Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231826

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



126 PM CDT

Saturday night through Friday...

Forecast concerns continue to be on periods of rain Monday night
through midweek. possibly a few thunderstorms.

High pressure will transit the area on Sunday in wake of Saturday`s
storm system. With surface high pressure over northeast Quebec,
northeast flow will maintain seasonally cool conditions in spite of
plentiful late March sunshine as readings hold in the 40s, with low
40s over snowpack covered areas and also near the lake.

We will transition to a moist southwest flow pattern early to mid
week as a deep upper level low currently off the Pacific Northwest
coast line will dig into an expansive trough across the western USA
on Monday. A surface low will organize in lee of the Rockies Monday
and lift toward Lake Michigan on Tuesday. The southwest flow will
advect well above normal precipitable water values toward the region
and will also push a warm frontal boundary from the Arklatex region
Monday to our area on Tuesday. Several periods of rain are
expected as a strong upper level jet lingers to our northwest.
The first will come Monday into early Tuesday with the warm
frontal surge and associated strong low level jet Monday night. A
second round is expected later Tuesday as the northern stream
portion of the deep western low pressure trough will steer a cold
front across the region Tuesday night.

The main concern will be rain on top of any areas that receive snow
this weekend, but this will look to keep basins elevated, especially
across east central Illinois and into northwest Indiana (Illinois,
Kankakee, Iroquois basins).

High pressure will move across the area Wednesday into Wednesday
night with perhaps some lingering 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 ahead of this slightly more baroclinic
front yields low confidence for precip for our area with chance
pops reasonable for the area. It appears late next week we will
transition to a west-northwest flow regime which will keep things
on the cooler to seasonal side.



For the 18Z TAFs...

Concerns are in increasing easterly winds, and then
pinpointing the expected sharp cutoff in heavy snowfall overnight
into Saturday morning.

Snow will develop from northwest IL through east central IL late tonight
and continue into Saturday, with a band of heavy wet snow expected
mainly well southwest of the terminals. A sharp cutoff to the
main precipitation shield is expected on the northeast side of
and will likely be near a rfd/mdw/vpz line, with current guidance
getting some light snow into rfd but not quite into the Chicago
terminals due to the low level dry east winds. There could be a
window from late tonight through mid Saturday morning (roughly
10z-17z) that will be a period to watch if any shifts in the snow
band occur. Very strong lift will suggest an increased
supercooled mid level cloud deck and at least some virga or some
occasional snow flakes. VFR appears to rule outside of the snow
band were IFR/LIFR will occur. At this time will keep the Chicago
area terminals dry for the above rationale.

The other concern will be the stronger east winds, which will
approach 30 kt, and maybe get slightly higher than that, with the
main ramp up occurring after 10z overnight and continuing through
the day.



1231 PM CDT

A ridge of high pressure across the Great Lakes will
slowly shift east tonight as low pressure moves from the central
Plains today to Tennessee Saturday as it weakens. The gradient
between these two systems will tighten tonight with easterly winds
increasing to 30 kts on the southern end of Lake Michigan. As the
low dissipates Saturday night...the ridge will build back across
the Great lakes region for Sunday and Sunday night. A trough of
low pressure will slowly move across the western lakes region
Monday through Tuesday night. Southeast winds will shift
southerly may increase to 30 kt ahead of this trough. A series of
generally weak cold fronts will follow for the latter half of the



IL...Winter Storm Warning...ILZ010-ILZ019-ILZ021-ILZ032-
     ILZ039...midnight Saturday to 4 PM Saturday.

     Winter Weather Advisory...ILZ008-ILZ011...1 AM Saturday to 4 PM

     Winter Weather Advisory...ILZ020-ILZ022-ILZ023-ILZ033...4 AM
     Saturday to 4 PM Saturday.

IN...Winter Weather Advisory...INZ010-INZ011-INZ019...4 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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