Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 242035

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
335 PM CDT Fri Mar 24 2017

330 PM CDT

Through Saturday...

Near-record to record warmth is present across most of the area
this mid-afternoon on gusty southwest winds. The pneumonia cold
front surging southward is evident on the TMKE radar, just into
northern Lake County, Illinois. Given the strength of
baroclinicity with this boundary, and the weakness in the pressure
field ahead of it, this should continue to surge. Kenosha Airport
dropped 20F in one hour and that was likely in less than 30
minutes. Similar if not larger drops are expected in eastern Lake
and northeast Cook County prior to 5 pm, and in downtown Chicago
near 6 pm based on current trends. Temperatures should drop into
the 40s within several miles of the shore.

Dew points ahead of the front are not particularly high,
characterized by mid 40s in northern Illinois. So not expecting
fog to rapidly form, but do expect a gradual lowering in
visibility throughout the night with some dense fog possible late
into Saturday morning, especially near the lake front.

The closed well-developed cyclone responsible for today`s warm
sector is situated across western OK this afternoon and will
gradually move east-northeast tonight. While some isolated
showers/storms are possible early this evening, an uptick in
coverage will arrive under stronger upper diffluence and within
increasing precipitable waters. The better chances look to be
across the northern and western forecast area, with the east and
south possibly remaining dry for a large part of tonight.
Elevated lapse rates are modest, and would expect there to be
isolated storms late this evening into overnight across the area,
but not seeing any signs for widespread or severe storms given the
slow movement of the low center.

Saturday continues to look like periods of showers with some
light rain/drizzle in-between, in an overall murky day. An
occasional rumble of thunder is possible during the day. The cold
front is expected to stall near I-80 later tonight and hold
steady through Saturday morning before probably trying to inch
north in the afternoon. The temperature discontinuity of the
boundary will be tempered some by showers/drizzle on Saturday,
however the stout east winds off the lake should keep far
northeast Illinois in the lower-mid 40s, while along/south of I-80
reaches the lower to mid 60s.



313 AM CDT

Saturday through Friday...

Northern Illinois and northwest Indiana have a wet weekend ahead
with active weather expected to continue through the upcoming
week. A closed 500mb low is progged to lift from southern MO
Saturday morning to northern Illinois midday Sunday. Showers and a
few thunderstorms will be possible at times throughout the
weekend as the low slowly makes its way across the region. Not
really any strong signals at this point on timing any peaks or
lulls in the precipitation, but would anticipate some waves in
intensity as shortwaves pinwheel around the low. The 500mb low
eventually opens up over the area on Sunday with the trough axis
passing overhead Sunday afternoon which should allow for
precipitation to wane from the west Sunday afternoon and evening.
At the surface, expect a stationary front to persist over far
northern Illinois on Saturday which will result in a temperature
gradient over the CWA with upper 50s into the 60s south of the
boundary while 40s and low 50s are in place to the north. The
front should lift back to our north by Sunday with upper 50s to
low 60s area-wide.

Another low pressure system is expected to track to our south
Monday into Tuesday followed by yet another low late in the
week. With storm tracks to our south next week and high pressure
building from the Canadian Prairies early next week to Quebec late
in the week, northerly flow will dominate the local area through
the week which will keep temperatures cooler, albeit still near
or slightly above seasonal norms, except near the lake where
onshore flow will knock temps down even more. Monday afternoon and
evening appears to be the best timeframe for showers during the
first low passage with chances ramping up again Thursday and
Friday with the second low.



For the 18Z TAFs...

Concerns with the airport forecasts are:

- Gusty south-southwest winds today turning north-northeast early
this evening.

- Low clouds and MVFR, possibly IFR visibility likely to move in
within a few hours after the wind shift this evening.

- Scattered showers this evening becoming more prevalent
overnight, with some thunder in the region.

- LIFR conditions, possibly to 200 ft, on Saturday morning and
probably into the afternoon, along with areas of showers and

Strong southwest winds today have already gusted sporadically to
over 30 kt at several area airports and such gusts are possible
prior to 22Z. These winds have held the cool front to the north
of the local area thus far, but the front should inch southward
later this afternoon into early this evening. Expecting that it
passes ORD sometime early this evening, but confidence is low on
precisely the hour when the wind shift to northeast occurs. It
could be as early as 23Z, and as late as 0230Z.

Cooler air will immediately move in behind the front but moisture
ahead of the boundary is not plentiful. Because of that, believe
that low cloud development, at least to the broken/overcast
level, could lag a few hours and have continued that approach. Do
think IFR is likely once clouds do develop later this evening,
with LIFR possible out the gate. LIFR cloud bases are expected
with high confidence Saturday morning, though just how low is
somewhat uncertain. As for visibility, confidence is lower in
this, as it will likely be dictated by rain shower intensity.

Showers are expected to expand over north central Illinois this
early to mid evening, and then into northeast Illinois late this
evening into overnight. Thunder is certainly possible but
confidence was not enough to include in the TAFs at this time.
Saturday should see arcs of showers rotating across the area,
with light rain and drizzle in-between, so that makes it
challenging for specifics on visibility.



313 AM CDT

Southerly winds are in place across much of the lake this
morning, but will begin to see this change over the next several
ours as a front is expected to move south down the lake. With the
passage of the front this morning, expect winds over the north
half to turn to the north but don`t anticipate an initial increase
in speeds. Do expect an increase though by early afternoon, but
will continue to monitor this situation as it is quite possible
for this increase to occur earlier. For the south half and
nearshore waters, southerly winds are expected and will likely be
on the higher side. Still anticipate winds hazardous to small
craft to occur today, with the Small Craft Advisory in place still
valid. Uncertainty does get higher later today into this evening,
owing to timing and placement of the front. Expect the front to
quickly push down the lake with northerly winds in place for much
of the lake including the Illinois nearshore, while the Indiana
side possibly stays more southerly. With the front possibly
stalling out near the southern end of the lake early Saturday
morning, winds may be highly variable in this location. However,
do think the Illinois nearshore could get into some stronger
northerly winds and then higher waves. So have extended the Small
Craft Advisory over the IL nearshore through tonight, with
stronger winds and higher waves likely persisting. Will need to
monitor this situation as well as these hazardous conditions could
last longer into Saturday morning, as well as the IN nearshore
possibly observing these hazardous conditions.



LM...Small Craft Advisory...IN nearshore waters until 7 PM Friday.

     Small Craft Advisory...IL nearshore waters until 5 PM Sunday.




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