Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 192346
AFDLOT

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

.SHORT TERM...
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.

MDB

&&

.LONG TERM...
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.

KMD

&&

.AVIATION...
For the 00Z TAFs...

646 pm...Current area of showers/thunderstorms will move southeast
of the terminals in the next 1-2 hours...as it slowly weakens and
most of this activity should remain south of ord. Dry overnight
with west/southwest winds under 10kts.

Similar pattern/set up Tuesday with colder air aloft and an upper
wave moving southeast across the area. Scattered thunderstorms are
expected to develop by late morning across northwest IL and
southern WI and spread southeast across the terminals through the
early/mid afternoon. Convection during this time period may be a
bit more organized than today with a more solid line of storms.
Strong/localized wind gusts and small hail will be possible with
the strongest storms. This activity will move southeast of the
terminals by late afternoon.

While outflow boundaries will affect wind directions at the
terminals...there appears the be a consensus among the models of a
shift in wind direction to the north or northeast either with the
storms or shortly thereafter. Only medium confidence from this
distance but opted to include the potential for these wind shifts
in this forecast. Assuming winds do shift north or northeast...
eventually wind directions should swing around easterly and become
light southerly by late afternoon or early evening. cms

&&

.MARINE...
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.

KMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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