541
FXUS63 KLOT 211117
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
617 AM CDT Fri Jun 21 2024

.KEY MESSAGES...

- Potential for strong to severe storms with damaging wind,
  large hail, and localized flooding late Saturday afternoon
  through Saturday evening.

- A brief reprieve in heat and humidity is expected Sunday and
  Monday, before it returns (with chances for thunderstorms) on
  Tuesday.

&&

.DISCUSSION...
Issued at 324 AM CDT Fri Jun 21 2024

Through Saturday night:

Ongoing isolated showers primarily north of I-88 should persist
for at least a few more hours in response to weak isentropic
ascent in the 5-7kft layer. Cannot completely rule out lightning
as marginal lapse rates yield up to 500J/kg of MUCAPE.
Meanwhile, low-level dry-air advection has limited the fog
potential across much of the area early this morning, though
haze or patchy and shallow fog are still expected through
sunrise. Also, persistent fog and low stratus advecting inland
of Lake Michigan will continue well into the morning from the
far south side of Chicago around the northwest Indiana shore.

Turning toward today through tonight, we remain precariously
close to a favorable axis of deeper low-level moisture and
correspondingly more unstable airmass on the northwest flank of
a persistent ridge. Upstream dew points in the mid 60s yesterday
amid weak S to SSW low-level flow today and added
evapotranspiration should yield afternoon dew points today in
the mid to upper 60s across much of the forecast area. These
values will be insufficient to erode a residual cap while also
limiting MLCAPE into the area, especially in the absence of any
appreciable forcing. The one exception will be where an axis of
low 70s dew points should begin to filter into the far
northwest CWA this afternoon and especially tonight as low-level
flow begins to veer. Isolated to widely scattered thunderstorms
are possible roughly northwest of a McHenry to Sterling line
this afternoon and across the counties adjacent to Wisconsin
tonight where weak WAA and any potential outflow boundaries
would support enough forcing to generate convection.

Saturday`s forecast has the potential to become rather muddled
as an abundant amount of upstream convection across the central
Great Plains tonight has the potential to eject a series of
convective waves into the weakening ridge before a main mid-
level wave crosses the area Saturday night. Conditional on
gradual low-level moistening Saturday morning as well as a low-
level focus of any outflow boundary from convection across
southern Wisconsin, regenerating scattered severe convection
with large hail and damaging winds as well as a low-chance
(10-20%) of flooding is possible north of I-88 during the
afternoon. Organized deep-layer forcing with the arrival of the
mid-level wave and surface front should generate a more
organized line of strong to potentially severe convection across
most of the remainder of the forecast area Saturday evening.
Only modest deep-layer shear may limit the overall severe
threat, but storm mode does favor the potential for some
discrete supercell structures eventually congealing into a
gradual SSE moving line with locally damaging winds. Veering
deep-layer and backward propagating shear vectors do yield some
minor flash flooding concerns later Saturday evening, though the
overall convective threat should remain progressive enough to
limit a more formidable flood threat.

Kluber


Sunday through Thursday:

In the wake of the cold frontal passage responsible for
thunderstorms Saturday evening, a welcome break in the heat and
humidity is expected to start next week. Highs on Sunday should
range from the low to mid 80s, while highs on Monday will range from
the mid to upper 80s. Locally cooler temperatures in the low to mid
70s can be expected near Lake Michigan (mainly northwestern Indiana
shore on Sunday, and the entire shoreline on Monday). While an upper-
level shortwave will race through the Great Lakes on Sunday, chances
for rain locally appear scant (<15%) owing to limited moisture. As a
result, the start of next week is shaping up to be pleasant.

Tuesday into Tuesday night, an upper-level shortwave will propagate
across the northern United States. The leading "ridge" portion of
the shortwave will arrive on Tuesday and allow for aggressive low-
level temperature and moisture advection into the lower Great Lakes.
Rising 850mb temperatures to +21 to +22C will support highs in
the low to mid 90Fs. If the moister side of guidance verifies
(e.g. GEFS, CMCE), heat indices may very well push above 100F.
When considering expected partly cloudy skies, a nearly maximum
annual sun angle, and southwest winds of only 10 to 15 mph,
forecast max wet-bulb globe temperatures top off in the mid to
upper 80s, as well. Now, the EPS remains steadfast that mixing
downward of low-level moisture will keep heat indices in check,
much like the past week. Regardless, Tuesday will be a day to
watch for potentially needing a Heat Advisory for part of our
area.

Now, anytime we`re talking about a potential need for heat headlines
this time of year, we often have to consider convection. While it`s
a little early to dig into the details for a Day 5 forecast, the
idea of an upper-level wave providing a glancing blow to a
plume of steep mid-level lapse rates (>8 K/km from 700-500mb), a
deep layer of moisture (PWATs approaching 2"), and an
associated extensive reservoir of instability (MUCAPE >3000
J/kg) raises alarm bells for MCS activity in the general region.
With the upper-level shortwave poised to propagate well to our
north near the US/Canadian border, areas from Wisconsin through
Lower Michigan should be the favored zone for MCSs (starting
Monday night, lasting through Tuesday). However, this is the
time of year when convection can verify farther south than
advertised in model guidance. Even if the most robust
thunderstorm activity remains to our north throughout the day on
Tuesday, at least scattered thunderstorms may accompany a cold
front dropping south through our region Tuesday night. Needless
to say, any thunderstorms (or associated southward-moving
outflows) on Tuesday would complicate the forecast for
temperatures.

Wednesday through the end of next week, ensemble model guidance
favors a pool of relatively cool low-level air spilling into the
Great Lakes region leading to lowering humidity levels and near
seasonable temperatures.

Borchardt

&&

.AVIATION /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 617 AM CDT Fri Jun 21 2024

Light and variable winds at press time should gradually become
southwesterly throughout the morning and increase to 8-10kt by
early afternoon. With a weak surface pressure gradient in place
and expected large temperature difference between the lake and
land by late morning, a lake breeze is poised to develop and
surge inland. At GYY/MDW, opted to advertise the wind shift with
the boundary as early as 17Z. Meanwhile, at ORD, the boundary
may stall or oscillate back and forth as southwesterly winds
provide a reasonable protest. Given low confidence in the timing
of the boundary reaching ORD, let alone the threat that the
boundary oscillates back and forth, will treat the wind shift
in a TEMPO. Meanwhile, at DPA/RFD, southwesterly winds are
expected to prevail this afternoon.

Tonight, winds will once again become light to variable.
Southwest winds will increase toward daybreak Saturday, and
become gusty from mid-morning onward. Occasional VFR cloud decks
will stream overhead throughout the TAF period.

There are two other items of note specific to individual
terminals. First, ongoing marine fog at GYY is expected to lift
within the next few hours as diurnal mixing commences.
Secondly, there remains a threat for isolated to scattered
showers and thunderstorms in the vicinity of RFD this afternoon
(for which a PROB30 has been maintained). In later TAF packages,
confidence in the occurrence of thunder at RFD may not be high
enough to earn an upgrade to a TEMPO or FM group.


Borchardt

&&

.CLIMATE...
Issued at 337 AM CDT Wed Jun 19 2024

Here are the daily high and warmest low temperature records
for Chicago and Rockford for today and tomorrow:

           Chicago
-----------------------------
Day:             6/21 6/22
-----------------------------
Record High:      101   97
Record Warm Low:   74   76

           Rockford
-----------------------------
Day:             6/21 6/22
-----------------------------
Record High:      100   97
Record Warm Low:   71   73

NWS Chicago

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002-
     INZ010-INZ011.

     Air Quality Alert until midnight EDT tonight for INZ019.

LM...Dense Fog Advisory until 11 AM CDT this morning for the IL and
     IN nearshore waters.

&&

$$

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