974
FXUS63 KLOT 200629
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
129 AM CDT Thu Jun 20 2024

.KEY MESSAGES...

- Widely scattered thunderstorms are possible this evening
  mainly along and north of a Dixon to Waukegan line which
  could produce locally gusty winds.

- Hot and muggy conditions will prevail through the end of the
  week, although cooler conditions are expected near Lake
  Michigan on Thursday and Friday.

- Periodic chances for thunderstorms will continue, though most
  hours will remain dry.

&&

.DISCUSSION...
Issued at 252 PM CDT Wed Jun 19 2024

Through Tonight:

Increasing coverage of cumulus this afternoon combined with
deep mixing has helped keep heat indices mostly in check so far
this afternoon with temperatures in the upper 80s to lower 90s.
Expect a couple more degrees of warming yet this afternoon with
peak heat indices in the 95-100 degree range.

Dry conditions prevail at this hour; however, attempts at
shower/storm development is already underway across portions of
northwest Illinois into south central Wisconsin ahead of an
approaching cold front. Expect this activity to gradually expand
in coverage as it shifts southeastward into the Rockford area
and along the Wisconsin-Illinois stateline. RAP mesoscale
analysis highlights a narrow axis of 1000-1500 J/Kg of MLCAPE
ahead of the front which intersects a region of DCAPE values to
1000 J/kg (inverted V soundings) with slow storm motions and
little shear (~15kts) suggests that any storms that develop
would be rather pulse-like and capable of strong downbursts
resulting in locally gusty winds to 50 mph for areas mainly
along and north of a Dixon to Waukegan, IL line. A very
localized gust to 60 mph can`t be fully ruled out.

Hit and miss isolated storm development still can`t be fully
ruled out south of these areas into Chicago and south of I-80
but this potential remains low with most areas expected to
remain dry. Accordingly will continue capping shower/storm
chances at 20% in these areas.

In the wake of the frontal passage over the lake, expect fog
development over the lake overnight into Thursday morning, this
could ooze inland particularly along the immediate shoreline. It
is possible that it pushes further inland, per the latest RAP,
but confidence in that scenario occurring is not especially high
and will let the evening shift get another look at that
potential.

Petr


Thursday through Wednesday:

A frontal boundary will be meandering across the cwa on Thursday
and will likely be the focus for at least isolated
thunderstorms with perhaps the best coverage along/north of I-80.
With northeast winds off of Lake Michigan, there may also be
enough convergence for isolated thunderstorms in the Metro area
as well. Overall confidence is low and current 30-40% pops
north to 20% south seem reasonable for now. Some of this
activity may linger into Thursday evening, but there should be a
gradual decrease in coverage with most of Thursday night dry.

High temps on Thursday may be back into the lower 90s for most
areas, especially south of the boundary mentioned above. Lowest
confidence for temps is generally from I-88 north and then
across then near the lake. Temps will be coolest near Lake
Michigan, perhaps only in the upper 70s/lower 80s right along
the shore. Where temps do make it to the lower 90s, heat index
values may be in the mid/upper 90s.

Most recent trends in guidance would suggest that Friday into
Friday night may end up mainly dry across the area. If any
precip did develop, probably near the IL/WI state line Friday
afternoon. Lowered pops to just slight chance (20%) on Friday
and removed pops for Friday night. High temps on Friday are
likely to be back in the lower to possibly mid 90s for most
areas, except cooler near Lake Michigan with northeasterly
winds. Heat index values would again be in the mid/upper 90s,
away from the lake.

Winds turn southwest Saturday and increase with gusts possibly
into the 30 mph range, ending any cooling near the lake. High
temps will reach the mid 90s for most locations but with
dewpoints in the mid/upper 60s, heat index values will peak in
the upper 90s to around 100.

Thunderstorm chances will be increasing Saturday afternoon into
Saturday evening as a cold front moves across the area. Best
timing may be after sunset which may limit instability and
coverage. Despite a larger scale feature, guidance still
showing a range of possibilities from the potential for heavy
rain to only scattered coverage. Blended pops are in the likely
range (70%) and this seems fine from this distance, though
duration is likely to only be a few hours at any one location.
Precip also looks to be progressive with perhaps some lingering
showers Sunday morning with most of Sunday likely ending up dry.
Dry weather is then expected through at least Tuesday morning.
By that time the models show another potential chance for
thunderstorms sometime from Tuesday afternoon into Wednesday
morning and while confidence is low, this has been a time period
the models and their ensembles have been highlighting for
potential thunderstorms.

The frontal boundary Saturday night/early Sunday morning will
bring slightly cooler and slightly drier air, but given peak
sun angle and mainly sunny skies, high temps Sunday and Monday
may still reach the upper 80s for most locations. cms

&&

.AVIATION /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 129 AM CDT Thu Jun 20 2024

Key Messages:

- Potential for low clouds and mist/fog early this morning and
  again early tomorrow (Friday) morning.

- Scattered showers and isolated thunderstorms expected this
  afternoon.


Fog is expected to develop along the Lake Michigan shoreline
overnight and may then creep inland and affect ORD, MDW, and/or
GYY -- most likely as low stratus, though some visibility
reductions can`t be ruled out from this either. Confidence in
MVFR or lower conditions prevailing from this is currently low,
so have continued to carry prevailing VFR conditions in the
TAFs for now and will continue to monitor how things evolve. In
addition, with the light winds, mostly clear skies, and moist
boundary layer, patches of radiation fog may also develop inland
and produce visibility reductions at DPA, GYY, and/or RFD before
dissipating after sunrise.

Slightly better large-scale forcing for ascent today will likely
lead to a greater coverage of showers and storms this afternoon
compared to yesterday. In particular, a frontal boundary that is
expected to stall out in the area today may serve as a focus
for convective development near the terminals. Thunderstorm
coverage may still only be isolated, though, due to a mid-level
warm nose inhibiting vertical cumulus growth, and the hit-or-miss
nature of these "pulse"-like storms lends low confidence to any
one TAF site being impacted by a storm as well, so have continued
to keep any TSRA mentions confined to PROB30 groups for now.
Needless to say, any deeper convection that develops this
afternoon will likely produce sub-VFR visibilities and gusty
winds. Shower and storm coverage will then dwindle this evening
with the loss of diurnal instability.

Similar to early this morning, the overnight into early Friday
morning time period should also be characterized by light winds,
mostly clear skies, and a moist boundary layer in the wake of
this afternoon`s and evening`s showers. Thus, it is possible
that patches of fog may develop near DPA, GYY, and/or RFD once
again and that marine-induced fog and/or low clouds may also
ooze inland and affect ORD, MDW, and/or GYY once again.
Confidence in sub-VFR conditions being observed early tomorrow
morning is currently low, though think that whatever happens
this morning could be a harbinger for what might happen tomorrow
morning.

Ogorek

&&

.CLIMATE...
Issued at 347 AM CDT Wed Jun 19 2024

With a low temperature of 78 degrees, Chicago broke its warmest
low temperature record for June 18th yesterday, while Rockford
tied its daily warmest low temperature record of 74. A few
additional temperature records (namely today`s warmest low
temperature records and Saturday`s high and warmest low
temperature records for Chicago and/or Rockford) may be
threatened over the next few days.

Here are the daily high and warmest low temperature records
for Chicago and Rockford for each day through Saturday, June
22nd:

             Chicago
----------------------------------
Day:           6/19 6/20 6/21 6/22
----------------------------------
Record High:    102  104  101   97
Record Warm Low: 78   78   74   76

             Rockford
----------------------------------
Day:           6/19 6/20 6/21 6/22
----------------------------------
Record High:     99  101  100   97
Record Warm Low: 76   73   71   73

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Air Quality Alert until midnight CDT /1 AM EDT/ tonight for
     INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.

&&

$$

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