Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240801

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
301 AM CDT Mon Apr 24 2017

250 AM CDT

For those of you who enjoy dry and warm spring days,
get out and enjoy the next few days.

The very dry airmass that has been in place will undergo very
little modification today as high pressure drifts only slightly
eastward and low level flow remains out of the southeast.
Southerly flow though does mean some warming will continue to
shove highs even farther upward today, though the southeast wind
combined with another lake breeze shifting things easterly across
the lake will maintain cooler conditions across the IL lakeshore
communities, with some possible across IN as well though much less
pronounced. This means more widespread 70s, but near 60 at the
lake. We will also see passing high clouds today, occasionally not
as transparent.

A very warm airmass will advect northeastward ahead of more active
weather to our west on Tuesday. The impact locally will be the
warmest temperatures of this stretch, but increased cloudiness as
longwave troughing across the west impinges on our region. Model
guidance still suggest southeast winds which will keep the lake
adjacent areas cooler, but the magnitude of inland cooling will be
much less.



250 AM CDT

Short term discussion was a foreshadowing of what will
be a bit of a pattern change that will not bring quite as dry of a
weekend that we just had. One look at water vapor pictures early
this morning depicts a very strong upper level jet extending well
out across the Pacific with an impressive swath of moisture
pointed at the west coast. And while our main moisture source will
not be this moisture (though we will indeed get some it), the
upper low associated with it and all of the upper level dynamics
will become the major players for our region beginning mid week
and then continuing through the weekend. This low will be able to
tap the Gulf of Mexico, which while at this point is still quite
dry, but that will certainly not be the case once a ridge of high
pressure becomes established across the southeastern United States
later this week.

First on the docket is a leading low pressure system that will
move from Iowa Tuesday evening to Wisconsin Wednesday, with a
secondary impulse in the southern jet stream that will eventually
merge with the leading northern low over north central Lake
Michigan late Wednesday. Wave 1 come Tuesday night into early
Wednesday, with relatively dry southeast flow in place, there
could be some light showers at best initially in spite of a very
strong low level jet focused into our far western CWA. Several
waves will stream northward ahead of the upper low which will
steer more gulf moisture toward the area eventually ahead of a
cold front. There appears to be a period later Wednesday and
Wednesday evening that will favor a better chance for showers and
thunderstorms across the area, as models have trended toward
keeping the cold front west of the area most of the day allowing
instability to build ahead of it. There will be decent shear ahead
of the front, and there would be a severe threat depending on how
much instability can get into our area. At this point the more
significant threat appears to farther south.

The cold front will pass through Thursday, with some lingering
rain expected Thursday morning before this initial upper low
finally kicks northward. After a brief breather later Thursday and
Thursday night, attention will turn to the longwave upper trough
that will develop across the west, and a large upper ridge will
position itself off the southeastern coast. Synoptically this is
a very favorable pattern for heavy rainfall somewhere across the
region with the deep layer southwest flow from the Gulf of Mexico
that develops in this regime, and likely severe weather (though
this is usually favored south).

Several waves of low pressure will ride northeast through our
region bringing occasional periods of showers and thunderstorms
through the weekend. The first and weakest one appears to be
Friday afternoon-ish time frame. The second one, with a bit more
gusto, will come Saturday morning as an elevated portion of a
northward moving warm front will move through. On Sunday the upper
low will move north through the nation`s heartland with a
corresponding surface low moving into southwest Wisconsin. This
will lift the surface warm front even farther northward, but
models disagree considerably in how quickly this will occur, and
this would play a big role in severe weather concerns and timing
of the next, and possibly the most widespread concern for heavy
rain with thunderstorms Sunday as precipitable water values creep
up into fairly high levels for late April with forecast soundings
showing deep moist saturated layers and relatively high freezing
levels. Widespread severe weather is not favored, especially
initially and through Saturday as the surface warm front (remnants
of the Thursday cold front) will still be well to the south, and
we will maintain a cool northeast wind keeping the low levels more
stable. But heavy rainfall will be a significant concern for the
area, possibly very heavy depending on how this all plays out.



For the 06Z TAFs...

1230 am...Only forecast concern this period are wind speeds/
potential gusts from mid morning through sunset.

Light northeast winds at the Chicago terminals early this morning
will slowly turn southeasterly through sunrise and then begin to
increase during the morning hours. The gradient will slowly begin
to tighten and prevailing speeds in the 10-15kt range look on
track. Low level mixing will likely allow for some higher gusts...
especially across northwest IL/rfd. But low confidence regarding
how widespread these will become further east. In addition to the
already southeast winds...a lake breeze is possible this afternoon
turning winds more easterly at gyy by early afternoon and possibly
at mdw/ord by mid afternoon. Speeds will diminish some with sunset
and remain southeasterly tonight.

Dry/vfr expected through the period...but some isolated/shallow
ground fog is possible through sunrise this morning in the usual
locations. Otherwise...sct/bkn high clouds. cms



215 am...A weak ridge of high pressure over the southern lakes
region will slowly move southeast today and tonight...while a
stronger area of high pressure remains over northern Ontario. The
gradient is expected to remain tight over far northern Lake
Michigan today and tonight with easterly winds 15-25kts expected.
Over the rest of the lake...east/southeast winds will increase to
15-25kt later tonight as low pressure over the western plains
moves northeast to the upper midwest tonight and dissipates. A
second area of low pressure will develop over the central plains
tonight and move northeast to the western lakes region Wednesday.
This low will then slow as it moves into Ontario Thursday. West/
southwest winds may increase to 30 kt Thursday as this low
departs. cms





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