072
FXUS63 KLOT 200555
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1255 AM CDT Fri Jun 20 2025

.KEY MESSAGES...

- A localized area of strong winds may develop in the wake of
  showers/storms late this morning into early afternoon.

- First heat wave of the summer expected this weekend into early
  next week with multiple day of highs well into the 90s and
  peak afternoon heat indices of 100-105F likely.

- Extreme heat watch issued for the city of Chicago and Cook Co.
  where special 3 day extreme heat warning criteria of peak heat
  indices of 100F+ are likely to be reached.

&&

.DISCUSSION...
Issued at 215 PM CDT Thu Jun 19 2025

Through Friday Night:

Regional water vapor imagery depicts implied northwesterly flow
from the northern Plains to the Great Lakes along the backside
of a departing upper-level trough. Partly sunny skies in tandem
with a residually moist boundary layer have allowed for cumulus
to develop this afternoon amidst temperatures in the lower 80s
and a westerly breeze.

Over the past 30 minutes or so, have noted pockets of increasing
cumulus growth including attempts for sustained convection
across DuPage/Cook counties. Suspect this type of behavior will
continue through the afternoon with a 20% chance of a storm at
any given location across northern Illinois and northwestern
Indiana. With some 50kt+ of flow at 500mb and relatively low
freezing levels near 12kft, cannot rule out a storm managing to
become strong and producing gusty winds at hail. Such a threat
should be quite isolated (5% or less chance at any given spot).
Will also have to keep an eye on the loosely-orgnaized cluster
of storms moving through central Wisconsin to graze northeastern
IL this evening with a threat for gusty winds.

With our area on the northern side of a broad surface high
centered to our south, tonight looks quiet locally with mostly
clear skies and overnight lows in the 60s. With that said, it
will be a busy night upstream of our area. Late tonight, a
subtle upper-level wave (currently progressing through the
Dakotas) should excite the development of a low-level jet across
the upper Mississippi River Valley beneath an eastward-
advecting plume of steep mid-level lapse rates. With time,
moistening at the base of the EML should allow for explosive
thunderstorm development across northern Minnesota but also
possible northern Iowa. Thanks to the aforementioned pocket of
dry air positioned across the Great Lakes, the orientation of
the instability axis should be more or less north to south along
the Mississippi River into early tomorrow morning. As a result,
the upscale growth (or development of GWAC) from overnight
convection should be guided toward eastern Iowa tomorrow
morning. With that said, west to northwest mid-level flow would
orient the stratiform region of any convection downstream into
southern Wisconsin and eventually northern Illinois after
daybreak.

While stratiform rain is usually not overly impactful, do have
to note a notable signal in model guidance for the development
of a pronounced wake low/gravity wave train on the eastern side
of whatever convective complex races into Iowa. If such a
gravity wave were to develop, strong descent in the downward
branch of the wave would allow for the low-level jet reach
ground, altogether supporting a "pop" of strong southerly winds
from late morning to early afternoon. HRRR guidance is most
bullish in depicting a nearly 8mb/100mile surface pressure
gradient from I-80 to the Wisconsin state line by noon tomorrow
with associated southerly wind gusts of 50 to 60 mph. Other
guidance, such as the HRDPS and experimental NSSL MPAS CAMs,
appear more reasonable with southerly wind gusts of 30 to 40
mph. This will be something to monitor closely tomorrow morning
as the overall character of the convection and associated
stratiform region can be assessed.

Outside any funny business with winds tomorrow, residual debris
cloud cover will limit the onset of diurnal heating and keep
temperatures in the mid 80s. At this point, don`t see much of a
threat for additional convection to develop in the afternoon
owing to a pronounced dry layer just above the ground (which
itself would be generated by the wake low/gravity wave in the
morning, should it occur).

Borchardt

Saturday through Thursday:

Medium range guidance is in excellent agreement on developing an
anomalously strong early season upper ridge over the Ohio Valley
into the mid-Atlantic region this weekend into early next week.
By Saturday night and Sunday, 500mb heights are forecast to peak
around 597-598dm, which is 2+ standard deviations above normal
for mid-late June. The strength and placement of this mid-upper
level ridge will shunt the jet stream well to our northwest,
from the northern High Plains east-northeast across Ontario and
Quebec. This should keep convection and associated ring of fire
100s of miles to our north and west this weekend, which does
increase confidence in the expected heat, as convection often
disrupts forecast heat near along the periphery of heat domes
like this one. By Monday, northwestern flanks of the upper ridge
look to get beat down just a bit by northern stream trough, but
still appears as though convection should remain well north of
our CWA, but will be a bit closer than over the weekend and
worth keeping an eye on in coming days.

ECMWF and GFS progged 925mb temps to climb into the upper 20s
to near 30C each day, which based on local 925mb temp climo
would support high temperatures in the mid to upper 90s. With
very little cloud cover expected Saturday-Monday, exactly how
hot temps get may be dictated a bit by how deeply mixed the
boundary layer gets during the afternoon.

On Saturday, GFS and ECMWF both keep boundary layer depth
capped off around 5kft, which would prevent afternoon dewpoints
from mixing out during the afternoon. Low level trajectory from
the southwest is from a region of the country and has
experienced abnormally wet conditions over the past 30 days. So,
despite limited contribution from evapotranspiration this early
in the season, the mid-upper 70s degree dewpoints being
forecast by guidance Saturday afternoon actually may not be all
that unreasonable. Tight pressure gradient between low-mid 990s
mb low pressure over the northern Plains and strengthening high
pressure to our east should result in strong southwesterly
winds, likely gusting over 30 to perhaps nearly 40 mph during
the day Saturday. Given the temps in the 90s and forecasted
oppressive dewpoints, the wind will not provide much relief from
the heat, but more likely act likely a blast furnace for anyone
venturing out during the afternoon.

While confidence isn`t high in this, by Sunday and especially
Monday GFS and ECMWF suggest boundary layer looks to become more
deeply mixed. If this occurs, it could result in dewpoints
mixing out and potentially ending up lower than currently
forecast. While this would likely result in slightly lower peak
afternoon heat indices, it would also likely mean ambient temps
could get a couple degrees warmer. Not even out of the question
that a couple locations could make a run at triple digit air
temps if mixing is really maximized.

Regardless of the exact high temperature, exact dewpoints, and
resultant heat indices, the forecast message is the same:
several days of potentially dangerous heat and humidity are
expected this weekend through at least Monday. Have issued an
extreme heat watch for Cook County and the city of Chicago where
extreme heat warning criteria is lower due to the combination of
Urban heat island effects and higher vulnerability to heat in
more urbanized area. For Chicago and Cook County, it appears
likely that the extreme heat warning criteria of 3 consecutive
days of 100F+ heat indices will be met, which combined with
oppressively warm conditions at night will offer minimal relief
from the heat. Outside of Cook County, most areas will likely
end up needing a heat advisory this weekend.

Upper ridge is forecast to weaken Tuesday into the middle
portions of next week which should allow the ring of fire
convection to work its way into our area, likely bringing
relief from more extreme temperatures heading into the middle of
next week.

- Izzi

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1255 AM CDT Fri Jun 20 2025

Key Messages:

- Period of -SHRA this morning into early afternoon (30% chance
  of TSRA at RFD).

- Strong SSW winds may develop in "wake" of -SHRA with localized
  gusts >40 kt possible.

Light southwest winds and dry conditions are forecast through
the overnight hours. Thunderstorms currently across Minnesota
and Iowa will continue to progress east and southeast toward the
area early this morning. This activity will decrease in
intensity as it approaches with the majority of the lightning
activity expected to remain largely west of the area (30% chance
near RFD and much lower into the Chicago metro). While in a
decaying phase, confidence was still high enough to convert the
PROB30 groups for VFR -SHRA to TEMPOs for the metro area
terminals mid morning into early afternoon and prevail -SHRA at
RFD through much of the morning.

We will also be closely monitoring surface observations in case
strong winds develop in the wake of these showers (and possible
isolated embedded storms). Hi-res guidance has been consistent
in such a scenario occurring and current storms across ND/SD/NE
have already exhibited characteristics of wake low wind
signatures. Wind gusts upwards of 35-45 kts would be possible,
with a narrow corridor of wind gusts >45 kt not out of the
question. This will likely have to be handled tactically,
similar to severe thunderstorms. Regardless if these winds
develop, winds will remain breezy out of the southwest through
the afternoon with gusts in the mid to upper 20 kt range.

A period of low-level wind shear will be possible this evening
and overnight as a 45-50 kt southwesterly low-level jet sets up
overhead. At least sporadic gustiness is expected overnight at
the surface with prevailing gusts >20 kt expected by daybreak
Saturday.

Petr

&&

.CLIMATE...
Issued at 114 PM CDT Thu Jun 19 2025

Temperatures this weekend into early next week could potentially
threaten record highs and record warm lows for Chicago and Rockford.
Here are the current records:

Chicago              High       Warm Low
Saturday 6/21     101 (1988)    74 (1923)
Sunday 6/22        97 (1988)    76 (1923)
Monday 6/23        97 (1930)    79 (1923)

Rockford             High       Warm Low
Saturday 6/21     100 (2022)    71 (1995)
Sunday 6/22        97 (1923)    73 (1908)
Monday 6/23 97 (1923) 74 (1908)

NWS Chicago

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Extreme Heat Watch from Saturday morning through Monday
     evening for ILZ103-ILZ104-ILZ105.

IN...None.
LM...None.

&&

$$

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