Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS63 KLOT 310721

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
221 AM CDT WED AUG 31 2016

216 AM CDT

Through Thursday...

For those of you eager for an end to the muggy and wet period, you
are in luck. High pressure continues to build southeast across
the Upper Midwest and into the Great Lakes over the next few days.
The leading cold front is moving through northeast Illinois early
this morning. Dewpoints are creeping down behind the front.

After daybreak the cold front will be south of the area across Central
Illinois, but dewpoints will be slower to fall. There are some
lower clouds across northwest Michigan that RAP guidance suggests
will bleed south down Lake Michigan this morning and impact
northeast IL and northwest IN for a time, just not clear how much
coverage will be. Model guidance is in pretty good agreement in
breaking out some scattered showers/isolated storms along and
ahead of a slightly more pronounced dewpoint gradient later this
morning closer to and southeast of Chicago, and then continuing
southeast across east central IL and northwest Indiana into the
early afternoon. HRRR/RAP forecast soundings, capped overnight
with the nighttime frontal passage depict a narrow window of
uncapped though very weakly unstable conditions near Chicago/Gary
and slightly more-so farther south, such that combined height
falls and density gradient will support this activity. Dewpoints
fall more rapidly from mid afternoon on and soundings dry out
quickly. It will be a breezy day, especially across the eastern
half of the area.

A secondary surge of cooler air comes in as a shortwave trough on
the backside of the eastward moving Canadian upper low digs over
Lake Michigan. Significant thermal differences between this air
mass and the warm lake will lead at least broken lake induced
clouds. Lake induced equilibrium levels are higher than depicted
yesterday, about 8-11k feet depending on your model of choice, but
the moisture still appears shallow enough preclude much more than
clouds or some sprinkles in spite of some hints of some weak qpf
(actually over 0.1 of QPF on the GEM, higher on the HiResARW/NMM)
and the GFS/NAM depicting lake induced CAPE of about 200-300 j/kg.
Therefore have introduced a non-measurable mention of
precipitation at this point as the thermodynamic/undirectional
wind profiles are marginally supportive. Nonetheless, it will
feel autumn- like Thursday morning.



216 AM CDT

Thursday night through Tuesday...

High pressure builds over eastern Canada and southward over Lake
Michigan for dry and sunny conditions with highs in the 70s. The
high retreats east Saturday as a tropical/subtropical system
slides northeast along the Atlantic coast. Southeast flow will
allow some temperature recoveries as the large upper ridge
holds overhead.

At this point model guidance suggests the upper heights will
remain high enough due to high amplitude blocking across the
northern Atlantic, which implies that most of the holiday weekend
will remain dry. The amplified southwest flow pattern across the
region will bring the return of warmth and humidity Sunday into
the early half of next week. Highs look to return to the mid 80s
with dewpoints back in the upper 60s to possibly low 70s.
Precipitation chances will eventually increase as a cold front



For the 06Z TAFs...

A cold front dropping across the region overnight will usher in a
cooler and more stable airmass that will be in place through the
end of the week. Light northwest winds are expected to be in place
overnight with primary concerns being fog or low stratus
developing as mid and high clouds gradually exit the area. While
some models are indicating MVFR vsby developing in the predawn
hours, a light north breeze may inhibit fog development though
cannot rule out some low stratus as cooler air behind the front
initially encounters the higher dewpoint air still in place. No
indication of widespread low stratus upstream yet and confidence
in either outcome is too low to include in the TAFs for the time
being. Winds will increase and shift to the NNE/NE Wednesday
morning with fairly steady-state conditions expected through the
rest of the day. Showers and thunderstorms are expected to develop
along the front late morning into early afternoon, but this will
be south of the terminals. Winds are expected to diminish around



300 PM CDT

A cold front will move down the lake tonight, reaching the
southern tip by early Wednesday morning. Northerly winds behind
this feature will initially gust to 15 to 25 kt, but a
reinforcement in the northerly push should occur Wednesday
afternoon through Thursday. Some gusts to 30 kt could occur in
that time frame. Waves will quickly build behind the frontal
passage. Small craft advisory winds and waves are expected along
the Indiana and a portion of the nearshore areas beginning
Wednesday afternoon. Further north of Wilmette Harbor, small craft
advisory conditions are likely to start there Wednesday night.
Choppy conditions will persist through Thursday followed by high
pressure for the weekend.



IL...Beach Hazards Statement...ILZ006-ILZ014...1 PM Wednesday TO 10
     PM Thursday.

IN...Beach Hazards Statement...INZ001-INZ002...1 PM Wednesday TO 10
     PM Thursday.

LM...Small Craft Advisory...LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...3 PM
     Wednesday TO 10 PM Thursday.




WWW.YOUTUBE.COM/NWSCHICAGO is the U.S. government's official web portal to all federal, state and local government web resources and services.