Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220936

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
336 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.



144 PM CST

...Wednesday night through Tuesday...

For Wednesday night and Thursday...

Wednesday night a cold front will move across the area under a weak
shortwave aloft.  It does not appear there will be sufficient
moisture along the front to support any precipitation, but behind
the front winds will turn northwest and eventually north and
northeast, ushering in cooler air for Thursday. Cooler is a relative
term for this unusual week, of course, so highs Thursday are still
looking to be about 15 to 20 degrees above normal values, even north
of the front.  The exception would be near the lake where cold
onshore flow will keep highs a few degrees cooler, likely in the mid

The Wednesday night frontal zone stretches between a low center
moving through the Great Lakes and a deeper low developing over
Colorado in the lee of the Rockies.  As the front stalls just to our
south Thursday morning, the Colorado low will continue deepening as
it drops toward the Texas panhandle and then begins to lift
northeast.  The northeast movement of the low will push the front
back toward the area late Thursday, along with increasing values of
precipitable water.  Weak mid-level shortwaves riding over the front
may support some scattered showers across the area Thursday
afternoon.  The better chance of rain and possible thunderstorms
comes Thursday evening and overnight into early Friday as warm
advection becomes more established at low levels and shortwave
energy increases aloft.

For Friday...

The biggest questions for this forecast period come Friday.  At some
point during the day our preview of mid-May departs with the passage
of a strong cold front and the reality of winter makes its return.
There is still considerable uncertainty regarding the timing of this
frontal passage, though that will make all the difference regarding
daytime highs and convective trends.

SPC has had portions of the area included in its Convective Outlook
for Friday due to a combination of the moderately buoyant warm
sector ahead of the front, strong southerly flow, and the strong
though mostly unidirectional shear. This would require the warm
sector to linger over the area for at least the first part of the
day. This is the scenario depicted in the GFS/ECMWF, in contrast to
the much faster NAM which would have the front well clear of the
area by mid-day. Given the GFS/ECMWF agreement and consistency but
accounting for the chance of a slightly faster trend, have thus
maintained highs in the mid 50s north and west, with mid 60s south
and east. The best chance of thunderstorms and severe weather would
be in the southeast portion of the area which will have more time to
destabilize before the front pushes through.

For Saturday and Sunday...

Highs in the 30s this weekend will be near average for late February
but feel like a harsh reality check after such a mild week.  To make
matters worse, it appears that very strong northwest winds and a
chance of snow could develop early Saturday in the deformation zone
behind the departing low.  Deterministic QPF from the GFS and ECMWF
is less than a tenth of liquid, so at this range this would appear
to be a quick shot of very light snow, perhaps with a bit of lake
enhancement across portions of NW Indiana.  Sunday looks dry under
the influence of an approaching ridge axis, though as the ridge
moves east by Sunday night another shot of light rain or snow is
possible with a weak shortwave aloft.

For Monday and Tuesday...

Milder air appears poised to return to the area for the first part
of next week.  Temperatures do not look quite to be quite as balmy
as we have been seeing, but ridging increases aloft and southerly
surface flow sets up behind the departing ridge and ahead of the
next approaching low.  Unlike these past few days, this warm
southerly flow also appears to bring Gulf moisture to the area,
increasing shower and thunderstorm chances for Monday night and




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-ILZ039 UNTIL 9 AM Wednesday.

IN...Dense Fog Advisory...INZ001-INZ002 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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