Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221021

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
421 AM CST Wed Feb 22 2017

336 AM CST

Through tonight...

The main concerns initially today will be with regards to the
dense fog across the area, and how quickly it will burn off this
morning. Visibilities thus far this morning have been very low in
some of this fog, with KRFD reporting down to 1/16th of a mile.
The main area that has been spared this dense fog thus far, aside
from the immediate Chicago area, has been over my southeastern
counties, where cloud cover has helped reduce the extent of the
fog. This may change, however, in the next couple hours as some of
the higher clouds shift out of this area. This could result in
the need to expand the current fog advisory for these areas
through mid morning. We will continue to monitor the latest
conditions to see if this will be needed. Otherwise, expect a
gradual improvement in the fog by mid to late morning.

The main story then becomes the potential to have another very
warm day across the region today. I did not make any big changes
to temperatures yet today, other than to lower them by a degree or
two. However, these record breaking temperatures into the lower
70s will largely depend on how quickly with burn off this fog and
low clouds, if it takes longer than currently anticipated, our
temperature forecast may end up being a bit too warm.



421 AM CST

Thursday through Tuesday...

A back door cold front, associated with a quick moving storm
system moving over the Upper Great Lakes, is expected to shift
southward over the area by Thursday morning. This will likely set
up a sharp temperature gradient across the CWA, with sharply
colder temperatures into the 40s over northeastern Illinois and
northwesterly Indiana near the lake, and temperatures again into
the low 60s over my far southern counties. Persistent and breezy
northeasterly winds off of Lake Michigan will likely result in
morning highs along the lakeshore, with temperatures quickly
falling into the upper 30s to low 40s and remaining there through
the day. Farther inland, temperatures could tag the upper 40s to
low 50s, before falling during the afternoon as the cooler lake
airmass oozes inland. Some lower clouds and or fog may also try to
move in off of the lake, this is something we will have to

Rain will also be developing over the area later in the day on
Thursday as the beginnings of our next significant storm system
begins to impact the area. This initial area of rain will develop
to the north of a northward shifting warm front in association
with increasing lower-level moisture convergence and frontogenesis
along the nose of southerly low-level jet. The rain will likely
intensify some over northern Illinois Thursday night as the best
moisture converge sets up over the area. Forecast guidance and
model soundings indicate the precipitable water values will top
1", which should help result in some health rainfall amounts over
portions of northern Illinois Thursday night. Thunderstorms will
likely become embedded within this area of showers Thursday night
as the strong warm and moist lower-level advection sets up a
corridor of steep lapse rates over the area. Deep warm cloud deeps
in excess of 7,000 feet should also result in rather efficient
rainfall production. Therefore, as a result, I would not be
surprised for areas north of I-80 to get 1 to 1.5" rain amounts
Thursday night.

Temperatures will also likely remain largely non-diurnal Thursday
night, with rising temperatures likely overnight as the surface
warm front begins to lift over my southern CWA. This should set
the stage for a very mild Friday (60-70) ahead of the eastward
shifting cold front. Ahead of this front very gusty south-
southwesterly synoptic winds are likely Friday as a ~990mb low
shifts east-northward into southern Iowa and southern Wisconsin.
Forecast guidance is still a bit uncertain on the exact behavior
of the surface low Friday afternoon, with some suggesting a
secondary low may develop, or the main low could hang back west
under the closing off mid-level disturbance. Either way, however,
it still appears that the setup of an eastward moving cold front,
impinging upon a modestly unstable and highly sheered environment
would favor a decent threat for strong to severe convection by
early Friday afternoon. The primary threat at this time looks to
be my eastern CWA, where convection is expected to initiate ahead
of the front, and within a zone of increasing ascent within the
exit region of strong upper-level jet.

The threat of convection is likely to quickly shift east of the
area by late afternoon as the mid-level dry slot moves overhead.
As a result, there will likely be a period free from precipitation
following the cold frontal passage. However, temperatures will be
on quick downward trend following this frontal passage. The next
batch of precipitation, likely in the form of snow looks to
impact the area either late Friday night or Saturday morning as
the storm systems deformation zone shifts over the area. This
period of snow could result in some minor accumulations, but
likely only to grassy surfaces given the warm ground temperatures.
Otherwise, the main story late Friday night into Saturday, will
be the sharply colder and very windy conditions. West-
northwesterly winds could approach 45 mph for a period into
Saturday morning.

The snow threat will end Saturday afternoon. The next system of
interest then comes into the picture late Sunday and Sunday night.
there is still a good deal of spread in model solutions following
Friday`s dynamic system, but it does appear that another
disturbance could try to spread a period of light snow over
portions of the area Sunday night.

Following this storm system, it appears the larger scale weather
pattern will remain very active, with another potential
significantstorm system to impact the area by next Tuesday.




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:
2/22: 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 06Z TAFs...

Primary aviation concern is fog potential tonight. Very moist
boundary layer, clear/mostly clear skies, and light winds should
provide for a very favorable set up for ground fog development
tonight. Already seeing fog developing in outlying areas and would
anticipate this fog to grow more widespread and dense through the
night. Urban heat island effects seems to be sheltering MDW to
this point with larger T/Td depression, so with 06z TAFs have
eased off vsby drop tonight there, though worth noting if dense
fog does indeed become widespread then with light SW wind it would
likely advect into MDW as well. LIFR cigs are accompanying the
dense fog and the stratus looks to be very close to ORD and MDW
at the moment. Ocnly stratus struggles to reach ORD and MDW so
have a TEMPO for LIFR cigs now, but expecting LIFR cigs if fog
develops at the terminals. Fog and stratus should burn off fairly
quickly Wednesday morning, with speed of fog dissipation dependent
on how widespread/dense/thick it becomes overnight. Once fog
mixes out look for VFR conditions the remainder of the TAF cycle.
Only other concern is wind, which could become a bit
stronger/gustier from the SSW than guidance suggests as temps warm
above what models are projecting. Cannot rule out gusts as high
as 30kt Wednesday afternoon.



232 AM CST

Headlines...Will let the marine fog advisory continue. Unsure how
much fog is over the lake, but given the amount of fog over land,
figure it is best to let the marine fog advsy ride.

Gusts over the NSH waters may approach 20-25 kt this afternoon but
thinking the majority of gusts will be 15-25 kt so will not be
issuing a small craft advisory.

Low pressure currently over South Dakota will pass over the lake
this evening. South winds will increase to 15 to 25 kt late this
afternoon ahead of the low. The low continues northeast across
Quebec and a second cold front slides south down the lake Thursday.
Winds become north to northeast at 30 kt.  The next strong low forms
over the plains Thursday afternoon and reaches the Mississippi River
Friday. Guidance is still struggling with the speed and strength of
the low, but models consistently have it passing over the lake
Friday night. East gales are possible ahead of the low Thursday
night through Friday afternoon.  Have just enough confidence to
issue a gale watch. Will hold off on a small craft advsy.

A break in gale force gusts is expected as the low moves over the
lake Friday night and then continues to Quebec. Northwest gales are
expected behind the low, and a high pressure ridge follows the low.
The center of the ridge stays well south of the lake over the Ohio
Valley or Gulf Coast. Guidance suggests that another low forms over
the plains early next week and reaches the western Great Lakes mid



IL...Dense Fog Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ023-ILZ032-ILZ033-ILZ039 UNTIL 9 AM Wednesday.

IN...Dense Fog Advisory...INZ001-INZ002-INZ010-INZ011-INZ019 UNTIL 9
     AM Wednesday.

LM...Dense Fog Advisory...NEARSHORE WATERS
     UNTIL 9 AM Wednesday.

     Dense Fog Advisory...LMZ777-LMZ779 UNTIL NOON Wednesday.

     Gale Watch...LMZ777-LMZ779...MIDNIGHT Friday TO 9 AM Friday.




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