Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 132006

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
306 PM CDT Thu May 13 2021

305 PM CDT

Through Friday night...

High pressure continues to create fair conditions with light winds.
The lake breeze has set up later than previous days, and is weaker,
presently just turning the winds near the lake easterly at around 10
mph. A broken cloud deck has developed in a broader region compared
to yesterday. These clouds will clear in the wake of the lake
breeze, but with limited strength the lake breeze will be hard
pressed to make it to north-central Illinois.

Temperatures will reach the upper 60s this afternoon, and even a few
low 70s for locations to the west. Low temperatures tomorrow will
mostly be in the 40s, so frost remains out of the forecast. The
warming trend continues with slightly warmer temperatures expected
Friday as our 850 mb temperatures increased from around 2 C to 4 C,
or even warmer to the west. Increasing cloud cover Friday as a short
wave moves through Friday night into Saturday. There will be
supporting dynamics for this system, with the left exit region of a
weak jet max combined with 850-700 mb frontogenesis and a 700 mb
vorticity max, so there is a decent chance for showers Friday night
into Saturday morning.



257 PM CDT

Saturday through Thursday...

Saturday: Stagnant CONUS-scale WNW flow aloft will continue into
the weekend as expansive surface high pressure across much of the
eastern third of the CONUS will suppresses low-level moisture
return to southwest of the CWA. There are increasing signs that,
amid mid- level moisture return, a convectively augmented wave
over the northern Plains Saturday evening will drift into the
region late Saturday morning into the afternoon. Have maintained
chance PoPs for showers during this time and slightly lowered
temps into the mid 60s away from the lake provided the increased
potential for more cloud cover.

Sunday through Tuesday: While fluctuations in guidance continue
through this period, the overall model suite is beginning to focus
on a more defined solution. The main focus will be on a
developing low-level baroclinic zone across the central Plains on
the southwest flank of the departing ridge in response to
increased troughing over the Great Basin. Weak Rex blocking will
redevelop during this time as a ridge slowly builds over the
northern Plains, cutting off a mid- level wave originating from
northwest Mexico. Given a persistent feed of low-level moisture
off the Gulf of Mexico, an E/W axis of heavy rain is becoming more
likely somewhere across the mid- Mississippi and Ohio River
valleys. The likelihood that a significant precipitation gradient
occurs over of just south of the CWA continues to increase, with
the overall ensemble suite favoring the axis of highest precip
across central Illinois. The main period of concern for our area
will be later Sunday afternoon into Monday morning.

Tuesday through Thursday: With guidance favoring a farther south
suppression of the boundary and upper ridging extending northwest
into the Great Lakes region, precip chances appear to be rather
low Tuesday through Wednesday. Even with the region north of the
boundary, moderating low-level temps under the ridge and the
potential for reduced low-level clouds should support seasonable
temps over the area during this time. On Thursday, a slow-moving
upper-low over the southern Great Plains will lift north into the
central Plains and mid-Mississippi Valley, leading to more
unsettled weather with increasing thunder potential across the
region to end the week.



For the 18Z TAFs...

High pressure remains in command with increasing 8000 ft broken
ceilings this afternoon. Winds have been light and variable,
generally trending from the west. The lake breeze will move
through MDW early afternoon, then ORD about an hour later,
bringing winds from the east and slightly stronger at around 9 kt.
The lake breeze will eventually work into DPA late afternoon /
early evening. Overall winds will go southerly tonight, and
eventually westerly once again by Friday morning, remaining light.
Lake breeze will set up again Friday early afternoon, but winds
are expected to remain light.





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