Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230324

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
924 PM CST Mon Jan 22 2018

924 PM CST

The main forecast concern for later tonight is the potential for
a band of snow to develop across portions of far northern IL into
early Tuesday morning.

Water vapor imagery early this evening indicates that the main
mid-level circulation is now beginning to shift northeastward
into far west-central IL (just north of Quincey, IL). The surface
low over eastern Iowa has become occluded over the past several
hours, with a new center of low pressure now developing over far
northeastern IL and southeast WI this evening. Given the occlusion
of the main storm system, a well defined TROWAL (TRough Of Warm
Air Aloft) structure has developed to the north of the main
circulation into southern WI. Ultimately this is setting up prime
conditions for the development of a rather intense area of mid-
level frontogenesis along the southern periphery of this TROWAL
where mid-level deformation is strengthening in association with
the arrival of the mid-level low. Radar trends across far northern
Illinois and far southern Wisconsin already indicate that this
process is underway as a nearly stationary band of precipitation

The main concern is that as dynamic adiabatic cooling continues
and colder air begins to wrap back into northern IL tonight that
this will result in the change over to a band of potentially heavy
snow into early Tuesday morning. Overall confidence on the trends
of this change over to snow remains low at this time, but guidance
indicates that the column should become sufficiently cold to
support a change over to snow around midnight over northern IL.
Given the presence of steep mid-level lapse rates above the
TROWAL, heavy snow will be possible across far northern IL later
tonight. The main question that remains is how much snow will
fall. There certainly is the potential to get a quick few inches
of wet snow near the WI state line, but amounts would likely
rapidly drop off to the south towards the I-88 corridor. I tossed
around the idea of putting an advisory up for my far northern
counties, but have opted to hold off for now and go with a
special weather statement. Given the potential impacts to the
morning rush, an advisory could be needed later tonight.

Given the above, I have increased snow amounts far north to around
2-3 inches, with a change over to snow expected around, or shortly
after midnight.



200 PM CST

Through Tuesday...

Low pressure is centered near Des Moines, Iowa this afternoon with
a warm front draped across far northern Illinois. In the
vicinity of the front, fog and drizzle will continue this
afternoon across far northern Illinois while farther south dry air
wrapping in from the southwest has allowed partial clearing west
of the I-57 corridor. Temperatures and dew points are into the
upper 40s to low 50s within the warm sector and are superimposed
by steep low to mid level lapse rates this afternoon which are
contributing to a corridor of seasonably strong instability.
Midday soundings from DVN and ILX show MLCAPE values around 150
J/kg while SBCAPE is closer to 600 J/kg. Latest run of the RAP
picks up on this axis of instability and shifts it east across the
CWA through the late afternoon and early evening ahead of an
advancing cold front, albeit with weakening instability as we get
past peak heating. Very strong winds aloft with 0-1km shear in
excess of 20kt and favorable effective SRH continue to prompt some
concern for a severe threat for any thunderstorms that make it
into the area this afternoon and evening, and an isolated tornado
remains a possibility. Convective initiation is already ongoing
near Springfield, IL with agitated cu field south towards St
Louis. These storms will move north-northeast through the late
afternoon and evening hours within the aforementioned instability
axis though confidence in how well they will maintain their
intensity is fairly low by the time they reach the local CWA.

Late this evening and overnight, 500mb closed low will move across
the region and allow deeper moisture to return resulting in
periods of light rain. Forecast soundings eventually cool enough
for precipitation to transition to snow during the predawn and
early morning hours Tuesday. Models have been fairly consistent
showing axis of highest QPF near and north of the IL/WI state
line. As precipitation changes to snow, could see a dusting/few
tenths of snow area-wide, but the best chance for any light
accumulations will be across the northern tier of counties where
up to an inch of snow is possible. Winds turn northwest behind
the departing low Tuesday morning and mid level moisture scours
out once more. Steep low level lapse rates redevelop in the
northwest flow and could result in a few additional flurries
during the late morning and afternoon. Temps will fall into the
low to mid 30s behind the cold front tonight and likely hold
fairly steady through the day tomorrow.



140 PM CST

Tuesday night through Monday...

Closed upper low continues to pull away from the region Tuesday
evening, with mid-level short wave ridging building briefly across
the area early in the night. Partial clearing of lower clouds is
possible during the evening in response to mid-level drying and
subsidence associated with strong H5 height falls (nearly 150
meters), though forecast soundings depict redevelopment of mid-
level clouds overnight as another mid-level short wave quickly
approaches. Clouds linger through the day Wednesday as the wave
propagates across the region, with a weak surface cold frontal
trough. Model soundings depict a relatively dry 900-700 mb layer
however, which suggests little precip threat despite cloudy skies.
Model 950 mb temps generally support daytime highs in the low-mid

Mid-level short wave and larger scale upper trough axis moves
east of the area Wednesday night. Medium range guidance is in good
agreement in building broad upper ridging east across the region,
with dry and milder conditions Thursday and Friday. Pattern is
progressive, with an upper trough developing eastward across the
central and northern Plains, and an area of surface low pressure
which tracks into western Ontario and trails a cold front through
the local area Friday night or early Saturday. Surface high
initially blocking gulf moisture return, and relatively quick
progression of upper flow, suggests somewhat limited precipitation
potential, especially with quicker cold frontal passage depicted
by the GFS. Slower ECMWF/GEM solutions would support a little
greater potential for rain especially across the southeastern part
of the cwa into Saturday. After 40`s Thursday, and temps likely
near 50 Friday, timing differences with the cold front add lower
confidence to the high temp forecast Saturday, with 45-50 degree
highs likely east/southeast of the advancing front.

Colder air spreads back in behind the front Saturday night,
though by the time precipitation ends in the southeast, model
partial thickness values would suggest precip type would likely
remain all rain. By Monday, even the slowest guidance (ECMWF)
moves the upper trough east of the area, with broad upper ridging
moving in aloft with weak high pressure ridge at the surface. Some
detail differences exist as expected by that time, though
generally it appears our colder air will result in temps around
average for late January. Pattern looks to remain progressive as
well, with milder air again early-mid next week.



For the 00Z TAFs...

Several forecast concerns/challenges this forecast period:

* Showers and Thunder this evening
* A band of light/moderate, possibly heavy, snow tonight thru Tue
* Returning IFR ceilings, with LIFR possible
* Wind shift to the northwest Tue AM
* Continued scattered snow showers Tue

Latest radar imagery showing area of showers and thunderstorms
continuing to move across much of northern Illinois. RFD currently
being impacted but expect the remaining terminals to see this
precip over the next hour, with thunder still a possibility.
However, ORD/DPA have the highest chances of observing this
thunder along with a possibility of small hail. Showers will then
linger for 2-3 hours but should diminish in coverage after that
point, while conditions remain mainly VFR. The arrival of colder
air and stronger forcing later tonight will support a window of
light to moderate snow, and have adjusted TAFs to account for
latest trends. This will start earlier at RFD, but then spread
east southeast soon there after. Have higher confidence of snow
now occurring, but do think there could be a window of heavier
snow. Have lowest vis falling to 1sm, but vis below 1sm is
possible. Will need to monitor this period and potential snow,
especially as snow accumulation will be possible at the terminals.
Ceilings will fall under this snow, with IFR a definite
possibility. LIFR will also be possible. Conditions should improve
by early Tuesday morning, but with winds shifting to the
northwest and with periodic snow showers continuing throughout the



155 PM CST

Main marine forecast concerns focus on deep area of low pressure
which will move across southern Lake Michigan tonight. In
addition, areas of locally dense fog were in place across southern
parts of the lake, where 40-50 degree dew point air has spread
north over the colder waters. Strong northeast winds currently
occurring across northern parts of the lake will gradually back
northerly and increase more solidly into gale range through early
Tuesday morning as the low moves through, and will increase to
gale strength over the southern portion of the lake Tuesday
morning as the low pulls off to the east. Gale warning headlines
are already up for the north half, though there winds are largely
just below gale force this afternoon. A gale warning is in effect
for the southern part of the lake starting early Tuesday morning,
and headlines for the entire lake will run through 00Z/6 pm CST
tomorrow. A dense fog advisory remains in effect into this evening
on the south end of the lake.

The low will eventually move off across the St. Lawrence Valley
Tuesday night, with weak high pressure ridging spreading across
the western Lakes Wednesday. This will allow winds to diminish to
less than 20 kts, and eventually back to the south-southwest by
early Thursday. Deep low pressure is expected to develop in the
lee of the Northern Rockies Thursday night, which will induce
increasing south-southeast winds. A period of southerly gales is
possible Friday through perhaps early Saturday, after which a cold
front will shift winds to the west.



LM...Dense Fog Advisory...LMZ777-LMZ779 until 3 AM Tuesday.

     Gale Warning...LMZ777-LMZ779...3 AM Tuesday to 6 PM Tuesday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742...6 AM Tuesday to 4
     AM Wednesday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...6 AM Tuesday to 10
     AM Wednesday.




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