003
FXUS63 KLOT 121116
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
616 AM CDT Fri Jul 12 2024

.KEY MESSAGES...

- Isolated showers/thunderstorms possible across far southeast
  cwa this afternoon. Isolated to widely-scattered storm chances
  develop for most of the area Saturday.

- Hot and humid conditions expected early next week. Heat
  indices will likely approach or exceed 100 degrees.

- Periodic thunderstorm chances Saturday through mid next week,
  including the potential for a couple nocturnal/early AM
  t-storm complexes.

&&

.DISCUSSION...
Issued at 407 AM CDT Fri Jul 12 2024

Through Saturday:

Monitoring patchy fog early this morning generally southeast of
a Chatsworth to DeMotte line. Some patchy dense fog is
possible, and a short-fuse dense fog advisory may be necessary
if visibilities lower over a widespread area there.

Early morning GOES water vapor imagery depicts an upper trough
over the western Great Lakes region, with an embedded vort
rotating southeastward along the southwestern shore of Lake
Michigan. This vort and the main mid-level trough axis are
progged to shift off to the east of the area by midday. Mid-
level height rises develop in its wake today, with subsidence
reflected at the surface by weak high pressure ridging which
settles across southern Lake Michigan and northern IL this
afternoon. Subsidence is also evident in forecast soundings with
warming of mid-levels, resulting in generally weak lapse rates
in the 900-600 mb layer. Subsidence is weakest over our
southeastern cwa however, which along with slightly higher
surface dew points in the mid-upper 60s this afternoon does
yield some weak instability of around 1000 J/kg MLCAPE by early
afternoon. While large scale forcing does not appear very
conducive for developing convection, weak but slightly
convergent low level winds may be the focus for a few isolated
showers or thunderstorms from east-central IL near the IL/IN
border eastward into northwest IN, and have maintained some
slight chance (<25%) pops there this afternoon. Elsewhere, dry
and warmer weather is expected with highs in the mid-80s except
along the Lake Michigan shore where onshore winds will keep
temps in the upper 70s.

The forecast area will be beneath mid-level short wave ridging
this evening, though another short wave trough currently
propagating southeast across the Northern Plains will approach
the area after midnight tonight. In advance of this wave,
warm/moist advection develops on a modest southwesterly 15-20 kt
low level jet, ahead of a northwest-southeast oriented warm
front which will near our far west/southwest cwa border by
morning. Most 00Z guidance soundings depict relatively dry
profiles (21Z RAP has noticeably deeper saturation in the
850-700 mb layer into our western cwa by 12Z Saturday), though
even shallow saturation in the right layer would potentially
support elevated convection within steepening lapse rate
environment aloft. Have added a slight chance (15%) thunderstorm
along/west of the I-39 corridor toward sunrise for that
potential, with some 00Z CAM signal for at least
isolated/widely-scattered elevated activity.

The Northern Plains short wave is then expected to slowly
transit the forecast area Saturday, while the surface warm front
develops east-northeastward into the area and 20-25 kt
southwest flow persists aloft. Warming/moistening low-levels
near and southwest of the warm frontal boundary, coupled with
steepening mid-level lapse rates, suggests fairly strong
destabilization during the day, with HREF sfc CAPE in excess of
2000 J/kg by afternoon. Forecast soundings do depict weak lapse
above the boundary layer (900-800 mb) maintaining some weak
capping, with 00Z HREF and latest 06Z CAM guidance only
depicting isolated development. Confidence is therefore somewhat
low in how things will play out, though certainly the potential
for at least isolated strong storms exists Saturday and have
maintained chance (~30%) pops for this during the day.
Thunderstorm/severe chances would be higher if greater
convection were to develop upstream to our west/northwest late
tonight/early Saturday and spread into the area.

Warmer and more humid conditions can be expected Saturday as
the warm front moves into the area. Highs in the upper 80s/near
90 are likely especially if storm coverage is minimal, with lake
cooling limited to the IL shore areas.

Ratzer


Saturday Night through Thursday:

Saturday evening, impressive EML will spread across the
northern Plains into the upper Midwest ahead of an advancing mid
level wave. While isolated to scattered thunderstorms will be
possible during the afternoon, a general lack of forcing looks
to put us in a bit of a convective lull during the evening hours
once the diurnal cap kicks in. However, coverage is expected to
build across the upper Midwest overnight as forcing increases
ahead of the approaching wave, in big part due to a quickly
strengthening LLJ sticking its nose into the region offering
some enhanced low level convergence near the base of the EML.
Deep layer shear also looks to build ahead of the system to a
respectable ~30 kt at 0-6 km. Most medium range guidance
develops an MCS to our northwest sometime in the evening which
could potentially take a dive southeastward into northern IL
overnight. Models tie the system to the sharp instability
gradient on the nose of the EML, but cannot quite agree on the
position/orientation of the gradient. Most guidance extends the
higher instability across WI which locks the more robust
convection just to our north. The Euro has been consistently the
most aggressive for us in that it drapes the instability
gradient southeastward across the CWA and tracks the MCS
accordingly right through northern IL. The NAM NEST has a
similar look to the instability gradient, but is less aggressive
with the convection.

That wave aloft will pass to our northeast by early Sunday with
a good deal of dry air feeding in behind it. This should keep
conditions mostly quiet during the day on Sunday following any
lingering convection into the morning. Most guidance maintains a
weak cap during the day on Sunday, but with the EML still
overhead there may be just enough low level moisture to fire up
a couple of isolated deeper afternoon storms.

The bigger concern on Sunday will be the heat. A pronounced
upper high will set up over the Intermountain West this
weekend which will allow for a hot continental airmass to
overspread the region. With the help of mostly to partly sunny
skies, highs on Sunday are forecast to reach the lower to middle
90s. Lower to middle 70s dewpoints will then pull peak
afternoon heat indices into the 100- 105F range. Luckily, this
synoptic pattern should break early next week. However,
conditions look favorable for another cooker on Monday as the
upper high extends into the central Plains before it retreats
back west and becomes more diffuse for the rest of the week.
Temperatures and dewpoints will be similar to Sunday but looking
like maybe a couple of degrees warmer on both fronts. This
would bring heat indices up to around 100 to almost 110F in
spots. Will have to eye Sunday and Monday closely in regards to
potentially issuing a heat advisory at least for parts of the
area. It`s also worth noting that southwest flow expected both
days will offer little reprieve from the heat near the
lakefront.

The speedy base of a large upper low will begin to swing into
the northern Plains and upper Midwest late Sunday into Monday.
A surface cold front will gradually get stretched across the
upper Midwest and bring a chance for scattered showers and
storms Sunday night into Monday, although the better chances
during this time are up north nearer to the front where we`ll
find the better moisture and especially forcing. The zonally-
oriented front is then expected to drop through the CWA Monday
night and Tuesday providing a more appreciable potential for
showers and thunderstorms. Medium range guidance builds a
impressive reservoir of instability ahead of the sagging front
with the GFS, Euro, and Canadian all in excess of 4,000 J/kg of
MUCAPE. Models also show shear building into Tuesday as the
trough base dips south. That said, the Monday night into Tuesday
timeframe will have to be one to watch for the potential for
severe weather. Conditions should cool and become overall
quieter Wednesday onward after the front moves away and the
upper trough axis pushes east.

Doom

&&

.AVIATION /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 615 AM CDT Fri Jul 12 2024

There are no major aviation weather concerns through the 12Z TAF
period

Light near-northerly winds will steadily veer to ENE during the
earlier half of this morning. Winds should remain under 10 kt
throughout the day. Direction veers to SE late tonight going near-
southerly for Saturday morning. Expect VFR through the period.

Doom

&&

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

&&

$$

Visit us at weather.gov/chicago