Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211827
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
127 PM CDT Sun May 21 2017

.SHORT TERM...
127 PM CDT

Through Monday...

Deep upper level low pressure across the upper midwest will
continue to stratocumulus deck across the region through the
evening and early overnight before diminishing. With cold
advection that will continue through the night and the lingering cloud
cover, expect temperatures at this point to largely hold steady
or even fall some through the afternoon. At the surface, high
pressure across the southern plains will shift to the lower
Mississippi Valley tonight while low pressure lingers across the
northern Great Lakes. This will maintain an occasionally gusty SW
wind through Monday especially during the daytime hours, though
cannot rule out occasional gusts overnight.

Clouds should thin by daybreak Monday which like today should
start off on the sunnier side. Several waves will rotate around
the back side of the low beginning Monday which will increase
clouds and bring the chance for showers and even a few
thunderstorms Monday afternoon. Confidence is high that there will
be at least scattered rain showers in northeast Illinois, and
possibly thunderstorms during this time frame, but uncertainty on
how much coverage precludes likely pops just yet. The
west/southwest wind on Monday will be a warmer wind then today,
thus expect temps to climb above today`s lower 60s readings.

KMD

&&

.LONG TERM...
332 AM CDT

Monday through Saturday...

Much of the weather this week will be influenced by the same
upper level system, with this system or at least a piece of this
upper level system remaining across the region. With initial sunny
skies in place Monday, will likely see mid/high could spreading
back over the area throughout the day. This will be as a stronger
piece of energy rounds the base of this system, and associated
surface reflection shift east southeast across the region. Monday
morning will be dry, but precip chances will increase in the
afternoon and Monday night as this system pushes overhead.
Instability does not appear overly great at this time, but there
may be enough for isolated storms. Forcing really increases on
Tuesday, with the highest chances for more widespread showers and
thunderstorms expected during this time. This system really closes
itself off, and although coverage of any precip not expected to
be as high Tuesday night into Wednesday, cloud cover and scattered
showery development is still likely to remain. This system should
finally exit to the east by Thursday but pattern appears to
remain active with yet another system possibly approaching on
Friday. Temps this week start off milder with 70s expected Monday.
As this upper level system settles back in temps in the 60s, and
quite possibly cooler in some locations, are expected Tuesday and
Wednesday.

Rodriguez

&&

.AVIATION...
For the 18Z TAFs...

Deep upper level low pressure across the upper midwest will
continue to steer low end VFR or higher end MVFR stratocumulus
deck across the region through the evening and early overnight
before diminishing. At the surface, high pressure across the
southern plains will shift to the lower Mississippi Valley tonight
while low pressure lingers across the northern Great Lakes. This
will maintain an occasionally gusty SW wind through most of the
period, especially during the daytime hours, though cannot rule
out occasional gusts overnight.

Several waves will rotate around the back side of the low and
bring the chance for showers and even a few thunderstorms Monday
afternoon. Confidence is high that there will be at least
scattered rain showers in northeast Illinois, but low on coverage
and medium on timing (per the ORD 30 hr TAF). Thunder chances are
too low to include at this distance but forecast soundings to
suggest the afternoon period could have a few thunderstorms.

KMD

&&

.MARINE...
259 AM CDT

Occluded front will sweep across the lake this morning shifting
winds to the southwest. Fairly stable conditions this afternoon
should keep wind somewhat decoupled over the open waters with a
somewhat lighter southerly wind expected when compared to the
stronger southwesterly offshore winds near the southern/western
nearshore. Stability decreases some tonight as cooler air filters
in, so expected stronger winds to develop over the open waters
tonight, probably peaking in the 25-30kt range for gusts after
midnight. Generally west/southwesterly winds likely until cold
front moves across the lake Tuesday shifting winds to the north in
its wake. Medium range models are showing some variability in
solutions with respect to the midweek storm system, but does look
like a potential for a period of fairly stout northerly winds in
the Tuesday night through Wednesday night time frame.

- Izzi

&&

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

&&

$$

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