997
FXUS63 KLOT 080555
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1255 AM CDT Sat Jun 8 2024

.KEY MESSAGES...

- Isolated to scattered rain showers expected today. A repeat
  of the washout last Saturday is not expected.

- Pleasant conditions are expected Sunday through the middle of
  next week with limited (15% or lower) chances for rain and
  seasonable temperatures.

- A shift toward warmer temperatures and higher humidity
  appears on track to arrive after the 2nd week of June.

&&

.UPDATE...
Issued at 959 PM CDT Fri Jun 7 2024

Incoming evening guidance suggests there may be an increasing
chance for a few thunderstorms tomorrow towards midday and into
the afternoon. While the severe weather threat remains low,
trends have been towards perhaps a bit more availability of
surface-based instability, particularly south of I-80 during
the afternoon, although the magnitude of instability will
depend on morning cloud cover/shower activity.

The initiating wave of interest appears to be embedded within a
zone of strong shear vorticity currently dropping south across
North Dakota late this evening. At the surface, a meso
low/troughing across South Dakota is forecast to scoot eastward
overnight and will be crossing the Mississippi River towards
midday tomorrow. While lapse rates are not steep upstairs, just
enough surface moisture may surge northward to carve out a
sliver of MLCAPE during the afternoon to the south of a surging
cold front. Noting a pretty big difference in both surface
dewpoints and air temperatures in the latest guidance, which
seems to explain the disparity in modeled CAPE. However, with it
not currently looking like morning showers and cloud cover will
be overly expansive, there seems to be a window for a theta-e
axis to build into parts of the area, mainly south of I-80 and
east of about I-55 during peak heating. With pretty impressively
veering surface-500 mb flow, deep (and low) layer shear doesn`t
appear to be lacking. Lots of "ifs" in this scenario, but looks
like we can`t entirely throw out the idea of a few more
organized stronger to potentially severe storms developing in
the aforementioned region tomorrow afternoon.

Otherwise, no big changes needed to the overnight grids.

Carlaw

&&

.DISCUSSION...
Issued at 219 PM CDT Fri Jun 7 2024

Through Saturday night:

National satellite fields augmented by RAP-analyzed pressure fields
indicate a decaying low pressure system in central Ontario and an
expanding high pressure system across the middle United States
beneath broad upper-level northwesterly flow. A pair of upper-level
shortwaves are evident, first over the central Plains and the second
near the Montana and Saskatchewan border. Meanwhile, closer to home,
our area is being treated to a delightful early June day with mostly
sunny skies, a northwesterly breeze, low humidity levels, and
temperatures in the mid to upper 70s.

Tonight, upper-level clouds will increase in advance of the leading
shortwave in the Plains. Initial thunderstorms now developing in
central Kansas are poised to grow upscale into a loosely organized
convective system overnight, which will likely move along the
instability gradient well to our southwest across Missouri.
Meanwhile, a few showers may attempt to develop near the
Mississippi River after sunset associated with ephemeral
filaments of frontogenesis. A residual dry
low-level airmass will be eager to chew up initial raindrops
throughout the night, so we will continue a dry forecast. A
passing surface pressure ridge will encourage winds to become
nearly calm overnight. Lows will range from the upper 50s to
lower 60s.

Chances for isolated showers will begin to tick upward toward and
after daybreak Saturday as weak WAA approach from the west
ahead of the secondary upper-level shortwave. The continued dry
airmass may continue to prove hostile for raindrops to survive
to the surface through the morning, but felt advertising
gradually increasing PoPs toward 20 to 30% by noon was a good
idea. A slightly better push of WAA during the afternoon (and
corresponding lowering cloud bases) should be enough to support
a few areas of scattered showers through the evening hours.
Overall, limited moisture and scant instability should limit the
coverage and intensity of showers and curtail more than an
isolated rumble of thunder. (In other words, tomorrow should not
be a repeat washout like last Saturday). Overcast skies will
limit highs to the low to mid 70s.

Tomorrow night, a weak cold front will slip through the area causing
southwesterly winds to turn northwesterly. Lows will dip to the low
to mid 50s.

Borchardt

Sunday through Friday:

We are entering what looks to be a pretty quiet period through next
week. High pressure will begin building into the area on Sunday
bringing sunny skies and temperatures in the mid-to-upper 70s.
Strong northerly flow will cause temperatures to be cooler Monday
and Monday night with highs in the upper 60s and low 70s and lows
potentially dipping below 50 degrees. However, temperatures
will rebound over the following days.

On Monday, a shortwave is expected to propagate through the backside
of the trough that will be pushing off to the east, however, a very
dry moisture profile should inhibit any precipitation from forming.
The next potential for precipitation comes Wednesday as a low tracks
through Canada with a cold front draped to its south. However, model
guidance is not in good agreement about the progression of the upper-
level shortwave/low, so PoPs remain low (15% and below).

A signal continues to be present in long range ensemble guidance for
a pattern shift to warmer temperatures by the end of the forecast
period. Temperatures look poised to push into the upper 80s and
perhaps even lower 90s by the end of the work week.

Carothers

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1255 AM CDT Sat Jun 8 2024

Key Messages:

- SHRA potential mid-late morning through early evening with
  associated MVFR CIGS/VSBYS

- Northeast wind shift with cold front mid-late afternoon

A pair of disturbances moving across the region are still on
track to bring periods of showers to the area today. Have sped
up the SHRA arrival time with this update based on the latest
hi-res guidance trends. Also opted to convert the prevailing
VCSH to -SHRA due to an anticipated broader time window for
scattered showers across the area. Ceilings will also gradually
trend down to MVFR as shower coverage increases along with
associated reductions in  visibility to MVFR in the steadiest
showers. IFR cigs/vsbys are not out of the question though
confidence is not high enough to include with this update.
While no TS is officially included in the TAF a few embedded
lightning strikes cannot be fully ruled out at the terminals.
The better potential remains mainly south of I-80.

Light and variable winds are expected early this morning,
eventually prevailing SSW after daybreak. A cold front will move
across the area mid-late afternoon. As it does so, a strong
northeast wind shift may occur across the Chicago area
terminals. Accordingly have introduced an 020 direction with
this update at ORD/MDW at 22Z. The timing of this wind shift
will be refined with later updates. Winds then gradually ease
and settle back into a NW direction after sunset.

Petr

&&

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

&&

$$

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