Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 070738

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
138 AM CST Tue Feb 7 2023

Issued at 138 AM CST Tue Feb 7 2023

Through Wednesday...

Key messages:

* Breezy today with falling morning temperatures

* Potential for fog development late tonight/early Wednesday
  morning over north central and far northern Illinois

A neutrally-tilted upper trough extending from western Ontario
through the Corn Belt is quickly racing east early this morning as
it is escorted by a 130 kt upper jet over the CONUS midsection.
The low-level response is a surface cold front that will move
east across the forecast area during the daybreak hours. The warm
sector ahead of this front is impressive, with temperatures up to
50F in Chicago (normal daily highs are low to mid 30s). Dew
points also are in the upper 30s to lower 40s, which is eroding
the remnant snow cover in far northern Illinois. As the front
sweeps eastward, westerly winds behind it will drop temperatures
about 10 degrees from 5 AM to 10 AM. A slight recovery will occur
this afternoon with scattered sunshine. Afternoon highs are still
on the plus side of normal with upper 30s to mid 40s.

High pressure of 1025 mb will quickly expand in by late this
evening. Light winds and forecast low temperatures 5F+ degrees
below forecast evening dew points indicate potential for patchy
fog development, especially over any residual snow pack. Some
high-resolution guidance hint at that area for reduced visibility.
It does not look as favorable of a setup as what occurred Sunday
night in north central and far northern Illinois, when areas of
dense fog occurred, as the snow pack then was both deeper and
colder. Will still continue patchy fog mention in the forecast,
and if locations see a dense enough fog there could be frost-
coated slick spots underneath it (i.e. freezing fog).

The southern jet stream will start to influence the area gradually
more on Wednesday, with a closed upper low starting to approach
from the Southern Plains. High clouds will thicken during the day
but it will still be warmer than normal. There is a fairly large
envelope of temperatures for Wednesday, maybe owing to cloud
thickness discrepancies in guidance. For instance the GFS MOS for
Chicago has a high of 50F and the NAM only 42F. We lean on the
cooler side of the mean at this time with the idea it should be
overcast in the afternoon. Some light rain may creep into the far
south late day. More on that system in the below discussion.



Issued at 237 PM CST Mon Feb 6 2023

Wednesday through Monday...

The primary messages during the long term include:

* Steady period of rain for the area Wednesday night into

* Chance continues for some accumulating wet snow across parts of
  far northern/northwestern IL prior to the precipitation ending
  on Thursday.

* Period of strong gusty winds plausible Thursday afternoon.

* Another period of rain/snow possible Friday.

Forecast guidance continues to come into better agreement with
the timing and potential track of a deepening storm system
expected to shift northeastward across the region Wednesday night
into Thursday. The expectation is for an area of low pressure to
develop northeastward into the Ozarks late Wednesday in response
to the eastward ejection of the cut off upper low setting up
across the Desert Southwest tonight. Thereafter, there is strong
model and ensemble support for the parent mid-level impulse to
quickly acquire a negative tilt (northwest to southeast
orientation) as it ejects northeastward into the Mid-Mississippi
Valley Wednesday night in advance of another digging impulse
across the Rockies. Strong upper level dynamics tied to this
ejecting wave, and complemented by coupling upper level jet
streaks, will support a rapid deepening trend with the
accompanying surface low Wednesday night into Thursday as tracks
northward across IL. Interestingly, model guidance is forecasting
a very impressive deepening trend (nearly 24 mb in 24hr) with the
surface low from 18Z Wed through 18Z Thu.

Also of note is the impressive northward moisture feed expected
with this deepening storm system. The warm and moist conveyor
belt looks likely to tap into the lingering Gulf moisture feed
from tonight`s northern stream system. This health moisture surge
should support precipitable water values topping out from 1.0 to
1.25", which is around 250 percent of normal for this time of
year. This in combination with some convective potential and the
presence of unseasonably deep warm cloud depths suggests the
potential for some heavy bouts of rain late Wednesday night into
early Thursday morning. As such, some notable rainfall amounts
look probable across the area with the potential for 1 to 2"
rainfall amounts.

While most of the precipitation from this system looks to fall as
rain, with periods of heavy rates probable, accumulating wet snow
is also possible along the northwestern periphery of storm system
late Wednesday night into Thursday. The ambient airmass across
the area will be relatively mild, so it will take strong dynamic
cooling in the systems deformation zone to result in an atmosphere
cooling enough to support snow. Ultimately the areas the see any
accumulating wet snow will largely be dependent on the exact
track the system takes into Thursday. With the potential favored
track being across far northeastern IL or far northwestern IN,
this would tend to favor eastern sections of IA into far
northwest IL and points northward into WI for accumulating snow
into Thursday. We thus continue to focus the main potential for
rain and snow on Thursday over far northwestern sections of
the area.

Also of note with this storm system is the threat for a period of
strong gusty winds, particular Thursday afternoon as the low exits
the area to our northeast. The system looks to generate a rather
strong pressure rise fall couple across the area Thursday
afternoon as it rapidly shifts to our northeast. Such patterns are
typically associated with rather strong gusty winds, potentially
in excess of 45 to 50 mph. This potential will need to be
monitored, especially for areas southeast of I-55.

The next trough will likely move over the area Friday. While there
continues to be spread in the guidance with how this system
evolves, it appears probable that a period of snow will occur
across parts of the area Friday. A shot of colder air with this
feature will push temperatures back into the lower 30s for highs
for Friday and Saturday. Thereafter, conditions look to moderate
back into the 40s as upper level ridging builds overhead for the
later half of the weekend.



For the 06Z TAFs...

Aviation Forecast Concerns:

* Gusty south-southwest winds gusting 25 to 34 kt becoming west
  toward morning and gradually diminishing during the day.

* Period of LLWS conditions into the pre-dawn hours with southwest
  winds 60-65 kts around 2000 feet AGL.

* Possible brief period of showers overnight with patchy MVFR

* MVFR ceilings expected to linger Tuesday, though gradually
  rising before scattering late.

Surface low pressure wave continues to lift northeast across
northeastern IA late this evening, with a trailing cold front
expected to push east across the terminals overnight. Southeast
winds have been gusting sporadically in the 25-30 kt range during
the evening have increasing some as of 05Z and this should
continue through 08Z as winds veer south-southwest. Winds will
eventually turn to the west behind the cold front early Tuesday
morning and then remain westerly through the day, with a gradual
diminishing trend. A strong low level jet of 60-65 knots at/above
2000 feet AGL will persist much of the overnight hours as well,
before weakening and shifting out of the area early Tuesday
morning. This will support continued LLWS conditions into the pre-
dawn hours.

Regional radar mosaic indicates scattered showers in the vicinity
of the cold front across eastern IA and western WI late this
evening. Scattered showers will spread east with the cold front
overnight, though best coverage looks to be largely north of the
terminals into WI. Showers will move out prior to sunrise, though
MVFR ceilings will likely linger (though they may be patchy
initially) into the day before lifting and scattering out late.

Skies will clear Tuesday evening, though some patchy high cloud
may spread north late in the night. Can`t rule out some patchy fog
development after midnight mainly west of the Chicago terminals,
though confidence is low at this time and this is beyond the end
of the 24 hour forecast for locations more likely to see fog.



LM...Small Craft Advisory...nearshore waters until 3 PM Tuesday.



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