bring a
wintry mix of precip to parts of central Illinois. Before the main
system ejects northeast towards the state Saturday night into
Sunday, a low amplitude shortwave to our northwest this evening
will track across the middle of the country on Saturday bringing
a round of snow to the lower Great Lakes with our northern
counties on the southern periphery of this steadier snowfall.

As the main system shifts northeast out of the southern Plains
Saturday night, precipitaiton will increase in coverage across our
area, starting out as snow, and then transitioning over to rain
over our southern counties Sunday afternoon, with more of a snow
to rain transition further north. There may even be a period of
light freezing rain or sleet as the warmer air rides north into
central Illinois Sunday morning, before changing to rain and snow
during the afternoon, with again the main threat for any significant
snow accumulation to our north. Some of our far northern counties
may see several inches of snow by later Sunday with less snow as
you head south.

No real significant changes seen in the latest 00z NAM with the
heavy snow axis still north of I-80 with the surface low track a
bit further west and northwest than earlier forecast solutions
which would push the heavy snow band even further north if this
track verifies. Will see if any changes occur with the other
model solutions later this evening. Current forecast is handling
the trends well this evening, so other than a few adjustments to
temperatures and the mention of flurries across the east for a few
more hours, no significant changes were made that would warrant an
early evening update.

&&

.SHORT TERM...(This evening through Saturday)
ISSUED AT 300 PM CST Fri Dec 9 2016

Ridge of high pressure extending from Saskatchewan southeastward
into the lower Mississippi River Valley continues to shift slowly
eastward this afternoon.  Persistent low cloud cover that has
blanketed central Illinois for the past couple of days remains in
place, although latest satellite imagery/obs show ceilings rising
along/west of the Mississippi River.  While forecast soundings
suggest the low clouds will gradually erode this evening, they
maintain a mid/high overcast through tonight into Saturday.

Once the ridge axis pushes further east, low/mid-level WAA will
commence Saturday afternoon.  A weak upper-level wave tracking out
of the central/northern Rockies will enhance lift sufficiently
within the warm advection zone to produce light snow across north-
central Illinois late in the day.  Models are in good agreement that
light snow will develop north of a Canton to Bloomington line by mid
to late afternoon.  Any daytime accumulations will be minor...only
amounting to a couple tenths of an inch at most.  Low temperatures
tonight will drop into the upper teens to around 20 degrees...while
highs on Saturday reach the upper 20s and lower 30s.

.LONG TERM...(Saturday night through Friday)
ISSUED AT 300 PM CST Fri Dec 9 2016

A warm advection pattern over northern Illinois will strengthen
Saturday night as a low pressure center strengthens over the central
Plains and moves toward northeast IL for late Sunday. This will
bring light but steadily increasing snow through Saturday night
north of I-72...then a warm layer aloft will produce a melting layer
resulting in potential for some mixed freezing rain and sleet in the
snow before warmer temperatures at the surface change the
precipitation type fully to rain Sunday afternoon and evening as far
north as I-74. The tail end of the system behind the warm sector
could see a return of snow Sunday night with little to no
accumulation expected for those areas that changed over to rain.
Total snowfall amounts continue to look like around 1 to 3 inches
from I-74 northward...with maximum accumulations trending down
sharply to the south...mainly 1/2 inch or less.

Dry zonal flow and high pressure will bring only slightly below
normal temperatures and dry conditions early next week before an
arctic cold front from Canada brings frigid temperatures for
Wednesday and late in the week. Afternoon forecast package trends
temperatures downward further with highs ranging from low and mid
teens along I-74 to low and mid 20s south of I-70 Wednesday through
Friday. Wind chill temperatures look to fall at least down to the
single digits...perhaps as low as -15 F north of I-74 in this
period. Chance for snow Wednesday as the front comes through...and
potentially Friday as well as another upper level wave swings
through. Amounts look light at this time.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1105 PM CST Fri Dec 9 2016

The MVFR cigs have finally cleared most of our TAF sites late this
evening. A large area of mid and high level clouds will continue
to stream in from the west tonight and gradually lower during the
day on Saturday as a weather system approaches from the west.
Still expecting VFR cigs through the day with cigs lowering to
3000-4000 feet across the north after 20z with the possibility for
a brief period of snow into the evening hours at KPIA and KBMI.
Most model data keeps the bulk of the steadier snows north of the
TAF sites but our northern TAFs will be close to the precipitation
so will add prob30s to both KBMI and KPIA after 20z. Further
south, still a question as to just when the snow will occur at
those sites, if at all. For now, will hold off mentioning in this
forecast issuance. Surface winds will remain light and variable
tonight and then gradually shift into the southeast at 10 to 15
kts by afternoon.

&&

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

$$

UPDATE...Smith
SHORT TERM...Barnes
LONG TERM...37
AVIATION...Smith



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