Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 182011

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

1010 AM CST

Still monitoring convective trends this morning, mainly across
central IL and central IN. However, focus turning more towards
the north where a wintry mix is likely today.

Latest radar imagery depicting the strongest development now
exiting the southern CWA and focusing more into areas away from
the CWA. Hail was likely limited to isolated reports of small
hail, with periods of heavy rainfall the more likely scenario.
Heaviest rainfall is ending, but with light to moderate rain still
likely today. As colder air moves across the area, have seen a
change over to a wintry of rain, snow, and sleet across portions
of northwest and north central IL. Cooling column will support
mainly a rain snow mix, however, think a brief period of sleet
will remain possible during the initial transition. Will likely
see this wintry mix expand into other areas in far northeast
Illinois, along and north of I-88, over the next 1-2 hours.
Confidence does lower with regards to this wintry mix getting into
other areas in Illinois today. Monitoring extent of deeper
moisture in place today, and timing of coldest air moving through
the area that will both affect the extent of the coverage of the
wintry mix. Do think it will be possible for other areas in
northern Illinois, between I-80 and I-88 to see at least a small
window. Overall, any snow accumulations should be minor and be
confined to grassy surfaces.

Also monitoring higher winds expected later today. Although
stronger winds likely area wide with gusts approaching 35-40 MPH,
think slightly stronger winds will be possible for areas near the
lake especially in northwest Indiana. Will keep an eye on wind
trends for these locations through midday. While strongest winds
should focus in northwest Indiana, a period of stronger winds
focused into the central and southern portions of Cook county
through midday could bring higher waves into these locations. If
this were to occur, some minor lake shore flooding could be
possible. Once again, at this time, think the stronger winds and
higher waves should be focused in northwest Indiana where the
current lake shore flood advisory is in place.



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.



209 PM CST

Sunday through Saturday...

After precipitation ends today, expect primarily dry conditions to
be in place across the region at least until near the end of the
upcoming week. Primary forecast challenge will be dealing with
fluctuating temperatures as a series of clippers track just to
our north over the upcoming week.

On Sunday, the strong low that impacted the region today will
continue to depart lifting across Quebec while high pressure
builds  across portions of the Great Plains and Midwest. Thermal
trough behind the low will move across the western Great Lakes
through the day Sunday and will keep temperatures on the cooler
side with highs topping out only near the freezing mark. Cold air
stratocumulus should be in place early in the day but there is a
chance we could see some peeks of sunshine later in the day as
corridor of stronger mid level height rises and subsidence behind
the 500mb trough axis pivots across the area and low level
anticyclonic flow increases.

Throughout the upcoming week, upper low will generally remain in
place over Hudson Bay while several waves pivot across the Great
Lakes region with the baroclinic zone undulating across the local
area. Forecast soundings indicate dry air will be in place through
the week, and the best forcing will remain to our north with the
passing waves, so do not anticipate any precipitation but
temperatures will fluctuate back and forth. The first wave will
move across the upper Great Lakes Monday with southwest flow out
ahead helping us to warm back into the upper 40s. Cold air behind
the wave will spill back across the region through the day
Tuesday. Depending on fropa timing, some areas will see morning
highs with falling afternoon temperatures. This makes temps a bit
tricky with highs in the mid to upper 30s northwest with earlier
fropa to mid to upper 40s in the southeast. Temperatures fall back
into the low to mid 20s area-wide Tuesday night into Wednesday
morning and only rebound to near the freezing mark Wednesday. This
cycle repeats itself late in the week as another wave digs across
the Great Lakes late Thursday followed by a third wave over the
weekend. This final wave is the furthest south and has the best
chance of producing precipitation across northern Illinois and
northwest Indiana Friday night into Saturday.



For the 18Z TAFs...

Main forecast concerns/impacts will be this afternoon into early
evening, as rain, snow, IFR/MVFR ceilings, and a gusty north
northwest wind will persist. Latest radar imagery depicting a
steady area of precip still moving over all of the terminals.
Expect this precip to beginning ending from west to to east over
the next couple of hours, continuing this afternoon into early
evening. Rain will transition over to a rain snow mix, with a
small period of all snow, before it departs today. Vis under the
heaviest precip should stay in the 2-3sm range, and don`t
anticipate any snow accumulation on pavements. IFR/MVFR ceilings
will slowly lift this afternoon into the evening but likely remain
in place slightly longer than previously forecast. Have extended
this MVFR cloud cover into tonight and early Saturday morning.
Stronger northerly winds will slowly turn more to the northwest by
mid afternoon, and stay this direction before diminishing through



209 PM CST

North winds have been steadily strengthening through the day today
as low pressure lifts from northeastern Indiana this afternoon to
Lake Ontario this evening. Expect strong gales to peak around 45
kt late this afternoon and evening then very gradually diminish
overnight dropping back below gale force early to mid Sunday
morning. Winds remain moderately strong to around 30 kt through
the rest of the day Sunday and will back to the southwest Sunday
night into Monday as another low moves east across the Canadian
Prairies. Winds will strengthen back to gale force Monday
afternoon and continue through much of the day Tuesday as the low
tracks just north of Lake Superior and then lifts to James Bay.
The pattern will remain active late in the week into the weekend
and cannot rule out additional periods of gales late in the



IN...Lakeshore Flood Advisory...INZ001-INZ002 until 3 AM Sunday.

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




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