Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 282107

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
307 PM CST Mon Nov 28 2016

257 PM CST

Through Tuesday evening...

A broad area of showers has spread across Illinois and Indiana
today ahead of an approaching frontal zone that is expected to
move through late this evening. These showers are associated with
a rich plume of moisture being advected off the Gulf between the
approaching front and a center of high pressure located just off
the mid Atlantic coast. Upper level support for the showers is
seen in several bands of shortwave energy, depicted by multiple
deterministic models. There has been a question whether the gap
between shortwaves aloft as seen in the models would be reflected
in a precip gap at the surface, and this seems to be what has
evolved from the area between RFD and UIN. Additional showers and
thunderstorms have been trying to develop across eastern Iowa in
closer proximity to the front, but these have been dissipating as
they approach the local area. Will thus continue the trend of a
short break in the precip for some areas this evening, then
increasing shower chances as the front moves through.

Rapid clearing behind the front, but persistent southerly flow,
will support very mild temperatures tomorrow before the next
frontal zone approaches later in the evening.




Coming shortly...


For the 18Z TAFs...

Showers and a few embedded thunderstorms were approaching the
terminal areas at TAF time. Opted to keep an hour of tempo TSRA
with slightly better cigs and vsbys before the period of
prevailing TSRA and lower conditions. The other factor in keeping
the initial 18z group was the wind direction which has been very
slowly trending more toward 160 degrees. Not completely sure winds
will be at 160 by 19z but stations immediately upstream already
have been reporting 140 to 160 so timing should be close. Also am
not sure about the break in activity shown after 21z. Was thinking
there could be a period of little to no rain, or perhaps just
drizzle, but now new convection is developing over eastern Iowa.
It remains to be seen if this will accelerate into the gap behind
the first wave of showers. IFR to LIFR should rapidly improve
behind the passage of a cold front toward late evening or shortly
after midnight.



222 AM CST

Headlines...Extended the gale warning for the southern open waters
through 12Z/7AM CST Tues morning, and to 21Z/3PM CST Tues for the
northern open waters.  Winds are struggling to reach gales in the
NSH waters so left NSH headlines as is.

The low over South Dakota will slowly weaken and shift to Minnesota
through Tuesday. Its cold front slides over the lake late this
evening. A second low forms over the middle of the lake late tonight
and weakens over Lake Superior before merging with the main low
Tuesday night. The second low will tighten the pressure gradient
resulting in another brief period of gales over the open waters
Tuesday. Decided to extend the current gale warning instead of
issuing another one.

The weakened main low passes over the Great Lakes mid week with
winds becoming west 15-25 kt behind the low.  West to northwest
winds are forecast through the weekend. A large high builds over the
plains Thursday night and then passes south of the lake this



     AM Tuesday.

     Gale Warning...LMZ777-LMZ779 UNTIL 6 AM Tuesday.




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