Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200538

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1238 AM CDT Tue Jun 20 2017

910 PM CDT

Thunderstorms have cleared the southeastern parts of the cwa as of
905 pm...with just a few showers lingering over the next 30-60
min over parts of NW Indiana south of the Kankakee River. Drying
in mid-levels per GOES vapor imagery and upstream 00Z soundings
will provide dry weather overnight, with clear to partly cloudy

Next wave digs into the region Tuesday, with redevelopment of
scattered showers and thunderstorms expected by late morning.



237 PM CDT

Through Tuesday...

One upper wave continues to track east-southeast across Wisconsin
early this afternoon. Bulk of shower and isolated thunder activity
continues to be across southern Wisconsin but scattered activity
has been gradually increasing in coverage across northern
Illinois. This activity seems to be developing along a subtle area
of convergence from near Rockford to just north of Valparaiso with
points to the south of this corridor seeing little development
thus far. Expect development to continue for a little longer this
afternoon. Have adjusted pops through the afternoon to better focus them
along and north of this line with values dropping off southward.
Chilly air aloft and associated steep lapse rates are supporting
small hail development with the stronger cells. So far have
received a few pea and dime size hail reports. Cannot rule out
something a little larger but this would be pretty isolated. A
second wave is generating showers across far southeast MN and this
will arrive late afternoon and exit early this evening. Will be
losing some of the diurnal instability as this arrives so coverage
should start to wane but did bump pops slightly into early
evening. Otherwise, clouds will decrease in coverage tonight
allowing lows to fall into the mid and upper 50s with readings
around 60 in the more densely urban areas.

Afternoon water vapor imagery shows another upper wave ejecting
southeast across far southwest Manitoba from a closed upper low
over southern Saskatchewan. This waves does appear pretty robust
on satellite with radar in this area showing some bands of
precipitation. Guidance is more excited about this wave and the
potential for showers and some thunder activity that earlier runs.
Instability appears to be weaker than today so thunder may be more
limited but do see decent potential for shower development as the
wave arrives by early afternoon. Have increased pops to high
chance with a small area of likelies in the north Chicago metro.
Temps overachieved today and tomorrow may be complicated by the
earlier timing of potential showers so adjustments may be needed.



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 06Z TAFs...

West winds increase by mid morning with gusts around 20 kt again.
Another upper level wave will drop into the region this afternoon
kicking off more convection. Thinking the early to mid afternoon
hours will bring the best chances for scattered thunderstorms
similar to yesterday. Thunderstorms may produce gusty winds and
wind shifts to the north so kept a TEMPO for TSRA and north winds
gusting to 30 kt. Also kept a two hour VCTS for lingering TS in
the late afternoon.

I have low confidence in how convection will impact the wind field
this afternoon and evening. Short term guidance has winds in
pretty much every direction. Thinking gusty north winds will
become southeast behind convection and associated outflow
boundaries. Wind speeds diminish in the evening and then slowly
back to northeast.



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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