Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 172259

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
459 PM CST Fri Feb 17 2017

142 PM CST

Through Saturday...

Daily warm temperature records have already been broken this
afternoon and more record breaking is probable on Saturday.

A deep, sub-985mb low across the Canadian Prairie provinces helped
to usher in the very mild air mass today. With the system`s slow
progression into early Saturday, the southwest winds will continue
tonight, providing evening temperatures more common for late
April. Have forecast low temperatures near daily record warm
minimums, which may be broken.

A weak cool front that is basically more of a wind shift, will
ease its way into northern Illinois midday Saturday, but this
should have little impact on temperatures. The 925mb temperatures
forecast midday Saturday are again at near to record values for
the time of year at 12C-14C which should support highs back at the
cusp of or into record territory. Thin high clouds may spread
into mainly the southern forecast area Saturday morning though
with little impact. So have further warmed the forecast as most
guidance seems once again too cool. The warmest highs of mid 60s
are forecast ahead of the surface wind shift in the eastern area.

Confidence in whether a lake breeze will develop Saturday is
low as boundary layer westerly winds look to be at the speed
threshold to prevent a lake breeze from moving inland. So while
confidence is low in a lake breeze developing, if it develops, it
would likely not progress very far inland and also be more of a
mid to late afternoon feature. Thus impacts on highs seem limited
and given a warm starting point, still have highs in downtown
Chicago forecast to reach 60 early in the afternoon.



326 PM CST

Saturday night through Friday...

No weather concerns through Sunday night from an impact
perspective, with concern next week on periodic precipitation
chances, and a chance for isolated thunderstorms late Monday
through Monday night. Throughout the forecast, focus was on
continued spring-like and likely at times record breaking warmth.
For reference to daily record warm temperatures, see Climate AFD
below. In general, highly amplified pattern with anomalous mid
and upper ridging over the central and eastern CONUS will remain
with us through Thursday, with changes finally on the way on

On Saturday night into Sunday morning, a weak surface ridge will
bring decent radiational cooling after a mild Saturday evening.
Favored cool spots in northern Illinois should dip down to or just
below freezing, with the rest of the area in the mid to upper 30s
for lows. Very weak flow will be in place at the surface and aloft
on Sunday thanks to the surface high pressure ridge, presenting a
very good setup for early lake breeze formation and passage. Highs
inland away from the lake should soar into the 60s once again
under full sun. But along the immediate lake shore and a few
miles inland, the early lake breeze passage is expected to limit
highs to the mid to upper 40s. Warming should be quick enough
until lake cooling arrives at O`Hare for another 60 degree there,
but it could be close.

In addition to the extreme warmth likely on Monday, attention to
start next work week turns to possible impacts from an elongated
positively tilted trough, with parent surface low well to the
north. This is where differences in the guidance creep in, with
GFS/NAM on faster side of envelope and ECMWF/GEM on slower side.
With strength of ridging over the area and amplification of it
likely ahead of the trough, do wonder if the slower solutions will
end up being closer to reality. However, it is far enough out to
bring in some chance PoPs into the northwest 1/3 to 1/2 of the CWA
Monday afternoon and Monday evening. With thermal profiles progged
on slower guidance at or above that what occurred today, 70 degree
readings are distinctly possible, especially south of I-80. With
that said, given uncertainty, favored largely mid to upper 60s
(low 60s far north), except for the Illinois shore likely kept
several degrees cooler (50s) by gusty southeast winds.

Rain showers are then likely to progress eastward across the CWA
later Monday night into Tuesday. In the period of shower potential
late Monday through Monday night, added in slight chance/isolated
thunder given decent low level jet, dew points into 50s and cold
enough temperatures at mid-levels for sufficient MUCAPE. There
will be literally no cooling of the air mass behind Tuesday`s
"cold" front passage, with another bout of readings well into 60s
area wide possible Wednesday. A change in the pattern will
finally be in the works late in the week, with the formation of a
strong surface low over the Plains as a deep trough carves across
the West. The unseasonable warmth will likely continue through
Friday as we probably stay on the warm side of the system, with
rain increasingly likely, followed by a return of much cooler
weather in the wake of the system beyond day 7.




142 PM CST

An unseasonably warm stretch that has already broke daily records
on Friday February 17th will continue, with probable more breaking
of records. Here are statistics on daily records.

Chicago:                         Rockford:
      High:      High Min:       High:      High Min:
2/18: 62 (1981)  45 (1981)       58 (1981)  38 (1997)
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 weather pattern provides limited concerns and those are only
low-level wind shear tonight and the possibility of a lake breeze
Saturday afternoon.

Southwest winds will show more gusts beginning by 19Z as mixing
deepens. The pressure gradient remains strong enough this evening
to keep surface sustained speeds at 10-12 kt though gusts should
be more sporadic. A stout southwest low-level jet is forecast to
develop tonight with 40-45 kt forecast between 700-3000 ft. Given
the jet within a 1000 ft of the surface and surface speeds easing,
have included LLWS in the TAFs for late this evening into
overnight. This should ease quickly during the pre-dawn hours as
the jet translates east of the area.

Westerly surface winds on Saturday will be less in speed than
today`s winds, presenting the potential for lake breeze
development Saturday afternoon. However, we expect enough of an
offshore component in the wind to keep the lake breeze close to
shore, thus not forecasting a wind shift at the TAF sites.



334 PM CST

No significant marine concerns until next week. Southwest winds
will increase to 15 to 25 kt tonight and then veer west and
diminish Saturday following a weak cold front passage. Light winds
favorable for lake breeze influence are favored for Sunday with
high pressure ridging over the region. Winds will turn southeast
synoptically Sunday night into Monday as the ridge shifts east and
low pressure over the Plains lifts north-northeast. Southeast
winds should increase to up to 25 kt Monday afternoon and then to
30 kt or so Monday night while shifting south. Very mild air over
the lake should result in stable conditions and limit gustiness
above sustained speeds. Winds will shift west and diminish on
Tuesday following cold front passage. Another strong low pressure
system will likely affect the region late next week with
increasing winds.






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