Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 181739

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1239 PM CDT Fri Aug 18 2017

1056 AM CDT

Minor updates this morning for lingering cloud cover across
northeast IL and northwest IN, and associated slower temp rise.

Surface low pressure was just north of Lake Huron late this
morning, with a region of slightly cyclonic west-northwest low
level flow extending across the western Great Lakes region. An
extensive stratocu field was evident in GOES visible imagery over
much of eastern WI and northeast IL/northwest IN, while generally
mostly sunny conditions were in place west of the Chicago area.
Modest height rises reflect large scale subsidence, evident at the
surface in weak high pressure ridge moving across the Mississippi
River with 1-2 mb rises. Subsidence, combined with diurnal warming
of the boundary layer, should continue to raise cloud bases and
scatter out cloud cover across the northeast cwa into early
afternoon. Have held forecast maxes about the same, though slowed
the warmup over mostly cloudy locations in hourly temp forecast.

Increasing clouds and developing showers expected this evening, as
a pair of mid-level short waves approach the area. Initial warm
advection precip with the initial wave has had a difficult time
holding together across IA per regional radar mosaic. While a few
showers from mid-level deck could spread in to the western parts
of the FA around 00Z, high-res guidance indicates decent agreement
in the focus for heavier showers/thunderstorms developing out of
southeast IA and spreading across areas especially south of I-80
later in the evening.



248 AM CDT

Through Saturday night...

As an upper low and associated sfc system lift out to the northeast,
short wave ridging aloft and a weak ridge of high pressure at the
surface will build across the region.  This will set up a mostly
cloudy with seasonable conditions with highs in the low 80s and
dewpoints in the low 60s.  West winds should be strong enough to
keep any lake cooling at bay, so the low 80s temperatures should
reach up to the lakefront.  The quiet weather will be relatively
brief as a pair of northern stream short waves dropping out of the
northern plains will cross the region late this afternoon and
tonight.  The lead wave should take a more northerly track, across
Wisconsin late this afternoon and this evening.  The strongest
forcing with this wave should remain well to the north, and the main
impact from this wave should be some increasing cloud cover this
afternoon for the northern portions of the CWA.  The trailing wave
is expected to take a more southerly track, moving across northern
IL late tonight and pushing into the eastern Great Lakes by tomorrow
afternoon.  This system should bring some showers and thunderstorms
to the area, possibly reaching the Rockford area during the early
evening hours and the Chicago Metro area overnight.  Some lingering
showers should end across nwrn Indiana in the early morning hours.
Surface high pressure will build across the region again on
Saturday, bringing mostly sunny skies and light winds.  Temps should
once again, reach the lower 80s, but the weak pressure gradient and
wind field should allow a lake breeze to develop by early afternoon,
turning winds onshore.  Lakefront locations, including downtown
Chicago should see highs only in the mid 70s.  Quiet conditions
should continue Saturday night with mostly clear skies, light winds
and lows in the upper 50s to lower 60s for much of the area.  The
urbanized areas of the Chicago Metro area should see temps only drop
into the middle to upper 60s.


248 AM CDT

Sunday through Thursday...

For Sunday, the sfc high will gradually build to the east, centered
over the Ohio Valley Sunday and then moving off the mid Atlantic
coast on Monday.  A system moving through central Canada will drag a
cold front across the upper Mississippi Valley on Sunday with weak
toughing setting up through the central plains.  this will lead to
winds turning more southerly and a return flow of low and mid level
moisture.  Even with the center of the high moving off the mid
Atlantic coast, general ridging will persist over the sern CONUS.
This will limit the srn progression of the cold front and low
pressure developing over the central plains, at the western extent
of the front will further limit the progression of the front, with
much of the guidance suggesting that the front should remain north
of the CWA.  Sunday is shaping up to be warm and mostly sunny with
highs in the middle 80s.  Into Monday, further warm advection is
expected with a persistent fetch of warm, moist air from the wrn
GLFMEX.  Aloft, zonal flow will set up with an upper low over Hudson
Bay and ridging over the sern CONUS.  This pattern will introduce
increasing uncertainty to the pcpn and cloud cover forecast for
early next week.  Temperatures will likely continue the warming
trend, with highs on Monday in the middle to upper 80s, with the
increased moisture and heating bringing increased instability
through the day.  Under zonal flow aloft, timing of individual waves
will be increasingly uncertain, so timing of possible showers or
thunderstorms will be a low confidence forecast.  However, the
general trend looks to be for increasing pcpn chances through the
afternoon and into Monday night.  The longer range models are
indicating that the pattern should become more progressive by
Tuesday, with broad longwave upper troughing moving into the region
forcing the frontal system across the region, bringing a more
organized area of showers and thunderstorms to the area on Tuesday
and Tuesday night.  For the remainder of the period, conditions
should trend a bit cooler, as persistent nwly flow aloft sets up
across the region, temperatures should only be in the mid to upper
70s...and even cooler near the lake as northerly surface winds


For the 18Z TAFs...

Primary aviation forecast concerns are with timing and coverage of
showers/thunderstorms tonight.

Surface low pressure continues to pull away to the northeast of
the Great Lakes region early this afternoon, while weak surface
high pressure develops eastward across the lower/middle
Mississippi Valley. Morning MVFR stratocu continues to lift and
thin, with VFR conditions expected the remainder of the day. Winds
also continue to diminish with the approach of the surface ridge
and the weakening of the pressure gradient.

Attention turns to the west of the area this afternoon, on weak
low pressure over southwest Iowa. This low will track east into
central IL tonight, in association with a pair of mid-level
disturbances moving through the flow aloft. Warm advection ascent
across IA has been too dry to maintain showers for the most part,
though high-res guidance is in very good agreement in developing
convection across southeast IA late this afternoon as well as in
spreading it east into parts of central/northern IL this evening
and overnight. The trend in most of this guidance has been to
focus the best potential for stronger showers, and thunderstorms,
south of the terminals tonight. Weaker scattered showers are
possible however, in association with the mid-level trough.

Rain and lower clouds should clear the terminals by Saturday
morning. Broad high pressure spreads into the area from the west
during the day, with the weak gradient providing light, generally
north-northwesterly surface winds. While the weak gradient lowers
confidence in specific direction, speeds should be less than 10
kts. Weak lake breeze will also likely push into ORD/MDW by late



151 AM CDT

It will be a gusty Friday over the lake with an anomalously deep
low for summer over far eastern Lake Superior early this morning
slowly moving northeast. The strongest west-northwest winds to 30
kt will be primarily over both the open waters and the eastern
shores of the lake. For the Illinois and Indiana nearshore areas,
breezy west-northwest winds will prevail through midday before
gradually easing. Near Small Craft Advisory conditions will be
seen this morning along the Illinois shore while likely reaching
it along much of the Indiana shore. Waves will be around 4 ft
along the Porter County Indiana shore, with lower wave heights
west along the Illinois shore as wind direction will be offshore.

A weak high pressure ridge will move over the lake this weekend.
For Saturday in the Illinois and Indiana nearshores, winds will
turn onshore by late morning or so, mainly through a lake breeze.
For Sunday, a lake breeze cannot be ruled out, though direction
is likely to remain south around 10 kt.

After south-southwest winds early next week, a turn to north is
presently forecast during midweek. It is possible when this
occurs, that there is a period of Small Craft Advisory along the
southern shores.






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