Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 272357

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
657 PM CDT Tue Jun 27 2017

203 PM CDT

Through Wednesday...

Quiet weather in place the rest of today under the influence of
high  pressure, but precip chances will increase through the day
Wednesday ahead of an area of low pressure working east across the
Upper Midwest.

This afternoon, high pressure is centered over Indiana with
ridging  extending into northern Illinois providing light flow
and fair conditions. A lake breeze is charging westward across the
Chicago metro pushing temps back down a couple degrees into the
upper 60s to lower 70s. Lake breeze should wash out later in the
evening with southerly flow overspreading the CWA behind the
departing ridge and ahead of low pressure over the plains.

Attention this evening and tonight will turn off to the west where
an MCS is expected to develop around the mid and upper Missouri
River Valley on the nose of a strong low level jet. Convection is
expected to spread east across Iowa and Minnesota overnight and
eventually reach Wisconsin and potentially northern Illinois
early tomorrow. Convection is expected to outrun any instability
and the better forcing associated with the LLJ is expected to
remain to our west and northwest with mid/upper level height
rises/ridging building overhead locally. Some of the models do try
to bring in a decaying area of precipitation across northern
Illinois while a number more are dry. Given the limiting factors
mentioned above, have lowered PoPs but do maintain some slight
chance/low chance PoPs mainly across the northern two tiers of
counties in Illinois to account for the uncertainty. If something
were able to persist into the CWA, there is not much of a severe
threat given the lack of instability.

Broad area of low pressure is expected to advance east across the
Dakotas and far upper Midwest through the day Wednesday with
strengthening southerly flow expected locally. Tightening pressure
gradient and deep mixing should be able to tap into strong mid
level winds which will result in gusty conditions from mid/late
morning through the afternoon. Expect a few wind gusts to top out
in excess of 30 mph with more frequent gusts of 25 to 30 mph. Dew
points are expected to gradually increase throughout the day,
eventually pushing above 60F, especially across the western CWA
where modest MLCAPE will eventually develop mid to late in the
afternoon. Hard to pin down specifics on convection tomorrow
before 00Z with no strong focus for ascent in the area, but with
broad upper difluence behind the departing upper ridge axis and
the potential for remnant outflow boundaries cannot completely
rule out isolated storms tomorrow afternoon. If any storms are
able to develop, there would be some threat for severe
thunderstorms given a strongly sheared environment with
cyclonically curved hodographs in place. Better chances arrive
tomorrow evening as discussed in the extended portion of the
forecast discussion.



236 PM CDT

Wednesday night through Tuesday...

Wednesday night through Friday looks stormy with a break from storms
over the weekend.  Active weather returns early next week.

Convection is expected to fire over Iowa Wednesday afternoon and
then shift east into the forecast area Wednesday evening and night.
Main concern is flooding as convection may be training along a
boundary.  PWATs are forecast to be a healthy 1.75-2 inches. CAPE
values are forecast to be around 500 J/kg with shear around 40 kt.
Therefore, a few storms may be strong to severe and capable of
mixing down stronger winds aloft, but the main threat will be

The low`s cold front stalls just north of the forecast area
Thursday. The atmosphere should recover from any morning convection
despite cloud cover, but the main question is how long will the
recovery take? Southwest winds will continue to usher in warm, moist
air. Forecast CAPE values are 2500-3000 J/kg Thursday afternoon with
shear around 30 kt. Model soundings feature a weak cap that erodes
in the afternoon. Despite fat CAPE, thinking damaging winds would be
the greatest risk given dry air aloft. PWATs remain at 1.5-1.75
inches so heavy downpours and flooding due to training and slow
moving storms is also a concern. While storms are expected after
18Z/1PM CDT, I think the best storm coverage will be after 00Z/7PM
CDT as an upper level wave approaches.

A surface low is forecast to form over the midwest Thursday
night/Friday morning and then pass over southern WI and Lake
Michigan.  More showers and storms are expected with the low.
Guidance features CAPE but also a decent cap. so unsure about the
severe potential. PWAT values are still high at around 2 inches, and
given two periods of possible heavy rain days before, this system
could bring more rain that we do not need leading to additional

We dry out Friday night into at least Saturday morning. Long term
models indicate precip is possible Saturday as the upper level
trough swings through. However, have low confidence in the strength
of the trough and how much precip will occur. Kept precip chances at
slight chance. Sunday looks similar with a dry start thanks to high
pressure. However, guidance again wants to bring showers into far
northern IL in the afternoon. Have even lower confidence in showers
Sunday afternoon. Both days will have highs in the upper 70s to mid

Guidance shows a lot of disagreement in how early next week will pan
out. The ECMWF keeps the pattern busy with a couple of upper level
systems bring rounds of showers and storms through Monday and
Tuesday.  The GFS, on the other hand, has high pressure over the
northern midwest keeping a low and the majority of its precip to the
south. Given the uncertainty, kept precip chances as is.



For the 00Z TAFs...

The concerns with the Chicago area TAFs are:

- Strengthening winds Wednesday morning near due south in
  direction turning south-southwest with gusts possibly near 30
  kt in the afternoon.

- Chance for showers and storms late Wednesday morning into the

- Higher potential for thunderstorms, some possibly strong, after
  dark Wednesday.

Dry high pressure over the region will begin departing overnight
with moisture and instability aloft working its way east into
Wednesday morning. There should be several areas of storm activity
across MN and IA overnight that will be fading as they move
eastward. Entertained the idea of rain mention at ORD and MDW
between 16Z and 19Z, but confidence was just too low with the
inherent dry air mass and the best forcing and focus to the
northwest of the area (though not by very far). Do think showers
will arrive to at the IL/WI border region area Wednesday morning,
including RFD, and there could be thunder activity with that.

The potential and timing of showers and storms for Wednesday
afternoon may depend somewhat on how any morning rain activity
pans out. There should be a decent amount of cloud cover either
way and that could limit instability, with the most likely
regeneration of showers and storms Wednesday afternoon over
central/eastern Iowa. These should work its way eastward with
storms likely at some point Wednesday evening and/or overnight for
the local airports. Some of these storms, or any earlier ones if
they were to develop in the area, have an environment where they
could bring strong gusts and heavy rain to IFR visibility. Any
storms that were to go through would likely temporarily shift
winds west/northwest.

As for prevailing winds, south winds overnight will start to
increase near daybreak and then more markedly by mid-morning.
While cloud cover may limit full potential of gusts, do expect to
see fairly regular gusts of mid to upper 20 kts. The direction
should be 170-190 degrees for much of the morning before locking
into more of a 190-200 degrees.



236 PM CDT

High pressure over the lake continues east through Wednesday
afternoon. Meanwhile the low over southern Saskatchewan will reach
Minnesota Wednesday evening. The pressure gradient over the lake
tightens in response to the high and low leading to gusty south
winds Wednesday. Gusts to 30 kt are likely, and occasional gale
force gusts are possible in the nearshore waters and over the
northern end of the lake. The lake looks too stable to support more
persistent gales, although winds will be around 50 kt 1500-2000 ft
above ground level. Will not be issuing a gale headline due to
stability concerns over the lake, but will have ocnl gales in the
GLF and NSH. Will issue a small craft advisory for all of the
nearshore waters Wednesday morning through most of Wednesday night.
Winds slowly diminish as the the low passes over the northern Great
Lakes Wednesday night, and the low`s cold front passes over the lake

A second low forms over the southern plains early Thursday morning
and passes over the southern or central portion of the lake Friday.
Winds will vary from southwesterly south of the low to northeasterly
north of the low.  Winds settle to westerly Friday night through
Sunday. I have low confidence in exact details early next week as
models disagree at how the pattern will evolve.



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...9 AM Wednesday to 6 AM Thursday.




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