Area Forecast Discussion
Issued by NWS Chicago, IL

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959
FXUS63 KLOT 172326
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
626 PM CDT Thu Aug 17 2017

.SHORT TERM...
224 PM CDT

Tonight and Friday...

Gusty west-southwest winds and isolated showers across far
northern IL are the main forecast concerns through the remainder
of the afternoon and very early evening hours. Breezy and drier
conditions expected overnight and much of Friday.

Stacked surface and upper level low pressure noted across
northern WI this afternoon. Tight pressure gradient associated
with the surface low was producing gusty west-southwest winds near
30 mph, and these winds will continue through the afternoon
before gradually diminishing this evening. Regional radar mosaic
also depicts scattered showers wrapping across northeast IA/far
northern IL and much of WI, though these are expected to mainly
affect areas of far northern IL along the WI border area through
the remainder of the afternoon. Loss of weak diurnal instability
with sunset, and gradual drying of the low levels behind the cold
front, should allow these to dissipate by early this evening.

The upper low and occluded surface system lift northeast away
from the area later tonight and Friday, with modest height rises
and subsidence developing across the forecast area during the day.
Weak surface high pressure ridge passes downstate, with enough
gradient lingering farther north for breezy conditions across the
area especially during the morning and early afternoon hours
before the gradient relaxes further. Subsidence and slightly drier
low level flow should allow for more sunshine than today, though
mid-level cloud cover will likely increase from the west late in
the day as the next upstream short-wave approaches. Overnight
temps tonight should dip to the low-mid 60`s across the region,
with highs on Friday generally between near 80 far north and the
low-mid 80`s central and southern parts of the cwa.

Ratzer

&&

.LONG TERM...
224 PM CDT

Friday night through Thursday...

Showers and storms are possible Friday night as an upper level
trough axis rotates through the region.  Showers may linger east of
I-55 Monday morning, but dry conditions are then expected for the
rest of the weekend.  Cooler air moves overhead with the trough
leading to highs in the low 80s away from the lake Saturday.

High pressure spreads over the region Saturday afternoon bringing a
warming trend with it.  Highs will be in the upper 80s to near 90 by
Monday.  I have low confidence in precip chances Monday.  Guidance
indicates a short wave trough will approach the region and my kick
off showers and storms. The ECMWF has the trough much further north
than the GFS, and storms should be closely tied to the location of
the shortwave. Kept a slight chance to chance of showers and storms
mainly due to the GFS` solution.

For those hoping to view the eclipse, it looks like cloud cover will
be increasing through the day. I kept partly clouding wording
going.

A cold front pushes down the lake and through the region Tuesday
evening/night, and we could see additional showers and storms ahead
of the front. The 12Z ECMWF is much more aggressive with vorticity
and precip Tuesday than the 00Z run was. The 12Z ECMWF and GFS also
have much more consistent front timing. While the 12Z ECMWF gives me
higher confidence, I still kept precip values in the chance
category. Quieter weather returns Wednesday. High temps are forecast
to be around 80 Wednesday and then in the upper 70s Thursday.

JEE

&&

.AVIATION...
For the 00Z TAFs...

626 pm...Gusty westerly winds will slowly diminish this evening
and into the 8-12kt range overnight and then increase again Friday
morning with gusts to 20kts...with directions turning more to the
west/northwest.

A few showers near the IL/WI state line will continue this
evening. With just isolated coverage and short duration...no
mention with this forecast. Overnight through Friday afternoon is
expected to be dry. A weak wave will move across the area Friday
night and may bring showers and few thunderstorms. Confidence is
low for timing and location so no mention with the 30 hour ord taf
but some precip mention with later forecasts may be need Friday
evening into early Saturday morning. cms

&&

.MARINE...
224 PM CDT

The Small Craft Advisory will continue through tonight.

Low pressure over northern Wisconsin will continue northeast through
the weekend. It`s cold front will pass across the lake this evening
causing winds to become west to 30 kt. Winds diminish Friday as the
low continues northeast across Canada.  A weak surface low will pass
south of the lake Friday night. Wind directions will vary across the
lake in response to the low.  High pressure will spread over the
lake Saturday evening and continue east to the Atlantic Coast by
Monday. Winds become south behind the high. A cold front is forecast
to shift down the lake Tuesday afternoon/evening.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...nearshore waters until 3 AM Friday.

&&

$$

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