Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210751
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
251 AM CDT Sat Oct 21 2017

.SHORT TERM...

Through Sunday...

251 am...Forecast concerns include precip timing/chances tonight
and Sunday as well as high temps today and Sunday.

Fairly thick mid/high clouds early this morning which are helping
to keep temps in the upper 50s/lower 60s. Mid deck also producing
some sprinkles across northwest IL and will add mention for early
this morning. This cloud cover appears to thin just a bit by this
afternoon and if that trend materializes...highs likely to reach
near the warmest guidance values in the upper 70s. As the gradient
tightens...southerly winds will become breezy today with gusts to
30 mph.

A line of thunderstorms is expected to develop well west of the
area later today and will weaken as it moves into northwest IL
later this evening. This activity will move across the western
cwa overnight in a weakening phase. Thunder will be possible but
still only slight chance mention. Its also possible that
additional showers will develop over the area during the early
morning hours.

A cold front will move across the area on Sunday and the best
chance for showers will likely be along/ahead of this front...
which will be dependent on frontal timing. Some of the hi-res
guidance suggests a lull in precip Sunday morning with activity
redeveloping across the eastern cwa Sunday afternoon. At this
point...not confident enough to make too many changes to pops but
some changes may be needed if current trends continue. And if
precip coverage is less Sunday morning...then temps are likely to
be warmer and could be in the lower 70s over the eastern cwa.
However...these highs would be midday/early afternoon with temps
likely falling by mid/late afternoon as the front moves across
the area. cms

&&

.LONG TERM...

Sunday night through Friday...

251 am...Main forecast concerns are two cold fronts and
associated cold bursts/precip.

There remains uncertainty regarding how fast precip will exit the
southeast cwa Sunday night or Monday morning. The Canadian remains
the most aggressive with precip across the region as an upper low
moves north across the Ohio Valley Monday...while the GFS moves
precip east of the area by Monday morning. Confidence is too low
to make any changes during this time period with some low pop
mention necessary until a consensus emerges. How this evolves then
affects the next system...another upper low diving across the
midwest on Monday and settles over the Great Lakes region Tuesday.
Much colder air will spread across the region with a chance of
showers as well as lake effect showers downwind of Lake Michigan.

As this upper low departs midweek...a short-lived ridge builds
across the region Wednesday night into Thursday which may make for
a milder day on Thursday before another cold front moves across
the area in the Thursday night time frame. cms

&&

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

South winds will again prevail throughout the TAF period ahead of
an approaching cold front expected to sweep across the area on
Sunday. Winds will become increasingly gusty mid to late Saturday
morning eventually topping out around 25kt throughout the
afternoon. VFR conditions are expected throughout the current
valid period, and expect the region to remain dry through at least
midnight local time tonight. After midnight Saturday night,
prefrontal showers and possibly a thunderstorm or two are
possible, thus have introduced a VCSH at ORD. The best chances for
precip on station will be beyond the current 30-hour TAF at ORD
during the day Sunday.

BMD

&&

.MARINE...
350 PM CDT

Southerly flow will continue tonight through Sunday morning as
high pressure is situated over the eastern half the U.S., and
while a deepening low moves through the northern U.S. and southern
Canada. During this time, speeds will likely steadily increase
with 15 to 25 kt increasing to 30 kt occurring first tonight over
the open waters. These higher speeds will bring an extended period
of hazardous conditions for small craft, likely lasting into
Sunday. The 30 kt winds will be observed over both the north and
south half on Saturday, and am monitoring the potential for a
period of gales soon there after. Have lower confidence of this
possibility given stability issues over the lake, but with the
possibility of these higher speeds and the potential for at least
sporadic gales, have included occasional gales in the open water
forecast. This is from late Saturday through Saturday night. Will
continue to monitor the possibility for more prevailing gales
though. A diminishing trend will be the trend on Sunday, while
winds turn more westerly.

Rodriguez

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743...9 AM
     Saturday to 4 PM Sunday.

&&

$$

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