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
FXUS63 KLOT 162013

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
213 PM CST Thu Nov 16 2017

131 PM CST

Through Friday...

Cloudy and dry tonight with rain spreading from west to east Friday

Stratus is rapidly clearing over eastern Iowa and central Wisconsin,
but I`m unsure how quickly stratus will clear over northern Illinois
and Indiana. The areas with the better chance of seeing sunshine are
west of a Waukegan to Peru, IL line. I decided to maintain a mostly
cloudy forecast with high temps this afternoon only in the upper
30s.  High pressure over Illinois will continue to shift east
tonight with winds becoming south by Friday morning behind a warm

I based low temperatures off of a cloudier forecast and raised lows
to around freezing away from the city.  If clouds clear out, actual
low temps may be cooler than currently forecast.

A surface low will form over Kansas Friday afternoon/evening.  The
pressure gradient will tighten over the midwest leading to south
winds gusting to 30 MPH.  Warm air advection will usher in warmer
air, but with thick cloud cover overhead, I`m unsure how much we
will warm. I made minor adjustments to the previous high temps which
has low to mid 40s across the forecast area.

Guidance is consistent with showers spreading across the forecast
area Friday afternoon. Coverage and intensity increase late Friday
afternoon into Friday night.



209 PM CST

Friday night through Thursday...

Primary challenges within the extended forecast are dealing with
the low tracking across the region Friday night into Saturday
and the cold air that overspreads the region in its wake.

A strong Pacific wave is progged to move onshore over northern
California early Friday and quickly push east across the mid
Mississippi Valley by around midday Saturday. At the surface, low
pressure will deepen in response and track from near Kansas City
Friday night to the local area Saturday morning. Southerly flow
and WAA in place out ahead of the low track Friday night will
result in widespread rainfall across the region with the potential
for a few embedded thunderstorms due to modest instability rooted
between 800-900mb. In addition, expect non-diurnal temperature
trends Friday night.

By midday Saturday, surface low will shift off to our east with
northerly flow spreading across the Upper Midwest and strong CAA
driving 850mb temps from around +10C to -2 to -4C locally. A
deformation axis and corridor of strong f-gen will then pivot
across the CWA late morning through the afternoon supporting
continued chances for precipitation. P-type becomes a concern at
this point as the column cools enough to support a transition over
to wet snow, particularly over western half of the CWA. There
looks to be a pretty narrow window of time that would support wet
snow or a rain snow mix before the better forcing shifts east of
the area, so do not anticipate much if anything in the way of
accumulating snowfall but cannot rule out some spots getting
dusting. High temperatures on Saturday will likely occur at
midnight (Friday night) or early in the morning with falling
temperatures during the day behind the low. Temps will eventually
bottom out in the 20s Saturday night into Sunday morning.

Second vort max is progged to dig across the western Great Lakes
early Sunday but by this point moisture is fairly meager over the
region and outside of an uptick in lake effect snow mainly over
MI/IN do not anticipate any precipitation with this wave. Surface
ridge axis builds overhead later in the day Sunday. Temperatures
will be cool, topping out in the low to mid 30s, but otherwise
quiet weather expected through the day.

Ridge axis shifts to our east Monday and Tuesday allowing
southerly flow to return as a clipper system takes aim on the
Great Lakes region. For now, models favor a track that keeps
precipitation well to our north, but do expect temperatures to
rebound back to near 50 Monday afternoon with non-diurnal trends
(falling daytime temps) possible again Tuesday as a trailing front
sweeps across the region. Dry but cool conditions are expected
Wednesday and Thursday as high pressure builds back across the
central U.S.



For the 18Z TAFs...

High pressure centered over northern IL will shift east through
this evening. Northwest winds will diminish and gradually veer to
the southeast by late this evening. MVFR stratus is holding on
strong and will likely continue through this evening. An area of
clearing over eastern Iowa is not growing, and there is a second
area of clearing over central Wisconsin. I have low confidence in
when MVFR cigs will become VFR. Cigs may not lift to VFR until
early Friday morning.

Overcast skies continue Friday morning ahead of the next low.
South winds will gust to 20-25 kt by mid morning. MVFR cigs and
rain showers spread from west to east in the afternoon. Some
models suggest IFR cigs are possible late Friday afternoon into
Friday evening, but I did not have enough confidence to go that
low yet.



209 PM CST

An active period of weather is expected over the upcoming week
with several periods of gales possible. In the near term, north
winds will continue to diminish today as ridge of high pressure
moves across the region. In its wake, southerly flow will
strengthen overnight and increase back to gale force across the
entire lake by mid morning Friday. Meanwhile, a deepening low over
the central Great Plains Friday will lift to far southern Lake
Michigan early Saturday. Expect strong north gales to overspread
the lake on Saturday and persist into Sunday morning when they
should finally diminish back below gale force. Another low will
move across the Canadian Prairies early next week and may result
in another period of gales Monday into Tuesday.



LM...Gale Warning...LMZ777-LMZ779...9 AM Friday to 9 PM Friday.

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

     Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...3 AM Friday to 3 AM Saturday.




WWW.YOUTUBE.COM/NWSCHICAGO is the U.S. government's official web portal to all federal, state and local government web resources and services.