Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240902

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
302 AM CST Tue Jan 24 2017

302 AM CST

Through today...

A weak ridge of high pressure will build across the region today.
However, skies should remain overcast through the day as warm
advection above the boundary layer sets up a relatively sharp
inversion this afternoon, effectively shutting off any potential
to mix down drier air and leaving a shallow stratus deck in
place through the day. In spite of the extensive cloud cover,
temperatures are still expected to remain relatively mild, as
broad upper ridging crosses the region. Max temps today should be
relatively close to the highs observed yesterday, generally
reaching into the lower 40s over much of the area. The weak winds
should turn more easterly to southeasterly this afternoon as the
sfc ridge axis shifts to the east and deepening low pressure moves
out over the central plains. An onshore wind component should
keep the Illinois lakefront a bit cooler with highs along the lake
likely to top out in the upper 30s.


241 PM CST

Tuesday night through Monday...

Primary focus early in the period is with low pressure system
moving through the upper Midwest Tuesday night and Wednesday, and
associated precip timing and eventually precip type as colder air
returns to the region by late Wednesday. This system is part of a
large scale pattern change to seasonably colder weather for the
remainder of the forecast period.

Surface low pressure is progged to be over the mid-Missouri River
Valley between Omaha and Kansas City by Tuesday evening, and is
forecast to track northeast across Iowa and into southern
Wisconsin by Wednesday morning before filling across the
northeastern Great Lakes Wednesday evening. Some model differences
remain, primarily with the speed of the low, with Canadian and
European models a little slower and slightly farther south with
the low than latest domestic guidance. Nonetheless, sensible
weather results are very similar between the various solutions.
The most organized forcing for ascent/precipitation production is
focused across the area Tuesday evening/early overnight hours,
when large scale 120-140 meter height falls, differential
vorticity advection and upper level divergence in left exit region
of 140 kt upper jet are juxtaposed atop the strongest low level
moisture flux on a 40-45 kt 925-850 mb jet. This forcing has to
overcome dry air initially noted in the 850-650 mb layer per
forecast soundings, with top-down saturation eventually producing
a deep layer of moist ascent particularly across north/northwest
portions of the cwa north of an occluding cold/warm frontal zone
which lifts into the area southeast of the surface low track
overnight. This results in the greatest potential for more
persistent precipitation across northwest/far northern IL and
north of the IL/WI border region overnight. While initial dry
layer could perhaps result in a very brief period of patchy light
sleet/snow early Tuesday evening far north/northwest as
precipitation develops, surface temps and wet bulbs are above
freezing and should support a quick transition to all rain as
column moistens and warm advection brings low-level column below
about 800 mb entirely above freezing. Of note as well, is the
presence of a region of steep mid-level (700-500 mb) lapse rates
of 8 C/km along the track of the upper low/vort which may be able
to support some isolated/embedded thunder across the northwestern
cwa where this elevated instability is co-located with the nose of
the low level jet forcing/warming and previously mentioned
stronger deep-layer forcing mechanisms. Models remain in agreement
in depicting some mid-level drying after midnight as low-level flow
veers more southwesterly and shifts best moisture flux to the
east/southeast of the cwa. This will likely end thunder threat
and decrease overall precip intensity to lingering light
rain/drizzle. Have generally utilized mos guidance 3-hourly temps
as we should see steady/slow rise trend overnight with upper 30`s
and lower 40`s likely more representative for non- typical diurnal

Surface low tracks just north of the cwa Wednesday morning and
begins to fill as it lifts out across the northeastern Great Lakes
during the day. Occluding cold front completes its transit of the
eastern counties, with low-level winds shifting westerly and
becoming gusty as cold advection steepens lapse rates. Lull in
precipitation intensity persists during the morning, though
trailing upper trough/deformation axis move across the area during
the afternoon and evening hours and brings deeper moisture with
it. Column cools with time from west to east, and though models
again depict some differences, thermal profile should become
marginal to support a mix of rain/snow or change to some light
snow across the west/northwest cwa by mid-late afternoon and by
evening in the east. Forcing is not particularly strong by this
time and suspect guidance qpf is a little on the high side for the
late afternoon/evening period, though some minor wet snow
accumulation appears possible especially after dark. Temps which
start the day in the upper 30`s to mid-40`s will see a slow fall
off through the afternoon, holding above freezing through sunset.
Cold advection, steepening low-level lapse rates and the tight
pressure gradient behind the departing low will make for blustery
conditions with west winds gusting around 25 mph.

As indicated above, a large scale trough evolves with the help of
the mid-week upper low and persists with several subsequent
weaker short-waves helping to reinforce the pattern through the
weekend. Overall this will produce a prolonged period of mostly
cloudy, blustery and colder weather with the potential for periods
of flurries or light snow showers. On the positive side, despite a
couple of reinforcing pushes of colder air, temperatures will
actually run near/just below average for late-January, and no
significant accumulating snow events are evident at this distance.
Daytime highs from the mid-upper 20s to around 30, and nighttime
lows in the teens and 20`s appear reasonable given pattern and
current medium-range guidance. Some respite is on the horizon for
Monday, as the upper trough edges east and is replaced by weak
mid-level and surface ridging ahead of a clipper-type system which
models depict Monday night/Tuesday. That system could bring snow,
but at least for Monday/Day 7 there looks to be a break in the
blustery winds and a slight bump in temps ahead of that system.



For the 06Z TAFs...

OVC conditions with MVFR or IFR CIGS likely through the TAF cycle.
Generally, expect to see a trend downward in CIGS tonight,
probably to IFR, with a likely upward trend expected by mid-late
morning Tuesday. Specific timing of category changes and smaller
time scale fluctuations in CIG heights are quite probable, but
also very difficult to predict. Some fog could develop again
tonight, but most likely scenario would be for VSBY to not get
much below MVFR. Potential will be on the increase for sct`d shra
and perhaps even a TSRA Tuesday night, have introduced PROB30 for
ORD after 06z, it`s possible precip may stay north of the ORD, so
only went with PROB30 for now.



241 PM CST

Areas of dense fog continue to be the primary marine concern in
the near term through Tuesday. Pattern change then leads to a
period of blustery west and northwest winds gusting 25-30 kt from
Wednesday into the weekend.

While low pressure moves off the mid-Atlantic coast this
afternoon, a weak pressure gradient lingers across much of the
northern and western Great Lakes region. Coupled with relatively
mild air with higher dew point temps, fog continues to persist
across the cooler lake waters particularly across the northern
half of the lake where widespread dense fog lingers. A marine
dense fog advisory remains in effect through Tuesday morning for
those areas.

Deepening low pressure will develop across the Plains and will
lift northeast across Wisconsin and Lake Michigan early Wednesday.
East-southeast winds will increase especially across southern
parts of the lake late Tuesday and Tuesday night, veering as the
low moves across roughly the central part of the lake early
Wednesday. The region will remain in an area of blustery cyclonic
flow behind the low Wednesday into the coming weekend, with west
and northwest winds gusting around 25 kts. Colder air will bring
lake effect snow to the downwind/east portions of the lake.






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