Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 251549

Area Forecast Discussion
949 AM CST Wed Feb 25 2015


9 AM surface map indicated 1006 MB Alberta clipper over eastern
Nebraska with a large area of snow northeast of this feature from
South Dakota into central and northeast Iowa. Main forecast issue
through this afternoon is timing of snow onset in the northwest.
Based on 12z ILX/DVN soundings and KDVN radar returns, there is
plenty of low to mid level dry air to overcome before precip
reaches the surface. Nearest sites reporting snow are Marshalltown
and Waterloo IA. As the system slides southeast, top-down
saturation will occur from the NW, and latest hi-res guidance
suggests snow will begin to reach the ground north of Peoria in a
3-5 PM time frame. Accumulations will likely begin quickly due to
cold surface temperatures and setting sun, in addition to strong
lift on the leading edge of the system. Before that, a period of
mostly sunny skies will affect the central and southern CWA later
this morning and afternoon, ahead of the increasing cloud cover.
Have raised maxes a few degrees due to temps already rising into
the mid/upper 20s over parts of the CWA.


.SHORT TERM...(Today)

08z/2am surface analysis shows 1004mb low over South Dakota, with
snow currently falling across the eastern Dakotas and far southwest
Minnesota.  Clipper system is expected to track southeastward to
near Kansas City by evening, resulting in snow spreading into
west-central Illinois later today.  00z KILX upper air sounding
shows plenty of dry air that will have to be overcome before precip
reaches the ground, so despite fast movement of system think snow
will hold off until after 21z.  Based on NAM time-height
cross-sections, it appears profile does not fully moisten at KPIA
until 00z, but this may be a bit slow given current speed of
system.  Have therefore gone with dry conditions across the board
through mid-afternoon, then have introduced PoPs across the Illinois
River Valley between 21z and 00z.  Once snow overcomes dry air, it
will rapidly begin accumulating, with up to 1 inch expected around
the Galesburg area by 00z.  Elsewhere around central Illinois, cold
and dry conditions will prevail with high temperatures mainly in
the middle to upper 20s.


.LONG TERM...(Tonight through Tuesday)

While clipper system will be dissipating as it tracks into the
region tonight, lift will be strong enough to support a period of
moderate snow across the western half of the CWA.  Time-height
cross-sections indicate strongest lift within the dendritic growth
zone between 00z and 04z at KPIA, then slightly later further east and
south.  Once profile moistens sufficiently, think snow will come
down at a moderate to heavy rate for a 3 to 5 hour time frame this
evening into the early parts of the overnight hours.  This will
result in a snow accumulation of 2 to 4 inches across the Illinois
River Valley, where a Winter Weather Advisory will be issued.  The
snow will be slow to spread southeastward across the remainder of
the area and will likely not reach the I-70 corridor until after
midnight.  Lift will weaken as well, so snow totals will steadily
decrease toward the Indiana border.  Am expecting around 2 inches
along the I-55 corridor...around 1 inch near I-57...and less than 1
inch in the Wabash River Valley.  Initial system will dissipate on
Thursday: however, a secondary short-wave currently over northern
Manitoba will drop southward into the area and provide enough lift
to keep light snow chances going through at least the first half of
the day.  Minor additional accumulations of less than one half inch
are anticipated.

Once the clipper departs, another shot of bitterly cold air will
arrive for the end of the week.  With clear skies and diminishing
winds over a fresh snow cover, have undercut guidance numbers for
low temps Thursday night with readings once again dropping below
zero in most locations.  Cold/dry weather will continue on Friday as
highs struggle to reach the teens.

After that, the weather becomes more active as a major pattern shift
gets underway.  As has been advertised for several days, the
persistent upper low over southern Canada will lift to the north,
allowing a southwesterly flow pattern to develop across the central
CONUS this weekend into next week.  This will result in warming
temps and a prolonged period of unsettled weather.  Models are still
having a hard time adjusting to the pattern change, although they
are now beginning to come into better focus concerning timing of
individual waves.  As flow becomes southwesterly, baroclinic zone
will lift into the Midwest and become nearly stationary for several
days.  As upper short-waves track along the boundary, periodic
precip chances will occur across Illinois from Saturday night
through Tuesday.  Initial wave of precip Saturday night into Sunday
will mostly be in the form of snow, with a few inches possible
across at least the northern two-thirds of the CWA.  Models are now
in good agreement that this wave will exit the region, leading to a
lull in the precip chances late Sunday night into Monday.  Have
removed PoPs on Monday accordingly.  After that, a much stronger
wave will approach from the southwest by Tuesday, potentially
bringing another round of precipitation.  Model solutions vary
considerably, with the GFS being much faster than either the ECMWF
or GEM.  Consensus brings low pressure across Illinois on Tuesday,
resulting in warmer conditions and the potential for rain or a
rain/snow mix.  Will have to keep an eye on the northern CWA as the
precip begins, as GFS forecast soundings suggest the potential for a
period of freezing rain late Monday night/Tuesday morning.


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)

VFR conditions will continue until 23Z-04Z then LIFR-IFR
conditions will develop as a system moves in from the northwest.
Cloud ceilings will gradually lower throughout the day, with IFR-
MVFR ceilings spreading from northwest to southeast late afternoon
through evening, while IFR-LIFR Visibilities in snow develop as
well. The poorest flight conditions are expected KSPI-KBMI
westward. Minor improvement is expected after 07Z-09Z as the
system moves southeastward and weakens. Winds gradually increasing
to E-NE 10-12 kts for the next 24 hours.


WINTER WEATHER ADVISORY from 4 PM this afternoon to 9 AM CST
Thursday FOR ILZ027>029-036-037-040-041-047-049-050.



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