Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
712
FXUS63 KLOT 220455
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1055 PM CST Tue Nov 21 2017

.UPDATE...
750 PM CST

Evening Update...

Clearing and colder conditions are expected tonight, in the wake
of a cold front which moved through the forecast area this
morning. Had made some earlier updates to hourly digital forecast
elements and text products, primarily to move precip out more
quickly in our southeast, and to adjust sky cover per satellite
trends. Otherwise, going forecast is on track and no major changes
are planned/needed this evening.

Early evening surface analysis depicts low pressure east of James
Bay, over western Quebec. A trailing cold front extended
southwest across Lake Erie and into the lower Ohio Valley.
Blustery northwest winds have been prevalent behind the front
including across the WFO LOT forecast area, thanks to a strong
isallobaric gradient (4-5mb per 3 hour pressure rises), and a
tight gradient ahead of surface high pressure building southeast
across the Plains. The strongest surface pressure rises were
moving east of the cwa however, which along with mainly clear
skies/stronger cooling should allow winds to continue to gradually
ease overnight. Modified arctic air mass spreading in from the
northwest, with dew points falling off to the lower teens across
the upper Midwest. Going lows in the low-mid 20`s (and a few upper
teens in our typical coldest spots) appear reasonable given
lingering northwest winds and observed upstream temps at this
hour.

Ratzer

&&

.SHORT TERM...
226 PM CST

Through Wednesday...

The cold front quickly pushing across the region extends from
northern Indiana through central Illinois at 230PM CST.  A band of
pcpn along the front is filling in and some pcpn will continue invof
the front and should exit the far sern portions of the CWA by late
afternoon or early evening.  Temperatures are dropping off in the
strong cold advection behind the front and expect that temperatures
should be in the upper 20s to lower 30s by shortly after sunset.
Skies should clear out from west to east through the remainder of
the afternoon and into the evening with much of nwrn IL already
seeing scattering clouds.  with clearing skies and diminishing winds
overnight, there should be a radiative cooling component to the
downward temperature trend along with the cold advection.
Temperatures tonight should drop into the lower to middle 20s across
area, with some isolated upper teens possible in sheltered low-lying
areas.  With high pressure building in from the west tomorrow, winds
should be relatively light through the day and even with ample
sunshine, temperatures tomorrow will only reach into the lower to
middle 30s.

&&

.LONG TERM...
252 PM CST

Wednesday night through Tuesday...

Some slight warming likely early in the period, as more defined
shortwave energy drops south across the region. Associated surface
trough does push across the region on Thursday but stronger
forcing will stay just to the north/northeast of the area, along
with any precip. Right behind this system, little to no CAA,
transitioning to more pronounced WAA on Friday ahead of an
approaching system. Warmer temps, likely above normal, return
Friday along with precip chances by late Friday into the start of
the weekend. Guidance continues to vary to the extent of any
precip but as a fairly strong shortwave swings through, some
scattered development will at least be possible.

Rodriguez

&&

.AVIATION...
For the 06Z TAFs...

No significant aviation forecast concerns through Wednesday night.

Strong surface high pressure continues to build southeast across
the Plains overnight in the wake of Tuesday`s cold frontal
passage. Cold, dry low level air spreading in on northwest winds
has allowed skies to clear across the region, except for some VFR
3500-4500 foot stratocu along the lake and downwind into northwest
Indiana and Michigan. This too will shift off to the east
Wednesday morning as low level winds back more westerly. Winds,
still above 10 kts at times late Tuesday evening, will continue to
diminish and back to the southwest by Wednesday afternoon. Speeds
will come up a bit again Wednesday night. Warm air advection will
spread lowering VFR cloud cover back across the terminals late in
the day and Wednesday night.

Ratzer

&&

.MARINE...
252 PM CST

The gales have ended across much of the lake this afternoon, and
have let the gale warning expire this afternoon. However, could
see a few gale force gusts continuing for a time this afternoon
and have included this mention in the forecast. Winds will remain
elevated through tonight, with winds to 30 kt expected. This will
provide continued hazardous conditions for small craft along both
the Illinois and Indiana nearshore waters tonight. However, some
diminishing trend tonight is expected along the Illinois side,
while waves remain elevated into Wednesday morning on the Indiana
side. As high pressure moves across the region Wednesday, this
diminishing trend will briefly continue for much of the lake.
Winds increase Wednesday night ahead of a trough of low pressure,
with a fairly active weather supporting another period of higher
winds Friday into the weekend when gales will once again be
possible.

Rodriguez

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...IN nearshore waters until 9 AM Wednesday.

&&

$$

VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE)

FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT:
WWW.FACEBOOK.COM/NWSCHICAGO
WWW.TWITTER.COM/NWSCHICAGO
WWW.YOUTUBE.COM/NWSCHICAGO



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.