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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
638 am CDT Fri Mar 23 2018

Short term...
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 quantitative precipitation forecast 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 quantitative precipitation forecast/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 radiosonde observation flights over the western Continental U.S.. 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.



Long term...

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 12z tafs...

638 am...forecast concerns include gusty easterly winds later
tonight into Saturday and the potential for light snow Saturday

Light northerly winds will shift more northeasterly in the next
few hours with speeds coming up to 10kts by early afternoon. The
gradient will slowly tighten with speeds slowly increasing later
this evening and overnight...into the 12-16 kt range. Speeds and
gusts will continue to increase Saturday morning...remaining

Snow is expected to develop from northwest Illinois through east central
Illinois late tonight and continue into Saturday. A sharp cutoff to this
snowfall is expected on the northeast side of the precip and will
likely be near a rfd/mdw/vpz line. Confidence for precip reaching
the Chicago area terminals remains low and opted to remain dry
with this forecast...even for the 30 hr Ord/mdw tafs. Its
possible some light snow mention may be needed...likely after
12z...with later forecasts. Confidence is a bit higher for rfd
that light snow may arrive late tonight and added prob mention
with this forecast. Due to the expected sharp cutoff and low
confidence to where it will occur...changes are possible with
later forecasts.

High clouds today will lower to a mid deck tonight and where any
snow develops...IFR vis/MVFR cigs are possible. Cms



420 am...a ridge of high pressure across the Great Lakes today
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 and southerly
winds ahead of this trough may increase to 30 kts early next week.


Lot watches/warnings/advisories...
Illinois...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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