Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 222307

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
607 PM CDT Sun Apr 22 2018

233 PM CDT

Through Monday...

Upper level low to the south will continue to slowly move east
through the period, with the bulk of any precip likely staying to
the south. However, much of the CWA will observe associated
mid/high clouds stream overhead tonight into Monday. The more
solid clouds had scattered earlier today and think this same
coverage will remain for much of tonight into Monday morning. As
this upper level system begins to take on a more east northeast
track, will see cloud cover increase into Monday afternoon.
Northern extent of the precip shield does try to inch close to the
southern CWA Monday afternoon, with low chance pops for scattered
light showers still appearing to be on track.



212 PM CDT

Monday night through Sunday...

Closed upper low is progged to be over the lower Tennessee Valley
Monday evening, eventually opening up and lifting out to the
northeast through the Ohio Valley Tuesday and Tuesday night as it
gets "kicked" by northern stream short wave trough digging across
the northern/western Great Lakes region. Vort energy wrapping
northward around the main mid-level circulation spreads mid-level
moisture into the dilatation axis over the area Monday night into
Tuesday, with a corresponding potential for some light rain across
the southeast half of the forecast area, especially along/east of
I-55. Pops peak Monday night, then gradually diminish from the
west during the day Tuesday as the system pulls slowly off to the
east. The northern stream "kicker" digs across the Lakes into
Tuesday night, with a cold front pushing across the area and
enhancing blustery north-northeast winds by Wednesday morning.

Surface high pressure then builds across the area Wednesday into
Wednesday night, with dry but cooler weather on persistent but
slowly diminishing north-northeast winds. After seasonable temps
around 60 Tuesday (though a little cooler in thicker cloud cover
and light rain southeast), cooler air mass looks to keep temps in
the 50`s in most area, and only in the 40`s near Lake Michigan.
Lighter winds and mainly clear skies within the ridge axis
Wednesday night will allow for favorable radiational cooling
conditions, with min temps falling into the mid-30`s for the most
part by sunrise Thursday.

Medium-range guidance digs another upper trough across the region
Thursday night into Friday, though shows considerable spread in
developing another closed upper low somewhere from the southern
Great Lakes to the lower Mississippi Valley. Given the model
differences with the details of this evolution, timing of the cold
front and associated precip chances are also a bit out of sync,
but appear at this time to be focused later Thursday or Thursday
night, with a slight warm-up Thursday ahead of the front. Slower
ECMWF and Canadian solutions would likely result in pops lingering
into early Friday as well. Upper trough then moves off to the
east of the area, though global guidance hints at another
disturbance moving through northwest flow aloft Saturday, which
trails a cold front into the forecast area Saturday night or early
Sunday. It does look like warming aloft would support milder
temps next weekend, likely well into the 60`s, though the location
of the cold front will determine the details of that, as winds
could be off the lake across the Chicago area at that time.



For the 00Z TAFs...

VFR with northeast winds, strongest during the day, through the

- Izzi


222 PM CDT

Only real marine concern in the coming days focuses on a period
of strong northerly winds Tuesday night into Wednesday, behind
sharp cold front. The combination of deepening low pressure along
the East Coast, cold low level advection, and high pressure
building southeast across the Upper Midwest, looks to support a
period of blustery north winds to around 30 kts during that
period. Some guidance suggests some marginal gale force gusts are
possible as well, though the general model consensus would suggest
30 kts is a good forecast at this distance. Winds look to
diminish Wednesday afternoon as the surface ridge axis spreads
southeast across the Lake.

Another surface low and trailing cold front approach from the
northwest Thursday, which may support a brief period of south-
southwesterly 20-25 kt flow during the day, with the potential for
another brief period of enhanced northerly winds Friday behind
the cold front.






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