Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 180721
AFDLOT

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

.SHORT TERM...
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.

MTF

&&

.LONG TERM...
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
Friday.

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.

Castro

&&

.CLIMATE...

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.

KMD/RC

&&

.AVIATION...
For the 06Z TAFs...

Very few concerns for aviation forecast through the TAF period,
namely a period of LLWS overnight tonight, and the low
probability of a lake breeze affecting MDW and/or ORD late
Saturday.

Low pressure over Lake Superior will track east through Saturday,
trailing a very weak cold front across the forecast area by
afternoon. Persistent southwest wind around 10 kt overnight will
shift west by morning, then northwest late in the day Saturday
with the passage of the weak cold front. Above the boundary layer,
southwest winds 40-45 kt will set up LLWS for the next few hours
into pre-dawn, before the low level jet shifts east.

Behind the cold front Saturday, winds will shift west-northwest
then eventually more north-northwest by evening. Some high-res
guidance depicts a weak lake breeze developing which mainly would
affect GYY with a north wind, and areas closer to the lake shore.
Low probability exists that the lake breeze could drift far enough
west and approach MDW, though majority of guidance does not
indicate this. Weak gradient should allow winds to settle to a
light north-northwest or variable direction after dark Saturday
evening.

Ratzer

&&

.MARINE...
120 AM CST

Southwesterly winds of 15 to 25 KT currently ongoing over the
lake, will ease today as a weak cold frontal boundary shifts over
the lake early today setting up light westerly flow. A surface
ridge of high pressure will build over the Upper Midwest through
the day, and this feature could allow wind speeds to ease enough
this afternoon over southern Lake Michigan to allow for lake
breeze development with light onshore winds over southern Lake
Michigan. Winds should then shift back southeasterly by late
Sunday as the surface ridge moves east and an area of low pressure
sets up over the Plains. While wind speeds will likely initially
remain light out of the southeast on Sunday, speeds should be on
the increase into Monday as the Plains low shifts northward into
into south central Canada. South-Southeasterly winds of 25 to 30
KT look probable Monday and Monday night. Winds then will turn
southwesterly and weaken following the passage of another cold
front. Later in the week there are signs that a significant storm
system could tack aim on the region. This could result in a period
of strong winds Friday into Saturday of next week.

KJB

&&

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

$$

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