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Area forecast discussion 
National Weather Service Lincoln Illinois
304 PM CST Sat Jan 31 2015

Short term...(tonight)
issued at 304 PM CST Sat Jan 31 2015

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
County Warning Area 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 County Warning Area 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)
issued at 304 PM CST Sat Jan 31 2015

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 Illinois 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 County Warning Area 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
County Warning Area with some sites likely getting below zero. Temps will then begin
to warm again for the beginning of the weekend.


Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1111 am CST Sat Jan 31 2015

Difficult forecast for this taf set, primarily with precipitation
types and associated timing. Radar and surface reports indicating
the leading edge of the rain to our west is near Kirk, a bit later
than earlier projections, and the rap/hrrr guidance continues with
a slower arrival. Have not gone that slow, but did delay the
arrival at all sites by a couple hours. Am uncertain on the
southern extent of associated MVFR shield, and kspi-kcmi may stay
VFR the remainder of the afternoon. All models are showing a
warmer trend with the lower atmosphere this evening and have kept
the precip as rain longer. Starting to receive more uniform
guidance that there will be a period of dry weather late evening
and overnight across the southeast half of the state. Trended more
toward a period of vcsh instead of predominate rain from kspi-kcmi
before the steadier precip arrives after 06z. The models then
begin to diverge with the thermal profile. Kpia/kbmi most likely
to be all snow the remainder of the forecast period, but have
mixed precip further south until a changeover early morning,
although the GFS would suggest all rain into early afternoon. LIFR
conditions likely to develop after 07z once the next wave of
precipitation arrives.


Ilx watches/warnings/advisories...
Winter Storm Warning from 9 PM this evening to 9 PM CST Sunday
for ilz027>031-036>038-040>048.

Winter Weather Advisory from 9 PM this evening to 9 PM CST
Sunday for ilz049>057-061.



Short term...geelhart
long term...auten

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