Area Forecast Discussion
Issued by NWS Central Illinois
FXUS63 KILX 042359
Area Forecast Discussion
National Weather Service Lincoln IL
559 PM CST Wed Dec 4 2013
ISSUED 220 PM CST Wed Dec 4 2013
Lots to talk about this afternoon with a change to winter almost
upon us. Models in general agreement with series of systems
through weekend. A Winter Storm Watch will be required for I-70
and southward for potential of 5-8 inches of snow.
SHORT TERM...Tonight through Friday
Sfc low now over North central Iowa will move northeast and swing
a cold front across Illinois overnight. Cold front is currently moving
across E Iowa and NW Missouri at this time with 15-25 degree
temperature drops and gusty northwest winds. Some partial
temporary clearing of low clouds and fog that has enveloped much
of the area today should be expected behind front. Much colder air
will blast into Illinois tonight and early Thursday with highs
20-30 degrees cooler than todays.
The front slows and stalls to our southeast as it becomes parallel
with the upper flow. A series of disturbances riding along the
front will produce several periods of precipition across southeast
Illinois late Thursday through Friday. Temperatures should be near
freezing by then. Although a brief period of freezing rain/sleet
at the onset is possible. Cold air should quickly change any
precip over to snow pretty quickly as 850 temps drop. NAM is a bit
slower in moving the coldest 850 temps in during the evening and
has a longer period of potential ZR/IP, but appears to be outlier.
Will follow closer to phase change timing suggested by ECMWF and
Although mixing ratios are running 4-5 g/Kg, isentropic lift is not
very impressive. Warm ground may also lower amounts slightly early
on, but intensity of snow should overcome this limitation by
middle evening. Even with tempering snowfall amounts some, ECMWF
low-level temps and HPC precip guidance give snow amounts
approaching 8 inches south of U.S. 50. Will be issuing a Winter
Storm Watch for I-70 and southward for 5-8 inches of snow through
At this time it appears that there may be a lull between systems
Friday morning before precip picks up again Friday afternoon as
the next disturbance moves through and finally pushes the front
and its associated precip further east and out of Illinois late
There looks like there will be a fairly quick cutoff to the
northwest of the precip shield as drier air advecting in will
erode the precip. Snow amounts will likely be light from
I-72/Danville northward with most locations northwest of a
Jacksonville/Lincoln/Bloomington line remaining dry.
LONG TERM...Friday Night through Wednesday
Colder air moves into the region behind the system dropping 850 mb
temps into the -7 to -11 C range. Wind chill temperatures will
likely range from around -5F along I-74 to just above zero south
of I-70 by Saturday morning.
As a trough digs along the west coast, we once again develop a
southwest flow above the cold boundary layer on Saturday Night and
by Sunday snow chances will once again manifest themselves with up
to an inch or two possible across much of the region in broad warm
advection pattern at 700-850 mb. Will have to watch this system
closely for potential for mixed precip with temperatures below freezing,
but for now will keep all snow.
Even colder temperatures are expected early next week as the
Hudson Bay Vortex deepens and brings another cold front across
Illinois Monday and Monday Night. 850 mb temps drop into the -10
to -15C range with wind chills below zero across the entire area
ISSUED 559 PM CST Wed Dec 4 2013
The cold front will continue its progression across Illinois this
evening. At 2330z, it had reached a line from PIA to SPI. Winds
will shift from south-southwest to west by 06-07z and eventually
NW by sunrise. Gusts will pick up behind the front with gusts to
near 20kt for a time as the pressure rises intensify. By midnight
the gusts may subside below 20kt but sustained winds will remain
in the 13-15kt range through sunrise. Gusts will increase to
around 20kt after sunrise on Thursday as turbulent mixing drags
mid-level momentum to the surface.
Wind gusts will subside after mid-afternoon as sunset
approaches, with wind directions becoming NW.
Cloud heights at the start of the TAFs may dip to MVFR behind the
front for a few hours. Then a dry push just above the ground
should help to dissipate the lower cloud deck enough to create VFR
conditions. VFR conditions should prevail the remainder of the TAF
period under a sustained push of dry air in cold high pressure.
WINTER STORM WATCH from Thursday evening through Friday
afternoon FOR ILZ062-063-066>068-071>073.