Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 190732

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
232 AM CDT Mon Jun 19 2017

232 AM CDT

Through Tuesday...

Primary short term concern is with convection today. Northwest
flow aloft is established over the region in the wake of
yesterday`s trough. GOES 16 water vapor imagery shows a series of
small ripples in the flow from the Dakotas southeast into IA, with
the most prominent shortwave over central/eastern ND. Sunday
evening RAOBs sampled some very chilly air aloft for mid-late June
that will be advected southeast into the area today along with the
more vigorous ND shortwave trough.

Unseasonably chilly 500 mb temps progged to drop to around -17C,
which SPC sounding climo for KDVN shows that very near record cold
values for the date. The very cold air aloft combined with
boundary layer heating is forecast to result in MLCAPE values
nearing 1000 j/kg this afternoon with little appreciable CINH. The
modest instability combined with increasing large scale ascent
ahead of the approaching shortwaves should result in a pretty good
coverage of scattered showers and thunderstorms this afternoon,
particularly over the NE half to 2/3rd of the CWA where I`ve
bumped pops to likely.

Effective shear values are not progged to be all that impressive,
so shouldn`t be much updraft separation. Despite the weaker shear
and skinny CAPE values, hard to rule out some small hail with the
more robust updrafts given the unseasonably cold temps aloft.

Convection should quickly fade early this evening as stronger
shortwave passes east and we lose daytime heating. Another
shortwave is progged to ride the NW flow into the area Tuesday
afternoon, but with warmer temps aloft and the resultant weaker
instability, expected coverage of showers to be much more sparse
than today with gridded pops reflecting that. Some of the higher
res models suggest that a back door lake enhanced front to send
temps falling Tues afternoon, this isn`t reflected in the grids
currently, but is something for subsequent shifts to consider.

- Izzi


232 AM CDT

Tuesday night through Sunday...

Upper flow across the CONUS should transition from the eastern
trough/western ridge with NW flow locally to more of a fairly
active quasi-zonal flow from mid-week into the weekend. Temps
should warm up later in the week as NW flow relaxes, but with
periodic bouts of showers and t-storms as a series of shortwave
troughs move across the region. Currently, better precip chances
look to be Wed night with warm FROPA, then later Thurs/Thurs night
with cold front then the next system looks to move through later
in the weekend. Will need to keep a half an eye on 93L as it moves
into the GOM this week as the ECMWF suggests it could move far
enough west to potentially get picked up by our weekend system,
which could boost moisture and rainfall potential. Still a LOT of
uncertainties and wildcard with that system and any potential
interaction with systems in the westerlies in the mid-latitudes.

- Izzi


For the 06Z TAFs...

Overall, expecting VFR conditions and west winds. Gusts to around
20 kt are possible from the late morning through the afternoon.

An upper level wave will likely force showers and thunderstorms
this afternoon as it moves over the region. Expecting scattered
coverage from about 16Z/17Z through the late afternoon. Therefore,
all TAF sites should expect ocnl showers and thunderstorms this
afternoon. Some models hint that coverage may diminish after
21Z/22Z so left the PROB30s for -TSRA as is in the TAFs. I want to
see how models handle the showers as far as coverage and timing
before possibly changing the PROB30s to TEMPOs for -TSRA.

Convection diminishes in the evening with west winds and VFR
conditions continuing tonight.



210 AM CDT

The low over Ontario continues north over western Quebec, and winds
remain westerly up to 20 kt through Tuesday. High pressure over the
Rockies moves over the lake Tuesday night resulting in light winds.
The high continues east with winds becoming south behind it.  Could
see a brief period of southwesterly 15-25 kt winds Thursday morning
as the pressure gradient tightens between the departing high and a
low over western Ontario.  The low`s cold front swings over the lake
Thursday. A secondary cold front may shift winds out of the north
late this week.






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