Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 160114

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
714 PM CST Wed Nov 15 2017

714 PM CST

Evening Update...

Made some minor tweaks to hourly temp trends behind cold front
this evening, and bumped winds/gusts a bit as well. Also increased
cloud cover a bit especially across the northern 2/3 of the cwa
through midnight.

Surface low pressure was analyzed over Michigan`s eastern Upper
Peninsula early this evening, with a trailing cold front now east
of the cwa across IN and the lower Mississippi Valley. A secondary
cold front was pushing across the forecast area as of 01Z/7 pm
CST, with gusty west-northwest winds to 30+ mph, temps dropping
into the mid 30`s and the return of an extensive low cloud deck.
Guidance has been struggling with the shallow stratus which
satellite shows extending back across eastern IA and southeastern
MN, and have bumped sky cover further based on satellite trends
into the overnight hours. Strengthening subsidence overnight, and
lowering of the subsidence inversion may eventually erode holes in
the cloud deck after midnight/toward morning, though forecast
soundings suggest some thin stratocu may linger especially across
the far north/northeast parts of the cwa overnight.

Surface high pressure ridge currently over the mid-upper Missouri
Valley is progged to spread east into the area through Thursday
morning, which will allow a gradual decrease in winds later
tonight. Persistent low level cold advection will allow 25-30
degree upstream air mass to reach the area by morning, so going
forecast mins appear ok. Did adjust hourly temps this evening to
show a sharper drop behind the secondary cold front. Otherwise,
aside from these tweaks to wind/sky and near term temp trend,
forecast appears to be in good shape.



134 PM CST

Through Thursday...

At 100 pm this afternoon, a cold front was in place from near UGN
south-southwest to near PNT. Light rain and drizzle will continue
out ahead of the front but will quickly taper behind the front with
a few peeks of sunshine developing across the western CWA this
afternoon. The front should clear the eastern counties of the CWA by
around 4pm this afternoon with dry conditions in place through the
rest of the day. RAP/NAM low level moisture profiles indicate
that low stratus may thicken again overnight and prefer the
cloudier model solutions based on current cloud cover over MN and
northern IA which has been gradually spreading southeast through
the day. Bumped up overnight lows a touch due to the expected
increased cloud cover. Gusty west to northwest winds will pick up
behind the front this afternoon and evening, peaking with gusts to
around 30 kt through late this evening. Cold advection through
the overnight hours will help to maintain steep low level lapse
rates as well as some gustiness through the overnight hours.

High pressure will build across the mid and upper Mississippi Valley
midday Thursday then slide east across northern Illinois and
northwest Indiana through the afternoon. Winds will veer to the
north and diminish as this occurs, and eventually switch to an east
to southeasterly direction behind the ridge axis during the
afternoon. Expect afternoon highs to top out around 40F.



318 PM CST

Thursday night through Wednesday...

Ensembles and their deterministic components are in reasonable
agreement regarding the handling of progressive mid-latitude flow
through late in the weekend.  Friday night into Saturday would
appear to have the most active weather in this timeframe, though
still in the form of rain, perhaps with a bit of thunder as well. A
50 kt low level jet ahead of an approaching frontal zone will bring
seasonably mild temperatures and considerable moisture...greater
than one-inch PWATs...into the region beginning late Friday. In
fact, temperatures Friday night are looking warmer than during the
daytime on Friday and Saturday. Forecast soundings are mostly moist
adiabatic Friday night and not indicating a lot of instability, but
with the pronounced moisture convergence and warm nose around 850mb
it would not be surprising to generate some convection, especially
in southern portions of the forecast area.  The frontal zone should
be clear of the area, or nearly so, early Saturday, but rain chances
may linger into the afternoon under the passage of the upper
shortwave energy trailing the front.

Apart from the early weekend activity the extended period looks
relatively quiet under the influence of weak upper ridging.  Looking
out toward the middle of next week and the busy Thanksgiving travel
days, there is nothing at this point suggesting significant issues
for the Midwest, at least in terms of a large midlatitude cyclone
making its way through the area.



For the 00Z TAFs...

The main concern is with lower CIGs through at least a portion of
Thursday. An expansive deck of primarily MVFR CIGs is rapidly
advancing southeastward, already at RFD, and should arrive at the
rest of the terminals between 00 and 01z. Gusty west-northwest
winds are accompanying the arrival of the lower clouds, with 30-35
kt observed at RFD. Gusts should approach 30 kt at the eastern
terminals for a few hours and could even be a bit over 30 kt at
times. Speeds and gusts will diminish later tonight.

Guidance is mixed on CIG trends into Thursday, however given the
expansive nature of the low CIGs evident on satellite, continue to
lean toward the more pessimistic guidance and maintained trends
from previous TAF. Wind off Lake Michigan during the afternoon
should keep the MVFR CIGs at GYY and it`s possible they could
spread back west. Confidence in CIGs is medium overall. Northwest
winds will flip over to light north-northeast (high confidence in
under 10 kt) by the early afternoon and then east to southeast by
Thursday evening as high pressure transits the region.



318 PM CST

A fairly active pattern will produce multiple periods of gales
through the weekend.  The passage of a cold front today is being
followed by a period of northwest gales through mid-morning
tomorrow.  Winds then relax and veer to southeasterly and southerly
during the Thursday evening and overnight hours. The approach of
another cold front will tighten the gradient and support another
period of southerly gales from Friday morning into Friday evening.
Following this front the winds relax and veer to northwesterly
overnight Friday into early Saturday, eventually increasing again to
likely gales beginning Saturday afternoon and persisting into early



LM...Gale Warning...LMZ777-LMZ779 until 6 AM Thursday.

     Small Craft Advisory...IN nearshore waters until 11 PM Thursday.

     Small Craft Advisory...IL nearshore waters until 4 PM Thursday.




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