Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 182329
AFDLOT

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

.SHORT TERM...
245 PM CST

Through Sunday...

Impressive, record-breaking warmth for mid-February continues,
with top five all time February temperatures having been
reached/tied today at Chicago and Rockford, including the second
earliest 70 on record for Chicago. A lake breeze has developed but
is hugging the lake shore as anticipated, with downtown Chicago
presently 51 degrees. Wind speeds will be diminishing fairly
quickly this evening enabling quicker radiational cooling than
last night, but still a plenty mild evening for February.

After lows in the mid to upper 30s, another quick rebound will
occur on Sunday. Have gone above all guidance for non-lake
adjacent areas with 925mb temperatures still near-record at 10C-
11C. Have highs near daily record levels again with mid to upper
60s for areas not near the lake.

For the lakeside counties, Sunday looks to bring a more
traditional lake breeze, which adds a challenge to the forecast
and reduces certainty in this area. The lake breeze should
progress through the lake adjacent counties by early to mid
afternoon given experience with a surface high directly over the
area and a stout temperature difference between land and marine
layers. For immediate lakeside communities including downtown
Chicago, temperatures are favored to remain in the 40s. For areas
near O`Hare and Midway, we presently have the lake breeze pushing
through in the early to mid afternoon, likely bringing a quick 10
degree drop or so after attaining forecast highs in the lower to
mid 60s.

MTF

&&

.LONG TERM...
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.

&&

.CLIMATE...
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.

&&

.AVIATION...
For the 00Z TAFs...

Aviation forecast concerns remain few this evening, and are
primarily focused on near term lake breeze pushing toward ORD and
MDW. VFR conditions to prevail through the period.

KLOT and TMDW radars depict lake breeze boundary surging south-
southwestward across Chicago at this hour, and should move across
both ORD and MDW by or shortly after start of the TAF period at
00Z. Mesonet obs behind the lake breeze boundary were generally NE
at 4-6 kts, though push noted on radar looks like a brief period
of around 10 kts likely based on propagation. Lighter NE winds
will likely last a couple of hours before winds go light/variable
later this evening as weak high pressure spreads across the area.

On Sunday, light/variable winds early will become east-southeast
as surface high pressure drifts off to the east. Easterly lake
breeze is likely and expected to push inland across ORD/MDW by
mid-afternoon with an east wind.

Ratzer

&&

.MARINE...
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.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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