Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221731

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1231 PM CDT Mon May 22 2017

309 AM CDT

Through Tuesday Night...

The "wrap around" stratocumulus deck is clearing out eastern CWA
at this time setting the stage for a sunny start to the day today.
This morning`s sunshine looks likely to be the last meaningful
amount of sunshine for at least several days (or more) as the
pattern takes a decided turn toward unsettled.

GOES-16 mid level (6.95 micron) water vapor imagery this morning
shows a bevy 7of vorticity maxima upstream from the western
Cornbelt to the northern plains and across the Rockies. One
shortwave over southern SD/northern NE is likely to pivot around
the slowly departing upper low east into the mid-Mississippi
Valley region this afternoon. Numerical models suggest that this
wave will dampen some this afternoon, however majority of short
range guidance suggests at least scattered showers will break out
this afternoon ahead of this feature, first across IA then
spreading east into northern IL late this afternoon. GOES-16
imagery shows ample dry air in the low/mid levels over the area,
so not sure how widespread rain will be this afternoon and largely
kept pops in the chance category.

A pair of strong, apparently closed, circulations are evident on
water vapor early this morning, one just NW of Lake Superior and
the other near the ND/MT border. Both of these features are
forecast to rotate around the large upper low and begin to phase
over the upper Missouri Valley tonight, eventually breaking free
of the current main upper low and closing off its own deep
tropospheric closed low over the Midwest. The new closed off mid-
upper level low is forecast to move gradually southeast toward the
lower Ohio Valley by Wednesday.

Tonight through Tuesday night look for waves of showers as small
impulses rotate around this upper low. Maintained a small chance
of thunder Tuesday afternoon as cold air aloft could allow for
sufficient instability to develop for some isolated thunderstorms,
particularly if there are any breaks in the clouds allowing for
better surface heating. If any weak convection develops Tuesday
afternoon, couldn`t rule out a bit of graupel or even "cold air"
funnels, particularly western CWA closed to the heart of the cold
air aloft and upper low.

At the surface, low pressure is forecast to develop over WI/IA
later this afternoon and tonight, then as upper low digs southward
guidance suggests that this low will meander south into northern
IL Tuesday with surface trough developing between this low and
another low over the southeast U.S. the result should be a
relatively mild day today with southwest winds and then eventually
turning colder near the lake Tuesday with a lake breeze developing
in the afternoon. The relatively moist air mass and fairly light
winds could allow for fog to develop Tuesday night, with marine
fog possible near the lake Tuesday afternoon into Wednesday.
Something later shifts will need to consider introducing into the

- Izzi


309 AM CDT

Wednesday through Sunday...

While models differ some wrt the details, guidance is in pretty
good agreement on depicting this surface low deepening to our
south/east Tuesday night into Wednesday as this unseasonably
strong upper low continues digging southward. Guidance has not
been in particularly good agreement, so confidence isn`t terribly
high, but latest runs would support the idea of rather
strong/gusty northerly winds and chilly conditions (by late May
standards) Wednesday and Wednesday night. If the cyclone deepens
as progged, then the TROWAL like band of moderate to locally
rather heavy rain being depicted wrapping around this cyclone
could result in even colder high temps Wednesday and a good
soaking rain. Best chances of this look to be in our eastern CWA,
but stay tuned because models often struggle handling cut off lows
like this one. Also, keep in mind that current forecast high
temps Wednesday could be significantly cooler if trowal sets up
over the area.

The upper low looks like it could finally move east of the area
Thursday with a brief break in the precipitation chances, however
right on the heels of the departing upper low medium range
guidance brings another shortwave into the region Friday bringing
a renewed threat of showers and t-storms. Another shortwave could
bring a subsequent round of showers and possible storms over the
weekend. Unfortunately, timing differences this far out require
carrying pops every period, but in reality Friday through Sunday
look unlikely to be a washout, rather just a short period or two
of some showers & storms.

- Izzi


For the 18Z TAFs...

Breezy southwest winds along with VFR conditions will be in place
through sunset. Primary forecast challenge for today will be
timing and placement of scattered convection from mid/late this
afternoon on. Some festering showers continue to move east across
Iowa early this afternoon and will move across northern Illinois
mid afternoon. Leading edge of this precipitation is expected to
continue to slowly erode, however, modest forcing will develop
starting mid afternoon and very weak instability in the area will
allow for some additional widely scattered shower development.
Models have been honing in on axis of low level convergence that
will serve as the primary focus of better precip coverage which
will stretch from west central Illinois into northern and
northeastern Illinois. As mentioned, instability is fairly weak
today, thus removed any thunder/vicinity thunder mention for the
terminals except for RFD where more marginal instability is

Expect a lull in precipitation overnight, but showers are expected
to increase in coverage again on Tuesday as series of pinwheeling
upper level disturbances rotate through the region. Confidence in
specific timing tomorrow is low, but expect off and on showers
throughout much of the day. Winds may also be a bit tricky
tomorrow with models suggesting a weak surface low developing
over northern Illinois resulting in backed, albeit relatively
light, winds during the afternoon. Ceilings are also expected to
lower to MVFR by early tomorrow morning and persist into the early



405 AM CDT

More organized surface pattern remains over the lake this
morning, as low pressure is situated just to the north in Ontario.
This is resulting in speeds of 15 to 25 KT over much of the lake
at this time, and expect this to likely persist through much of
today. Do anticipate some slight increase later today, especially
for the nearshore waters where winds of 20 to 25 KT, and possibly
to 30 KT, will be observed. With hazardous conditions for small
craft returning, have issued a Small Craft Advisory for mainly
this afternoon into the early evening. Surface pattern relaxes
tonight, with speeds diminishing throughout the night. Pattern
then gets complicated later tonight into Tuesday, likely through
mid week, as new low pressure develops over the upper Mississippi
valley tonight and then slowly moves through the Great Lakes
region. After a few periods of lighter winds, winds will turn more
northerly Wednesday and likely increase. This will likely return
hazardous conditions for small craft.



LM...Small Craft Advisory...nearshore waters
     until 9 PM Monday.




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