Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211944

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
144 PM CST Tue Feb 21 2017

144 PM CST

Through Wednesday...

Short term forecast centers on fog potential tonight and record
breaking warmth on Wednesday.

A weak cold front has basically stalled out across the area this
afternoon. The front is only discernible by a subtle wind shift
and a remnant dewpoint gradient. Lower clouds along this boundary
will slowly erode this afternoon being held in place a bit longer
by southwest flow ahead of the front. Tonight these and upper
clouds from low pressure to the south should ultimately dissipate
as high pressure will move in. Light winds, mostly clear skies,
and lingering moisture in the form of elevated dewpoints and
earlier rainfall would suggest the formation of fog. While the
setup is not perfect for widespread dense fog with a slowly
increasing pressure gradient overnight and a decent low level jet
forming, localized dense fog is certainly a possibility as
suggested by MOS and RAP/HRRR guidance. At this point the favored
area is in the lingering moisture gradient boundary draped
southwest to northeast through NE Illinois, and under the high
which will be across Central Illinois. Weak SW flow may advect
some of this northward.

Model guidance still appears too low for Wednesday, and we have
maintained the going forecast of 70s area wide with generally
clear skies and with the thermal ridge basically maximizing right
on top of the area. The day will start of mild with some fog and
low clouds, but once we erode them, a quick warmup will ensue.
Gusty southwest winds will allow the warming all the way to the
lakefront. If we really do warm as forecast, mixing will likely be
greater and therefore a greater dewpoint drop would be expected,
which would bring increased fire weather concerns.



144 PM CST

...Wednesday night through Tuesday...

For Wednesday night and Thursday...

Wednesday night a cold front will move across the area under a weak
shortwave aloft.  It does not appear there will be sufficient
moisture along the front to support any precipitation, but behind
the front winds will turn northwest and eventually north and
northeast, ushering in cooler air for Thursday. Cooler is a relative
term for this unusual week, of course, so highs Thursday are still
looking to be about 15 to 20 degrees above normal values, even north
of the front.  The exception would be near the lake where cold
onshore flow will keep highs a few degrees cooler, likely in the mid

The Wednesday night frontal zone stretches between a low center
moving through the Great Lakes and a deeper low developing over
Colorado in the lee of the Rockies.  As the front stalls just to our
south Thursday morning, the Colorado low will continue deepening as
it drops toward the Texas panhandle and then begins to lift
northeast.  The northeast movement of the low will push the front
back toward the area late Thursday, along with increasing values of
precipitable water.  Weak mid-level shortwaves riding over the front
may support some scattered showers across the area Thursday
afternoon.  The better chance of rain and possible thunderstorms
comes Thursday evening and overnight into early Friday as warm
advection becomes more established at low levels and shortwave
energy increases aloft.

For Friday...

The biggest questions for this forecast period come Friday.  At some
point during the day our preview of mid-May departs with the passage
of a strong cold front and the reality of winter makes its return.
There is still considerable uncertainty regarding the timing of this
frontal passage, though that will make all the difference regarding
daytime highs and convective trends.

SPC has had portions of the area included in its Convective Outlook
for Friday due to a combination of the moderately buoyant warm
sector ahead of the front, strong southerly flow, and the strong
though mostly unidirectional shear. This would require the warm
sector to linger over the area for at least the first part of the
day. This is the scenario depicted in the GFS/ECMWF, in contrast to
the much faster NAM which would have the front well clear of the
area by mid-day. Given the GFS/ECMWF agreement and consistency but
accounting for the chance of a slightly faster trend, have thus
maintained highs in the mid 50s north and west, with mid 60s south
and east. The best chance of thunderstorms and severe weather would
be in the southeast portion of the area which will have more time to
destabilize before the front pushes through.

For Saturday and Sunday...

Highs in the 30s this weekend will be near average for late February
but feel like a harsh reality check after such a mild week.  To make
matters worse, it appears that very strong northwest winds and a
chance of snow could develop early Saturday in the deformation zone
behind the departing low.  Deterministic QPF from the GFS and ECMWF
is less than a tenth of liquid, so at this range this would appear
to be a quick shot of very light snow, perhaps with a bit of lake
enhancement across portions of NW Indiana.  Sunday looks dry under
the influence of an approaching ridge axis, though as the ridge
moves east by Sunday night another shot of light rain or snow is
possible with a weak shortwave aloft.

For Monday and Tuesday...

Milder air appears poised to return to the area for the first part
of next week.  Temperatures do not look quite to be quite as balmy
as we have been seeing, but ridging increases aloft and southerly
surface flow sets up behind the departing ridge and ahead of the
next approaching low.  Unlike these past few days, this warm
southerly flow also appears to bring Gulf moisture to the area,
increasing shower and thunderstorm chances for Monday night and




A highly anomalous warm stretch that has already broke daily
records will continue to threaten records this week, including the
all time February warm records for any time of the month on
Wednesday. Here are daily records.

Chicago:                         Rockford:
      High:      High Min:       High:      High Min:
2/21: 67 (1930)  47 (1930)       64 (1930)  42 (1930)
2/22: 68 (1922)  52 (1930)       61 (1922)  48 (1930)

All-time February Record Highs:
Chicago:  75  2/27/1976
Rockford: 70  2/25/2000

For more, including consecutive days of such warmth this time of
year, please see our web top news headline.


For the 18Z TAFs...

Concerns center on fog potential tonight. Smaller concerns on
lingering MVFR cigs this afternoon and weak onshore wind shift
later today.

A weak cold front has essentially stalled out across northeast
Illinois this afternoon. MVFR cigs will continue to erode from the
west with weak low level drying occurring, but they may remain a
bit more stubborn than has occurred this morning. Dewpoints remain
fairly high tonight, and with weak high pressure passing overhead
and some clearing occurring, fog development is likely. Some
guidance is bringing conditions down pretty hard overnight once
the fog develops. Confidence on timing and how low things will go
at this point is not very high, but 1/2 mile or lower conditions
not indicated yet in the TAF are certainly possible late tonight
into the start of the day on Wednesday.

This evening there could also be some marine fog that may bleed
inland a little bit, but the lake breeze push does not appear to
be very strong at this point, so we do not feel that any of this
would get to ORD/MDW. There could wind shift to an east component,
though confidence on direction is low, but confidence is high that
speeds would remain less than 8-10 kt and could just be variable.

Southwest winds will pick up tomorrow, so after some lingering
morning fog, concerns would be minimal.



144 PM CST

Mild and moderate southerly flow will become reestablished by
tonight and persist through most of Wednesday.  A low pressure
center then crosses the northern half of the lake late Wednesday,
veering winds to northerly by midday Thursday.  Winds continue
veering to northeast by Thursday night and begin to increase ahead
of a larger and stronger low pressure system approaching from the
Plains.  This low also will cross the lake on Friday, and although
timing is uncertain, a period of gales appears likely both Friday in
the mild easterly and southerly flow ahead of the low, and more so
on Saturday in the cold northwest flow behind it.






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