Flood and Hail in Illinois, July 5, 2017

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
233 PM CDT Wed Jul 5 2017

.SHORT TERM...
217 PM CDT

Through Thursday...

Warm and muggy conditions will persist through Thursday over
northern Illinois and northwest Indiana, along with the continued
threat for a few isolated thunderstorms. This afternoon,
temperatures into the low to mid 80s and dewpoints into the upper
60s to low 70s is contributing to moderately unstable conditions
across the region. MLCAPE axis of 1500-2000 J/kg stretches from
southwest WI south into west-central Illinois, then eastward
towards west central Indiana. Within this corridor remains the
greatest chance for a few isolated thunderstorms throughout the
rest of the day. Winds aloft remain weak, so the severe threat
with any storms this afternoon is low and limited to primarily a
localized wind threat. Storm motions will be slow though, likely
10 mph or less, and could result in locally heavy rainfall and
ponding of water.

Through the day Thursday, a cold front is expected to move south
across the far Upper Midwest with strong instability progged to
develop out ahead of the front including over northern Illinois
and northwest Indiana as very steep mid level lapse rates advect
across the area. Through at least 00Z, forcing is lacking over the
local area, outside of any convergence that may develop near the
lake if a lake breeze is able to develop. Lack of forcing will
keep PoPs low through 00Z, but if an isolated storm is able to get
going certainly wouldn't be able to rule out a severe threat.
Deep layer shear gradually increases through the day as upper jet
noses towards the region. While, the 0-6KM shear increases to
25-30kt in the 21-00Z time frame, much of contribution comes from
0-3KM which increases to around 20-25kt with large cyclonically
curved hodographs. The better thunderstorm coverage and more
likely severe threat lies beyond 00Z as discussed in the long term
portion of the forecast discussion.

Temperatures tomorrow are expected to be a little warmer than
today, topping out in the upper 80s and a few spots may tag 90.
With dew points still in the upper 60s and low 70s, this will
result in heat indices pushing up into the mid 90s. Initially
light flow early in the day may allow a lake breeze to develop,
but little inland progress is expected and increasing west flow
through the day would likely push the lake breeze back over the
lake by mid to late afternoon.

Deubelbeiss

&&

.LONG TERM...
217 PM CDT

Thursday night through Wednesday...

The large scale upper flow pattern will be going through a
transition Thursday night as a large scale upper trough develops
across the Great Lakes and remains in place into early next week.
Active weather looks to accompany this transition Thursday night but
once the trough is in place, this should lead to primarily pleasant
conditions for most areas most of the time, there will likely be
several more subtle features that will need to be watched for that
could result in isolated-scattered thunderstorm activity through the
weekend. Details follow.

The main cold front associated with a developing surface low across
central Wisconsin Thursday evening will move across the local area
late Thursday night and early Friday morning. The moist and unstable
airmass will remain in place into the evening as a westerly low
level jet strengthens modestly and this may support scattered
thunderstorm development well ahead of the approaching front. With
the surface low moving across central Wisconsin and the frontal
passage being better aligned diurnally, would expect that the bulk
of the convective activity will be north of the forecast area and
approach the local area in a weakening state as it may not arrive
until very late in the evening or even during the early overnight.
Activity well ahead of the front, should it even develop, may stand
a better chance at being strong to severe, but it may be limited in
coverage.

Winds shift northwesterly behind the front Friday morning bringing
in cooler and gradually less humid air. Winds look to turn north and
northeasterly and a convergence area/lake enhanced boundary over
inland areas near the lake would be a likely result Friday
afternoon. Forecast soundings show a range of possibilities in terms
of instability but there is decent agreement in a shortwave trough
dropping across the area late Friday afternoon and early Friday
evening. The combination of these factors points to a little better
chance for some isolated to scattered showers and thunderstorms from
late afternoon through mid evening with the favored area being along
the lake enhanced boundary and points south and east. Will add low
pops to northeast IL and NW IN sections of the forecast area
including the Chicago metro.

Surface high pressure builds in for Saturday with cooler temps aloft
as well. Highs will be a few degrees cooler and dewpoints several
degrees lower. An upper trough axis is still expected to swing
across the area but the earlier signal for isolated showers has
shifted southwest of the area with the latest guidance. Shifts like
this are pretty typical in this type of flow pattern. The upper
trough remains in place for Sunday with another embedded shortwave
expected to cross the area as upper ridging over the central U.S.
nudges eastward. This will lead to slightly warmer temps but also
some low chances for scattered storms. The upper flow attempts to
break down a bit early next week but an active northwesterly air
stream looks to remain in place locally supporting more low chances
for shower and storms.

MDB

&&

.AVIATION...
For the 18Z TAFs...

Synoptically southeasterly flow across the region early this
afternoon will turn easterly for ORD/MDW and northeast at GYY early
this afternoon as a lake breeze moves inland. Winds will remain
easterly at those terminals behind the boundary for the remainder of
the afternoon, the trend towards the southeast and south this
evening but becoming light and possibly variable at times.

A light westerly gradient will develop tomorrow ahead of an
approaching cold front pushing into the upper Midwest. Very unstable
conditions will be in place throughout the afternoon, and cannot
rule out isolated thunderstorms, but better chances will be tied to
the frontal passage later in the overnight hours beyond the current
TAF period. Light flow early in the day may allow a lake breeze to
develop once more. With west winds gradually increasing through
the afternoon, think the boundary will struggle to push inland and
do not currently anticipate any wind shift at the terminals, but
the boundary could help produce an isolated thunderstorm or two
just east of ORD/MDW.

Only other minor concern would be the potential for late night/early
morning patchy fog producing MVFR conditions. Given the light flow
and high dewpoints in the area, have opted to include a tempo for
MVFR vsby at RFD and DPA tomorrow morning where the likelihood of
this occurring is greatest.

BMD

&&

.MARINE...
230 PM CDT

Surface low pressure of 29.9 inches will slowly move northeast up
the Ohio River valley into Friday. Another low pressure system of
29.5 inches will track southeast across southern sections of Hudson
Bay tonight with a trough axis trailing it to the southwest. A
secondary surface low is expected to develop along this trough
across central Minnesota Thursday morning with a pressure of around
29.8 inches and track east across central Lake Michigan early
Friday. Southerly winds will prevail ahead of this system but
turn sharply north and northwest as it passes into Friday morning.
High pressure of 30.1 inches will then spread across the northern
Plains and Upper Mississippi River valley Friday night before it
shifts southeast into the lower Ohio Valley Saturday. An active
pattern sets up late in the weekend through early next week with
low pressure systems tracking eastward across the lake likely
setting out a frontal boundary which may bisect the lake. High
pressure may build across the lake by the middle of next week.

MDB

&&

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

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Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
1221 PM CDT Wed Jul 5 2017

ILZ003>006-008-010>014-019>023-032-033-039-INZ001-002-010-011-019-
061730-
Winnebago-Boone-McHenry-Lake Illinois-Ogle-Lee-De Kalb-Kane-
DuPage-Cook-La Salle-Kendall-Grundy-Will-Kankakee-Livingston-
Iroquois-Ford-Lake Indiana-Porter-Newton-Jasper-Benton-
1221 PM CDT Wed Jul 5 2017 /121 PM EDT Wed Jul 5 2017/

THIS HAZARDOUS WEATHER OUTLOOK IS FOR NORTH CENTRAL ILLINOIS...
NORTHEAST ILLINOIS AND NORTHWEST INDIANA.

.DAY ONE...This Afternoon and Tonight.

WEATHER HAZARDS EXPECTED:
  Limited Thunderstorm Risk.

DISCUSSION:

Isolated thunderstorms are again possible well south and west of
Chicago this afternoon.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

Thursday...
  Elevated Severe Thunderstorm Risk.
Friday through Tuesday...
  Occasional low chances for thunderstorms.

DISCUSSION:

Severe storms are possible late Thursday into Thursday night. At
this time the better chances remain north of our area, but a
strong to severe storm threat does exist, particularly into north
central IL. Strong winds and large hail would be the main
hazards.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will not be necessary through tonight.

GENERAL STORM MOTION OF THE DAY:

Slow moving storms, generally moving northeast at 10 mph or less.

&&

LMZ740>745-061730-
Winthrop Harbor to Wilmette Harbor-
Wilmette Harbor to Northerly Island-
Northerly Island to Calumet Harbor-Calumet Harbor to Gary-
Gary to Burns Harbor-Burns Harbor to Michigan City-
1221 PM CDT Wed Jul 5 2017

THIS HAZARDOUS WEATHER OUTLOOK IS FOR THE ILLINOIS NEARSHORE
WATERS OF LAKE MICHIGAN AND THE INDIANA NEARSHORE WATERS OF LAKE
MICHIGAN.

.DAY ONE...This Afternoon and Tonight.

No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

Thursday and Friday...
  Limited Thunderstorm Risk.
Sunday through Tuesday...
  Limited Thunderstorm Risk.

&&

LMZ080-261-362-364-366-563-565-567-669-671-673-675-777-779-868-
870-872-874-876-878-061730-
Lake Michigan from Michigan City IN to St. Joseph MI 5 NM
offshore to Mid Lake-
Lake Michigan from Seul Choix Point to Rock Island Passage 5 NM
offshore to Mid Lake-
Lake Michigan South of a line from Seul Choix Point to the
Mackinac Bridge and north of a line from Charlevoix MI to South
Fox Island 5 NM offshore-
Lake Michigan from Charlevoix to Point Betsie MI 5 NM offshore to
Mid Lake-
Lake Michigan from Point Betsie to Manistee MI 5 NM offshore to
Mid Lake-
Lake Michigan from Rock Island Passage to Sturgeon Bay WI-
Lake Michigan from Sturgeon Bay to Two Rivers WI 5 NM offshore to
Mid Lake-
Lake Michigan from Two Rivers to Sheboygan WI 5 NM offshore to
Mid Lake-
Lake Michigan from Sheboygan to Port Washington WI 5 NM offshore
to Mid Lake-
Lake Michigan from Port Washington to North Point Light WI 5 NM
offshore to Mid Lake-
Lake Michigan from North Point Light to Wind Point WI 5 NM
offshore to Mid Lake-
Lake Michigan from Wind Point WI to Winthrop Harbor IL 5 NM
offshore to Mid Lake-
Lake Michigan from Winthrop Harbor to Wilmette Harbor IL 5 NM
offshore to Mid Lake-
Lake Michigan from Wilmette Harbor IL to Michigan City IN 5 NM
offshore to Mid Lake-
Lake Michigan from Pentwater to Manistee MI 5 NM offshore to Mid
Lake-
Lake Michigan from Whitehall to Pentwater MI 5 NM offshore to Mid
Lake-
Lake Michigan from Grand Haven to Whitehall MI 5 NM offshore to
Mid Lake-
Lake Michigan from Holland to Grand Haven MI 5 NM offshore to Mid
Lake-
Lake Michigan from South Haven to Holland MI 5 NM offshore to Mid
Lake-
Lake Michigan from St. Joseph to South Haven MI 5 NM offshore to
Mid Lake-
1221 PM CDT Wed Jul 5 2017

THIS HAZARDOUS WEATHER OUTLOOK IS FOR THE OPEN WATERS OF LAKE
MICHIGAN.

.DAY ONE...This Afternoon and Tonight.

There are low chances for thunderstorms today.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

With the exception of Friday through early Saturday, low chances
of thunderstorms will linger into early next week.

&&

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