Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250121 AAA

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
821 PM CDT SAT SEP 24 2016

817 PM CDT

The forecast for the rest of tonight and Saturday morning is well
on track. Despite the clear sky right now, the increasing pressure
gradient should keep enough of an east wind to prevent any
widespread fog threat. In addition, gradually increasing moisture
in the 850-925mb layer may favor clouds late tonight which would
also hinder a dense fog threat.

Looking ahead to Sunday, not much new in the 18Z guidance from
what the day shift had assessed earlier. Think that any warm
advection induced showers in the morning would be isolated and
likely more favored north of I-88, with most communities to remain
dry. The concentrated forcing just ahead of and along the sharp
convergence of the front will remain the 2-3 hour focused period
of showers and storms, with an axis or two of clustered
showers/storms moving eastward. Have added some higher resolution
into the forecast rain timing, and the overnight shift should be
able to further add upon that.

While most areas probably will see rain late in the day into
Sunday evening, the potential for some more robust gustier storms
still looks to be where the juxtaposition of the return of CAPE
and downdraft CAPE (DCAPE) and incoming strong kinematic fields
is. Obviously pre-frontal cloud cover will play into the
instability part of the equation, and could limit strong strong
storms altogether. The area though presently more favored is
along/north of I-80, mainly in north central Illinois mid and late
afternoon. Several convective-allowing models have an overlap in
that area, albeit brief, of stronger winds or indications of storm
sustainability, such as updraft helicity. Even further east,
showers/storms would have potential to be gusty (35+ mph) through
the evening, though may lose a little oomph after dark.



130 PM CDT

Through Sunday night...

Water vapor imagery depicts a highly meridional pattern in place this
afternoon as a deep upper low (bringing snow to the higher
elevations of the mountain west) continues to spin across Wyoming.
Strong ridging is in place downstream of this deep upper low. At
the surface a cold front extends in a N-S direction across the
high plains, with our lingering stationary front extending NW-SE
across the central plains into the Ohio Valley. A narrow ribbon of
warm moist air extends through the plains ahead of the front, and
this airmass will arrive tomorrow across our area ahead of our
first fall-like storm system.

Locally, with the ridge in place has kept stratus in place for
many areas thus far, though some decent erosion to the stratus
deck across the area this afternoon. The low level thermal
inversion will remain in place as surface heating is slow and we
turn flow at 5000 ft around to southerly through the afternoon but
with heating now occurring on the cloud deck edges and a warm air
mass aloft, forecast soundings do suggest that we will eventually
able scatter out the clouds.

With high pressure sinking southeast into the eastern Great Lakes
and low pressure migrating east-northeast across the Upper
Midwest, winds will shift southeasterly overnight and weaken some
allowing for some patchy fog or low clouds once again. Otherwise
There is an elevated warm front ahead of the low that will lift
through late tonight into early Sunday that could trigger a shower
or storm across north central (move favored) and northeast IL.
Coverage should be low. Better chances for precipitation will
arrive Sunday later afternoon and evening. There could be some
showers in the prefrontal moist airmass during the day with a
prefrontal trough approaching, though there looks to be some
capping in place into the early afternoon from earlier cloud

Better chances for precipitation will arrive Sunday later
afternoon and evening once the front and upper trough axis arrive.
A moist and at least marginally unstable airmass will develop in a
narrow corridor across northern IL ahead of the cold front. Model
guidance is in good agreement with the development of a strongly
forced line of showers and thunderstorms moving through the area
in the evening. The upper and lower forcing is not juxtaposed with
the front leading ahead of the upper trough, but the coupled
forcing is sufficient to warrant likely/categorical shower and
thunderstorm chances in the evening. With modest instability and
0-6km shear values of 35-40 kt due to increasing southwest flow
ahead of the cyclone, some of the storms may have a little kick to
them, though the greatest instability may precede the shear.
Northern IL remains in a marginal risk for severe storms which
seems to fit the situation. There is some lingering elevated
frontogenesis behind the main push of storms/surface front just
that showers may continue overnight.



246 PM CDT

Monday through Saturday...

Cooler more seasonable conditions will be in place to start off
the week, with a warming trend expected to round out the week
and continue into next weekend. Main forecast challenge will be
dealing with a closed upper low expected to meander over portions
of the Great Lakes through at least the first half of the week.
The low is currently over eastern MT/WY and is progged to reach
the western Great Lakes by Monday. Cooler and much drier air
wrapping around the backside of the low is expected to arrive by
Monday. Steep low level lapse rates and a moderately strong
pressure gradient will drive strong winds on Monday and continue
to expect some gusts into the 30 to 35 mph range Monday afternoon.
High temps will only be mid to upper 60s. The low center over
northern Lake Superior Monday evening will only move about 150
miles to eastern Lake Superior by Tuesday evening, so forecasted
conditions on Tuesday will be very similar to Monday.

Details become a little more murky from Wednesday on as the 12Z
ECMWF now suggests the upper low will dive southward bringing
chances for precip and continued cooler weather. This is a
departure from earlier runs which were more in line with the
GFS/GEFS which suggests the low will eventually open up and depart
east with upper ridging building into the mid section of the
country. Did not introduce any PoPs just yet based on the 12Z
ECMWF run, and will wait to see if more models trend this way
before doing so.



For the 00Z TAFs...

Easterly winds this afternoon will turn south-southeasterly during
the day Sunday in advance of an approaching cold front. The main
weather concern during the period will be timing the thunderstorm
chances with the cold frontal passage later Sunday. Currently, it
appears the best timing for storms will be later Sunday afternoon
at KRFD (21-00 UTC) and early Sunday evening (00-03 UTC Mon) at
the Chicago area terminals. These are the times I have included in
the latest RFD and ORD TAFs. Similar timing will likely need to be
added to the other TAF sites with the 06 UTC issuance. Expect the
possibility for a period of lower visibilities in heavy rain with
the storms and the possibility of a period of gusty winds. Winds
will then turn westerly Sunday evening following the cold frontal



246 PM CDT

This afternoon, Lake Michigan sits between a ridge of high
pressure over portions of Ontario into the central Great Lakes
and a low centered over the Dakotas. Modest east flow in place
today will gradually veer to the southeast and increase tonight as
the low lifts into the Canadian Prairies. Southerly flow will peak
around 30 kt Sunday into Sunday evening as a trailing cold front
approaches, then winds will turn westerly to 30 kt Sunday night
and continue through Monday. Cold air behind the front will
promote deep mixing and cannot rule out a few gale force gusts
Monday afternoon and evening, but confidence in any significant
duration of prevailing gusts is not particularly high at this time
so will not issue a headline. In addition to the strong winds,
lake effect showers and possibly some thunderstorms also appear
favorable Monday through midweek.






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