Small Craft Advisory 77, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
222 PM CDT Tue Sep 26 2017

.SHORT TERM...
200 PM CDT

Through Wednesday...

One last hurrah with the current late season heat event is ongoing
this afternoon, but we are about to take the leap back into fall.
ORD did hit break the record high earlier today, now for the
seventh day in a row. Showers have developed across north central
Illinois ahead of a cold front currently in northwest Illinois.
The main upper forcing will pass off to our northwest, and thus
deeper saturation for more widespread activity is limited. This
should act to keep showers more in the isolated to scattered
regime. There is a narrow instability axis ahead of the front,
thus an isolated storm is possible, though with the cu field
filling in, capping will set in farther downstream.

The weather pattern change will be noticed here through the
afternoon and evening as the front moves in. There is a healthy
stratus field back behind the cold front, and we will have some of
these clouds plus a healthy mid level cloud deck also. Expect
somewhat breezy northwest winds, though most upstream sites have
topped only out around 20 mph or so. Additional shortwave energy
will propagate northwest along the frontal zone keeping some
chances for showers into the evening, though coverage will be
fairly low.

Wednesday will feel much different across the area in spite of the
fact that temperatures will be peak around seasonal normals. After
some morning clouds, expect a dry day with partly cloudy skies.

KMD

&&

.LONG TERM...
222 PM CDT

Wednesday night through Tuesday...

Cooler, more seasonal, weather is expected later this week and into
this weekend. However, the chances for precipitation through much
of the period looks to remain low.

After a pleasant day on Thursday, a mid-level trough is expected
to dig southeastward across the Western Great Lakes Region into
Friday morning. This feature should push another cold front
south over the area Thursday night. Even cooler air in the wake of
this front should result in temperatures on Friday only in the 60s
across much of northeastern Illinois, and into the low 70s inland.
Lake effect showers are likely to largely remain east of the area,
though some of these showers and clouds could impact far eastern
portions of the area in northwestern Indiana into Friday morning.
Otherwise, expect a surface high to build over the Great Lakes
late Friday and into Saturday, resulting in continued dry and
seasonal weather.

Later in the weekend, the surface high should shift towards New
England, and as it does so, the low-level flow should turn
southerly again. This should allow for a warming trend and a
return to above average temperatures into early next week. The
only chance for rain during the period looks to be late Monday
through Monday night as a frontal system looks to move across the
area. Otherwise, forecast guidance is insistent in rebuilding the
upper ridge across the central portion of the country by the
middle of next week. This may result in yet another period of
well above normal temperatures for the later half of next week.

KJB

&&

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

Concerns are on shower development ahead of an approaching cold
front, along with maybe a thunderstorm mainly along the western
terminals. There is some chance for MVFR cigs behind the front
also.

In spite of a decent cold front that will move through this
evening, upper level forcing for precipitation is fairly weak, and
will be the strongest west of the Chicago terminals. Therefore
expect the shower activity across Iowa to move with the front, but
likely diminish in coverage. RFD does stand a smaller chance of
thunderstorm, but the window appears small and short lived and
thus we have held the VCSH mention instead of VCTS. Expect a
northwest wind shift with the front, with occasional gusts into
the mid teens before they settle to around 10 kt or so. MVFR
ceilings could briefly occur overnight, and there may be some weak
showers, but again very low coverage. VFR will dominate much of
Wednesday on continued northwest wind.

KMD

&&

.MARINE...
222 PM CDT

A cold front will push across the lake this evening. In its wake,
expect winds to increase up around 25 KT out of the northwest
overnight into Wednesday. The winds will gradually ease into
Thursday and back more west-southwesterly for a period late
Thursday. However, another cold front is expected to drop down the
lake Thursday night, with a return to northerly winds up around 30
KT into Friday. Given the amount of cool air expected to shift
over the warm lake, waterspouts will also be possible in
association with lake effect showers Thursday night into at least
Friday morning. A surface high is then forecast to shift over the
lake on Saturday.

KJB

&&

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

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