Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 01/26/17
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
759 PM CST Wed Jan 25 2017
748 PM CST
Current forecast for precipitation type transition is right on cue
with the RAP/HRRR suggesting a rain or drizzle for a short time,
even some sleet, to transition to snow. This transition to snow is
approaching the Chicago metro area, but may initially be slowed in
the heart of the city briefly until low level temps cool and weak
deep saturation. Road temps and marginal surface temps have
limited accums largely to grassy surfaces in our northern and
western counties where snow has been the main precip type for a
few hours, but expecting some slushy accums on less traveled
After a leading band there may be a brief lull, but as cold air
filters in from the north, a secondary trough will rotate
southward overnight. The precipitation shield, largely of the snow
variety, is filled in through central Wisconsin, and this will
move through our area later, with lighter snows lingering into
the early portion of the day Thursday.
304 PM CST
As was anticipated, focused ascent across northern Illinois has
allowed drizzle to overspread much of northern Illinois, while
upper level low lifts overhead and surface low departs to the
northeast. Patchy dense fog has also developed over far northern
Illinois and while I do think this may linger for a time this
evening, don`t think it will become too widespread. With continued
large scale ascent in place and more focus per lingering trough
axis over northern Illinois, should continue to see precip shield
blossom over northern Illinois over the next several hours with
the stronger development initially staying focused north of I88.
Have once again bumped up timing for transition from liquid to all
snow per latest trends. Latest short term forecast soundings,
AMDAR soundings, and surface obs all in fair agreement with
placement of colder air pushing through the region. Would
anticipate as soon as stronger precip shield develops here in the
near term, that mixed precip could quickly then be observed before
a transition to all snow occurs this afternoon for these areas.
While light snow is generally anticipated, there still remains a
possibility for periods of light to moderate snow in these
locations this evening. Expect precip to further blossom late this
afternoon/early evening into the I88 corridor with similar trends
of a small window of rain before the transition to rain/snow and
then all snow occurs in the 00-02Z time frame. Light snow
anticipated in this corridor and although smaller windows for
light to moderate snow are possible, think this would be rather
brief and isolated.
Large scale ascent will be present over much of the CWA tonight,
including areas south of I88 in Illinois and northwest Indiana.
However, it does appear that the deformation axis will likely
glide more east northeast. This should minimize the amount of
light snow in these locations to the south. For most locations, a
5-7 hour window of at least light snow will be possible tonight.
Have similar snowfall amounts forecast with 1-2 inches over far
northern Illinois, half inch to an inch for areas along the I88
corridor, and then under a half inch for areas to the south. With
warmer pavement temps, think any accumulation/impacts on the roads
will be limited. However, for areas in northern Illinois where the
more intense snow could occur, can`t rule out some slushy
conditions especially more into this evening.
205 PM CST
Thursday night through Wednesday...
Seasonable conditions return to the region throughout much of the
extended timeframe along with periodic chances for light snow
showers or flurries.
Area of low pressure lifting across the region today is expected
to merge with another low currently off the New England coast
then meander over the Canadian Maritimes through the latter part
of the work week and through the weekend. This will leave the
local area under persistent cyclonic low level flow and northwest
flow aloft along with steep low level lapse rates at least through
the weekend. This will result in mostly cloudy to cloudy
conditions and periodic chances for light snow or flurries as a
series of low amplitude waves rotate across the region.
Deep upper troughing over the eastern CONUS will begin to shift
east and flatten early next week with weak upper ridging
developing overhead on Monday. Strong mid level height rises and
surface high translating across the area should provide dry
weather for Monday and at least a chance to see some sunshine.
Forecast details become a little less clear thereafter as upper
ridge begins to buckle with another approaching upper level
disturbance/Albert Clipper Monday night into Tuesday. 25.12Z run
of the GFS shifted the low track farther south and came in faster
than previous runs while the ECMWF maintains a more northerly
track with less sensible weather for the local area. Could see a
brief return of well above normal warmth ahead of the wave on
For the 00Z TAFs...
* -DZRA changing to a period of steady snow this evening.
* Associated CIG and VSBY impacts.
* Additional flurries/snow showers late tonight through Thursday.
* CIG/VSBY trends late tonight through Thursday.
An upper level disturbance will result in a period of steady snow
likely at all terminals this evening, with somewhat lower
confidence at GYY. Prior to changing to snow, the eastern
terminals will see light rain, drizzle and light fog with
occasional IFR VSBY possible. Based off observed temperatures and
aircraft soundings, MDW and GYY should take longer to go over to
all snow. Radar trends suggest that IFR VSBY is likely in the
snow, with temporary LIFR possible, especially at RFD, where 3/4SM
was maintained in the TAF. Cannot rule out LIFR VSBY at the
eastern terminals, but confidence is not high enough to mention in
the TAF. Mild ground temperatures are expected to limit
accumulations on runways, with perhaps a wet slushy accumulation
possible, especially at RFD and DPA.
The lead disturbance will move east overnight, followed by
another wave dropping south from Wisconsin, that could bring a
period of steadier snow showers in the predawn hours. The overall
pattern through Thursday with an upper level trough and steep low
level lapse rates supports at least periodic flurries/light snow
showers. While VSBY impacts are currently not anticipated, will
need to monitor observational trends on Thursday. West-southwest
winds this evening will shift to gusty northwest later in the
evening and then remain west-northwest for the rest of the
205 PM CST
Area of low pressure currently over far southern Lake Michigan
will shift to the eastern Great Lakes tonight then merge with
another low currently off the New England coast late Thursday into
Friday. The low is expected to meander over the Canadian Maritimes
through the remainder of the week and through the weekend which
will result in a prolonged period of moderate northwest to west
flow of 25 to 30 kt that will persist into early next week. A
small craft advisory will be in effect for the Illinois nearshore
zones from mid evening through Thursday afternoon and for the
Indiana nearshore zones from late this evening into Friday night.
Weak surface ridge will translate across the region on Monday
with winds turning southwest behind the high later Monday into
Tuesday. Meanwhile, a clipper low will drop into the Upper
Midwest/western Great Lakes or just north of the Great Lakes
Tuesday, though there remains considerable uncertainty in track
and timing details at this distance.
LM...Small Craft Advisory...ILLINOIS NEARSHORE WATERS UNTIL 6 PM Thursday.
Small Craft Advisory...INDIANA NEARSHORE WATERS UNTIL 10 PM Friday.
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