Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 132346

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
646 PM CDT Fri Oct 13 2017

248 PM CDT

Through Saturday...

Today has been sunnier than expected leading  to slightly warmer
afternoon temperatures outside of the Rockford area. Cloud cover
will increase though through this evening as a cold front over
central WI and northern IA will stall over northern IL this
evening. The front should settle just north of O`Hare to Amboy,
IL. Showers spread across areas northwest of I-55 this evening,
and a few embedded thunderstorms are possible to the north of the
front. Showers and isolated to scattered embedded thunderstorms
continue to spread across much of the forecast area overnight.
CAPE values are minimal so I have low confidence in thunder
coverage. Severe storms are not expected.

PWATs are forecast to be 1.5-1.75 inches overnight so some areas
could see upwards of a half inch of rain tonight.

The front surges north as a warm front Saturday. Morning showers
and storms should dissipate. There may be a short dry period late
Saturday morning, but kept a chance of showers in the forecast in
case scattered showers linger.

An upper level wave moves through forcing another round of showers
and storms. Models differ on CAPE location but agree that CAPE
will likely be minimal. CAPE values may be slightly higher than
currently forecast if we get peaks of sun, but I`m expecting CAPE
values to be much lower than we`d typically like for severe storms
given a cloudy forecast. Shear remains favorable at around 60 kt.
If any storms pulse up, damaging winds are the most likely hazard.
High PWATs could lead to additional heavy rain. The highest
rainfall forecasts are for areas north of I-55, which could see
another 0.5-0.75 inch of rain Saturday. Higher rainfall totals
would be north of I-88, and could be locally higher under



248 PM CDT

Saturday night through Friday...

A powerful cold front will sweep across the region Saturday night
into Sunday morning. Surface low is projected to track from eastern
Iowa Saturday evening to southern Wisconsin Saturday night with a
trailing cold front sweeping across most of the CWA overnight. Out
ahead of the low track, a strong low level jet will transport
unusually high PWat air of 1.7-1.9 inches over the local area
driving heavy rainfall. An axis of one to two inches of rainfall
appears likely with locally higher amounts plausible. Where exactly
this sets up remains a bit uncertain at this point, but somewhere
along or just north of the IL/WI state line is favored depending on
where the strongest convergence ahead of the 60kt+ LLJ aligns.
Within the warm sector, lapse rates are fairly weak and unfavorable
diurnal timing will contribute to overall weak instability. Still
guidance shows 200-300 J/Kg MLCAPE possible ahead of the cold front
superimposed by very strong deep layer shear. Despite the lack of
any appreciable instability, given the strong kinematic environment,
any convective elements would bring some risk of strong and
potentially damaging winds and bears close watching as the event

Prefrontal 850mb temps of 15-17C Saturday will fall to around -1C by
midday Sunday behind the front. The strong cold advection will
result in steep low level lapse rates. Meanwhile, as precipitation
along the front exits the CWA to the east Sunday morning, high
pressure will build across the Great Plains resulting in a strong
gradient and isallobaric response. GFS indicates a corridor of 6-
7mb/3hr pressure rises will move across northern Illinois mid to
late in the morning. This combination of factors will result in
strong and gusty winds across the region. Expect winds gusting in
excess of 30 mph Sunday morning, very gradually tapering in
magnitude through the day as the gradient relaxes and the strongest
pressure rises move to our east. In addition Sunday, expect the area
to remain locked under stratus cloud cover with moisture trapped
atop the shallow cold airmass by a strong inversion. Expect early
morning high temperatures Sunday with falling or steady non-diurnal
temperatures trends through the day.

Ridge axis will move across the local area Monday then settle to our
east Tuesday through the end of the upcoming week. Seasonably cool
temperatures on Monday will return to above normal conditions
Tuesday through the end of the week as southerly flow dominate on
the western periphery of the ridge. No significant chances for
precipitation are noted as upper ridge gradually strengthens over
the eastern CONUS.



For the 00Z TAFs...

The concerns for the Chicago area airports through Saturday are:

- Winds with mainly an easterly component likely through mid
  Saturday morning, with some variance in speeds

- Showers likely by overnight with isolated thunder in the area

- Periods of showers likely again Saturday afternoon into Saturday
  night, with increasing chances for thunder after dark

- Winds becoming near due south Saturday afternoon

A warm front is developing across northern Illinois with showers
traversing north of this feature near the IL/WI border through
the evening. Some of these will close in on ORD and MDW but the
best chances for those airports are overnight as an upper
disturbance moves overhead. Isolated to possibly scattered thunder
is likely in the region. Winds will likely be below 8 kt, with
some variance in direction but should be mainly if not entirely
from the eastern half of the compass.

Confidence in a minimum in coverage of showers Saturday morning is
fairly high, but there is increasing confidence in IFR ceilings
being an issue. These are likely along and north of the warm
front, which is expected to be just south of ORD and possibly MDW
too. The warm front will ease northward during Saturday and as it
passes the Chicago airports winds will turn to southerly. Speeds
will increase, especially after dark Saturday and some gusts will
be occasionally seen.

Periods of showers are favored during Saturday afternoon,
especially along/northwest of a PIA to ORD line. It is challenging
to say what thunder coverage would be in these showers as
instability is somewhat marginal. Ahead of the cold front later
Saturday evening into the early overnight, a band of convection is
likely, some of which may be gusty.



248 PM CDT

Low pressure is expected to track across Lake Michigan this weekend
bringing a period of strong gales as a trailing cold front sweeps
across the region. A leading warm front will begin to lift across
Lake Michigan this evening and tonight with strengthening
northeasterly flow expected across the northern 2/3 or so of the
lake while southerly winds overspread the south end of the lake. By
Saturday evening, expect south to southeast winds to be in place
across most of the lake, increasing to around 30 kt. A few southerly
gales are possible by Saturday evening ahead of the low, but gales
will become widespread and potentially strong Saturday night into
Sunday as winds veer around to the west and northwest behind a
trailing cold front. Expect wind gusts to peak around 40 to 45 with
the potential for a few occasional 50 kt gusts with any showers that
help mix down stronger winds aloft. A Gale Watch has been hoisted
for the open waters, though there may yet need to be refinements to
timing before the event begins. Winds should ease back below gale
force mid to late Sunday afternoon or early Sunday evening.



LM...Gale Watch...LMZ777-LMZ779...7 PM Saturday to 7 PM Sunday.




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