Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230807

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
307 AM CDT Wed Aug 23 2017

307 AM CDT

Through Thursday...

The one noteworthy change with the short term forecast was to
mention a chance for lake effect showers (yes lake effect in
August!) into lake adjacent counties late tonight and Thursday

A cool morning to start this Wednesday, with far outlying
locations in the lower 50s. Just across the Chicago metro area
alone temperatures range from 53 at ARR to 67 downtown as of
3 am, and that range of difference is likely on several upcoming
nights. High pressure of 1018 mb will build eastward over the area
this afternoon. With just a bit of cold advection at the 850mb
level, low-level lapse rates will support at least scattered
cumulus. Based on upstream highs yesterday and climatology with
850 mb temperatures, highs should fall out in the mid 70s.

The strong upper trough across eastern Canada and the Great Lakes
region will continue to move eastward tonight, with a backside
short wave impulse seen on upstream water vapor dropping southeast
over the area late tonight into Thursday morning. This will be
dampening some as it arrives, and by itself looks to at most
support patchy sprinkles. However, 850mb air temperatures dipping
to 6C over lake temperatures around 70 (21C) will support
convective clouds and potential for lake effect showers into
Illinois and Indiana lakeside counties. While moisture magnitude
for deep cloud depths is somewhat shallow, lake induced CAPE
values around 500 J/kg will likely support some showers along a
forecast north-to-south convergent axis. How wide this is quite
uncertain. Also of note for those immediately along the shore is
rare August potential for waterspouts. While again moisture depth
is a bit shallow, there is a window late tonight into mid-morning
Thursday where lake-induced equilibrium levels are forecast to
top 20,000 ft. If the convergent axis is pronounced, the
conceptual pattern is there for low-level focus and stretching of
lake induced updrafts.

Drier air should advance in early Thursday afternoon. Have
continued to lean toward raw guidance, with highs likely climbing
no higher than 70 near the lakefront (and only that warm because
that is the water temperature), while a few degrees warmer



307 AM CDT

Thursday Night through Tuesday...

Thursday night continues to look brisk for late August. Given this
morning`s temperatures and what appears a more favorable cool
setup Thursday night, still see some outlying areas making a run
at mid 40s for daybreak Friday.

The eastern North America long wave trough heads east into the
weekend, with a weakness in the upper westerlies over the area
the rest of the week into the weekend. Global guidance is in
general agreement of a weak upper low inching its way eastward
from the Upper Mississippi Valley during the weekend and
potentially providing some chance of showers Sunday-Tuesday. In
addition, what is expected to become tropical cyclone Harvey in
the western Gulf of Mexico is hinted at by global guidance to
possibly further stall the pattern early to middle next week in
the eastern half of the country. Quite a bit of uncertainty, but
nothing screaming for large rain chances over our area, and
thunder potential looks low. Have forecast temperatures in the
extended about 5 degrees below normal.



For the 06Z TAFs...

Only forecast concern is a possible lake breeze Wednesday

Under clear skies and with high pressure spreading across the
middle Mississippi Valley, the lower levels are decoupling from
winds aloft and sfc winds have diminished to nwly at 5-10kt for
the Chicago area terminals and lgt/vrbl for ncntrl IL arnd RFD.
Northwesterly winds will increase through the morning and into
early tomorrow afternoon, with some gusts into the 15-20kt range
likely from mid morning to early afternoon. Wind speeds/gusts
will begin to diminish during the mid/late afternoon as high
pressure continues to spread east, weakening the pressure gradient
over nern IL/nrn IN. A lake breeze is expected to develop...moving
likely moving through GYY and MDW by mid/late afternoon. Have
maintained the wind shift to nely at 00z at ORD, but confidence
remains low on the chances for the lake breeze to move through
ORD. In any case, winds will likely be less than 10 kt regardless
of direction. High pressure will continue to spread east tomorrow
night, keeping cigs/vis at vfr through the period, with light
winds tomorrow night.


216 PM CDT

North-northwesterly winds of 15 to 20 KT will continue down Lake
Michigan into Wednesday as a stout area of low pressure continues
to shift over northern Quebec and a surface ridge of high pressure
builds over the Upper Midwest. With these winds, expect waves of 4
to 7 feet over southern Lake Michigan with hazardous conditions
for small craft into Wednesday for the Indiana near shore waters.
Expect the winds over the lake to gradually abate for a short
period on Wednesday before another cold front shifts southward
down the lake Wednesday night night into Thursday morning. As this
front passes, expect a bit of resurgence in some of the northerly
winds over the lake for a period. A few showers, and possibly an
isolated water spout may also be possible over Lake Michigan
early Thursday morning with the frontal passage. However, with
time expect the surface ridge of high pressure to build eastward
over the lake late in the week and into the weekend. This will
result in a period of lighter winds, which should gradually become
southeasterly late in the period as another area of low pressure
takes shape over the Upper Midwest.



IN...Beach Hazards Statement...INZ001-INZ002 until 4 PM Wednesday.

LM...Small Craft Advisory...IN nearshore waters until 4 PM Wednesday.




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