Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 241927

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

215 PM CDT

Through Tuesday...

Generally quiet weather expected through tomorrow.

For the remainder of the afternoon, main concern will focus on
dewpoints, and in this case, how low dewpoints will be. As of
early this afternoon, sfc dewpoints were in the upper 20s to low
30s as sfc temps rise into the low to middle 70s. This will allow
afternoon min RH values to drop to the upper teens to lower 20s
percent range. While this will be pleasant for most people, for
those planning on burning field brush will need to exercise
caution as the low relative humidity values, combined with winds
gusting to 15 to 20 kt could cause some fire spreading issues. For
tonight, there should only be some increasing mid and high level
cloud cover. Combined with winds of 5 to 10 mph, radiative cooling
should be tempered by the increasing cloud cover and low level
moisture increasing overnight as winds turn to more southerly. By
daybreak tomorrow morning, sfc dewpoints are expected to rise to
the middle to upper 40s. So, overnight lows should be in the lower
50s. For Tuesday, weather conditions will remain quiet, with
strengthening south winds further allowing for a continued
moisture increase through the day with little in the way of
afternoon drying or mixing down of drier air. Highs tomorrow
should be in the middle to upper 70s, with dewpoints in the upper
40s to lower 50s. Temperatures along the lakefront, from Chicago
northward will likely remain a bit lower as a slightly east of
south wind directions should allow for lake cooled air to spread a
short distance inland, keeping highs there in the lower to middle


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 18Z TAFs...

Main concern for the period will be wind speed/direction.

Weak high pressure is moving across the region and the light sely
winds have increased through the morning. With a weak gradient in
place, expect winds to shift to more ely as a lake breeze
develops. As of issuance time, there is no sign of the lake breeze
on radar, but anticipate that it should develop over the next
couple hours. With a modest push of cooler air, some ocnl gusts
are possible, so have added a tempo for gusts to 20kt for a few
hours following the passage of the lake breeze. Winds should drop
off with sunset, but there should be strong enough of a gradient
to keep 5 to 10 kt winds overnight, with direction veering to sely
overnight and more sly tomorrow as the high pressure slides to the
east and low pressure approaches the middle/upper Mississippi
Valley. As low level moisture begins to increase overnight into
Tuesday morning there may be some patchy/shallow ground fog
possible in sheltered, low-lying areas.



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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