Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 192017

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
217 PM CST Sun Feb 19 2017

215 PM CST

Through Monday...

Another unseasonably warm day today with record setting high temps
at ORD and RFD. A lake breeze pushing quickly inland will put the
brakes on much more warming this afternoon for far nern IL and
nwrn IN, though inland locations could still see a couple more
degrees of warming this afternoon under sunny skies. With mostly
clear skies likely into the overnight hours, radiative cooling
should be rather quick, but with sfc dewpoints on an upward trend,
already in the lower 40s over much of the area and expected to
increase through the night, more widespread fog development is
likely tonight, with some sheltered, low-lying areas possibly
seeing some dense fog. Overnight lows should be in the middle to
upper 40s, which is still 10 to 15 degrees higher than seasonably
normal high temps for this time of year. Tomorrow should be
another unseasonably warm day. After any morning fog burns off,
expect that temperatures should reach into the middle 60s to
around70F across much of the area. This would be sufficient to
break records again at ORD and RFD. However, with strengthening
SSELY winds tomorrow as sfc ridging moves off to the east and a
sfc trough and associated cold front push east across the plains,
any lake cooling will likely be confined to the IL shore from
downtown Chicago northward...where temps should top out in the
middle 50s.

As the cold front tracks ewd overnight and tomorrow. Increasing
cloud cover is expected to build in from west to east. Chances for
pcpn ahead of the front should be on the increase through the
afternoon. There should be adequate instability and forcing along
and just in advance of the front to carry slight chance TS with
the pcpn, though coverage of any TS is still a question mark.


328 AM CST

Monday through Saturday...

Spring-like warmth continues through Friday.  The next low makes it
way from the plains tonight to James Bay Tuesday night. Still
expecting precip ahead of the cold front Monday afternoon into
Tuesday morning.  The GFS remains the fastest piece of guidance with
precip east of the region by Tuesday morning. Since other guidance
members are slower, kept a chance of rain through Tuesday morning.
The best chance for measurable rain will be Monday night. Kept a
slight chance of thunder Monday evening as forecast soundings still
feature a little elevated CAPE. Not expecting widespread
thunderstorms nor severe storms.

Not expecting much cooling Monday night, and the record warmest
minimum temperatures may be broken.  Slightly cooler air aloft moves
in, but thinking we will still see highs in the 60s Tuesday. In
fact, I may be too cool with high temps. Wednesday will be warmer
with highs again in the 60s to low 70s.

A cold front moves through Wednesday night and cooler air aloft
moves in limiting high temps to the 50s or 60s away from the lake.
Precip spreads over the region Thursday afternoon and evening ahead
of the next low`s warm front. Showers and storms are expected, but
confidence is only medium regarding how long rain persists into
Friday as there may be a break. Guidance shows differences in the
low`s path with some models taking it right over northern IL and
others have it passing over IA and southern WI.

All models still feature a stout cold front with a cold air push
Friday night. If wrap around persists long enough behind the cold
front, could see some snow, but not overly confident in
accumulating snow especially given rain and recent warm spell. The
big difference will be in temps. Highs on Friday are forecast to
be in the 50s to low 60s, but then only in the 30s Saturday.




245 PM CST

A highly anomalous warm stretch that has already broke daily
records on Friday February 17th and Saturday February 18th will
continue, with potential for more breaking of records. Here are
daily records.

Chicago:                         Rockford:
      High:      High Min:       High:      High Min:
2/19: 65 (1930)  51 (1994)       63 (1930)  42 (1994)
2/20: 64 (1930)  49 (1930)       61 (1983)  46 (1930)
2/21: 67 (1930)  47 (1930)       64 (1930)  42 (1930)

For more, including February warmth on consecutive days, see our
web top news headline or Public Information Statement.


For the 18Z TAFs...

A ridge of high pressure across the area will result in light and
variable winds across the region into this afternoon. Differential
heating between land and a relatively cool Lake Michigan, combined
with the weak pressure gradient will allow for a lake breeze to
develop. There is a better chance for the lake breeze to penetrate
inland far enough to reach ORD/MDW, turning winds to easterly. But
even if the lake breeze does not push very far inland, wind
direction should, synoptically, turn more easterly. With a weak
gradient in place, expect winds to remain under 10 kt through
early evening. As low pressure lifts northeast through the
northern plains and into Canada, with a cold front trailing to the
south, the gradient should strengthen through the night, with
light sely winds developing. Low level moisture will be on the
increase across the area tonight and into early Monday. As a
result, there is still some concern that a period of restricted
cig/vis will develop over the area early Monday morning as a
surface warm front begins to lift over the area. Have maintained
the going forecast for a tempo ifr cig/vis at ORD/MDW with the
potential for more dense fog over more rural area and have gone
with tempo 1/2sm at RFD/DPA/GYY. However, a period of lower vis
cannot be ruled out at ORD/MDW. Conditions should improve by late
morning. The next concern will be pcpn associated with a the cold
front approaching from the west. latest guidance suggests that any
pcpn will not reach RFD until after 18z tomorrow. Have added a
prob30 for shra in mvfr cigs durg the extended 24-30 hr forecast
period for ORD per the latest guidance, but there does remain some
concern that the pcpn may not reach the Chicago area terminals
until early tomorrow evening.


138 AM CST

The main periods of interest through the forecast period will be
Monday and Monday night, then later in the week as a series of
storm systems impact the region.

A surface ridge of high pressure will build over the lake later
this morning, and this will result in a period of light and
variable winds, before becoming east-southeasterly by tonight.
Southeasterly winds are expected to increase markable Monday
afternoon and night as potent area of low pressure shifts
northward into South Central Canada. While the airmass over the
lake will be quite mild and stable, wind speeds of 25 to 30 KT
appear likely by Monday evening as the pressure gradient
strengthens ahead of an approaching cold front. The cold front is
expected to shift over the lake by early Tuesday morning,
resulting in a period of lighter westerly winds on Tuesday.

By mid to late week, the weather pattern is expected to become
active over the region again, with at least two weather systems
expected to have an impact on the Great Lakes Region. The first
looks to move over the region by Wednesday evening. Ahead of this
low, southerly winds will likely increase for a period Wednesday,
then a another cold frontal passage Wednesday night will result in
northerly winds over the lake by early Thursday. Finally, an even
more impressive storm system could impact the region Thursday
night and Friday. This second storm system could be a strong low
that tracks northeastward from the Plains to the Great Lakes by
Friday afternoon/evening. Given the potential strength of this
storm system, a period of east-northeasterly gales will be
possible into Friday, followed by the potential for high end
west-northwesterly gales (~45 KT) late Friday into Saturday.
While there is uncertainty in the timing and track of this storm
system, this is certainly a storm system to keep an eye on in the
coming days.






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