Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 271838
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
138 PM CDT Tue Sep 27 2016
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 a bit lower today than yesterday and forecast soundings
due show a drop off dewpoints with height such that dewpoints in
the low 20s to near 30 are expected through at least mid
afternoon. Red flag criteria includes 10 hour fuel moisture values
of 8 percent which is the one hold out that may not be met.
Yesterday Midewin hit 9 percent. So we are walking a fine line
here, but given very similar forecast conditions today to
yesterday we may consider a red flag after partner collaboration
or handle with an sps. Decision coming soon.
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
257 AM CDT
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.
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.
With the abundant sunshine and temps warming into the upper 60s
to near 70 coupled with good mixing, afternoon minimum RH values
have reached 20-25% mainly south of interstate 88. Red flag
conditions have been met other than official 10 minute fuel
moisture values are not quite there (based on the Midewin ob
only), but due to brush fires in central IL yesterday, fuels at
least in IL are more susceptible and conducive to burn.
Conversations with partners in IN suggest fuels are still more
moist from recent rainfall, and this is consistent with higher
fuel moistures in IN observations and forecasts. Therefore have
limited the Red Flag Warning to IL south of I-88 where if fires
were to start, rapid growth and spread would be possible.
For the 18Z TAFs...
Main adjustments for the 18z TAFs were to slow down the arrival of
precipitation a little bit this evening, and then introduce some
low end MVFR clouds tonight and even some IFR late tonight into
Northeast IL and northwest IN are just south of the main cloud
shield associated with the upper low. Deep mixing will therefore
continue through the afternoon as the cloud shield will be slow to
arrive until late this afternoon. It will gradually bleed in from
the north so expect some decrease in the west winds.
Expect there to be a period of rain showers/drizzle overnight
tonight. Upstream observations in Wisconsin are solidly into the
MVFR category, so confidence is high on low MVFR cigs, and even
medium high on occasional MVFR visby, possibly brief IFR visby.
Several guidance sources suggest IFR cigs in the morning, and
while we have not seen it yet upstream, it is certainly plausible
as we start the day. The main batch of rain moves through
overnight into early Weds, but with the deep upper low spinning
across the area additional showers are expected in the afternoon,
with lake enhancement also. Cigs gradually lift to low VFR but
could be MVFR at times in shower activity.
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
IL...Red Flag Warning...ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-
ILZ022-ILZ023-ILZ032-ILZ033-ILZ039 UNTIL 7 PM Tuesday.
LM...Small Craft Advisory...NEARSHORE WINTHROP HARBOR TO GARY IN UNTIL 7 PM
Small Craft Advisory...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL 10 PM Wednesday.
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