Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 201217

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
617 AM CST Mon Feb 20 2017

254 AM CST

Through tonight...

The main weather concern early this morning is centered around
the potential for a period of dense fog through mid to late
morning. A corridor of upper 40 degree dewpoints is currently
streaming northward across central Illinois, and the concern is
that as this better moisture runs into the cooler air farther
north into northern Illinois, that areas of dense fog will quickly
develop over the next couple hours. Visibilities just south of my
CWA are down to 1/4 mile, so expect some of these lower
visibilities to develop northward. As a result, a dense fog
advisory will likely be needed for at least portions of the area
over the next couple hours. However, before making a final
decision, I want to watch trends for the next hour or two to get
an idea of exactly how much of the area will need to go into an
advisory. At the present, it appears the primary dense fog
potential may stay to the west and south of the immediate Chicago
area, where warmer temperatures are currently present.

Otherwise, the other main story again today will be potentially
the 3rd day in a row of record breaking temperatures. While there
is some small concern that the morning fog and persistent high
cloud cover today could limit the extent of the warmth across the
area this afternoon, the overwhelmingly warm start to day, and
the continued warm temperatures aloft should not make it too hard
to warm back into the 60s this afternoon. As a result, I have
decided not to make any changes to the high temperature forecast
for today. However, a south-southeastward wind component off of
Lake Michigan could result in ribbon of cooler temperatures along
the lakeshore this afternoon.

A weak surface cold front is expected to shift across the area
tonight. This feature combined with the approach of a mid-level
disturbance, should support a period of rain showers across the
area tonight. The main changes to the going forecast was to slow
the timing of the showers down, making today dry across most of
the area. The best timing for showers will likely not be until
after 7 PM for north central Illinois, and not until after 10 pm
to 12 am tonight for the Chicago area. While there is a modest
chance for some thunderstorms with this activity, overall lapse
rates look marginal for widespread storms, so any thunderstorms
will likely be widely scattered.



405 AM CST

Tuesday through Sunday...

The main story for Tuesday, but especially Wednesday, will be the
continued near record to record warmth across the area. While
temperatures could be a touch cooler across the area on Tuesday
following the Monday night Pacific cold frontal passage, the
airmass over the area will still remain very mild for late
February. As a result, with clearing skies expected during the day
high temperatures will again likely top out in the 60s. We may,
however, fall just short of the record high on Tuesday. This is
mainly because the record is a bit higher, than the previous few
days. By Wednesday we may see our warmest temperatures of the
entire stretch. Another Pacific system is forecast to shift
eastward across the Upper Great Lakes during the day, and this
should set up a return to dry and breezy southwest winds. Current
forecast guidance suggests that 925 MB temperatures up around 15C
will overspread the area, which given the warmth of late would
support temperatures into the low 70s, though given the dry
southwest winds, even warmer temperatures into the mid 70s may not
be out of the question. The current record warmest high for the
entire month of February is 75 degrees for Chicago. Gusty
southwest winds and warm and dry conditions on Wednesday could
also result in a high fire danger across the area.

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 cooler temperatures on Thursday. Onshore
flow could easily keep temperatures in the lower 40s near Lake
Michigan, with 50s farther inland.

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
could 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.

While there area still some uncertainties in the exact track and
timing of this storm system on Friday, there is moderate to high
confidence in a significant storm system impacting the region
Friday. The current thinking is that a ~990 MB surface low will
track northeastward across southern Wisconsin on Friday. This low
will likely drive a warm front back north across the area late
Thursday and into Thursday night. Expect a period of showers and
storms develop over the area during this time as the nose of a
low-level jet sets up over the area into early Friday. Another
round of storms then looks like a good likelihood on Friday as the
strong dynamic system drives a cold front eastward across the
area. Given that the storm system could produce a strong kinematic
field over the area in combination with a warm and unstable
prefrontal airmass, severe weather will be possible across the
area Friday. As a result, this potential will have to be watched
closely in the coming days.

Strong synoptic winds, potentially gusting to or above 50 MPH are
also possible with this storm system late Friday and Friday night.
Colder air will move in over the area Friday night into Saturday,
and as this occurs, the precipitation could end as a little snow
into early Saturday.



350 PM CST

A highly anomalous warm stretch that has already broke daily
records will continue to threaten records this week. Here are
daily records.

Chicago:                         Rockford:
      High:      High Min:       High:      High Min:
2/20: 64 (1930)  49 (1930)       61 (1983)  46 (1930)
2/21: 67 (1930)  47 (1930)       64 (1930)  42 (1930)
2/22: 68 (1922)  52 (1930)       61 (1922)  48 (1930)

For more, including all-time February warmth and consecutive
days of such warmth this time of year, please see our web top
news headline or Public Information Statement.


For the 12Z TAFs...

MVFR vsby is at all of the terminals now and guidance suggests
dense fog of less than a mile well south of the terminals should
continue northward. The cirrus shield may be inhibiting the fog
and low cigs` progression. LIFR cigs are tied to the fog, so
thinking fog vsby and fog will drop around sunrise. Have higher
confidence in vsbys less than one mile at DPA and RFD. Good news
is mixing should lift cigs and vsby rapidly in the mid to late
morning. VFR conditions will coincide with gusty southeast winds.
Max gusts will be this afternoon and evening with southeast to
south winds gusting to 25 kt. Could see higher gusts if we mix
better than forecast. 45-50kt winds are expected about 2000-3000
ft AGL this evening, which may result in LLWS for a few hours. If
gusts are higher, LLWS will be less impactful.

A cold front moves through tonight with showers and isolated
thunderstorms. Confidence is medium in thunderstorms occurring but
low in how many and exactly where. Therefore, only have rain in
the TAFs. MVFR/IFR cigs also move in along with the rain. Winds
become westerly behind the front and rain ends Tuesday morning.



203 AM CST

Headlines...Issued a small craft advisory for this afternoon and
evening for hazardous wind gusts across all of the nearshore waters.
Low end gales may occur over the northern half of the lake this
evening, but do not have enough confidence to issue a gale warning.

The pressure gradient tightens as the high currently over the Ohio
Valley reaches the east coast, and the low over the Dakotas moves
over Manitoba this afternoon. Southeast to south winds increase and
become hazardous to small craft. Low end gales are possible over the
northern half of the lake, but unsure how stable the surface layer
will be. If the surface layer is weaker than currently forecast,
gales will be more prevalent and a gale warning will be needed.
Winds weaken and become west behind the low`s cold front Tuesday.

The next low passes over the northern end of the lake Wednesday
evening and winds become northwest behind it.  The bigger story is
the strong low that will impact the midwest later this week.
Guidance features a 990-992mb low passing over the lake Friday
night. Have medium confidence in gales with an easterly component
ahead of the low Friday, but high confidence in northwest gales
behind the low.  It`s still too early for exact details and a
headline. The low continues to Quebec Saturday while a surface high
passes south of the lake over the weekend.



IL...Dense Fog Advisory...ILZ019-ILZ020-ILZ021-ILZ022-ILZ023-ILZ032-
     ILZ033-ILZ039 UNTIL 10 AM Monday.

IN...Dense Fog Advisory...INZ001-INZ010-INZ011-INZ019 UNTIL 10 AM

LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...NOON Monday TO 3 AM Tuesday.




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