Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220809

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
309 AM 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 06Z TAFs...

VFR ceilings still in place but clearing line is steadily pushing
east across the area at this time. Expect this clearing line to
reach the terminals in the 7-8z time frame, with clear skies then
likely through much of the morning. Anticipate only mid/high
clouds for much of the day, until the next chances for precip
arrive later this afternoon. While VFR ceilings return this
afternoon, expect scattered precip to develop around the
terminals as well. Have not made any big changes to precip timing
in the TAF including the possibility for thunder. Will monitor
trends this morning, but at this time, conditions don`t look
overly favorable for any widespread thunderstorm development
across the terminals. Scattered showers will likely linger into
the evening. Monitoring for possible lowering of ceilings to MVFR
later tonight, but at this time, setup would favor continued VFR
ceilings. West southwest winds in place with some sporadic gusts
still occurring. Do think gusts will diminish and speeds lower as
this clearing line moves overhead early this morning. Then
anticipate speeds/gusts to increase once again later today.



350 PM CDT

A cold front will continue to push across the lake through this
evening, with southwest to west-southwest winds generally up to
25 kt through mid day Monday. Winds will turn southerly on Monday
over the open lake due to new low pressure forming over the Upper
MS Valley and stable air mass. The surface low will meander
gradually southeastward through Wednesday morning while slowly
deepening. The low could then further deepen and retrograde
northward on Wednesday afternoon into Wednesday night. The track
and strength of the low is uncertain and will affect the strength
of northerly winds on its backside into Thursday. The potential
exists for stronger northerly winds than current forecast (up to
25 kt) with a deeper farther west low and vice versa with a
weaker farther east low pressure. The northerly winds will
gradually give way to close out the week as weak high pressure
ridging moves over the region, with generally lighter speeds and
east to southeast direction for southern and central portions of
the lake.



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...noon Monday to 9 PM Monday.




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