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000
FXUS63 KILX 020255
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
855 PM CST Sat Mar 1 2014

.DISCUSSION...
ISSUED 852 PM CST Sat Mar 1 2014

Band of snow that was to our northwest thru most of the day has
finally made it into west central and central Illinois and we
should see it fill in much more across the area over the next
3 to 4 hours. Preceding the snow, we have had areas of freezing
drizzle over east central and central Illinois as the cloud top
layer was supporting mainly supercooled water droplets. However,
after about a 1 to 2 hour period, ice crystals are introduced
resulting in a transition over to snow, similar to what is
occurring now over parts of west central and central Illinois.
Latest HRRR model suggests the transition zone from freezing
drizzle, sleet and snow will settle to along or just south of I72
by midnight, with snow to the north. Snowfall totals in the band
to our northwest from this afternoon thru early this evening have
ranged from 2 to 4 inches, so see no reason we shouldn`t see
similar reports in our north late tonight into Sunday morning.

Have already updated the zones earlier this evening to address
the timing of the snow and freezing drizzle across the north.
The remainder of the forecast is in good shape so am not planning
on any additional zone updates at this time.

Smith
&&

.AVIATION...
ISSUED 535 PM CST Sat Mar 1 2014

Timing and coverage of snow into the TAF sites and affect on cigs
and vsbys overnight and into Sunday morning the main forecast
challenge this period. Large area of snow was pushing into far
west central and northwest Illinois late this afternoon and
expect the snow to overspread the entire area by 05z. We may see
a mix of sleet and possibly a brief period of freezing rain as
the precip begins at SPI and DEC but expect the mix to change
over to snow rather quickly late this evening. Once the snow
develops, expect cigs to drop to IFR to LIFR with vsbys at times
around one half mile in moderate snow late this evening and
thru the early morning hours. Past few model runs have suggested
the northern periphery of the snow will progress a bit faster
southeast Sunday morning and end at PIA and BMI by afternoon.
Will trend in that direction but keep the snow going with a
gradual improvement in vsbys further south thru Sunday afternoon.
Surface winds will be out of the north to northeast at 12 to
17 kts with an occasional gust around 22 kts tonight into Sun.
morning, with winds backing more into a north to northwest
direction at 12 to 17 kts by Sunday afternoon.

Smith
&&

.PREV DISCUSSION...
ISSUED 310 PM CST Sat Mar 1 2014

SHORT TERM...Tonight through Tomorrow night...
Storm developing over the west coast, expected to move in and move
along an elongated piece of energy hovering with the quasi
baroclinic zone across the Midwest. The initial impact of
precip/snow for the FA will be frontogenetic in nature before
changing over to more of a deformation zone. Details are changing
as far as the duration of the precip, now ending sooner...the
thermal profiles, altering ptype...and the locations of the best
heavy snowfall potential. All models have been backing off of
precip at times, while still pointing to banding and mesoscale
enhancements to the snowfall. Enough borderline conditions in a
couple of time frames to raise concerns for impact based warnings.
Changed the product headlines across the area to reflect this. One
major concern is the changing thermal profile in the SE. Should
the evaporative cooling eliminate the very subtle warm layer in
the mid levels (GFS is far more subtle and shallow than the NAM),
the QPF will be even more dedicated to snow. This would result in
higher amounts. This, in addition to the proximity to the greatest
chance for mixed precip and signif ice potential, is the reason for
the Winter Storm Warning. Though criteria may be missed as this is
over more than a 24 hr time frame...impact and collaboration have
resulted in that warning decision. Elsewhere...the QPF is
currently outside of the warning criteria over any 24 hr pd and
trending down. That being said, a lot of snow is coming.
Isentropic lift is weak at best over ILX, more signif to the
south.  Fn vectors maxed over saturated 1000-500mb RH supporting
the frontogenetic forcing from 03z-12z over ILX. At that point the
movement stalls somewhat as the sfc system begins to develop over
the southwest and move NE through the region. Where exactly the
storm stalls will greatly impact ILX. Too far south and the totals
may need to be knocked down even further. A little further to the
north and the weak deformation region will shift. HPC snow totals
not in complete agreement with the QPF, and as a result have kept
the warning area as conservative as possible. Situation will be
very subject to adjustments through the overnight hours. Last
couple runs have brought the precip to an end sooner...with the
NAM and the GFS both out of the CWA by 12z Monday morning.

LONG TERM...Monday through Friday...
More cold air for the extended. Mon-Thursday still well below
freezing. Although Friday is showing signs of a warm up, the WAA
looks a bit weak at this point. Brief waves bringing quick shot of
precip possibly on Wed across the north. Otherwise, dry beyond
the current storm.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WARNING from Midnight tonight to 6 AM CST Monday
FOR ILZ054>057-061>063-066>068-071>073.

WINTER WEATHER ADVISORY until 6 PM CST Sunday FOR ILZ027>031-
036>038-040>043-047>053.

WINTER WEATHER ADVISORY from Midnight tonight to 6 AM CST Monday
FOR ILZ044>046.

&&

$$