Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 242344
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
544 PM CST Sat Feb 24 2018

.SHORT TERM...
213 PM CST

Through Sunday...

Precipitation this afternoon remains fairly light as we are in
between waves, and deep moisture is not present.  Warm advection
does pick up a little bit more later this afternoon, and there are
no concerns for frozen or freezing precipitation. Better chances
for organized rain will be tonight. The disturbance tonight is
pretty dynamic out ahead of a cold front. Precipitable water
values will get above an inch in some spots, thus expect a good
batch moderate to briefly heavy showers. It appears these will
come in waves, the first along and southeast of Chicago as a
moisture surge arrives, with the second organized round associated
with the upper trough and cold front that will shift from
western Illinois northeastward across northern Illinois late this
evening into the overnight hours.

High based elevated lapse rates do also warrant a low thunder
mention mainly during the mid to late evening hours. Instability
progs mainly graze our SE areas that were hardest hit with rain
earlier this week, but cannot rule out an isolated strike
elsewhere. At this time generally think QPF will top off around
0.25", but some spots could see up to 0.5" or so with any thunder
or heavier shower. These amounts generally do not correspond to
any river rises but could briefly slow the decline.

Precipitation will exit quickly overnight, with concern shifting
to at least a brief period of fairly gusty winds overnight into
early Sunday, with breezy/windy conditions lingering for a time.
The surface low associated with the front will strengthen overhead
and as it continues northeast tonight. Therefore good pressure
rises and modestly stronger lower level wind field will support sw
wind gusts to around 40 mph. Locally stronger gusts could briefly
occur in the pre dawn hours if the low clouds lift a bit quicker
resulting in slightly deeper mixing. Breezy conditions continue
on Sunday with clearing skies and highs back up into the 40s with
the late February sunshine.

KMD

&&

.LONG TERM...
232 PM CST

Sunday night through Saturday...

Dry conditions appearing likely at the start of the period
through early/mid week. Expect the air mass to moderate early in
the week, with above normal temps expected. Next highest chance of
precip looks to be with the arrival of a potentially stronger
system, around midweek. Still some variability among model
guidance with this system, but at this time, precip is appearing
to be mainly rain. Given the possible strength of the system and
returning moisture, can`t rule out thunder. Will need to monitor
trends on the back side of this system as guidance is indicating
the possibility for precip to linger with the arrival of colder
air. Can`t completely discount this, as this system closes itself
off over the area, while additional upstream energy pushes
overhead.

Rodriguez

&&

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

IFR cigs and MVFR vsby have arrived at all of the terminals.
Another wave of rain over central IL will continue to shift
northeast. I have high confidence in the rain reaching MDW and
GYY, but lower confidence in the rain reaching ORD and DPA.

More rain showers will shift west to east across the terminals
early this evening ahead of a cold front. All terminals will
receive rain, and vsby may drop to around 2SM with the heavier
showers. IFR cigs will continue through midnight, and may drop to
around 600 ft at times.

Southeast winds will gust to 25 kt this evening as the low moves
into the region. A couple of embedded thunderstorms are possible,
but I did not include thunder in the TAFs. I have low confidence
in TS coverage, exact location, and timing. Numerous
thunderstorms are not expected at this time.

Winds shift to the WSW behind the cold front after midnight. Gusts
will be between 30-35 kt right behind the front, and east to
around 30 kt early Sunday morning. Cigs slowly lift and scatter to
VFR as well. Wind gusts dissipate late Sunday afternoon as the
pressure gradient finally relaxes.

JEE

&&

.MARINE...
232 PM CST

Stronger winds still anticipated this evening through Sunday, as
low pressure moves across the region while deepening. Expect winds
to quickly ramp up early this evening, with gales becoming likely
over much of the open waters. During this time, speeds do
increase over the nearshore waters, but will remain below gales.
East southeast winds are expected during this time, but with this
low quickly pushing to Lake Superior, winds will turn to the
southwest later tonight into early Sunday morning. As the winds
quickly turn, speeds/gusts will also quickly increase with higher
end gales continuing over much of the lake and now including the
nearshore. By Sunday morning, anticipate gales to continue, and
will likely see some slight increases over the northern part of
the lake. It`s during this time, that a period of occasional storm
force winds to 50 kt will be possible. The window of these winds
appears to be on the smaller side, and so have not issued a
headline to address this. Will keep the gale warning going, but
include mention of occasional storm force winds in the open water
forecast. Gales should end from south to the north on Sunday, with
gales continuing Sunday afternoon into Sunday evening for only
the north half. Winds will continue to diminish Sunday night into
Monday morning.

Rodriguez

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...nearshore waters
     until 3 AM Sunday.

     Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...3 AM
     Sunday to 3 PM Sunday.

     Gale Warning...LMZ777-LMZ779 until 3 PM Sunday.

&&

$$

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