Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221107
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
607 AM CDT Mon Jul 22 2019

.SHORT TERM...
256 AM CDT

Through Tonight...

A drier and cooler regime continues to advect into the area.
Today may see some early morning sprinkles or a light shower or
two. Otherwise the noteworthy weather item is gusty winds from
Lake Michigan resulting in high waves along beaches bringing
dangerous swimming conditions and probably minor immediate
lakeside flooding in northwest Indiana and possibly in northeast
Illinois.

The pesky slow-moving surface low from yesterday is over eastern
Ohio early this morning, with its trailing boundary now well
downstate Illinois and Indiana. A welcome-to-many 1027 mb high
pressure is building southward over the Upper Plains. In the mid
to upper levels, a short wave is digging into the Great Lakes and
will provide northwest flow aloft by late morning. This trough has
little moisture to work with, but there have been spotty light
showers or sprinkles with this upstream. So could see some prior
to 10 a.m., but after that point the clouds should clear out
sharply from northwest to southeast as dry advection increases.
The 850 mb temperatures will drop to 10-11C by afternoon, which
upstream yesterday correlated to mid 70s highs, and think we`ll
see that plus a couple degrees. Dew points are around 60 now and
should fall well into the 50s by early afternoon -- that`s 20-25
degrees below what they were 48 hours ago.

The north-northeast surface winds early this morning will see a
fairly quick increase this morning due to a tightened pressure
gradient and just increasing pressure, as well as developing
mixing. The emphasis in the higher speeds will be from over the
frictionless lake, where 25-30 mph gusts are likely along the
immediate shore. With the favorable fetch, waves should readily
build into northeast Illinois and northwest Indiana shoreline
areas. The wave angle of approach relative to the shore will favor
classic rip currents, especially on the unprotected shore areas.
The Beach Hazards Statement will continue for this.

The lake remains at anomalously high water levels. Because of
that, comparing with recent events is needed for assessment of
lakeshore flood potential. This high wave episode looks to be
just a tad less in northeast Illinois than the July 6-7 event
that brought 5-8 ft waves and minor lakeshore flooding such as
Chicago`s bike path. So have considered a Lakeshore Flood Advisory
for there, but the longer duration of the perpendicular flow and
highest winds looks to be into northwest Indiana, resulting in 5-8
ft occasionally higher waves. Have issued a Lakeshore Flood
Advisory for this area and will watch buoy trends this morning to
see if this needs to be expanded for Cook County, which presently
looks to see more 4-6 to 4-7 ft with temporarily higher for about
a 9 hour period today. It can be difficult to get down to a 1-2
ft precision in the wave forecast, especially during gusty wind
episodes. Again the high lake levels are key as traditionally
these wave heights would not result in a concern for lakeside
flooding.

Cooler air will continue to filter into the area tonight as
anticyclonic low-level flow takes over. For outlying locations,
lows look to fall into the mid-upper 50s.

MTF

&&

.LONG TERM...
256 AM CDT

Tuesday through Sunday...

A long wave trough will have temporarily established itself over
eastern Canada by Tuesday morning. A 75 kt upper jet speed maximum
riding down its backside will point into the Great Lakes region
midday Tuesday and an associated short wave should be on its
leading edge. The main path of this and its cold pocket aloft will
be over Michigan but a glancing blow of forcing will drift over
the northeast half of the CWA on Tuesday afternoon. High-
resolution guidance, including convective allowing, have been
regularly showing some precip with this near the low-level focus
of the lake breeze on Tuesday afternoon. This guidance also is
more moist with the boundary layer than global models, which have
not been showing much of any QPF with higher clouds bases and
stretched out/thin CAPE profiles. For now along with NWS
Milwaukee, we have added a slight chance of showers on the
immediate western side of the lake. If spotty showers were to
materialize, cannot rule out a couple thunderstorms. Otherwise
Tuesday will see highs around 80 in most locations with a quite a
bit of sun.

Upper heights will then gradually build Wednesday into Thursday
and favoring sunshine. Wednesday will have a lake breeze again,
and there are still a handful of model solutions showing spotty
QPF on this, but given negligible forcing aloft and less low-
level wind speed for convergence, do not have any mentionable
afternoon rain chances. Temperatures will inch up a couple degrees
each day in this pattern with high confidence. An upper low/short
wave trough is forecast to progress eastward along the Canadian
border with the Upper Midwest during/near Friday. There is decent
convective potential with that in the Upper Midwest, but looks
more cold frontal driven with a more channeled/limited moisture
return ahead of it. This is not ideal for a progressive MCS or
pattern of such, so continue to keep Friday and Saturday dry in
the forecast. The associated cold front is forecast to lay out
west-to-east and right now that looks to be a little to our north
for the middle to latter part of the weekend. Temperatures will
continue to warm if that is the case, and would expect 90 to be
reached in some locations by Saturday and probably pretty close to
it already on Friday.

MTF

&&

.AVIATION..
12Z TAFs...

Quiet weather expected at the terminals for the period. As high
pressure spreads across the Upper Mississippi Valley and western
Great Lakes, drier air will spread into the region while keeping a
nly-nely pressure gradient in place. Expect conditions to be dry
and vfr. A locally stronger gradient near Lake Michigan should
keep winds generally nely and increase through the morning, with
sustained speeds of arnd 15 to 20 kt with gusts of 20 to 25 kt in
the afternoon.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement...ILZ006 until 11 PM Monday.

     Beach Hazards Statement...ILZ014 until 5 AM Tuesday.

IN...Beach Hazards Statement...INZ001-INZ002 until 5 AM Tuesday.

     Lakeshore Flood Advisory...INZ001-INZ002 until 5 AM Tuesday.

LM...Small Craft Advisory...IL nearshore waters until 11 PM Monday.

     Small Craft Advisory...IN nearshore waters until 5 AM Tuesday.

&&

$$

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