Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 251844

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
144 PM CDT SUN SEP 25 2016

136 PM CDT

Through Monday...

Warm and humid conditions are in place early this afternoon ahead of
an approaching cold front. Temperatures have warmed into the mid to
upper 80s with dewpoints in the upper 60s across the area
contributing to moderate instability. Modified sounding from ILX
indicates just over 1000 J/kg SBCAPE in place across portions of the
area which initialized well on latest RAP guidance. With minimal CIN
in place, cu field has become increasingly agitated across much of
central IL and far northwest IL with scattered showers and
thunderstorms developing across those areas. Hi-res convective
allowing models continue to show widely scattered to scattered
showers and thunderstorms shifting across the remainder of the
forecast area through the late afternoon and evening hours. Deep
layer shear will likely be a limiting factor for a more organized
severe thunderstorm threat. Values of 15-20kt are in place south of
I-80 though do increase to 25-30 kt as you head northwest corner of
the CWA. With steep low/mid level lapse rates, cannot rule out an
isolated pulse severe wind threat, but overall am not impressed with
the severe setup.

Cold front will sweep across the region this evening and tonight
with winds shifting to the west/northwest and remaining breezy
overnight. Guidance suggests some precip possible immediately behind
the front as the upper wave moves across the region, but soundings
show the low levels quickly drying out behind the front so not sure
it will amount to much.

Cooler and blustery conditions will be in place on Monday. Should
see plenty of sunshine in the dry airmass outside of some scattered
afternoon cold-air stratocu development. Cold airmass will promote
deep mixing with top-of-the-channel winds of 30-35kt resulting in
strong wind gusts at the surface, especially during the afternoon
hours. Expect widespread wind gusts of 30-35 mph during the
afternoon, and cannot rule out a few sporadic gusts pushing 40 mph.
Afternoon highs will only be in the mid to upper 60s.



139 PM CDT

Monday night through Saturday...

Overall this period is on the drier side with seasonally cool
conditions with the best chance Tuesday night into Wednesday. The
deep upper low will only slowly move southward on Tuesday, though
it will get shoved with a bit more earnest later Tuesday and
Tuesday night. Therefore the day should start off as continued
cool and breezy with more sun then clouds, but late in the day
expect cloud cover to increase along with increased chances of
rain showers. These patterns tend to afternoon diurnal increases
in the chance of showers, but core of the cold temperatures aloft
will be moving through overnight into early Wednesday and
therefore would just suggest either clouds or more scattered
showers. As the low moves south a secondary surge of colder air
will move through which could make Wednesday the coolest day of
the week with some chance of showers, especially south of the lake
and across Northwest Indiana where the better chance of
instability showers along with some lake effect/enhancement
possible. There is also some hint of lake convergence later
Wednesday and Wednesday night which could lead to an uptick in
lake effect showers given a decent thermally driven instability
signal. Upper level heights will be increasing during this time
such that the equilbrium levels remain low enough to preclude any
thunder with any possible activity, but there could be some lake
effect continuing into early Thursday.

The general trend is a very slow moderation in the airmass back to
normal levels for late September, though there are some
significant model discrepancies regarding how quickly the low
exits the region. NCEP/GEM ensembles suggest that ridging will
slowly move in but the upper low will become blocked to some
degree. There is some spread in these members, and the EC would
bring continued showers Fri/Sat after the low initially pulls
away. but the farther western track of the EC is not favored at
this time as it is at the western envelope of guidance and even
west of its ensemble mean. Therefore the trend is for generally
dry conditions and a temps near climatological highs in the upper
60s to low 70s with a cool northeast wind into the weekend. Long
range models hint at another warm up beyond this period once the
low does kick out east but this could take a little time.



For the 18Z TAFs...

A slow moving cold front will push across the region this evening
and tonight with showers and thunderstorms expected out ahead,
and strong winds behind. Mother nature starting to show her hand
early this afternoon with prefrontal showers developing from
central WI into far northern IL while the cumulus field is
starting to become more agitated across portions of central IL.
Hi-res convective allowing models have been fairly consistent
showing some scattered development mid-afternoon affecting the
terminals, though timing and coverage details have varied.
Additional convective development this afternoon could be very
near or over the terminals so will not have much time to further
hone in on timing details. Additional showers are possible with
the front this evening and possibly even some rain for a couple
hours behind the front, though drier air is eventually expected to
arrive behind the front.

Outside of the influence of precip, SSE winds late this
morning/early afternoon are expected to veer to the SSW by mid
afternoon with gusts to around 20 kt possible. Expect SW flow to
continue into the evening hours until the cold front sweeps
through with winds turning W/WNW behind the front. Even stronger
winds are expected tomorrow with mid to high 20 kt gusts possible
and may even tag 30 kt at times.



139 PM CDT

The first fall-like system will move across Lake Michigan tonight
resulting in a more active period across the lake to start the
week with higher winds and waves expected for several days. Low
pressure across central Canada will send a strong cold front
across Lake Michigan tonight. As the low sinks south toward Lake
Superior Monday, a high pressure ridge across the plains will
extend eastward across the Ohio River Valley. A seasonally strong
pressure gradient coupled with a warm lake will easily result in
westerly winds to 30 kt, and even some occasional gale force
winds, most notably Monday and Monday night. The low will only
slowly sink southward through the Great Lakes through midweek.
During this time winds will shift to more northwesterly but will
still remain in the 20-30 kt range through early Wednesday. The
wind field appears on the stronger side then that normally seen
for waterspout development, but several guidance sources suggest
that as the low nears and begins to pull away midweek that there
could be better convergence of the wind field across southern Lake
Michigan and support waterspouts. Depending on how quick the low
pulls out, an extended period of northerly winds may continue
elevated waves on the south half of the lake. High pressure will
bring a calmer period late week into the weekend.






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