Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 241802

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1202 PM CST Tue Jan 24 2017

302 AM CST

Through today...

A weak ridge of high pressure will build across the region today.
However, skies should remain overcast through the day as warm
advection above the boundary layer sets up a relatively sharp
inversion this afternoon, effectively shutting off any potential
to mix down drier air and leaving a shallow stratus deck in
place through the day. In spite of the extensive cloud cover,
temperatures are still expected to remain relatively mild, as
broad upper ridging crosses the region. Max temps today should be
relatively close to the highs observed yesterday, generally
reaching into the lower 40s over much of the area. The weak winds
should turn more easterly to southeasterly this afternoon as the
sfc ridge axis shifts to the east and deepening low pressure moves
out over the central plains. An onshore wind component should
keep the Illinois lakefront a bit cooler with highs along the lake
likely to top out in the upper 30s.


336 AM CST

Tonight through Monday...

The return of precipitation remains on track for tonight,
mainly for north of I-80. The forecast changes were subtle with
still a chance for brief non-accumulating sleet or a wet snow in
the far north this evening, before temperatures warm into daybreak

A well-defined upper short wave over western Colorado early this
morning will eject ahead of the main western U.S. longwave trough
today. A strong surface low of around 993mb will very slowly move
from near Kansas City early this evening to the IL/WI state line
by midday Wednesday. Return warm advection ahead of this is short
duration but strong and compact tonight due to the approach of a
160kt upper southwesterly jet. The warm advection comes with
good moisture flux on a 55 kt low-level jet, and do expect
precipitation to blossom across southern WI/northwest IL and
maybe part of north central/northeast Illinois during the
evening. The frontogenesis between 600mb and 750mb is forecast to
increase this evening near the IL/WI state line, and if modest
precipitation can develop there, that would be the most favored
area and time to have a little mix with partially melted snow or
even a brief wet snow. Surface temperatures should be no cooler
than the mid 30s in the north thanks to increasing southeast
winds, and that is even taking into account wet bulb cooling, so
accumulation is unlikely.

The best rain coverage timing for north of I-80 looks to be
between 11 pm and 5 am on the high-resolution guidance, as strong
mid-level positive vorticity advection is juxtaposed with the
warm/moist advection. Further south, the deep layer saturation
struggles, so maintain only chance of rain. In addition, the short
duration but strong warm advection actually generates 800mb to
600mb lapse rates in excess of 8C/km. So continue the chance of
thunderstorms in the north during that late evening/overnight
window too. Rainfall amounts are expected to be mainly under one
quarter of an inch tonight, with isolated higher possible near the
WI state line.

As the warm sector moves into the area, temperatures will slowly
rise overnight and into the 40s Wednesday morning for most of the
area. The system dry slot will move in aloft Wednesday morning,
putting a temporary end to precipitation except for possibly
drizzle. The fog threat does not look particularly high due to
wind speeds staying up over the CWA. As the low moves past, wind
speeds will sharply increase from the northwest late Wednesday
into Wednesday evening, with gusts in excess of 30 mph looking
likely for part if not all the CWA through Wednesday night.

The short wave continues to hold its own as it ejects further
northeast Wednesday evening, thanks to the long wave trough
sliding east right behind it. The wraparound moisture from the
system cold conveyor belt, within a broad deformation area, is
forecast to wrap into northern Illinois by late day Wednesday, and
then across most of the area Wednesday evening. Rain should see a
transition to snow if this forecast pans out. Most of the model
guidance gives light QPF, but conceptually there is enough
synoptic forcing for an axis of more moderate precip to clip
northern Illinois Wednesday evening. Cannot rule out some places
in the northern/northwestern CWA getting around one inch or a
little more of snow. Given the gusty winds and light snow expected
for most of the area, it will be a noteworthy change from the past
couple weeks whether snow accumulates or not.

More typical January weather then ensues through the weekend with
upper northwest flow dominant under a developing eastern U.S.
upper trough. The 850mb temperatures are forecast to drop to -13C
by Thursday Night and be reinforced through the weekend. Low
amplitude short waves will likely support some flurries or
possible snow showers. Gusty northwest winds will basically be
consistent through much of that time. Highs look to be in the mid-
upper 20s most of those days, and could be a little on the mild
side given the northwest flow. One weather element that will not
change is the clouds, with overcast skies likely to remain
steadfast much of this period.



For the 18Z TAFs...

Main forecast concerns/challenges this period are with low
ceilings, a rain snow or sleet mix this evening, and then a window
of thunderstorms late tonight into early Wednesday morning.

Dry across the terminals at this time with ceilings staying around
1KFT. Although these ceilings may stay in place for a time early
this afternoon, do anticipate a slow upward trend this afternoon
into the evening. However, approaching precip could complicate
this trend. Confidence is lower with precip trends with the
initial round of precip arriving late this afternoon and early
evening, currently noted on radar across Iowa and west central
Illinois. It does appear possible though for a small window of
precip to move through, once again late this afternoon into the
early evening. Did move up timing and will continue to monitor
trends as its possible for an earlier arrival. Drier air in place
over northern Illinois and northwest Indiana should help slow down
this arrival time though. Have maintained just rain for most of
the terminals with the exception of RFD, but this drier air could
also allow for a rain snow or rain sleet mix. Where confidence is
higher, RFD, have included this rain snow mix and even kept this
mix in longer into tonight with the possibility of cooler air
staying in place longer. Rain is then anticipated to be the only
precip type for the remaining terminals tonight into Wednesday
morning. Confidence is higher for a window of embedded thunder to
move across the terminals overnight, and have included VCTS with
this possibility. Ceilings likely to lower back to IFR once again
later tonight, and likely stay low for most of Wednesday morning.



328 AM CST

Areas of dense fog should continue over the northern portions of
the lake through this morning as winds remain relatively light
over the lake and the unseasonably warm, moist airmass remains in
place. The sfc ridge will move off to the east today as low
pressure deepens over the central plains and tracks to the
northeast. The low is expected to track across southern Wisconsin
and Lake Michigan early Wednesday. East-southeast winds will
increase especially across southern parts of the lake in
advance of the approaching low and then become more variable as
the low crosses the central portion of the lake early Wednesday.
Following the passage of the low, strengthening nwly winds will
develop, increasing to arnd 30 kt by Wednesday night as colder air
is drawn across the lake. Brisk nwly flow should persist through
the remainder of the week as high pressure slowly builds over the
cntrl and srn plains and a broad, deep low parks over nern Canada.





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