Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 010131

Area Forecast Discussion
731 PM CST Sat Jan 31 2015


Initial surge of precipitation associated with approaching winter
storm system has been mainly rain: however, there have been
pockets of snow and sleet mixed in as well. Made a quick update to
the forecast to include a rain/snow/sleet mix along the I-74
corridor through mid-evening. Radar trends suggest first band of
precip will lift into northern Illinois later this evening,
leading to a lull in the precip, especially for areas east of the
I-55 corridor. Further west, precip currently over Missouri will
gradually work northeastward and overspread the entire area after
midnight. A rain/snow mix will initially occur, but will change to
all snow along/north of a Danville
line by dawn. No changes in the expected snow totals for later
tonight into Sunday, so current Winter Storm Warnings and
Advisories remain in place.


.SHORT TERM...(Tonight)

Latest surface map shows the low pressure with our storm system
organizing across Colorado early this afternoon, with a frontal
boundary arcing northeast along the Iowa/Minnesota border to just
north of Lake Huron. Upper wave is digging across the
Montana/Wyoming Rockies, as an upper low continues to spin just
south of Arizona. Leading edge of the precipitation shield with this
storm system has edged into the forecast area this afternoon. Some
sprinkles reported as far east as Clinton, but precipitation remains
light as there is a fair amount of dry air at the surface to
overcome. Radar mosaics showing the more organized rain back in
central Missouri and Iowa.

Forecast remains a challenge for tonight. Most of the current
guidance is suggesting precipitation being all rain much of the
evening, with a transition toward snow occurring across the northern
CWA late evening. Complicating matters is the indication by most of
the models that there will likely be a period of dry conditions
across the south half of the forecast area this evening, before
strong isentropic lift moves in from Missouri after midnight and
rapidly moistens the lower atmosphere again. The GFS came in on the
warmer end of the guidance with mainly rain into Sunday morning as
far north as I-72, while the remaining guidance rapidly cools the
column as the precipitation begins, with a switchover to snow. Have
kept a mix in the grids along I-72 past midnight before
transitioning to snow there between midnight and 3 am, with the
southeast CWA remaining mainly rain. Amount-wise, think that 3 to 5
inches of snow is likely north of a Havana to Bloomington line
tonight, with 1-2 inches along I-72, although this area remains
concerning due to the potential mixture lasting longer. Also
complicating things is temperatures above freezing in many areas,
which could limit some of the accumulations unless the snow is
falling heavy enough.


.LONG TERM...(Sunday through Saturday)

Main focus for this forecast period continues to be the winter
weather system that will be moving across the area Sunday through
Sunday night. Over most of the area, precip should have changed over
to all snow, mainly along and north of Quincy to Paris line. Just
south of this line, precip will still be a mix of rain and snow, and
then south of I-70 it should be all rain. As the low pressure area
tracks across southern IL during the afternoon, colder air will
advect into the area and the southeast counties will change to a mix
of rain and snow, while areas north of I-70 will be all snow. Models
seem to be in fairly good agreement with the track of the system,
but differ on the temp profile during the morning hours, especially
for areas surrounding the I-72 to Danville corridor. NAM is cooler,
while the GFS is warmer and would mean more rain than snow. This
continues to be the tricky part of the forecast since the temp
profile plays into the amount of snow/snow accumulation those areas
will have. Looking at current snowfall grids, appears that Piatt,
Champaign, and Vermilion counties could reach warning criteria
snowfall Sunday, so will be adding them into the current winter
storm warning. This coincides with the eastward movement of the low
pressure area and lines up well with Indy. Counties just to the
south of this area will still remain in an advisory as the mix of
rain and snow will likely keep snowfall totals below 6 inches. Then
the 500mb trough will move across the area Sunday evening and then
bring an end to the snow by midnight.

The system will also be deepening some as it moves across the area,
and with colder air advecting into the system, the gradient will
become tighter, which means that the winds will increase and become
gusty for Sunday afternoon through Sunday night. Therefore, there
could be areas of blowing and drifting snow in the north Sunday
afternoon and then along and north of Quincy to Paris line for
Sunday evening. As the winds decrease overnight, the blowing snow
will become patchy. The areas of blowing and drifting snow will be
where most of the snow will have fallen. Believe the blowing and
drifting will be possible, even though the initial snowfall might be
heavier due to the amount of moisture. The snowfall during Sunday
afternoon and evening should be lighter given that the water content
should be drier than the earlier snowfall. Wind gusts should reach
between 30 and 35 mph during the afternoon and into the evening.

After this system, dry and cold weather is expected for Monday.
However, a clipper system is forecast to drop into the area and
bring a little bit of more snow to northern parts of the CWA for Mon
night through Tue night. Any snowfall looks to be less than one half
inch since moisture for this system will be very limited. Beyond
this, conditions will be dry through the rest of the week.

Temps will still be on the warm side Sunday, but then become much
colder for Sun night as colder air advects into the area behind the
system. Temps will remain cold for Monday and Mon night behind the
system but then warm slightly for Tue as the clipper system moves
through the area. Once the clipper moves through, colder air will
return for Wed night through Friday. Overnight low Wed night and
Thur night will be in the single digits for the northern half of the
CWA with some sites likely getting below zero. Temps will then begin
to warm again for the beginning of the weekend.


.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)

Initial band of precipitation associated with the approaching
winter storm is currently lifting through the area late this
afternoon. Most of the precip has been in the form of light rain:
however, some higher reflectivity echoes seen on radar lifting
northward across the I-74 corridor have resulted in a brief
change-over to snow at both KBMI and KCMI. Have included RASN at
both sites accordingly. Based on radar timing tools, it appears
this initial precip area will lift north of the central Illinois
terminals by 01z, followed by a lull in the precip for much of the
evening. Have therefore ended the predominant precip after 01-02z,
with only VCSH and higher VFR ceilings expected until later this
evening into the overnight hours when the main area of precip
arrives. NAM continues to spread RASN from west to east, beginning
at KPIA by around 04z, then further east to KCMI by 09z. The
precip will quickly change to all snow overnight into Sunday
morning, with visbys reduced to less than 1 mile and ceilings
dropping below 500ft. The heaviest snow will shift east of the
area by mid-afternoon, so have improved visbys slightly after
20-21z. In addition, strong N/NE winds will develop on the back
side of the departing system by late Sunday afternoon, especially
at sites along/west of the I-55 corridor. Stronger winds will be
delayed until after 00z at the eastern terminals.






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