Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 180946

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
346 AM CST Sat Nov 18 2017

345 AM CST

Deepening low pressure system moving through the region today,
presenting plenty of near term forecast concerns. Precip trends,
including morning thunderstorms and midday/afternoon rain/wet snow
mix, falling temps and very windy conditions all the main foci
for the next 12-18 hours. Lake Shore Flood Advisory will be issued
for Indiana shore of Lake Michigan for this afternoon and
tonight, due to strong north winds producing large waves leading
to potential flooding and beach erosion.

Surface low pressure along the KS/MO border early this morning,
will deepen in response to strong mid-level height falls
associated with a strengthening upper trough which will propagate
across the region today. The low is progged to lift quickly
northeast across central IL and into northern IN by midday, and to
Lake Erie by early this evening. Strong forcing, including
isentropic ascent of warm/moist air atop strengthening surface
stationary/warm frontal trough in advance of the low and enhanced
by upper divergence in left exit region of 140+ knot upper jet
streak, is expected to result in rain expanding across the area
again early this morning. Current radar and lightning detection
depicts thunderstorms occurring across the southern tier or so of
cwa counties where RAP mesoanalysis indicated 500+ J/kg of MUCAPE
along and south of the stationary frontal zone. Thunderstorm
potential is expected to persist across these southern counties
this morning, before instability is shunted off to the east of the
area with the passage of the surface low late this morning.
Locations south of a Pontiac-Kankakee-Rensselaer line may see
1-1.50 inch rain amounts from this convection.

Various model guidance depicts deformation zone precipitation
lingering across the forecast area into this afternoon in
association with the elevated baroclinic zone north of the surface
front and low track, with cooling of the thermal column expected
as northerly winds increase and pull colder air in behind the
deepening low. Morning temperatures in the low 40`s to mid 50`s
north to south will fall into the mid-upper 30`s across most of
the area by afternoon, with forecast soundings supporting a
mix/change to some wet snow especially across northern IL portions
of the cwa during the late morning and afternoon. Ground/air
temps are expected to remain mild enough, with surface wet-bulb
temps in the mid 30`s such that little if any accumulation is
expected, perhaps a slushy tenth or two on some grassy surfaces
where precip is briefly intense enough. Precip will taper off and
end from west to east this afternoon into early this evening.

The other notable weather concern will be the development of very
windy conditions by late morning/midday, as northerly winds
increase to 20-30 mph with gusts to 40-45 mph in the wake of the
deepening low. Not only will these winds make for uncomfortable
wind chills in the 20`s, but they will also result in large waves
building on southern Lake Michigan, which will likely cause lake
shore flooding issues along the south (Indiana) shore this
afternoon into tonight. Based on this, a Lakeshore Flood Advisory
for Lake and Porter counties from noon today through 3 am CST
Sunday morning.



211 PM CST

Saturday night through Friday...

Primarily dry conditions are expected throughout the extended time
frame, but temperatures will be quite variable as a series of lows
move across the northern Great Lakes region.

Saturday night, deep area of low pressure will be over the central
and eastern Great Lakes region with precipitation now east of the
local area but cold air arriving behind the cold front. 850mb
temperatures will bottom out around -10C Sunday morning but the
thermal trough will quickly shift off to the east with
temperatures moderating through the day Sunday. Strong subsidence
behind departing 500mb trough axis will overspread the area
through the day. Forecast soundings indicate stratus will be in
place early in the day Sunday, but subsidence helping to dry the
column and surface ridge axis building in from the west may allow
some sunshine later in the day.

Monday, another vort is progged to dig across the Canadian
Prairies and reach the Great Lakes region midday Tuesday.
Attendant surface low will stay over Canada, and precipitation
remains favored north of the local area so will maintain a dry
forecast through this time frame. Southwest flow out ahead of the
wave will help temperatures further moderate, reaching back into
the mid to upper 40s Monday and Tuesday. Expect a reinforcing
shot of cold air to push across the region Tuesday evening and
Tuesday night putting the area back into a deep freeze Tuesday
night with colder air remaining in place through early Friday.
Another clipper-like system will take a similar though slightly
more southerly track across the region Friday into Saturday with
temperatures undulating once more. This system may also provide
the next decent chance for precipitation in the area.



For the 06Z TAFs...

There are multiple aviation concerns through Saturday. The
overnight/early Saturday morning hours will be characterized by
drizzle and occasional light rain. CIGs and VSBY will continue to
trend downward as low pressure approaches and winds diminish.
Concern continues for fog potential overnight, with highest
confidence at DPA and RFD where temporary 1/2SM VSBY was
maintained. Increasing north winds by or a bit after daybreak
today should end the fog threat. Expecting CIGs to drop to LIFR
area wide over the next few hours, with some uncertainty on
improvement time on Saturday, likely mid day or thereabouts.

Additional TSRA development this morning through mid day should
stay south of the terminals. However, an area of steady rain will
develop and move into the area on the northwest side of
strengthening low pressure during mid morning. Periods of low MVFR
to IFR VSBY are likely with the rain. As this occurs, northerly
winds will increase with gusts in the 25-30 kt range, and 30-35 kt
range at GYY. Colder air moving in could potentially allow snow
to mix in, with a changeover to snow possible before the precip
ends, but confidence is low. Confidence is also low-medium on
exact precip end time. Only maintained PROB30 for RA/SN at RFD.
Will need to monitor trends for this at the other terminals.
North-northwest to northwest winds will stay strong and gusty
through Saturday evening, with gusts likely to 30 kt or even
higher at times, especially at GYY. CIGs are expected to remain
in MVFR range, possibly improving sometime by early Sunday.




150 am...A cold front will move south across Lake Michigan early
this morning shifting winds northerly. The gradient will quickly
tighten as low pressure moves from Missouri this morning to near
Lake Erie this evening. These northerly winds will increase to
gales by late morning with 45kts likely on the south end of the
lake this afternoon. The gales will slowly diminish from north to
south late tonight into early Sunday morning. A large area of high
pressure will move across the southern U.S. Sunday into Monday.
As it passes south of the lake...northwesterly winds Sunday
morning will shift southwesterly Sunday night. Low pressure will
move across Ontario Monday night and the gradient will tighten
ahead of this low with a period of southwesterly gales expected
late Monday afternoon through Tuesday morning. A trailing cold
front will move across the lake Tuesday afternoon and then large
high pressure will move across the region next Wednesday. cms


IN...Lakeshore Flood Advisory...INZ001-INZ002...noon Saturday to 3 AM

LM...Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779...9 AM Saturday to 6 AM Sunday.




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