Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 281804
AFDILX

Area Forecast Discussion
NATIONAL WEATHER SERVICE LINCOLN IL
1204 PM CST Sat Feb 28 2015

.UPDATE...
ISSUED AT 1056 AM CST SAT FEB 28 2015

Made minor adjustments to the forecast for this afternoon and this
evening for the timing of the advancing snow. The latest radar
mosaic and infrared satellite loops show the snow slowly advancing
into central and southeast Missouri. The 12z Lincoln upper air
sounding indicated a lot of dry air that will need to be overcome
below 5,000 feet. NAM forecast soundings indicate that this dry
air will moisten quickly in central IL by late this afternoon.
However, this may be a bit too quick, so pushed back the beginning
of the snow until early evening in most of the forecast. The
exception will be in west central IL where late afternoon should
see the beginning of the snow.

A quick look at the latest 12z forecast runs of the NAM and GFS
indicate that the strongest isentropic lift should tap increasing
low level moisture to result in the higher snow amounts south of a
Springfield to Danville line - and along/north of I-70 - where 5-6
inches of snow looks likely. Should be a sharp cutoff in southeast
Illinois, especially from Flora to Lawrenceville where a mix with
freezing rain is expected toward daybreak. This will temper snow
accumulations there to 2-3 inches. The same amounts are expected
north of a Canton/Peoria-El Paso line where the weaker upper level
support is anticipated.

&&

.SHORT TERM...(Today through Monday)
ISSUED AT 254 AM CST SAT FEB 28 2015

First of two significant winter weather systems poised to begin
spreading across the area later today. Latest water vapor imagery
showing an upper trough along the Montana/North Dakota border, with
a strong southern stream jet across the southern tier of states.
Upper low over Oregon will be sinking southward into California,
which will help orient the southern stream into a southwest-
northeast configuration in time, with various disturbances riding
along it toward our area.

Latest guidance indicates the morning hours will remain dry, but
rapid development of light snow is expected to our southwest toward
midday as isentropic lift strengthens into the afternoon. The
forecast was updated during the evening to hit the PoP`s harder
during the afternoon, and have kept this basic philosophy with some
minor refinements. Bulk of the snow threat will be from this evening
into Sunday afternoon. However, the models have continued with their
slight southward trend in the 00Z runs, shifting more into the I-72
to I-70 corridor. Will issue a winter weather advisory for most of
the area except the extreme northwest CWA during this period. While
we cannot rule out some areas seeing just over 6 inches, the
duration of it (about 24 hours) would be too long to justify a
winter storm warning. However, the general thought is more of a
broad 4-6 inch total, with lesser amounts across the northwest and
southeast parts of the CWA. There also starts to be a precip type
question on Sunday. Forecast soundings north of I-70 generally
support all snow, but a rain/snow mix is more likely along and south
by late morning as temperatures rise just above freezing. Have also
added a short period of freezing rain toward Flora and Lawrenceville
during the morning before temperatures rise above freezing.

Any lingering precipitation should exit the southeast CWA during the
evening on Sunday, with Monday dry as an area of high pressure moves
overhead.

&&

.LONG TERM...(Monday night through Friday)
ISSUED AT 254 AM CST SAT FEB 28 2015

An active split flow pattern to amplify into a large central U.S.
trof by the middle of the week as energy from the northern stream
phases with the southern stream upper wave. Very dynamic subtropical
jet will aid in the strong moisture transport northward into the
Midwest starting late Monday ahead of our next weather maker with
precipitable water values by Tuesday in the 1-1.25 inch range, some
2-3 standard deviations above normal for early March. This will be a
concern for area rivers and streams as the snow melt combined with
0.50 to 1 inch of rainfall will lead to significant rises along many
of our basins by midweek.

Strong warm advection/isentropic ascent will lead to a rapid increase
in clouds late Monday night with precipitation breaking out late
evening or just after midnight. Models have trended a bit slower with
the onset of the precip but all forecast soundings continue to suggest
a period of sleet and freezing rain will develop from southwest to
northeast late in the evening and encompass most of our forecast area
by morning. Ice accumulations Monday night still look as if it will
average a tenth of an inch or less with temperatures slowly rising
overnight. Have most of the area above freezing by 15z/9am Tuesday
morning with rain expected for the remainder of the day with even
some isolated thunder possible in the south as forecast soundings
across southern Illinois indicate some elevated instability during
the afternoon hours as the deepening surface low tracks to our north
dragging a cold front through our area by evening.

The strong surface low will push well to our north but the boundary
will start to slow down as it becomes parallel to the upper flow
late Tuesday night into Wednesday. Highest POPs will shift into
southeast Illinois Tuesday night into Wednesday when models indicate
another wave to push east across the southern Plains and at the same
time an intensifying jet with the right rear quadrant and enhanced
upper level divergence/lift over the Ohio River valley during the
day Wednesday. In response to the upper wave and jet lift, models
indicate a surge northeast with precip again by Wednesday afternoon
which would bring the threat for snow to southeast Illinois into
the late afternoon or early evening hours before the lift departs
Wednesday night. GFS is a bit more progressive with the wave and
does not show as much precip building back to the north as the
ECMWF. For now will continue to support the more conservative look
to the GFS with just chance POPs over southeast IL during the day
with just slight chances for light snow Wednesday night as the
system pulls away from our area.

After that, colder air moves in for a brief stint as high pressure
settles over the region on Thursday with our coldest overnight lows
Thursday morning when we will see single digits CWA-wide. The cold
air will not hang around very long as the surface high slips off to
our east by Friday as a northern stream shortwave tracks to our
north for the end of the week. This will bring a southerly flow back
into our area with afternoon temperatures by Friday rising back into
the 30s, with 30s and 40s by Saturday before a cold front sweeps
across our area Saturday afternoon. No significant rain or snow is
expected with the late week shortwave.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
ISSUED AT 1204 PM CST SAT FEB 28 2015

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.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
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.

&&

$$

UPDATE...Miller
SHORT TERM...Geelhart
LONG TERM...Smith
AVIATION...Miller





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