Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 282045

Area Forecast Discussion
245 PM CST Sat Feb 28 2015

.SHORT TERM...(Tonight)

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

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)

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 18Z TAFS through 18Z Sunday afternoon)

Backed off a bit on the timing of the advancing snow and
associated lower visibility and ceilings for central and eastern
IL TAF sites. The 12Z ILX sounding indicated plenty of dry air
that will need to be overcome below 5,000 feet AGL this afternoon.
Many of the echoes on the Midwest mosaic radar approaching the MS
River were aloft due to the dry air. Thus, have adjusted the start
of the light snow in VFR conditions between 21-23Z. As the lower
atmosphere moistens, MVFR ceilings and visibility are anticipated
after 00z in central IL and toward 02z in eastern IL.

A weakening pressure gradient should keep wind speeds around 5
knots or less through much of the snow event this evening through
Sunday morning. Many of the short range models are indicating that
the snow may start to taper off around mid-morning. However, will
keep IFR conditions in place because of the uncertainty involving
lift with a 150kt jet max moving into north central IL. As a
result, the GFS and NAM both produce a narrow band of snow and low
ceilings/visibility between I-72 and I-70 by late Sunday morning.


WINTER WEATHER ADVISORY from 6 PM this evening to 6 PM CST
Sunday FOR ILZ029-031-036>038-040>057-061>063-066>068-071>073.



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