Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 191830

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


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


130 PM CDT

Tuesday night through Monday...

We will get a break from the northwest flow wave train Tuesday
night into early Wednesday as the upper low across Ontario finally
gets dislodged. Showers and storms from the afternoon will
dissipate surface high pressure will expand across northeast IL
and northwest IN. The high will quickly shift over the lake on
Wednesday with a synoptic and likely lake enhanced onshore flow
pattern expected for Wednesday. The upper air pattern will shift
back to a fast zonal flow pattern with high pressure centered over
the desert southwest, with an active northern jet stream that
will spread across the northern tier of the country. Warm air
advection behind departing high will expand eastern extent of the
heat dome to our west back closer to our area. This will allow
much of the area to warm back in the low to mid 80s with a dry
day, though it will be much cooler at the lakefront.

A speed max in the westerly flow across the northern plains will
lead to strong to severe storms across the middle Missouri valley
in the afternoon. The low level jet will strengthen overnight
ahead of the system`s surface low into early Thursday allowing a,
while veering and becoming pointed toward the midwest and
eventually the western Great Lakes. This will allow a warm front
to lift northward and become the focus for nocturnal convection. Global
guidance indicates the better shower and storm chances will lie
along and north of the front which at this time appears to be
along and north of the Wisconsin border, though some showers and
or storms maybe able to sneak into northern IL. The better severe
threat is west and north.

The front will stall out in the region on Thursday, and this
boundary along with a southeastward advancing cold front ahead of
upper level low pressure across the Upper Midwest will serve as
the focus for another round of showers and thunderstorms. The
pattern and time of day does not scream severe weather, but with
near or above 2" precipitable water values and the fairly
unidirectional mid level flow parallel to the lower level winds
and frontal boundary, very heavy rainfall is possible in this
pattern. It is uncertain whether this axis will remain pointed
into Wisconsin or if it will shift south into Illinois, at this
point the moisture transport appears to weaken in our area, but
it is something to keep an eye on.

The cold front will move south Friday which will eventually shift
showers and storms out of the area, with some disagreement in
models as to how quickly this will occur whether it is in the late
morning or not until evening. The frontal position, still too
difficult to pin down at this distance, will be pivotal in
determining storm chances, but it will be in the region. Temperatures
will take a dip back down Friday into the weekend. Model
agreement heads downhill this weekend with the handling of the
progression of a upper level trough across the northern tier of
the country, so a model consensus for now keeps low chances for
showers and storms through the weekend. The pattern does support
this general idea with upper low around, fast zonal flow or
possibly northwest flow.



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



130 PM CDT

Deep low pressure over western Quebec will continue
slowly off to the northeast, which will maintain westerly winds up
to 20 kt through Tuesday. High pressure over the northern Rockies
will move over the lake Tuesday night into early Wednesday, which
will lead to lighter/lake breeze circulation winds. The high will
quickly shift east later Wednesday. A warm front will lift north
to portions of the lake early Thursday. We could see a brief
period of southwesterly 15-25 kt winds Thursday morning as the
pressure gradient tightens between the departing high and another
seasonally deep low over western Ontario. The low`s cold front
will pass over the lake Thursday night. A secondary cold front
will approach Saturday into Sunday, with generally high pressure
will holding to the west and low pressure to east into next week.






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