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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
1125 PM CST Mon Dec 9 2019

Short term...

248 PM CST

Through Tuesday...

Main forecast concerns are with the passage of a strong cold
front late this afternoon/early this evening. A brief band of low
visibility in moderate snow is possible across far northern IL,
with gust west winds dropping temperatures quickly this evening.

Early afternoon surface analysis shows 993 mb low across northern
lower Michigan. A strong cold front trails from the deepening
low, crossing the Mississippi River into western Illinois. Sharp
temperature drop noted across the frontal zone, with mid-40s in
place across the cwa as of 2 PM, dropping quickly into the low-mid
20s west of the river across eastern Iowa. Impressive look to the
upper wave on GOES vapor imagery, with the low level frontal zone
also a bit more intense than guidance had depicted over the past
couple of days. Stronger frontogenetic forcing, focused around 850
mb, has also been producing a narrow but intense band of
primarily snow behind the front across Iowa this afternoon. Rapid
drop in visibility within this band to 1/4sm or less had produced
many accidents across IA, with a 50-car pileup on I-80 near dsm
earlier. While high-res guidance trends continue to indicate a
gradual weakening and erosion from the southwest as the band moves
east across northern Illinois late this afternoon, felt it prudent to
fill in a Winter Weather Advisory for far northern Illinois in agreement
with dvn and mkx. For simplicity, ran the headline though 6
PM/00z, though main impacts from snow band look to be only on the
order of 30-45 minutes as it moves through any given area. Thus
headlines can be cancelled for counties from west to east as band
moves through and precip ends. Still some concern especially
farther east that shallow moisture/lack of ice nucleation will
prevent much in the way of snow have mentioned more of a rain or
rain/snow mix into the Chicago Metro area and points south.

Otherwise, gusty west winds and strong cold advection are the
story tonight, with temperatures dropping into the teens and low
20s across the area. Wind gusts of 30-40 mph during the evening
will slowly diminish overnight, though blustery and cold
conditions linger into Tuesday. The combination of wind and cold
will produce wind chills in the single digits (negative single
digits across north central il) tonight and only in the teens at
best Tuesday. Skies do look to clear out late tonight, providing
welcome sunshine for Tuesday, with very dry moisture profiles in
the Arctic air mass.



Long term...
124 PM CST

Tuesday night through Monday...

Mostly clear skies beginning Tuesday night should allow temps to
drop quickly into the teens and single digits across the area.
Increasing cloud cover from an approaching weak clipper system
early Wednesday morning could limit additional cooling. Despite
this, northwest winds will likely result in wind chills values in
the single digits to just below zero.

There is a brief period where we could see some flurries/light
snow across the area Wednesday morning as the aforementioned weak
clipper system moves through. The combination of strong 850
baroclinicity and associated isentropic lift/frontogenesis could
support a brief period of light snow, although uncertainty remains
on the overall intensity/coverage due to model variability and
being under a fairly dry airmass. Currently have a mention of
flurries across most of the area by Wednesday morning. If
confidence increases, it may be necessary to include some light
snow accumulations given that ground temperatures will be
sufficiently cold to limit melting. Highs Wednesday will again be
in the 20s and lows drop into the teens.

Temperatures begin to rebound Thursday as the surface high and
cold air shift to the north and east and southerly flow returns.
Highs on Thursday will be near normal in the mid 30s, and slightly
above normal by Friday and Saturday with highs in the upper 30s
to 40s. The overall pattern remains active during this period,
with slight precip chances Friday night through Sunday. Multiple
weak upper level disturbances track across the area and appear to
blend together in the forecast due to model variability. Will
continue to monitor trends as this gets closer to better determine
timing of rain/snow potential.



for the 06z tafs...

MVFR ceilings have been slow clear thus far tonight and satellite
trends would support pushing the forecast clearing time later than
in previous tafs. As colder air spills in, could see a few
flurries overnight, but unlikely to reduce visibility or accumulate, so
left flurries out. Otherwise, will likely maintain gustiness
through the night, with gust frequency quickly fading toward
sunrise. Cannot rule out a sporadic gust or two during the day
Tuesday, especially in the morning, but unlikely to see prevailing
gustiness during the day.


Lot watches/warnings/advisories...
Lm...Gale Warning...Gary to Michigan City in until 2 am Tuesday.

Small Craft Advisory...lmz744-lmz745...2 am Tuesday to 3 PM

Small Craft Advisory...Winthrop Harbor to Gary in until 9 am



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