799
FXUS63 KLOT 080202
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
902 PM CDT Sun Jul 7 2024

.KEY MESSAGES...

- Spotty showers and perhaps an isolated thunderstorm remain
  possible through midnight, then chances (30 to 40 percent) for
  scattered showers and thunderstorms may increase again after
  midnight across mainly the northwest 2/3 of the area.

- A cold front will move over the area on Monday providing
  another chance for scattered showers and thunderstorms.

- Post-Tropical Depression Beryl will move through the Ohio
  River Valley or Lower Great Lakes region Tuesday night into
  Wednesday night with a corresponding threat for heavy rain and
  gusty winds.

- Heat and humidity levels will increase toward the end of next
  week and continue into the weekend.

&&

.UPDATE...
Issued at 901 PM CDT Sun Jul 7 2024

Widespread thunderstorm chances appear to be decreasing for the
overnight hours, though scattered showers and isolated
thunderstorms will remain possible. Lowered pops through
remainder of the evening hours for mainly isolated showers, and
kept chance pops/slight chance thunder after midnight mainly
across the northwest 2/3 of the cwa.

Evening water vapor imagery continues to show an upper trough
across the upper Midwest, eastern Dakotas and central Plains. A
mid-level short wave was noted rotating slowly northeastward
across the lower Missouri into the mid-Mississippi valley, while
there was some evidence of a remnant MCV over west-central IL
associated with a rapidly decaying convective complex. Locally,
afternoon storms driven by warm/moist advection atop a
southward-sagging outflow boundary have dissipated with sunset
and the exhaustion of low-level instability. Through the
remainder of the evening hours, spotty light showers will likely
continue, as modest 15-20 kt southwest 925- 850 mb flow
continues to provide ascent atop the stable surface layer.
Lightning threat appears non-zero but pretty low through
midnight. Scattered shower/isolated thunder threat may increase
a bit after midnight as weak low-level theta-E advection
persists and the lower Missouri Valley short wave lifts
northeast across the area, and mid-level lapse rates steepen
slightly especially across the northwest 2/3 of the forecast
area. Did ease back on pops a bit (though kept them in chance
range) as 00Z CAMs (HRRR, RAP) have cut back markedly on
convective coverage overnight, though was hesitant to lower too
much after midnight given weak but persistent signal for
continued modest ascent and weak elevated instability.

Ratzer

&&

.DISCUSSION...
Issued at 241 PM CDT Sun Jul 7 2024

Through Monday:

Primary forecast/challenge remains thunderstorm chances, timing
and coverage through Monday night. Current activity continues to
gradually increase in coverage but lightning has been isolated
as poor lapse rate and warmer air aloft have limited more robust
updrafts. However, as coverage continues to increase, expecting
a gradual increase in lightning activity through mid/late this
afternoon. An outflow boundary from earlier convection currently
across Kane/DuPage/Cook Counties could also serve as a focus
for new development over the next few hours. While no severe
weather is expected, the strongest storms will be able to
produce gusty winds, perhaps locally as high as 40 mph as well
as brief heavy rainfall. Instability decreases to the
southeast, with the southeast third or so of the cwa, likely
staying dry this afternoon.

Additional thunderstorms are developing across northern MO and
far southeast IA and this activity may move northeast into parts
of the area this evening, especially across northwest IL, though
there remains quite a bit of uncertainty as the models are
struggling with timing/location. After a lull in the precip
during the early evening, have increased pops again by mid/late
evening across the northwest, but low confidence for timing and
coverage. How this activity evolves this evening will influence
precipitation trends both overnight and on Monday. There will
likely be a lull in the precipitation on Monday, perhaps in the
morning with new development possible in the afternoon along an
approaching cold front. There is quite a bit of uncertainty for
coverage Monday afternoon and opted to maintain pops in the
40-50% chance range, but these may end up being too high.
Regardless of precip coverage Monday afternoon, precip is
expected to end from northwest to southeast Monday evening as
the front moves through the area.

Current temps in the mid 80s are likely the highs for today with
temps falling in the 70s where precipitation is occurring. Highs
on Monday could be similar today, lower/mid 80s but if precip
does materialize with scattered coverage that may hold highs a
few degrees cooler. Lows tonight and Monday night expected to be
in the mid/upper 60s, warmer in Chicago. cms

Monday Night through Sunday:

Tuesday night through Wednesday night, approaching upper-level
troughing from the northwest and a narrow ridge over the
southeastern United States are expected to guide what will be
post tropical depression Beryl into the Ohio River Valley or
Lower Great Lakes region. The exact track and strength of the
system will ultimately depend on the amount of phasing that can
take place with the low-level circulation and upper-level
trough, with more (less) phasing supporting a more northwesterly
(southeasterly) track. The latest forecast from the NHC appears
in line with available ensemble model guidance, and depicts the
center tracking generally from just east of St. Louis Wednesday
morning to Detroit Thursday morning. Of course, the impacts of
the system will be felt beyond its center, and may include
parts of northeastern Illinois and northwestern Indiana
(particularly on Wednesday afternoon and evening).

Along and to the northwest of the track of the system, there
will be a pronounced threat for heavy rain given the overlap of
synoptic-scale lift provided by the phasing upper-level trough
(coupled upper-level jets and frontogenesis) and the
northeastward advection of tropical-quality moisture (PWATs
>2.25"). While the thermodynamic and moisture profiles will not
be overly supportive of thunderstorms, at least modest upright
instability will support a threat for occasional convective
enhancement to otherwise tropical/warm-rain processes. Pattern
recognition, deterministic model guidance, and individual
ensemble member QPF supports a threat for 24-hour rainfall
totals to range from 2 to 4 inches (locally 6 inches) in the
heart of the band of heavy rain. Even with antecedent dry
conditions, such rainfall totals over a short period of time
would lead to flooding in low-lying areas and rapid rises in
river and stream levels.

Conceptually, there will likely be a sharp northern edge to the band
of heavy rainfall with conditions changing from overcast skies to
tropical downpours over short distances. Much like determining the
edge of a swath of heavy snow in the wintertime, determining
the edge of the heavy rainfall from merging synoptic and
post-tropical systems is challenging more than, frankly, 12
hours out. With that said, ensemble model guidance continues to
favor areas along/southeast of I-55 as having the highest threat
for heavy rainfall as the system moves by. With that said,
adjustments northward or southward in the path of the system, as
well as any more or less phasing with the approaching upper-
level trough, will correspondingly shift where and at what
amount rain falls.

Outside of heavy rain, the system will also bring a threat for
strong winds around the perimeter of the circulation. The range
in potential wind speeds will ultimately depend on the track and
strength of the system, with a middle-of-the-road approach
favoring sustained speeds of 10 to 15 mph and gusts to 25 mph
along and southeast of I-55. The 12Z NAM is a reasonable worst
case scenario with sustained speeds of 20 to 25 mph and gusts to
45 mph along the perimeter of the center of the system. With
the path of the system currently favored to pass south of Lake
Michigan, the northerly component of winds down the spine of the
lake will likely lead to dangerous swimming conditions,
particularly Wednesday afternoon and evening.

The post-tropical system is expected to quickly leave the
region Wednesday night, though remnant upper- level troughing
may nevertheless support continue showers and a few
thunderstorms into Thursday. Looking toward next weekend,
ensemble model guidance supports the development of an upper-
level ridge across the central United States with the "downward"
branch oriented into the Great Lakes. Such a pattern will
support rising temperatures and humidity levels, as well as
chances for episodic thunderstorm clusters.

Borchardt

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 631 PM CDT Sun Jul 7 2024

The primary aviation weather concerns for the 00Z TAF period include:

* Intermittent, mostly non-impactful showers this evening and
  tonight

* A potential for additional thunderstorms Monday afternoon and
  evening

The thunderstorm threat has just about ended for the evening and
precip coverage has become much more scattered. Pockets of showers
will continue to move across the area through the remainder of the
night. This rain will be mostly non-impactful, but any heavier
shower that falls on an airfield could knock vsbys down for a brief
period. And while the thunder threat will remain very low, an
isolated lightning strike or two cannot be ruled out tonight.

We may find a few isolated showers floating around during the
morning on Monday. Chances build into the afternoon and early
evening when isolated to widely scattered thunderstorms are
anticipated, primarily across the Chicago metro and points southeast
with lesser coverage up near RFD.

Meanwhile, NE winds near 10 kt will subside to closer to 5 kt and
veer to SE through the early evening. Direction could get squirrelly
at times INVOF nearby showers. Winds may end up going light and
variable overnight, but direction looks predominantly SE or SSE.
Some enhanced flow just off the surface may provide the isolated
gust or two overnight. Winds will take on a SW direction during the
early morning and remain as such throughout the day. Expect wind
speeds tomorrow mostly near or under 10 kt with gusts into the
teens.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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