Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240844

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
344 AM CDT Mon Jul 24 2017

343 AM CDT

Today through Tuesday night...

A couple of quiet weather days are in store for northern IL and
northwest IN to begin the week. Early morning GOES-16 water vapor
imagery depicts a mid-level short wave trough propagating to the
east, just north of Lake Huron. Surface low pressure associated
with this feature was analyzed over Lake Erie, with a cold front
trailing west across far northern IN then northwest across far
northern IL. This front will continue to push south across the
area this morning, with breezy north-northeast winds bringing
cooler and somewhat drier air. IR satellite imagery does indicate
an extensive area of low clouds upstream in the low-level cool
advection regime behind the front, and this cloud cover will
continue to spread into the forecast area this morning. Forecast
soundings suggest diurnal temp climb should raise cloud bases and
thin/scatter much of this cloud cover by early afternoon. With
cooler low-level air spreading in from the north, and onshore
winds off Lake Michigan, afternoon high temps are expected to
range from the low 70`s along the immediate lake shore, to the
mid-upper 70`s farther inland. Northerly fetch and winds gusting
over 20 mph this morning, building waves on Lake Michigan will
result in an increased rip current/swim risk at Lake Michigan
beaches today.

Surface high pressure builds southeast across the western Lakes
by this evening, subsidence promoting mostly clear skies and light
winds overnight. Despite dry column, moist ground conditions may
support patchy ground fog in spots by early tuesday morning, with
decent radiational cooling potential. Overnight min temps expected
to range from the mid-upper 50`s away from the urban core of
Chicago, to the low-mid 60`s in the city.

Surface high pressure moves east of the area Tuesday, though
weaker surface ridging lingers across the area with persistent
height rises aloft. Some moderation in temps is expected, though
southeasterly surface winds will support a lake breeze and
maintain a bit of lake cooling across far northeast IL. Forecast
sounding moisture profiles indicate mostly sunny skies, with low-
level model thermal fields supporting afternoon highs in the lower
80`s in most areas, except mid-upper 70`s for IL locations near
Lake Michigan.

Warm advection flow becomes better established Tuesday night, as
low level winds above the boundary layer turn southwesterly in
response to height falls developing across the Upper Midwest in
association with a vigorous short wave trough traversing southern
Canada and the International border region. Guidance keeps focus
for precip well west and north of the cwa through the night,
closer to a cold front which stretches from the northern Lakes and
Upper Mississippi Valley into the Central Plains by Wednesday



343 AM CDT

Wednesday through Saturday...

Warm and humid weather conditions return briefly Wednesday into
early Thursday, with attendant thunderstorm potential. Mainly dry
weather is expected late in the week, as medium-range guidance
depicts an expansive area of surface high pressure developing
across the region Friday into Saturday.

A vigorous mid-level short wave is expected to propagate across
the northern Great Lakes Wednesday, with it`s associated surface
low pressure center tracking across Ontario and a cold front
trailing into the local area Wednesday night. Low-level winds turn
southerly ahead of the front Wednesday, resulting in return
moisture feed into the region. Forecast soundings generally depict
a capped profile during the day, with the greatest potential for
thunderstorm development expected to be focused along/ahead of the
approaching cold front Wednesday night. Deep layer 0-6 km bulk
shear increases to 30-40 kts, with PW`s increasing to 2.50 inches
per GFS, suggesting the potential for organized strong/severe
storms and heavy rainfall threat overnight. Thunderstorm/heavy
rainfall threat continues Thursday morning, and possibly into
early afternoon across the south/southeast, as the front slowly
sags southeastward across the remainder of the forecast area.

Amplifying upper trough then moves east of the area Thursday
night, with broad ridging developing aloft across much of the
central CONUS states. Surface high pressure spreads across the
Upper Midwest and Great Lakes region into Saturday, setting the
stage for a return of drier and slightly cooler weather for the
end of the period.



For the 06Z TAFs...

Concerns with the aviation forecast are gusty north to northeast
winds today and MVFR ceilings this morning.

A cold front has moved south through the TAF sites as of 0530Z,
swinging winds northeast, and for Chicago area sites providing
occasional gusts. There is the chance that winds could swing back
to 350-360 degrees for a few hour period centered on/a little
after daybreak. Confidence is high that winds will want to go back
north-northeast by mid-morning, and then should ease late in the
day before diminishing this evening.

The northeast flow has brought in MVFR clouds. While some holes
are possible this morning, most of the a.m. should see
broken/overcast. Cloud bases will show variability between 1000
and 2500 ft, but confidence is medium-high that they will remain
out of the IFR category. Bases should gradually lift through
midday before quick scattering is anticipated during mid-



130 AM CDT

A cold front has pushed south of the lake early this morning with
a stiff northerly wind now prevailing over the lake through
midday before easing. Gusts of 20-25 kt will be common this
morning, especially across the central and south. Buoys indicated
a quick chop had built in the Illinois and Indiana nearshores
behind the frontal passage early this morning, and while waves may
slightly drop from this through daybreak, they are expected to
re-increase with the swell of a long north-northeast fetch. Will
go ahead and issue a Small Craft Advisory for mainly waves today.

Winds turn south by Tuesday morning and remain as such before a
cold frontal passage most favored early Thursday morning. There
are some indications on model guidance that a low pressure may
develop along this front, either over or near the southern part of
the lake. By some point on Thursday during the day in the wake of
this system, northerly winds will once again prevail. The
pressure gradient is enough to drive 30 knot northerly winds over
the open water, and if the pattern unfolds as forecast, Small
Craft Advisory conditions will be met again in the Illinois and
Indiana nearshores.



IL...Beach Hazards Statement...ILZ006-ILZ014 until 10 PM Monday.

IN...Beach Hazards Statement...INZ001-INZ002 until 10 PM Monday.

LM...Small Craft Advisory...nearshore waters
     until 10 PM Monday.




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