360
FXUS63 KLOT 151730
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1230 PM CDT Sat Jun 15 2024

.KEY MESSAGES...

- Multi-day stretch of hot and muggy conditions beginning on
  Sunday. The highest heat indices (100-105 degrees) are
  currently expected on Monday.

- Daily chances for thunderstorms will return Sunday and
  continue into next week, though coverage is expected to be
  isolated to scattered at best.

&&

.DISCUSSION...
Issued at 313 AM CDT Sat Jun 15 2024

Through Sunday:

Today should be quite similar to yesterday weather-wise as
surface high pressure continues to dominate over the Great Lakes
and the axis of an amplifying upper-level ridge scoots east of
the Mississippi River. High temperatures should once again top
out in the mid 80s at most locations within our forecast area,
though southeasterly return flow will keep temperatures cooler
along the Illinois lakeshore, where highs are unlikely to reach
80F. There should be more high-level cloud cover today as a
convectively- enhanced shortwave begins to crest the
aforementioned ridge, but no precipitation is expected.

Tomorrow (Sunday) will mark the beginning of our upcoming
stretch of hot and muggy weather as a tightening pressure
gradient between the departing Great Lakes surface high and a
strengthening surface low over the northern Great Plains induces
breezy southerly flow that will begin to more rapidly transport
warmth and moisture of Gulf origin into the Midwest. Dew points
should be at quite comfortable levels in the low-mid 50s to
start off the day, but by the late afternoon, the combination of
moisture advection and evapotranspiration from crops will make
it feel a lot more humid outside with dew points approaching and
potentially even reaching the 70 degree mark. Just how high
they will climb will depend on how much mixing occurs during the
daytime, which NCEP and global guidance are still somewhat at
odds about. However, even the HRRR model, which is notorious for
often being too aggressive with mixing dew points out,
advertises 70+ degree dew points being achieved Sunday afternoon
outside of the Chicago metro in its now in-range 00Z run, so
felt comfortable nudging forecast dew points for Sunday upward a
few degrees in our gridded forecast database. Assuming that
these higher dew points are realized, air temperatures are
likely to top out near 90F/in the low 90s, which would translate
to heat indices peaking in the mid to upper 90 degree range
Sunday afternoon.

While most of the area should remain dry on Sunday, there is a
lower-end chance for showers and maybe even an isolated storm
or two to be observed in our northern counties during the
daytime. Whether that comes to fruition or not appears to be
largely dependent on how convection across Iowa plays out
tonight. Some model solutions have been depicting the
development of an MCV from the Iowa convection that ends up
lagging behind the main shortwave trough and tracks into
southern Wisconsin after daybreak tomorrow. This kind of outcome
would be the most auspicious for showers and/or storms being
observed in northern Illinois tomorrow, though even if a
trailing MCV did not develop, the main shortwave may pass by
just close enough for some of its associated shower activity to
graze part of our CWA. At any rate, PoPs in the slight chance
range best characterize Sunday`s precipitation potential at this
time, so have maintained those along and north of I-88 with
this latest forecast package.

Ogorek


Sunday Night through Friday:

Hot and muggy conditions are expected on Monday, although
questions remain regarding temperature and dewpoint trends
through the day. While there`s high confidence peak heat indices
will near or locally/briefly exceed 100 degrees, how we get
there still remains a bit unclear (is it highs in the upper 90s
and dewpoints in the mid 60s, or highs in the mid 80s and
dewpoints in the mid 70s?). The GFS continues to be a moist
outlier with a notably shallower PBL depth compared to the
ECMWF, Canadian, UKMET. Even the NAM, which has a propensity to
severely undermix is advertising higher air temperatures and
lower dewpoints compared to the GFS. For these reasons, and
based on how strongly we`ve been mixing recently, continue to
generally toss the GFS solutions on Monday. This results in a
warmer high temperatures in the mid and upper 90s and mid-upper
60s dewpoints driving peak heat indices near 100, locally a bit
higher. While conditions will be hot and uncomfortable, these
conditions aren`t uncommon in our area in June and the latest
guidance continues to point towards things settling just under
Heat Advisory criteria on Monday.

Regarding thunderstorm chances: suspect the GFS continues to
depict a bit too much in the way of convection owing to its less
deeply mixed PBL which is driving higher dewpoints and less
afternoon MLCIN, particularly on Monday. Obviously, if we do
indeed fail to mix as deeply as the rest of the global guidance
indicates, a bit more in the way of "airmass" and pulse-type
afternoon activity would be possible. Additionally, there really
don`t look to be any significant large scale forcing mechanisms
nearby, so capping things to isolated/widely scattered seems
like the way to go. Surface-700 mb theta-e deficits nearing 30 C
might support a gusty downburst wind threat, but again storm
coverage is not expected to be significant.

Tuesday may feature some slightly better chances for scattered
showers and storms as a plume of deeper moisture emanating from
the central Gulf of Mexico surges northward as an anomalously deep
high builds a bit farther east across southern New England. In
addition, there`s a potential for increased mid-level cloud
cover with this moist plume, and it seems like most guidance is
suggesting high temperatures may "cool" a few degrees from
Monday`s as a result. The GFS may once again be advertising too
much in the way of convection with resulting highs only in the
upper 70s/to near 80. The blended offering of highs near 90
seemed like a good middle ground.

On Wednesday, a cold front is expected to press southward into
Iowa and Wisconsin and will serve as a focus for greater
coverage of showers and storms, some of which may meander into
our forecast area. This boundary looks like it`ll stall or
become quasistationary into Friday which will keep some degree
of storm chances running, primarily in our north and northwest.
Upper-level winds are not expected to be all that strong, and
while a southward-oozing convective complex can`t be ruled out
during this period, conditions don`t currently look supportive
of organized severe MCS development.

Carlaw

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1230 PM CDT Sat Jun 15 2024

Key Aviation Impacts for the current TAF period:

- Gusty southwest winds Sunday

- Chance (30 percent or less) for scattered showers and storms
  around KRFD Sunday morning

A broad swath of high level clouds will continue to move over
the airspace, but VFR skies and southeasterly winds are
expected. The occasional gust up to 20 knots or potentially the
direction flops to 180 or 190 once or twice are possible, but
are not expecting to be prevailing.

Winds will become out of the southwest as a low level jet aloft
increases. Better mixing should keep LLWS out of the TAF and
allow for wind gusts to make it to the surface between 20 to 25
knots through Sunday.

While the bulk of the storms currently in Iowa and Minnesota
will move northeastward into Wisconsin overnight and into
Sunday, there should be just enough instability and moisture
present for the chance for a shower or thunderstorm near KRFD so
a PROB30 group was introduced into the TAF. If the showers over
KRFD do materialize, it would not be surprising if a rogue non-
impactful shower made it to the Chicago terminals, but thunder
is not expected so the the TAF was kept dry for the time being.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Air Quality Alert from 1 AM Sunday to 1 AM EDT Monday for
  INZ019.
LM...None.

&&

$$

Visit us at weather.gov/chicago