Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 010540
AFDILX

Area Forecast Discussion
NATIONAL WEATHER SERVICE LINCOLN IL
1140 PM CST Sat Feb 28 2015

.UPDATE...
ISSUED AT 858 PM CST SAT FEB 28 2015

Light to moderate snow continues to fall across the area, with
lighter amounts northwest of I-55. However, another moderate area
of snow will be making its way into northwest Illinois soon. Temps
have already fallen to forecasted lows or just above them, so will
be updating the forecast to account for lower temps overnight.
Rest of forecast looks ok for now. Will not be updating anything
related to the pops/wx or WSW, right now. Update grids and worded
forecast will be out shortly.

&&

.SHORT TERM...(Tonight)
ISSUED AT 244 PM CST SAT FEB 28 2015

Snow was just starting to move into west central Illinois early this
afternoon, although a pretty substantial wedge of dry air in the
lower atmosphere will need to be overcome before the snow starts
reaching the ground in central and especially eastern Illinois. Am
expecting to this to occur across the forecast area by early
evening.

Strong high pressure extending from the New England states to the
upper Midwest will keep a cold easterly flow across the forecast
area through the night. The snow will mainly be a result of a
southwest low to mid level flow overrunning the cold surface air. The
short range models all indicate a weak surface trough extending from
southwest MO into southeast IL overnight, with no real distinct low
pressure center.

Nevertheless, the location of this weak surface feature, combined
with moderate isentropic lift should focus the heavier snowfall
between 03-09Z (9pm through 3 am CST) in an area roughly just south
of Springfield-Danville on the north end and close to I-70 on the
south end. Nearly all of the short range are in agreement on this
scenario both in amounts and location. Thus, there is high
confidence that 3-5 inches of snow will occur in this area by
daybreak. Snow amounts should be less farther to the northwest of
the best forcing, where 1-2 inches is anticipated by daybreak along
and west of the Illinois River valley.

The lack of a substantial surface low, and high pressure nosing back
across north central Illinois should result in very light wind
speeds overnight. Thus, blowing and drifting snow should not be a
concern with this system.

&&

.LONG TERM...(Sunday through Saturday)
ISSUED AT 244 PM CST SAT FEB 28 2015

Snow should be ongoing across most of central and southeast Illinois
to start the day Sunday. The stronger isentropic lift should have
ceased by the daytime hours Sunday, but some weak impulses riding
the strong baroclinic zone and the associated frontogenesis will be
able to keep the snow going for much of the day. The baroclinic zone
has shifted a little further to the south in the latest model progs
which will push the maximum snow axis south, likely just a little
north of the I-70 corridor. Still expect a maximum of 4-6 inches
between the I-72 and I-70 corridors, with lower amounts to the north
and south. There are model signals that banded snow is possible
within the heavier snow area, which would likely push snow totals
over 6 inches locally, but it is too tough to pin point exactly
where this threat will occur.

The system departs a little more quickly than previously
anticipated, so the snow should end from northwest to southeast
across the forecast area Sunday afternoon into the early evening.
High pressure will then build across the area Sunday night into
early Monday night, supporting quiet but still cooler than normal
weather. However, this high will not be accompanied by the frigid
temperatures we have seen with the Arctic highs of late.

Our next storm system is progged to arrive later Monday night into
Tuesday night. The precipitation should largely hold off until after
midnight Monday night/Tuesday morning, which is a good thing since
the strong WAA aloft should quickly have the precipitation falling
in liquid form. While it will take a few hours, likely after sunrise
Tuesday for much of the area, for surface temperatures to climb
above freezing, the window of opportunity for ice accumulation
appears short. In fact, the WAA aloft is so significant that most of
the icing will likely occur on the frozen ground/snow as opposed to
trees and power lines. All areas will transition to rain Tuesday
morning, with a few thunderstorms possible across southern Illinois.
Total rainfall amounts may approach 1 inch in some areas, which may
pose a flooding risk when combined with rapid snow melt.

This system should quickly depart Tuesday night, also faster than
previously anticipated. Rapidly falling temperatures behind the
system`s cold front should quickly have any lingering precipitation
change to snow Tuesday night. Some earlier model runs had an
additional disturbance push the cold front back north on Wednesday,
providing another shot of light snow, at least across the southeast
half of the forecast area. However, the model consensus has been
backing away from this potential of late. Have maintained some low
snow chances Wednesday for now, but if recent trends persist,
Wednesday will end up being dry.

Another fairly chilly area of high pressure will build across the
region for Wednesday night into Thursday night, supporting
widespread single digit low temperatures, and high temperatures
struggling to reach 20 degrees. However, southerly return flow will
develop on Friday, which should help temperatures recover to near
normal levels for early March.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
ISSUED AT 1140 PM CST SAT FEB 28 2015

MVFR conditions will continue at BMI and PIA, but some IFR
conditions will be possible, so will have a TEMPO group for those
two sites. Then MVFR conditions will continue through the morning
hours. IFR and LIFR conditions will continue at SPI/DEC/CMI
overnight and into the morning hours. Models continue to indicate
another round of moderate snow overnight, so will keep TEMPO group
for these three sites for next 2 to 3 hrs. Could be another round
of moderate snow toward morning, but timing and location will
remain uncertain given the short range model differences; but will
keep IFR conditions through the morning hours. Then most of the
snow should be east of the area and believe conditions will become
MVFR. Based on model trends, believe snow will end during the
afternoon hours, though MVFR cigs will still be around. Toward
midnight, lower clouds should scatter out and leave a high cirrus
deck. Winds will be light southeast to south overnight, then
become light and variable as the system moves toward the east.
Then by afternoon and into the evening, winds will become westerly
with speeds less than 10kts.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY until 6 PM CST Sunday FOR ILZ029-031-
036>038-040>057-061>063-066>068-071>073.

&&

$$

UPDATE...Auten
SHORT TERM...Miller
LONG TERM...Bak
AVIATION...Auten





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