169
FXUS63 KLOT 271605
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1105 AM CDT Tue Sep 27 2016

.UPDATE...
1050 AM CDT

Main issue for the daytime period is the fire weather concern.
Yesterday we had a few isolated locations that tapped relative
humidity values in the 20-25 precent range with 20 ft wind speeds
around the 20 mph threshold. Most locations had rh values that bottomed
out 25-35 percent. Dewpoints and relative humidity values are
tracking pretty similarly today. Red flag criteria includes 10
hour fuel moisture values of 8 percent. Yesterday Midewin hit 9
percent. So we are walking a fine line here, but given very
similar forecast conditions today to yesterday with most locations
expected to hover in the 25-30 percent range, will likely handle
the situation with an SPS for areas along and south of I-88.

Othwerwise lower clouds will arrive through the afternoon and
evening from north to south with models in good agreement
regarding a gradual uptick in shower activity this evening and
overnight.

KMD

&&

.SHORT TERM...
257 AM CDT

Through tonight...

Skies remain near cloudless across Northern IL/Northwest IN early
this morning, with a continued southwest wind preventing temps from
radiating too quickly and hovering in the lower 50s. Expect temps to
fall into the mid/upr 40s away from Lake Michigan prior to daybreak
and remain in the low/mid 50s closer to the lake. Main concern today
will again be on the deep mixing progged to provide strong
subsidence and push dewpoints down to the mid/upr 30s along and
south of I-80. With increasing cloud cover this afternoon from the
north, dewpoints may only be able to fall to around 40 further
north. Fuel moistures remain dry, and as dewpoints fall and surface
temps warm into the upr 60s/arnd 70 in the southern half of the
forecast area, afternoon minimum RH values will approach red flag
conditions. At this time will hold off on any headlines.

By late afternoon guidance continues to prog clouds sliding south
across Southern Wisc, and quickly shielding much of Northern IL by
00z with precip arriving shortly behind the clouds and spreading
south after 00z. Cold air advection will persist through tonight
with winds turning northwest, and expect temps to steadily radiate
into the upr 40s.

Beachler

&&

.LONG TERM...
257 AM CDT

Wednesday through Monday...

Wed through Thur: Main feature in the medium range will continue to
be on the shortwave rotating over the Western Great Lakes, and
progged to drift south/southeast by Thur over the Ohio Valley. This
is expected to produce considerable solar shielding Wed and likely
limit the surface from warming. Some of the guidance has been
indicating highs may struggle to push beyond the upr 50s. Have
nudged temps cooler to around 60, but could see this needing to be
further reduced given the limiting aspect of heating and continued
chances for periodic rain showers. With the Lake Michigan waters
still very mild, coupled with much cooler air flowing over the lake
and some convergence, it is likely some lake induced instability
could create some thunder Wed for areas near Lake Michigan.

Wed ngt broad surface ridging across Western Ontario and the Upper
Midwest will begin to eat away at the back edge of the stubborn
shortwave over the Great Lakes, and should help to erode some of the
thick cloud cover. The gradient does tighten late Wed ngt/early
Thur, which could help to expedite the erosion of clouds; however,
this may only be short-lived as the cutoff low will be retrograding
from the Ohio Valley to Lower Michigan later in the day Thur. This
will bring precip chances back west as well, with the far western
forecast area possibly being influenced by weak diffluence and
keeping the precip shield focused in the eastern two-thirds of the
forecast area Thur. Temps will start to moderate Thur with less
cloud cover, although confidence on temps rebounding to the upr 60s
to near 70 is not high at this point. So could see this too needing
to be trimmed back a degree or two.

Fri through Tue: Heading into the weekend, ensembles continue to
maintain some weak blocking across Ontario/Quebec limiting the
departure of the cutoff shortwave from the Great Lakes. Through at
least Sat and possibly longer into Sun, guidance is indicating this
feature will linger before weakening and becoming absorbed in the
longwave pattern as troughing begins to develop across the Southwest
CONUS Sun/Mon. As the western trough develops, this will lead to a
downstream weak ridge across the Great Lakes as the cutoff low
finally departs the region early next week. Precip chances will
begin to decrease late weekend into early next week, with temps
steadily returning to near seasonal conditions in the upr 60s to
lower 70s, and possibly the mid 70s.

Beachler

&&

.FIRE WEATHER...257 AM CDT

Another well mixed day is expected today, with dewpoints beginning
the day in the upper 30s to lower 40s. By late morning winds will
be gusting from the west between 20-25 mph and possibly
approaching 30 mph by midday through the afternoon pushing
dewpoints further into the lower to middle 30s. With the abundant
sunshine and temps warming into the upper 60s to near 70,
afternoon minimum RH values will approach 25-30% mainly south of
interstate 80. So near red flag conditions are possible again
today. At this time will hold off on any headline issuance.

Beachler

&&

.AVIATION...
For the 12Z TAFs...

642 am...Forecast concerns remain wind speeds/gusts today and
showers/mvfr cigs tonight into Wednesday morning.

West/southwest winds will steadily increase this morning with
gusts into the mid to perhaps upper 20kt range from mid morning
through at least early afternoon. Cloud cover approaching from
the north now looks like it will arrive sooner than previously
expected...and when it does speeds/gusts will diminish with
speeds dropping to 10kts or less after sunset. Wind directions
will turn more northwest tonight and then more to the north/
northwest Wednesday morning...after sunrise.

Low pressure just north of Lake Superior will move south across
Lake Michigan tonight. As it does cloud cover will spread south
across the terminals...first as vfr cigs this afternoon but
eventually these cigs will lower to mvfr with showers developing
this evening. Its possible cigs may dip into ifr especially toward
sunrise Wednesday morning but confidence too low to include ifr
cigs with this forecast. cms

&&

.MARINE...

219 am...Low pressure north of Lake Superior this morning will
move south across Lake Michigan tonight and then to the mid
Atlantic region Wednesday evening. Strong westerly winds will
slowly diminish from north to south as this low approaches. There
could still be some gale gusts early this morning...but appears
prevailing gales will be ending so will be letting the gale
warning for the north half of Lake Michigan expire.

Winds will diminish tonight as they shift to the northwest but as
the low moves south...winds will shift northeasterly overnight
into Wednesday morning. High pressure building across the upper
midwest and Ontario will tighten the gradient with northeast winds
increasing to 30 kts Wednesday and possibly continuing into
Thursday. The low pressure is expected to move back west toward
the southern Great Lakes region Friday into Saturday and if this
does occur...there will be a prolonged period of northeast flow
into this weekend.

As this low moves across the lake combined with showers and cold
air aloft...there will be a chance of waterspouts later today and
continuing through Wednesday. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Small Craft Advisory...NEARSHORE WINTHROP HARBOR TO GARY IN UNTIL 7 PM
     Tuesday.

     Small Craft Advisory...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL 10 PM Wednesday.

&&

$$

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