Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230831

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
331 AM CDT Fri Mar 23 2018

330 AM CDT

Today through Saturday...

Main focus is on the potential for significant winter weather
tonight and Saturday, specifically with the potential for several
inches of wet snow from Iowa into portions of west-central
Illinois. Have elected to upgrade Lee, La Salle and Livingston
counties to a Winter Storm Warning, while maintaining the
remainder of the Winter Storm Watch. This is due to higher
confidence in significant snow accumulations west and southwest of
the LOT cwa, and the expectation that there will be a very sharp
gradient along the northern edge of the snow area.

Storm system which will affect the area starting tonight is just
now moving onshore across southern California early this morning,
per GOES water vapor imagery. This disturbance is progged to move
quickly across the Rockies by this afternoon, and onto the Plains
by early this evening. The short wave then propagates into the
mid- Mississippi Valley overnight, then dampens across the Midwest
through the day Saturday. This system taps moisture spreading
quickly northward from the western Gulf, which combined with
strong mid-level frontogenetic forcing, steep lapse rates above
600 mb, and the presence of an area of negative EPV above the
vertical frontogenetic ageostrophic circulation, should result in
liquid equivalent precip amounts in excess of an inch in the axis
of heaviest precip. A couple of things make this forecast a bit
tricky for the LOT forecast area however. One, is that model
guidance has generally shifted a bit south/southwestward with the
heaviest QPF axis, with highest amounts generally just west and
southwest of the cwa. Secondly, easterly low level winds develop
across the area today and tonight as the surface low tracks from
Kansas into the lower Ohio Valley into Saturday, drawing very dry
low level air into the area from the Great Lakes region, where
surface dewpoints are only in the mid-teens at this hour. Thus,
while heavy precipitation is likely across parts of the region, it
appears that the northern edge of the precip shield will have a
very sharp cutoff. Some differences remain in model guidance as
well, with the NAM more energetic and a little to the north of the
general consensus, while the GEM is farther to the south. This
lends somewhat low confidence with respect to the details of the
greatest QPF/snow axis, and just how far north accumulating
precipitation occurs across the cwa. Using a consensus approach,
the current axis of heaviest snow would be roughly from just north
of the Quad Cities, southeast through the Peru and Pontiac areas,
with amounts of 6"+ expected. 50-70 miles north of that track,
there may be little or no accumulation! Based on this have elected
to upgrade the southwestern part of the cwa to a Winter Storm
Warning, while maintaining the existing watch elsewhere. I suspect
that some of that may end of as an advisory or just being able to
be trimmed away later today, as the mid-level wave should be
better sampled by the 12Z RAOB flights over the western CONUS. If
the heavy snow axis does not shift farther south, an additional
county or two south of the Chicago area could be added to the

In addition to the potential for heavy wet snow, isothermal
profiles around 0 deg C in forecast soundings across the southern
cwa would support some sleet/freezing rain mixing in later tonight
into the morning hours of Saturday, and precip may begin as light
rain late this evening before dynamic forcing/wet-bulbing turn
things over to freezing/frozen precip. Precipitation should begin
to decrease in intensity Saturday afternoon as the wave begins to
dampen/shear, with precip ending most areas by evening as the
system pulls away to the southeast.




Saturday night through Thursday...

328 am...Forecast concerns include temperatures through Monday
and then a periods of rain Monday night through midweek.

Have generally maintained lower temps across the southwest cwa
where snow is expected to fall Friday night/Saturday. North of
where the precip falls...both low temps and high temps may be
higher than expected. Bumped high temps up some across the
northern cwa Sunday with at least partial sunshine...though temps
will remain cooler near the lake.

High pressure will shift east of the area on Monday as low
pressure over the plains slowly lifts northeast across the
western lakes region Monday night through Tuesday evening. There
will likely be waves of precip along this trough/frontal boundary
and there remains uncertainty regarding how much qpf will fall.
With return flow from the gulf...precipitable water values
increase to around 1 inch during this time. Thus rainfall amounts
could easily approach an inch or more. If this heavier rain
materializes over areas that receive several inches of snow early
this weekend...there could be some flooding risks. But overall
confidence is low from this distance. Instability also increases
during this time period with some weak cape and introduced slight
chance thunder.

High pressure will move across the area Wednesday into Wednesday
night with perhaps some linger showers Wednesday morning. A cold
front will be moving across the midwest/Great Lakes region
Thursday into Thursday night as low pressure moves northeast from
the Ohio Valley to New England. Some differences in the models for
how much precip develops on the northwest side of the system and
ahead of the front yields low confidence for precip for our area
with chance pops reasonable for the southeast cwa. cms


For the 06Z TAFs...

1222 am...No forecast concerns this period. Light east/northeast
winds overnight will turn more easterly Friday with speeds
increasing into the 10-12kt range. Some gusts are possible across
northwest IL and rfd by mid/late afternoon and then gusts are
expected to slowly increase by late evening into Saturday morning
when gusts into the 20-25kt range are possible.

A patch of mid clouds will slowly move south of the Chicago
terminals early this morning. There may be additional mid clouds
through the morning along with increasing high clouds that will
lower to a mid deck later this evening and into early Saturday
morning. There could be few/sct cu today as well.

Precipitation from an approaching storm system is still expected
to remain southwest of rfd through the end of this forecast...06z
Saturday morning as well as remaining southwest of ord/mdw through
the end of their 30 hour taf periods. Precip may remain southwest
of the Chicago terminals for the entire event. Based on current
trends...will hold off any mention at rfd with this forecast. cms


228 PM CDT

High pressure across the lake will support relatively light and
somewhat variable winds through late Friday at which time a low
pressure system approaching from the Plains will start to increase
the gradient and turn winds easterly. The low moves well south of
the lake, through the Tennessee Valley, early in the weekend.
Meanwhile a ridge positioned from the western Great Lakes into
eastern Canada will help support ongoing easterly flow into early
next week.



IL...Winter Storm Warning...ILZ010-ILZ019-ILZ032...10 PM Friday to 4
     PM Saturday.

     Winter Storm Watch...ILZ008-ILZ011-ILZ020-ILZ021...10 PM Friday
     to 4 PM Saturday.

     Winter Storm Watch...ILZ022-ILZ023-ILZ033-ILZ039...1 AM Saturday
     to 4 PM Saturday.

IN...Winter Storm Watch...INZ010-INZ011-INZ019...1 AM Saturday to 4
     PM Saturday.

LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742...1 AM Saturday to 4
     PM Sunday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...4 AM Saturday to
     10 AM Sunday.




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