Area Forecast Discussion
Issued by NWS Chicago, IL

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102
FXUS63 KLOT 242326
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
626 PM CDT Sun Sep 24 2017

.SHORT TERM...

Through Monday...

206 pm...Temps have reached the lower 90s at many locations this
afternoon with a lake breeze slowly moving inland in northeast IL.
Similar to Saturday...coolest temps behind the lake breeze likely
to be confined to right along the lakeshore. This lake breeze
will likely remain near the Indiana lakeshore and not move too far
inland there. Dewpoints have dropped into the upper 50s in a few
locations and these slightly lower dewpoints will likely allow low
temps to drop into the lower 60s away from the city Monday
morning and a few upper 50s are also possible in the usual cool
spots. And while these lows will be a few degrees cooler than this
morning...there isn`t as strong of a fog signal overnight. There
may still be some patchy/shallow fog in the usual locations but
confidence is too low to include in the forecast for now.

Monday will be similar to today with temps warming quickly during
the morning and then leveling off in the upper 80s though its
possible some areas...including the city starting warmer in the
morning...will tag 90. A lake breeze is also expected...keeping
the lake shore areas cooler. There should be more of a cu field
by early afternoon and some of the high res models are showing
isolated showers developing by late afternoon across northwest IL
and the western cwa. Confidence is low regarding coverage and if
activity does develop...it may remain west of the cwa thus
maintained a dry forecast. cms

&&

.LONG TERM...
206 PM CDT

Monday night through Sunday...

Late season heat lingers through Tuesday, before a pattern change
brings a transition to cooler weather by mid-week. Global models
are trending toward the development of a deep upper trough across
the western Great Lakes by the end of the week, which may result
in well below average temperatures for the region by next weekend.
Rain chances continue to appear fairly minimal through the
period.

Fairly strong mid-level low pressure will emerge from the western
CONUS long-wave trough early in the period, lifting northeast
across the Northern Plains and moving into Ontario through
Wednesday. Persistent southerly flow will maintain very warm
conditions across the forecast area through Tuesday, in advance of
a slow-moving cold front which is progged to push across the area
Tuesday afternoon through Tuesday night. Low-level thermal ridge
progged with 925 mb temps in the +21 to +24C range across the cwa
would support afternoon high temps in the mid-upper 80`s Tuesday
afternoon, with coolest areas northwest where cloud cover will
likely be greater. Best potential for showers/thunderstorms would
appear to be across the west/northwest parts of the forecast area
Tuesday afternoon, closer to the cold front during timing of more
favorable diurnal instability. Model qpf diminishes markedly
along the front further east Tuesday night, suggesting much of the
forecast area will not see significant rain before the front
exits the region.

Much cooler air then spreads into the area Wednesday behind the
front, as winds turn northwesterly, and cool/dry advection takes
hold. Thermal gradient behind the front supports a range in temps
Wednesday from around 70 north to the mid-upper 70s far southeast,
and lows in the mid-upper 40s away from urban Chicago Wednesday
night. Similar conditions persist Thursday, though perhaps a
degree or two cooler as dry high pressure spreads across the
region.

Guidance is in good agreement in digging another mid-level short
wave southeast into the trough across the area Thursday night,
with a cold front moving across the area early Friday morning. A
few showers are possible with the cold frontal passage early
Friday, though again the potential for significant rainfall
appears quite low. The 12Z ECMWF and Canadian depict a much
deeper trough/closed low developing across the western/central
Lakes with this feature, while the operational GFS maintains a
more progressive open trough. A few GEFS ensemble members do
indicate more amplified solutions more along the lines of the
EC/GEM however, lending weight to the potential for a short-lived
stronger push of colder air Friday-Saturday. H8 temps around +4C
in both EC/GEM would likely support lake induced convective
showers over Lake Michigan and into northwest Indiana late Friday
into early Saturday. Progressive trough moves quickly east of the
area late Saturday however, which should allow some moderation in
temps Sunday after cool high pressure moves through Saturday
night. For now, have coolest temps Fri-Sat with highs in the 60`s
and lows in the lower 40`s. Could potentially be chillier, in the
30s in some spots early Sunday morning depending on location of
surface ridge and good radiative cooling setup.

Ratzer

&&

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

Other than some possible MVFR patchy ground fog during the
predawn hours Monday in the outlying areas (possibly affecting
DPA), look for VFR conditions through the period. Lake breeze will
probably result in wind shift at ORD/MDW/GYY again Monday
afternoon, though given the background synoptic gradient flow, it
looks possible that the lake breeze passage may be a bit slower
than it was today, possibly as late as 23z at ORD/MDW.

- Izzi

&&

.MARINE...
206 PM CDT

Period of relatively quiet marine weather continues to start the
week, with modest south winds persisting through Tuesday. Low
pressure moves through the northern Lakes Tuesday afternoon
however, and trails a cold front which pushes across Lake Michigan
Tuesday evening. Northwest winds behind the front will increase
to 20-25 kts for a time Wednesday into Wednesday night, eventually
becoming north and gradually diminishing early Thursday. The long
north-northwest fetch and push of much cooler air across the warm
lake waters will build waves especially on the southern part of
the lake, which will likely eventually require small craft
advisory headlines.

Winds quickly turn back to the south and increase by late Thursday
as another low and cold front approach from the northwest.
Stronger northerly winds approaching 30 kts are expected by
Friday, gradually easing into Saturday before high pressure builds
in Saturday night.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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