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444
FXUS63 KLOT 161503
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1003 AM CDT Sun Jun 16 2024

.KEY MESSAGES...

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- There is an increasing threat for severe weather and even
  flash flooding this afternoon and evening mainly across
  northern Illinois.

- A stretch of unseasonably warm and humid conditions will begin
  today and continue through the end of the week. Heat indices
  will be in the upper 90s to around 100 degrees at times.

- Chances for showers and storms from Monday onward are
  relatively low (30% or less), with expected limited coverage.
-- End Changed Discussion --

&&

.UPDATE...
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Issued at 1003 AM CDT Sun Jun 16 2024

Regional radar and satellite imagery depicts a well-organized
mesoscale convective vortex (MCV) near the IA/MO border along
the eastern side of a plume of low-level moisture that extends
toward the Mississippi River. In spite of modest height rises
in Lower Great Lakes behind a synoptic shortwave,
ongoing/increasing convection near the core of the MCV suggest
it will maintain integrity (if not further strengthen) as light
southwesterly upper-level flow pushes it east-northeastward
into northern Illinois this afternoon. With the axis of low-
level moisture poised to slosh northeastward in pace with the
MCV, prospects for thunderstorms in our area this afternoon are
on the rapid rise.

Based on the current motion vector of the MCV and rate of
heating/moistening (to erode a modest subsidence inversion in
the wake of the synoptic shortwave), thunderstorms appear poised
to erupt near or west of I-39 in the 2 to 4 pm timeframe (if not
earlier near the Mississippi River) and spread eastward across
much of northern Illinois through the evening. MLCAPE >2000
J/kg, DCAPE >1000 J/kg, PWATs >1.5", and a pocket of effective-
layer shear >35kt (aided by a compact 500mb speed max induced by
the MCV) will support "upscale growth" into a QLCS with severe
intensity that carries a threat damaging winds in excess of 60
mph. In addition, modest augmentation to the low-level wind
field ahead of the MCV may allow for a tornado with any bowing
segments or embedded supercell structures, as well. Incoming 12Z
CAM guidance favors areas along/north of I-80 toward the
Wisconsin state line as the preferred zone for thunderstorms
this afternoon, though observations of the track of the MCV show
a motion a hair further to the south of model guidance suggest
that areas south of I-80 are not in the clear. All things
considered, made some notable changes to our gridded database to
feature likely PoPs (55+) for thunderstorms this afternoon
centered along the I-88/80 corridors (a hair further south than
model guidance) keeping in mind adjustments to the corridor of
severe weather are possible as the MCV nears.

One other thing to monitor is the threat for flash flooding.
Convection forced by slow-moving MCVs within airmasses
characterized by high moisture-driven instability can produce
prolific rainfall rates, and any training/regeneration of
thunderstorms can lead to highly localized but high-impact
flash flooding. Such a threat will come down to mesoscale
details of low-level confluence at the intersection of the
western side of the cyclonic circulation and any continued
southwesterly low-level synoptic flow. The 06Z NAMNest shows an
apparent "worst case scenario" with regenerating convection
along the backside of the MCV, leading to a swath of 4"+
rainfall amounts along and north of I-80. However, other
available guidance (including incoming 12Z guidance) shows a
more progressive scenario with limited backbuilding. For now,
will continue to assess the structure of the MCV to determine if
the flash flood threat will materialize.

Updated products have been sent.

Borchardt
-- End Changed Discussion --

&&

.DISCUSSION...
Issued at 339 AM CDT Sun Jun 16 2024

Through Monday:

Convection is ongoing early this morning from northeast Kansas
through Iowa and into western/central Wisconsin as a shortwave
trough tracks northeastward across the region. The current
trajectory of the closest precipitation in eastern Iowa should
keep the bulk of it west and north of our forecast area, though
as an associated outflow boundary propagates eastward and
isentropic ascent continues, additional cells may pop up south
of the existing convective activity and track into the northern
half or third of our CWA this morning. Therefore, have continued
to carry chance and slight chance PoPs in the gridded forecast
for this possibility, with the highest probabilities centered
near/over the Rockford metro.

One thing that should not be overlooked as it pertains to
today`s forecast is the possibility of the convection currently
in northeast Kansas evolving into an MCV that tracks into or
near our forecast area this afternoon. Some model guidance had
been hinting at this possibility yesterday, but the signal for
this has been inconsistent from run to run in the models that
have depicted such a scenario, which has lent low confidence to
this solution verifying. Notably though, the 05Z HRRR jumped
back onto the idea of an MCV tracking into northern Illinois
this afternoon, and the 06Z run doubled down on that idea. The
06Z RAP advertises a similar outcome, albeit with a weaker MCV.
It`s hard to put much stock into any one of these model runs
when they have been bouncing back and forth on this like they
have been, but if these latest HRRR and RAP runs are the
beginning of a trend, then significant adjustments may need to
be made to this afternoon and evening`s forecast and to our
messaging as if a relatively robust MCV makes it into northern
Illinois, then part of the area would likely see thunderstorms
this afternoon and/or evening, and these storms would have a
decent chance at becoming strong to severe given that there should
be plenty of instability (about 2000-2500 J/kg worth of MLCAPE)
around during this time. For now, just added some slight chance
PoPs for this afternoon and evening to account for this low
confidence possibility.

Regardless of how today plays out precipitation-wise, today
will mark the beginning of a stretch of hot and muggy weather as
the axis of a strong upper-level ridge stalls to our east and
increasingly breezy southerly flow serves to advect warmth and
moisture of Gulf origin into our CWA. Dew points will climb from
comfortable 50-55 degree readings this morning to much more
humid values in the mid 60s to about 70 by mid-afternoon, while
temperatures rise into the upper 80s and low 90s across the
area. Peak heat indices are thus expected to top out in the mid
90s at most locations this afternoon.

Going into Monday, the upper-level ridge will remain locked in
place, allowing for ample sunshine to further warm the newly
arrived air mass. High temperatures are thus likely to end up
being a few degrees warmer on Monday than they will be today.
There`s still some uncertainty as to how much dew points will
mix out on Monday, with HRRR forecast soundings suggesting that
the 850-700 mb layer will be quite a bit drier than it will be
today, possibly allowing for more efficient mixing to take place
and scour out dew points more readily. Nevertheless, continued
warm/moist advection off of southerly winds and
evapotranspiration should offset this mixing to some degree and
keep dew points from plummeting lower than the mid 60s,
affording us another humid day. The degree of mixing that occurs
shouldn`t have a substantial effect on heat indices as lower
dew points would mean that air temperatures would likely end up
being a little higher, but our latest forecast grids have peak
heat indices getting to about as high as 100 degrees, with some
potential for them to end up being as high as 105 degrees if dew
points struggle to mix out. Either way, it looks like we will
fall shy of meeting Heat Advisory criteria.

Lastly, it remains possible that isolated to scattered pulse-type
thunderstorms could be seen tomorrow afternoon. Most CAMs are quite
bullish on thunderstorms developing and expanding in coverage as new
convection bubbles along the outflow that they spit out. However,
large-scale forcing for ascent looks pretty nebulous tomorrow, and
it appears that we will see slight geopotential height rises as
well, so the thinking is that the CAMs are a little overcooked with
their output, and have continued to carry just slight chance
PoPs for now with the expectation that their coverage should remain
isolated to widely scattered at best in our CWA. Nevertheless,
wherever storms do develop, their rain-cooled outflow will
provide temporary local relief from the heat and humidity, but
there will also be some potential for strong to perhaps locally
damaging downburst winds to occur as deep updraft cores collapse
in an environment characterized by meager deep layer shear.

Ogorek


Monday Night through Saturday:

The northern fringes of a deeper moist plume advecting off the
Gulf of Mexico will arrive in the area on Tuesday, and PWATs look
to build towards 1.75-2 inches through the day. At least some
increase in mid-level cloud cover looks to accompany this
advancing moist plume, and this may curtail mixing a bit compared
to Monday. This should result in slightly higher dewpoints and
lower high temperatures, driving peak heat indices in the mid to
upper 90s. Somewhat gustier southerly winds may help take some of
the edge off though, and this is reflected with slightly lower
wet bulb globe values compared to Monday. Large scale forcing
still won`t be significant, but low-amplitude vort lobes within
the southwesterly steering flow will likely continue parading
around the periphery of the sprawling east coast high. Isolated to
widely scattered mainly afternoon shower/storm chances will
continue, as a result.

Through Tuesday night, a southward-advancing cold front to our
north looks to be pretty active across Iowa, Minnesota, and parts
of Wisconsin with widespread convection. While it`s possible some
of this activity tries to ooze into our northwest into Wednesday
morning, thickness gradients and LLJ orientations seem to point to
any upwind propagating portions of MCSs building preferentially
into southern Iowa, while the forward propagating components scoot
across central/northern Wisconsin.

On Wednesday, the aforementioned cold front will have sagged
further southward into southern Wisconsin as outflow from
overnight activity pushes closer to our region. With diurnal
thunderstorm activity igniting in a bit closer proximity, higher-
end chance PoPs across northwest Illinois seem appropriate during
the afternoon. The thermodynamic/kinematic parameter space
suggests pulse convection will be the norm, with heavy rain (due
to slow cell motions) and gusty winds as the main threats, if
activity manages to sneak south of the Wisconsin state line. With
the core of the parent upper ridge expected to be situated near
Long Island, highs on Wednesday will probably be near where they
were on Tuesday although there is a decent spread across the
guidance suite. The latest gridded forecast shows heat indices
generally in the mid and upper 90s again.

On Thursday, guidance suggests the east coast high may attempt to
build farther west which would increase thicknesses locally and
support a higher ceiling on air temperatures compared to the
previous two days. However, it`s possible the portion of the
quasistationary boundary/cold front across southern Wisconsin
starts to move southward, with some acceleration as a lake
breeze/lake-enhanced boundary. Latest indications are this
probably wouldn`t occur until later in the day and for a small
part of our forecast area across northeast and northern Illinois,
but there`s quite a bit of uncertainty regarding this evolution.
Ahead of wherever the front ends up during the afternoon, eyeing a
potential for temperatures to surge a bit within the thickness
bump ahead of the synoptic front, along with dewpoints to locally
pool which could result in a corridor of notably higher peak heat
indices. Pinpointing where (and if) this will set up remains too
unclear to try to paint this in the grids for Thursday, but
something we`ll be keeping an eye on.

Looks like another toasty day on Friday as the front lifts back
northward, but we`ll eventually push a reinforcing front through
here on Saturday or Sunday. This will bring increased chances for
thunderstorms to the area along with a cool down into early next
week.

Regarding heat headlines: while this continues to look like an
extended stretch of anomalously warm and humid conditions, it
still looks like we`ll end up largely under local Heat Advisory
criteria with peak heat indices generally near or under 100
degrees. However, if these values creep up a bit more,
particularly Wednesday and Thursday, there could be an argument
for an advisory due to the extended nature of 100+ values. We`re
not seeing a strong enough signal for this right now, however.

Carlaw

&&

.AVIATION /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 620 AM CDT Sun Jun 16 2024

Aviation weather concerns are:

- Potential for showers and storms this morning into the early
  afternoon, mainly INVOF RFD, but the threat could extend into
  the Chicago-area terminals.

- Winds becoming gusty and veering to the southwest through the
  morning and afternoon.

- Another window for storms late this afternoon and evening.
  Low confidence at this time, but PROB30s added to the TAFs.

Winds will become gusty quickly this morning as mixing into the
base of a low-level jet occurs. Expecting prevailing 180-190
directions at the Chicago-area terminals by mid-morning.

Showers and storms have developed near the Mississippi River at
this time and will push into the vicinity of RFD through 7-10
AM. It remains a bit unclear whether this activity will develop
far enough southward to bring TS chances into the Chicago-area
sites. The latest indications are that the best TS chances will
slide north of ORD, DPA, and MDW, but it might be a close call.
The main time window of interest is about 14-18z.

Another round of storms may develop late this afternoon and
evening, although confidence in this round is also quite low as
an MCV currently spinning across northwest Missouri tracks
towards the region. Earlier added PROB30 groups to account for
this secondary activity. If storms do fire, strong wind gusts
would be possible in addition to localized heavy rainfall and
small hail.

Finally, if storm coverage is extensive, winds would likely
acquire an ESE or SE direction for a period this evening, but
this level of detail can`t be added at this time. Winds would
then gradually return to the southwest through the night. VFR
conditions and SW winds would then prevail through the rest of
the TAF period.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Air Quality Alert until 1 AM EDT Monday for INZ019.

LM...None.

&&

$$

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