Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 280559
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1259 AM CDT Wed Jun 28 2017

.UPDATE...

920 PM CDT

No real big changes in the near term/tonight. Main updates this
evening were to increase pops Wednesday morning, primarily across
northern Illinois. Hires and short term guidance indicating that
upstream development tonight will have a higher chance of
reaching northern Illinois by Wednesday morning. Will begin to see
convection blossom over MN/IA over the next several hours ahead
of approaching short wave energy, with a veering LLJ assisting in
bringing this into mainly WI by early Wednesday morning. However,
WAA will support developing precip along the southern end, some
of which will get into portions of northern Illinois. Lower
confidence as to how far south this will get, but do think areas
along the WI/IL border will likely observe precip. It`s quite
possible that this precip may extend down to the I-88 corridor
though. Instability axis is currently situated well west of the
CWA, but anticipate instability to be on the increase through
midday Wednesday especially across north central Illinois. The
precip that does move through Wednesday morning though, will
likely be limited to showers with embedded thunder, in a weakening
state. Don`t anticipate a severe threat Wednesday morning.

Rodriguez

&&

.SHORT TERM...
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.

Deubelbeiss

&&

.LONG TERM...
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
flooding.

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
flooding.

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
80s.

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.

JEE

&&

.AVIATION...
For the 06Z TAFs...

A challenging forecast for this TAF cycle with multiple concerns
and confidence on the lower side, especially this afternoon
through this evening. Concerns include SHRA and TSRA potential and
strong southerly winds. A shower and thunderstorm complex over
Iowa and Nebraska should progress eastward through the early
morning and weaken into Illinois as it outruns better instability.
Have the highest confidence in embedded thunder at least in the
vicinity of RFD along with prevailing SHRA activity for a few
hours. Have added a few hours of temporary SHRA to ORD/MDW/DPA in
late morning through early afternoon, but withheld any thunder
mention for now. The showers should be elevated in nature, so at
this time not anticipating a wind shift behind them.

A tight pressure gradient will result in strong southerly winds
(180-200 direction) this afternoon and likely into the early
evening as well with gusts at least near 30 kt and possibly
higher. Forcing for additional thunderstorm development in the mid
afternoon is appearing to be fairly subtle, keeping confidence
low, but with some signal in the guidance added PROB30 mention. It
is very difficult to determine what if any influence outflow
boundaries will play a role in TSRA trends this afternoon through
tonight. Models are indicating a stronger upper disturbance for
TSRA this evening and early overnight, so added mention in RFD TAF
and maintained previous timing from ORD TAF and included it in
other eastern TAF sites. If gustiness can drop this evening and
overnight, then LLWS may need to be added. Wind direction will be
veering to southwesterly with time.

Castro

&&

.MARINE...
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
Thursday.

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.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...9 AM Wednesday to 6 AM Thursday.

&&

$$

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