Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

FXUS63 KLOT 191801

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

1259 PM CDT

Isolated to scattered showers and thunderstorms continue to
gradually increase in coverage across far northern Illinois and
northwest Indiana. Upper wave moving across Wisconsin is resulting
in the most concentrated area of activity over southern Wisconsin
but enough ascent and instability is in place over northern
Illinois and northwest Indiana to keep activity going for a few
hours yet. Have trimmed pops south and west where activity is
expected to be minimal and maintained highest pops north and east.
Hail and heavy downpours are the primary threats though some gusts
are possible as well. A secondary area of showers over far
southeast Minnesota may make its way to the area late this



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 18Z TAFs...

Isolated showers and thunderstorms are starting to increase in
coverage across northern IL and NW IN as an upper wave moves
southeast across Wisconsin. Expect scattered showers/storms to
continue for the next few hours as the wave passes. A secondary
area of activity over far southeast MN may arrive late this
afternoon so may need to extend VCSH or even VCTS mention a little
longer. The strongest cells will likely reduce vsby below 2 miles
if they pass over the terminals. May also see some wind gusts in
to the 25-30 kt range. Otherwise expect VFR with cloud bases
around 5000 ft or better for the most part.

A similar scenario is expected tomorrow with another trough
passing during the early afternoon. Have included a PROB30 for
thunder after 18z with scattered activity possible. Guidance
suggests that winds will shift north or northeast with a weak
front working down the lake later Tuesday afternoon. Will need to
keep an eye on this but have not included this in the latest ORD



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.






WWW.YOUTUBE.COM/NWSCHICAGO is the U.S. government's official web portal to all federal, state and local government web resources and services.