Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221908

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
208 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


205 PM CDT

Tuesday night through Monday...

The unsettled pattern will continue into Wednesday as deep low
pressure across the corn belt region will shift south to the Ohio
valley and lower Mississippi. Areas of showers will pinwheel
around the periphery of the upper low. The core of the coldest
temperatures aloft will drift to our south and west which would
suggest the thunder potential here is small, but 700 mb low and
just to our east suggests the deformation band of precipitation
associated with this system will get into at least a portion of
our area, with model guidance depicting more reasonable agreement
across the I-57 corridor southeastward across northwest Indiana.
The Kankakee and Iroquois river basins which are somewhat swollen
may be impacted by a period of moderate rain for a time as the
trowal axis extends back into the area and contains a modest PWAT
axis in excess of 1.1" PW. While individual model disagreement
exists, GFS ensemble measure of predictability is fairly high with
this system, but certainly these upper lows can provide fits for
forecasts with minor shifts in track having significant impacts on
the location of the moderate to possibly heavier rainfall, with
the EC and NAM being on opposite envelopes regarding the western
extent of precip.

While some lighter precipitation will linger Wednesday night, the
low will slowly pull east across the Ohio valley. While we get
some weak height rises on the back side of the low which will lead
to column drying and some warming, we maintain a northerly low
level flow which will keep things a bit cooler for Thursday and
its possible we still have some lingering lower level cloudiness
into the day.

Normally in an amplified pattern the big ridge to our west brings
a break from the cool and showery weather. Well we will get a
ridge from the west, but it appears to be dampening as it slides
eastward. We will warm up due to southerly flow but our dry period
may be somewhat short lived thanks to another deep upper level
low currently well off in the Gulf of Alaska that will slide
southeast across the northern Rockies Friday. Energy ahead of the
low will interact with southern stream jet energy leading to
another chance of showers and storms later Friday with unsettled
weather continuing into the holiday weekend with the upper level
low making more headway into the region. Depending on the Friday
system areas north could once again get stuck in cool northeast
flow for the first half of the weekend with a warm front draped
across our southern areas and a big temperature disparity, but
model predictability for the weekend low is not that great.
Therefore the weekend forecast confidence while favorable for
unsettled conditions is still not super high on specifics thus
the broad chances of rain at times.



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



205 PM CDT

Breezy southwest winds courtesy of high pressure across
the Ohio valley and Low pressure north of Lake Superior, most
notably across the nearshore waters, will ease as a secondary area
of low pressure across the corn belt will shift to the south and
west of the southern tip of Lake Michigan through Tuesday. Weak
high pressure will build across north central Ontario, which will
allow a weak cold front to shift south across Lake Michigan
beginning tonight across the north half, and eventually across the
south half Tuesday night. The low south of Lake Michigan will
strengthen through mid week which will lead to a several day
period of increasing northerly winds across the entire lake. Gales
appear unlikely though expect winds to pick back up to 25 kt or
briefly higher especially Wednesday, which will build modest
waves across southern Lake Michigan and lead to hazardous
conditions for small craft.



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




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