153
FXUS63 KLOT 170532
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1232 AM CDT Sat Aug 17 2024

.KEY MESSAGES...

- Scattered thunderstorms late this afternoon and evening,
  mainly near and north of I-80. A few storms may become strong
  to severe with a threat for damaging winds and large hail.

- Scattered showers and isolated to widely scattered t-storms
  on Saturday, then isolated to widely scattered showers, and
  perhaps a few isolated t-storms on Sunday.

- Long duration of dangerous swimming conditions and hazardous
  boating conditions on southern Lake Michigan Sunday-Monday.

&&

.UPDATE...
Issued at 1040 PM CDT Fri Aug 16 2024

Thunderstorms have cleared most of the forecast area as of late
evening, with the exception of our far south and southeast
counties roughly southeast of a Pontiac to Valparaiso line.
These too, should exit the area by or shortly after midnight
tonight. The threat of severe weather has decreased where storms
remain, thanks to low- level diurnal cooling and convective
outflow, though locally heavy downpours and isolated wind gusts
to 50 mph remain possible. Otherwise, skies have become partly
cloudy where storms have exited through increasing cloud cover
is expected to develop from the north overnight.

Ratzer

&&

.DISCUSSION...
Issued at 322 PM CDT Fri Aug 16 2024

Through Saturday Night:

A clear "swirl" pattern is evident in GOES water vapor and
visible satellite imagery this afternoon over the Upper Midwest,
where a vertically-stacked low pressure system is centered at
this time. Along the southern periphery of the low pressure
region, an embedded vorticity maximum can be made out.
Immediately downstream of this vort max, scattered showers and
thunderstorms have blossomed across northeast Iowa and southern
Wisconsin, and the expectation is for this convection to
continue to increase in coverage and intensity as the vort max
steers it our way into an uncapped and increasingly unstable air
mass. Quality low-level moisture has largely remained in tact
here into this afternoon with dew points generally in the upper
60s to low 70s as of this writing, so feel pretty confident
that we`ll get at least a few storms in our forecast area late
this afternoon into this evening, mainly in areas near and north
of I-80, which are positioned closer to the core of the
mid/upper-level low and better large-scale forcing for ascent.
Dry mid-level air has thus far caused our inbound convection to
struggle to take full advantage of the 1000-1500 J/kg of MLCAPE
analyzed in RAP objective mesoanalysis, but if storms can
eventually manage to overcome this dry air, then increasing mid-
level flow going into this evening could enhance shear profiles
enough to support some storm organization, perhaps allowing for
maybe one or two strong to marginally severe storms to
materialize and track across the northern half of our CWA. Most
of this evening`s showers and storms should then depart our area
by midnight, though some straggling spotty showers could hang
around for up to a few hours longer.

The mid/upper-level low will still be in our vicinity come
tomorrow, likely centered over the Lower Peninsula of Michigan.
Its close proximity to our forecast area in conjunction with
steepening low- and mid-level lapse rates should foster another
period of diurnal convective showers across our CWA beginning as
early as the late morning, with peak coverage not expected until
the mid-late afternoon. While lapse rates will not get
particularly steep above 650 mb (where forecast soundings
indicate that a warm nose will be present), equilibrium levels
should still easily exceed the -20C level, so the deepest
convective cells tomorrow will likely contain lightning for a
relatively brief period of time before their updrafts come
crashing down amidst an overall lack of deep-layer shear.
Overall shower coverage should be scattered and generally
greatest across the eastern half of our forecast area, but with
us being only on the periphery of the stronger large-scale
forcing for ascent, the factors inhibiting convective growth and
storm longevity should keep thunderstorm coverage isolated to
widely scattered at best. Shower/storm coverage should then wane
after sunset with the loss of diurnal instability, though lake
effect processes will likely lend a hand in keeping a lingering
stream of broken rain showers going into northwest Indiana
through the night. The daytime showers, associated cloud cover,
and northwest winds should keep high temperatures capped at
about 80F tomorrow.

Ogorek


Sunday through Friday:

The mid-upper level low pressure that will bring diurnally
driven showers and some storms on Saturday will shift into the
eastern Great Lakes on Sunday. While large scale forcing will be
steadily diminishing (mid-level height rises), weak embedded
impulses on the west side of the upper low, lingering mid-level
moisture, and modest destabilization inland of an advancing
lake shadow may yield the development of isolated to widely
scattered showers (highest PoPs 20-40%).

Shower activity should favor inland areas east of I-39 in the
early to mid afternoon. Mid-level moisture will likely quickly
dry out by the late afternoon, causing lingering showers to
dissipate. Regarding thunder chances, generally poor mid-level
lapse rates (5.5C/km or less) are forecast, so with diminishing
forcing and warmer air aloft, expectation is a few isolated
thunderstorms (~20%/slight chance) at most inland. Notably, the
instantaneous lightning flash density product from the 12z ECMWF
kept almost our entire area devoid of any implied lightning.

The slow movement eastward of the weak surface low pressure
associated with the upper low, seasonably strong expansive high
pressure building in from the north and northwest, and relatively
cool air over the warm lake (modest lake induced instability)
will result in an extended period of full fetch brisk northerly
winds Sunday through Monday. Expecting to need a Beach Hazards
Statement and Small Craft Advisory for this period barring a
significant forecast change.

A pseudo-blocky pattern will develop next week, keeping us in
dry northerly mid-level flow aloft between strong ridging over
the southwest and pronounced troughing over northeastern North
America. This will bring dry weather, comfortable temperatures
(slightly cooler by day lakeside), and low humidity through the
work week. Late summer heat may finally make inroads into the
region during the last work week of August.

Castro

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1232 AM CDT Sat Aug 17 2024

Aviation Forecast Concerns:

- MVFR ceilings developing around daybreak and continuing
  through midday.

- Isolated to widely scattered showers and thunderstorms again
  this afternoon into the early evening, especially for Chicago
  metro terminals.

- Low-confidence potential for winds to shift northeast for
  Chicago terminals later this afternoon/evening.

While all is quiet and VFR across the terminal airspace early
this morning, there is a fairly extensive deck of MVFR CIGs
currently noted across southeastern MN and into adjacent areas
of northeastern IA and west central WI. This area of cloud cover
is expected to develop southeastward over the next several
hours, before overspreading the terminial airspace across
northern IL by daybreak this morning. These MVFR CIGs then are
likely to persist overhead through much of morning hours,
before diurnal improvement gradually pushes bases back into the
VFR category sometime around midday.

As we head into this afternoon, an upper level low will
continue to drift slowly east-southeast across the western
Great Lakes. As it does, a couple of smaller mid-level
impulses wrapping around its western periphery will be steered
right across northeastern IL later today. These should each help
foster at least isolated to widely scattered diurnal showers
and thunderstorms again this afternoon, especially across
eastern WI/northeast IL including the Chicago terminals. While
TSRA coverage may not be very high, forecast soundings do
indicate enough instability during the afternoon and early
evening hours to continue the going message of a PROB30 for TS
and a prevailing VCSH.

Westerly winds tonight are expected to become northwest to
north-northwest during the day as the surface low gradually
progresses eastward into towards Lake Huron. A couple of models
suggest a shift to northeast is possible for KORD/KMDW/KGYY
later this afternoon as a lake breeze tries to develop and push
inland. However, confidence in this remains very low at this
distance. It appears the ability of this wind shift to make it
all the way inland to ORD and MDW may ultimately become
dependent upon the ability for any outflow from this afternoons
convection to drive the lake breeze boundary inland. Given the
low confidence with this wind shift at this stage, we have opted
to leave a formal mention out of the TAF at this time.

KJB

&&

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

&&

$$

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