052
FXUS63 KLOT 220552
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1252 AM CDT Sat Jun 22 2024

.KEY MESSAGES...

- Potential for strong to severe storms with damaging winds,
  torrential downpours 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 305 PM CDT Fri Jun 21 2024

Through Saturday night:

The remainder of the afternoon will remain very warm and humid,
especially away from the cooling influences of Lake Michigan.
Expect low to mid 90s inland, with 70s to low 80s closer to the
lake. Capping is expected to curtail thunderstorm develop across a
majority of the area through the remainder of the. The only
exception being areas across far northern IL near the WI state
line, where capping is weaker thanks to a corridor of higher
surface dew points around 70. Accordingly, a few isolated showers
and storms cannot be ruled out in this area late this afternoon
into early evening. However, expect the better focus for these
storms to largely remain north of the WI state line into this
evening.

Warm air advection will increase tonight along an strengthening
southwesterly low-level jet across much of the Corn Belt, as
low pressure shifts out across the Mid-Missouri Valley. This is
expected to foster an significant increase in the areal coverage
of elevated thunderstorms tonight, particularly across IA
eastward across southern WI. While the primarily focus for these
storms will be north of the area tonight, it does appear that a
period of showers and some scattered storms may develop into
northern IL after midnight tonight. Severe weather is not
anticipated with these storms, though a high PWAT environment
(values around 2") will support the potential for some torrential
downpours with any storms.

Another very warm (hot) and humid day is on tap for the area on
Saturday, as temperatures areawide top out in the low to mid 90s.
However, unlike today, offshore gusty southwest winds up to 30
mph on Saturday will curtail any chances for lake cooling near
the shore, so expect these very warm conditions to extend all the
way to the lake.

The threat of thunderstorms looks rather low during most of the
day on Saturday as we await the arrival of an upper trough and
surface cold front expected late Saturday into the evening. While
this is the case, severe storms are expected to develop to our
west-northwest Saturday afternoon as the moist airmass in advance
of the approaching cold front destabilizes through the day.
A favorable kinematic environment with effective shear values up
to 40 kt will support organized storm structures and clusters
capable of producing strong damaging wind gusts. Damaging wind
gusts are likely to remain the primarily threat with these storms
as they shift eastward across southern WI and northern IL Saturday
evening. Rather poor mid-level lapse rates in combination with a
very high PWAT airmass (up to 2.25") suggests the threat of hail
will be rather low outside of any well developed supercell
structures. Torrential downpours will also accompany these storms
given the near record PWATs. Accordingly, even short instances of
training storms could result in localized corridors of 2-3"+
amounts and possible flooding. This will especially be problematic
if it occurs over the urban areas in and around Chicago and
Rockford.

The arrival of these storm clusters could be as early as ~5pm
Saturday afternoon across northwestern parts of the area
(including the Rockford area), and as early as 6 or 7 pm in and
around the Chicago metro area. This threat will then come to an
end from northwest to southeast later in the evening.

KJB


Sunday through Friday:

Behind the frontal passage early Sunday morning, temperatures and
humidity finally turn more seasonal, with highs back in the 80s
Sunday and mid 80s to 90 Monday...with upper 70s to low 80s along
the lakeshore. Northwest winds arrive on the backside of the frontal
system, lingering into the start of the new work week, before sfc
high pressure drifts over the Great Lakes. While this sfc high
drifts into the region, a rapid moving upper level low will track
across the northern Great Lakes into the ne CONUS. With sfc high
pressure around during this timeframe, limited moisture at the sfc
should prevent any precip from developing. The sfc high will then be
ushered quickly to the mid-Atlantic coast Tuesday as the next
frontal system deepens across the upper Midwest.

Southwest winds return Tuesday ahead of the main frontal boundary,
bringing a brief return of hot and humid conditions to the region.
Heat indices are currently forecast in the upper 90s to low
100s, with high temps in the mid 90s. The low pressure system
driving this frontal boundary will track eastward across
southern Canada, while the trailing boundary will drape through
from the Great Lakes into the eastern Plains. Dependent on
moisture axis flowing into the region during Tuesday, could
produce early pre-frontal showers and storms across portions of
the Midwest, with focus on southern Great Lakes into the mid-
Mississippi River Valley. The main wave of showers and storms is
forecast to track through the area late Tuesday evening into
early Wednesday morning. Some model depictions are showing a
weak (almost non-existent) upper level wave developing as this
system moves out of the southern Great Lakes into the Ohio River
Valley region, with a slow southward progression Wednesday
could occur. This would linger precipitation chances through
Wednesday morning for areas south of the I-80 corridor.

After the front pushes out of the region, the remainder of the week
looks dry with seasonal conditions prevailing across the region.

Baker

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1252 AM CDT Sat Jun 22 2024

Aviation Key Messages Include:

- Scattered showers and a few thunderstorms will move near
  RFD/DPA/ORD this morning (high confidence).

- Breezy southwest winds with gusts of 25-30kt will prevail from
  mid-morning through the evening (high confidence).

- A line of thunderstorms will sweep across the terminals this
  evening (high confidence) accompanied by northwesterly winds
  gusting 25-35kt (medium confidence), and trailed by MVFR cigs
  (medium confidence).

Discussion:

Through 12Z:

Ongoing scattered showers and thunderstorms over northwestern
Illinois will continue moving slowly northeastward throughout
the night. Opted to introduce targeted TEMPO groups between
07-09Z at DPA/ORD for TSRA. The showers and thunderstorms may
graze the 10-mile vicinity ring of MDW, but for now, opted to
keep all precipitation out of their TAF this morning. Meanwhile,
at RFD, will go out the gate with VCTS through 09Z as the
current cells appear to be on a trajectory to just miss the
airfield.

Outside of showers and thunderstorms this morning, a slowly-
moving outflow boundary with a northeasterly wind shift is in
the process of stalling along a line from Elgin to MDW.
Confidence is low on whether it will actually reach MDW, though
if it does, speeds should be less than 5 kt (given a forward
motion of the boundary of a mere 3 kt). Finally, will have to
keep a close eye on an area of marine mist and LIFR/IFR cigs
lagging behind the outflow boundary near PWK. The forward
progress of the BR/IFR cigs is slowing, so confidence is
cautiously growing that it will not reach ORD, let alone MDW.
Regardless, trends will be monitored throughout the night (visby
may lower to 2-3SM and cigs may lower to 400-500ft if the bank
does reach ORD).

After 12Z:

Southwest winds will become increasingly breezy after sunrise
as the low-level pressure gradient gradually packs overhead.
Confidence is high in gusts over 25kt, and medium in a few gusts
nearing 30kt, particularly this evening. The expectation is
that all terminals should be dry from daybreak through at least
mid-afternoon. However, will have to keep an eye on ongoing
thunderstorms in northwestern Iowa for signs that they build
southward into the feeding LLJ and roll along the Wisconsin
state line from mid-morning through mid-afternoon. In fact,
there may be a somewhat continuous west-to-east line of
convection near the Wisconsin state line at some point this
afternoon. All things considered, felt introducing a 6-hour
PROB30 for TSRA at RFD from 16-22Z was appropriate given a low
confidence but high impact event.

This evening, a line of thunderstorms is then expected to sweep
southeastward across the terminals (generally between 23Z to 05Z
across the airspace). Confidence is high enough in
thunderstorms to convert the inherited PROB30 groups into
targeted TEMPO groups, albeit maintaining longer than needed
4-hour long windows to account for continued wiggle room in the
time of arrival of the line. Later TAF packages should offer
refinements in the timing (to be 2 hours or so) at all
terminals. A northwesterly wind shift with gusts of 25 to 35kt
appears likely with the line of storms. Gusts may even locally
exceed 40kt (particularly at RFD). In addition, visibility may
drop to 2 miles or lower in the most vigorous convective cells
given very high environmental moisture content. Confidence is
high that there will be thunderstorms this evening, and medium
in the magnitude of wind gusts and drops in visibility as they
arrive.

Finally, cigs will attempt to build downward into MVFR behind
the line of storms this evening and prevail through the
remainder of the TAF period. In fact, pockets of IFR may
develop, as well. Confidence is medium on cig trends after 00Z.

Borchardt

&&

.CLIMATE...
Issued at 337 AM CDT Fri Jun 21 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...None.
LM...Dense Fog Advisory until 4 AM CDT early this morning for the
     IL nearshore waters.

&&

$$

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