Area Forecast Discussion
Issued by NWS Central Illinois

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS63 KILX 011751

Area Forecast Discussion
1151 AM CST Sun Feb 1 2015


Latest surface map showing low pressure center over south central
Missouri late this morning. The warmer air continues over the
entire CWA and temperatures are above freezing everywhere, but
surface reports indicating the precipitation has remained largely
snow north of I-72. Steadier rain across southeast Illinois has
moved off into Indiana, with a dry slot coming through, but
another surge of snow is coming back into Springfield. Have added
some rain/snow mixture as far north as I-72 for this afternoon as
temperatures won`t be going anywhere for a few more hours, and
we are now getting reports of mixed rain/snow on the west side of

The HRRR tracks the low up along the I-70 corridor this afternoon
and won`t exit until around mid afternoon, so have updated the
wind trends across the eastern CWA to hold back the ramping up of
the winds until evening. The ongoing question through the day
though will be with the extent of the deformation zone currently
extending from eastern Nebraska/northwest Missouri eastward across
the Midwest. Latest RAP focuses the extent of it more across the
northern third of the CWA this afternoon, while the NAM and the
HRRR is further south near . The RAP track may be a bit too far
north based on where it analyzed the low at the start of the 15Z
run, so will favor the HRRR/NAM solution. Have not made a lot of
changes to the snow totals across the north, but sharpened them
downward further south due to the mixture. No changes to the
headlines at the moment, but some adjustments on the south end may
be required soon, specifically with the advisory.


.SHORT TERM...(Today)

Much as anticipated yesterday, it has been a struggle to get
significant snow accumulation going thus far across the forecast
area given marginal surface temperatures and low level thermal
profiles. Waves of precipitation have spread mainly across the
northwest half forecast area (mainly north of a Springfield to
Danville line) since last night. Predominantly dry conditions have
been seen across the southeast half of the area.

For the moment, surface observations suggest most of the
precipitation in the area has changed over to snow, although areas
closer to the I-72 corridor are still seeing a mix. Strong
WAA/isentropic lift is expected to persist through the morning,
along with good upper divergence courtesy of jet coupling. This will
result in widespread precipitation to start the day across most of
the forecast area. Some of the more recent model guidance is
suggesting a warmer low level thermal profile across much of the
forecast area this morning which should be able to keep a rain,
snow, or sleet mix going between the I-72 and I-70 corridors, with
rain expected south of the I-70 corridor. The further north and
longer lasting mix will cut snow totals some across the advisory
area, as well as at least Champaign and Vermilion counties in the
warning area. Since uncertainty still exists regarding the ultimate
storm total snow amounts, do not plan to alter current headlines at
this time. However, it is looking increasingly likely that totals
will be lighter than expected south of a Rushville to Bloomington
line, since little if any accumulation has occurred in this area
thus far.

Temperatures and the low level thermal profile will finally cool
starting this afternoon as the surface low pushes off to the east.
The low is currently centered over eastern Kansas, and it is
expected to reach central Indiana by this evening. This cooling will
eventually result in precipitation changing over to snow over most
of the area by late afternoon, with the main exception being some of
our counties south of I-70. However, the best forcing will already
be east of the area by this afternoon, and precipitation intensity
much lower. In fact, it is looking increasingly likely that a dry
slot wrapping around the system will push into the eastern 2/3 of
the forecast area during the afternoon which would greatly curtail
or even end precipitation chances for a time. Forcing within the
deformation zone behind the dry slot is not very robust and should
not be a prolific snow producer as it crosses the area starting late
this afternoon into tonight.


.LONG TERM...(Tonight through Saturday)

Later this evening, northern tier of the state still experiencing
some light snow as the system pulls away to the east.  Both the GFS
and the ECMWF slower with the progression of the system and lingers
the accumulating snowfall after 00Z. As the sfc low moves eastward,
the tightening pressure gradient in the wake will result in gusty
winds going into the overnight hours.  Northerly winds up to 20-25
mph and gusting to 30...and in excess of 30 mph early in the
evening...will create a potential for blowing snow.  Will keep the
blowing snow mention as the last portions of snowfall may be a lot
fluffier after 00Z.  But the bulk of the snow in this event will be
heavy and wet, so blowing snow may be limited.  Cold air moving into
the region behind this system will drop the highs for Monday down
into the teens and 20s...albeit briefly. After a quick clipper
system bringing some light snow to the northern third of Illinois
Monday night, another abrupt change to the pattern sets up a warmer
Tuesday. Southerly winds, a switch to WAA  brings the highs up 24
hrs later on Tuesday into the 30s.  Tuesday night models again hint
at another quick clipper, though the wave is not as well represented
in the models and consistency is lacking.  Not willing to go too
high above low chance at this point.

Remaining forecast is relatively dry with temps dropping again for
Wed and Thur with the pulses moving into the synoptic trof aloft
deepening the cold air. Then warming back up as the sfc ridge shifts
east and southerly winds and WAA pick back up again.  All in all, a
rather progressive and active pattern.  Another system starting to
develop for Sat night next weekend, but models have plenty of
disagreement so far in the track.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)

Dry slot has exited the TAF sites, and the next band of
precipitation is about to come into KCMI. Have seen visibilities
lower to around 1/2SM or so with the heavier snow bands, although
some mixed rain/snow will be occurring early this afternoon from
KSPI-KCMI. As low pressure moves through central Illinois early
this afternoon, southeast winds will be trending more northeast to
north. Rapid increase in winds is expected due to the strength of
the current system, and gusts over 20 knots will be affecting KPIA
by mid afternoon and spreading over the remaining sites through
00Z. Gusts of 25-30 knots are likely much of the night, and with
lingering light snow through the evening, some blowing snow will
be possible. Have only added this to KPIA/KBMI, as the locations
further south have had less snow and what has fallen is very wet.
Winds and clouds will diminish Monday morning, although MVFR
conditions will likely linger at KDEC/KCMI the remainder of the


WINTER STORM WARNING until 9 PM CST this evening FOR ILZ027>031-




AVIATION...Geelhart is the U.S. government's official web portal to all federal, state and local government web resources and services.