Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 182007

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
207 PM CST Sat Feb 18 2017

1005 AM CST

Another record warm and sunny mid-February day is unfolding as

The 925mb temperatures observed on area 12Z raobs this morning
were again at daily record territory of 12C-13C. While this proxy
for today`s top of the mixed layer will not be warming like
yesterday, the warmer starting point than yesterday provides 10
a.m. temperatures a tad above 24 hours ago. Highs should then fall
out similar, with mid 60s and some spotty upper 60s expected
east, after a little nudge up on the going forecast temperatures.

The forecast lake breeze timing and inland impingement has not
been changed, with it mainly occurring near the shore during mid-
afternoon and easing its way inland after 3-4 pm, potentially
quite slowly before fading early this evening. The wind speed
behind the wind shift looks to be light, and this would keep
marine layer induced cooling from reaching too substantial of a
rate, including in downtown Chicago, where forecast temperatures
are expected to dip into the upper 40s by 6 pm.



254 AM CST

Through tonight...

Another spring like day is expected with temperatures well above
normal.  A weak cold front currently over eastern Iowa will pass
over the region today. Winds will become light out of the northwest
behind the front. 925 mb temps will be cooler than yesterday, but
low to mid 60s seem reasonable for highs across the area today. we
will once again make a run at record high temps, and see the climate
section below for more details.

A lake breeze may form this afternoon but expecting it to stay on
the immediate lake shore. If it does come on-shore, it should be
after we reach our high temps for the day.

Calm winds and clear skies overnight will help temps fall into the
30s tonight.



205 PM CST

Sunday night through Saturday...

The high amplitude upper level pattern will become more progressive
by Sunday night as the axis of the upper ridges moves across the
upper midwest with longwave troughing moving across the Rockies and
over the western plains.  Swly flow will persist though Monday as a
cold front pushes across the plains.  Unseasonably warm conditions
will persist through much of next week.  Low pressure is still
expected to drop out of the northern rockies Sunday night and lift
northeast across the northern plains and into south central Canada
Monday.  As the low continues to track northeast, an associated cold
front will push across the region Monday night into early Tuesday
morning.  The longer range guidance has trended toward the slower
solution indicated by previous ECMWF runs and the the general model
consensus is for the the frontal passage to occur a little later
than previously anticipated.  However, there is little change to the
going forecast as far as pcpn and temperature trends, with a swath
of showers pushing into the Rockford area late Monday afternoon or
early evening and then steadily spreading east Monday night and
Tuesday.  There should be sufficient forcing and instability along
and ahead of the front to carry at least a slight chance for
thunderstorms.  However, the strongest forcing will be to the north
of the wisconsin border while the strongest instability should be to
the south, across the lower Mississippi and Ohio Valleys. Therefore,
confidence is low in the coverage and timing of thunderstorms
impacting the region.  Highs in the 60s should continue through at
least mid week.  There is some potential for lake cooling impacting
the max temps along the lake front, keeping temps close to the lake
a little lower.

Temperature trends will change little through early next week with
highs in the lower to middle 60s across much of the area.  Another
southern stream system is expected to lift across the central plains
Wednesday night and Thursday.  The longer range guidance is in good
agreement on the development and track of the system, there are
characteristic timing differences between the GFS and ECMWF in the
speed of the system.  The GFS is the faster solution, which is
fairly typical, while the ECMWF is trending 12-18 hours slower.
Following the passage of this system, temperatures should trend back
to closer to seasonal normal levels by late next week with lows in
the middle 20s to around 30 F and highs in the middle to upper 30s.


1145 AM CST

An unseasonably 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 statistics
on 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...

The only forecast items of interest with the TAFs are wind
direction and speed.

West to northwest winds will continue this afternoon and will
keep an expected lake breeze from moving quickly inland. It is
possible ORD and MDW briefly flop light northeast between 00Z and
03Z, but the speeds should be less than 7 kt if that occurs.

A high pressure ridge will be directly over the area Sunday
morning and then move slowly east. This will favor a southeast
wind, which will become more easterly or northeasterly at Chicago
area airports thanks to a lake component and likely a true lake
breeze. Confidence in specific timing of this lake breeze passage
is low-medium, with confidence in speeds around 8 kt being



205 PM CST

A ridge of high pressure will build over the Upper Mississippi
Valley and western Great Lakes tonight, allowing brisk west winds to
diminish overnight and Sunday as the ridge axis moves across Lake
Michigan. Winds should become somewhat light and variable on Sunday,
but then shift to ely-sely through the day on Monday as low pressure
lifts through the northern plains and into south central Canada. As
the low continues to lift northeast, an associated cold front will
sweep across the lake late Monday night and early Tuesday morning.
Sely winds may increase to arnd 30kt ahead of the front and then
shift to wly on Tuesday following the frontal passage.  An extended
period of generally southwest winds will set up over the lake
through midweek as weak high pressure builds over the lower
Mississippi Valley and into the Ohio Valley.  Another low and
associated cold front are expected to cross the lake by Thursday. A
colder airmass associated with high pressure building out of Canada
should spread into the western Great Lakes by Friday.





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