Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211755

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1155 AM CST Tue Feb 21 2017


Rain south and east of Interstate 55 will continue to diminish as
drying aloft occurs behind a departing upper level wave. This wave
will outrun a weak and slow moving cold front, and this coupled
with weak surface high pressures the front to linger. Some subtle
low level drying occurs behind this front which has allowed areas
across north central Illinois to see an erosion of the lower
clouds and fog, but expect some of these lower clouds and patchy
fog to linger a bit longer farther east as this dewpoint drop
does not come as quickly and is a bit more subtle and we maintain
a sw flow along and ahead of the front.

Records may be a bit harder to reach today but it will still be
plenty mild out there, and we certainly will warm once these
clouds clear so records remain in jeopardy.. Expect with
lingering moisture there would be some fog around tonight,
possibly briefly dense near the lakefront and bleeding a bit
inland later this evening. It will again be mild tonight as the
thermal ridge will approach from the west.



254 AM CST

Through tonight...

An area of rain showers will continue to gradually shift east-
northeastward across the area this morning. The threat of thunder
is extremely low with this activity, and has not had a history of
producing any for several hours, so I have removed the mention
from the forecast. This line of showers should end by daybreak in
and around the Rockford area, and by mid morning in the Chicago
area as a weak cold front shifts over the area. However, some
showers may linger across my southeastern counties through midday
before ending. Otherwise, expect cloudy skies to gradually give
way to mostly sunny skies from northwest to southeast throughout
the day.

In spite of a cold front moving across the area this morning,
there is no cold air behind it. In fact, temperatures yet again
will be pushing record territory this afternoon. Overall, I only
made minor tweaks to temperatures today, with highs expected to
be in the middle to perhaps a few upper 60s over the area. This
would place highs at both Chicago and Rockford right around their
record highs, which are 67 and 64, respectively.



353 AM CST

Wednesday through Monday...

The main story for Wednesday, you guessed it, record breaking
warmth. For potentially the 6th day in a row the area could be
setting record high temperatures. It is even likely that the
warmest conditions yet in the stretch will occur Wednesday, and
this could come near or even break the all time warmest
temperatures for the month of February. Southwest winds are
expected to become breezy out of the southwest during the day as
another fast moving Pacific system shifts eastward across the
Upper Great Lakes. Forecast guidance continues to suggest that
925 MB temperatures will warm back up around 15C over the area,
which given the warmth of late would easily support temperatures
into the low to middle 70s. This is especially true given the
expectation for sunny skies and dry conditions. Gusty southwest
winds and warm and dry conditions could also result in an elevated
fire danger across the area. Given there will be a corridor of
very dry air above the forecast boundary layer, if the southwest
winds are able to effectively mix into some of this drier air the
afternoon dew point temperatures could fall into the 30s or low
40s over northern Illinois, and this would result in relative
humidity values falling below 30 percent.

Another cold front, associated with the Upper Great Lakes storm
system, is expected to shift southward over the area for Thursday.
This will likely set up slightly cooler temperatures on Thursday.
The most pronounced temperature change, however, looks to be for
areas near the lake shore, due to the persistent onshore flow
on Thursday. Highs look to range from the lower to middle 40s
near Lake Michigan, to the mid to upper 50s inland, and into the
low 60s over southern portions of the area.

The main forecast focus later in the week then quickly turns to
the likelihood of a strong storm system expected to impact the
central CONUS late Thursday through Friday. This storm system
is likely to have a significant impact on the area in terms of a
threat of severe thunderstorms Friday, along with a period of very
strong synoptic winds Friday and Friday night.

Showers and Thunderstorms are likely to begin developing over the
area by late Thursday, and continue Thursday night as the low-
level jet begins to nose into the area. This activity will not
poss a severe threat, but some decent rainfall amounts up to or
even in excess of an inch could occur over northern Illinois into
early Friday morning.

A Non-NAM evolution is favored for this storm system on Friday.
The 06Z NAM is too fast with this system, and is a fast outlier.
Given the parent mid-level wave may develop into a closed
circulation over the Mid-Missouri Valley by Friday morning, a
slower progression is certainly the way to go. Non-NAM forecast
guidance continues to forecast a ~990 MB surface low shifting
east-northeastward across southern Wisconsin on Friday. This would
drive the surface warm front northward over the area by Friday
morning. This will allow temperatures to once again warm well
into the 60s across much of the area, with some 70s even possible
in some areas. However, of more concern will be the potential for
a round of severe thunderstorms during the day as an eastward
moving cold front begins to shift across the area during the early
afternoon. This setup certainly points to the potential of cold
season severe thunderstorms over the area; namely a low CAPE, but
very high shear environment, that could support strong damaging

Strong synoptic winds, potentially gusting to or above 50 MPH are
also possible with this storm system. Gusty southerly winds during
the day will likely shift west-northwesterly Friday night in the
wake of the cold front. Strong Isallobaric pressure rises in the
wake of this 990 MB low could result in the strongest winds late
Friday night. However, this timing will ultimately depend on the
actual timing of the larger scale storm system itself.

These strong northwest winds will quickly transport much colder
air (temps in the 30s) over the area for Saturday, and as this
occurs, it appears that another period of precipitation, likely in
the form of snow, could occur over the area as the mid- level
deformation zone shift over the area. So, Saturday will be a night
and day difference from the Spring-like weather we look to see on




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.



124 AM CST

Headlines...Will let the small craft advsy go with the overnight
update as scheduled. Gales are still expected late this week, but
not enough confidence in the details to issue a watch now.

A surface low is moving over northwest Ontario. Its cold front will
cross the Mississippi River early this morning, and then pass over
the lake mid to late morning. A surface ridge builds in behind the
front so winds will weaken rapidly today. By late this afternoon
winds will vary from south over the northern half of the lake to
north or northeast over the southern half. A weak lake breeze is
expected late.

The next low takes shape over the Dakotas this evening and passes
over the western Great Lakes Wednesday evening.  Southwest winds
increase to 15 to 25 kt Tuesday night ahead of the low and then
become north to northeast 10-20 kt behind the low Thursday.

The active pattern continues as the next low forms over the plains
Thursday afternoon and then reaches the lake Friday evening.
Guidance still has differences in the low`s exact path and
intensity; however, gales are likely to occur with this system.  Have
low confidence in northeast gales ahead of the low Thursday night
and Friday, but have very high confidence in gales behind the low
Friday night into Saturday.  As the low continues northeast, a
surface high shifts over the southern Mississippi Valley over the
weekend.  A second surface high is progged to move over the western
Lakes early next week.






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