226
FXUS63 KDVN 190551
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1251 AM CDT Thu Jun 19 2025

.KEY MESSAGES...

- Scattered showers and isolated storms will continue into
  tonight.

- First significant heat episode of the summer for Saturday into
  early next week, with heat indices potentially topping 105 on
  multiple days along with high confidence in very warm nights,
  which may result in prolonged heat stress impacts

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 255 PM CDT Wed Jun 18 2025

Through Tonight:

As of early this afternoon, SPC mesoanalysis and surface
observations had a compact ~1004mb low situated over west-
central Illinois near Peoria. Aloft, a 500mb trough was located
over the Mid to Upper Mississippi Valley, providing added lift
to the region which has led to widespread steady rain across
much of the outlook area within a deformation zone NW of the
surface low. There have been occasional isolated storms
developing within the larger light to moderate rain shield. The
slow movement of the convective cells will result in locally
heavy rainfall of 1"+. Outside of the heavier downpours,
rainfall totals are likely to hold near or under 0.25". The
threat for severe thunderstorms across even the far southeast to
east- central counties is quickly waning as the better
instability (MLCAPE of 500+ J/kg) slides to the southeast of
the area through portions of central Illinois, following along
and east of the surface low track. For this evening into
tonight expect areal coverage of the rain to decrease as the
surface low shifts well to the east. However, we`ll still be
left with lingering scattered showers and isolated storms
through approximately 10 PM to midnight tonight, mainly focused
on a NW wind shift.

Thursday:

A ridge of high pressure will build into the region from the
west as warm air advection increases aloft. Temperatures will
reach the mid to upper 80s and expect mainly dry conditions
through the day. Late Thursday night a strong LLJ (850mb winds
near 50 kts) is forecast to tilt into the northwest portion of
the outlook area. 850mb convergence and persistent WAA may
result in the development of scattered elevated storms after 3
AM into early/mid Friday morning (20-40%), favored north and
west of the Quad Cities. SPC has maintained the Marginal Risk
(Level 1 of 5) for severe storms to the north of I-80. At this
time, the primary threats are hail and gusty winds.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Issued at 255 PM CDT Wed Jun 18 2025

Some lingering effects of clouds or rain-cooled air from any
Friday morning convection (noted above) may stunt temperatures a
bit Friday. That`s been taken into account in northern and
eastern parts of the forecast area, while highs around 90 and
heat indices in the upper 90s in the far southwest.

Attention then turns to a conceptually favorable setup for a
multi-day heat episode. This is summarized by an upper level
ridge expanding over the region Friday afternoon and peaking in
500 mb heights Saturday-Monday. The setup keeps the stronger
belt of upper level westerlies and primary baroclinic zone to
our north, especially during the weekend, so thunderstorm
chances and clouds to disrupt heating look to both be
negligible. The low-level thermal ridge is at its peak
magnitude on Saturday, with 850 mb values between 23-26C. These
are right at record values for June using the DVN 30-year
sounding climatology. The pull of higher dew point air from the
south is well agreed upon. There are some uncertainties tied to
specific dew point values noted below, but overall, a humid air
mass should be in place Saturday afternoon. Factoring these
together and looking at various anomaly tools, this checks a lot
of boxes for a very unusual early summer stretch of heat.

Now digging into the details more and considering some of the
spectrum of possibilities for dew points and temperatures. For
dew points, the mid 70s axis that was in Illinois today will be
"squished" southward before returning northward late this week.
Almost all available guidance shows mid 70s dew points during
Saturday and Sunday afternoons, with little in the way of range
from the National Blend of Models (NBM) membership. This is
still early in the mature crop-provided evapotranspiration
cycle, but we have had a fairly wet few days, so locally not
seeing a strong signal for mixing out dew points...maybe 2-3F?
As for temperatures, the NBM 50th percentile is right around 95
for highs Saturday-Monday, as well as mid 70s for lows those
three mornings. With high dew point air, its sometimes tough to
achieve upper bounds (97+) which about one quarter of guidance
are showing. So feel comfortable with messaging highs of 93-96
and mid 70s lows. It`s important to note that a few degrees
difference in dew points and temperatures can result in a 5-6F
difference in heat indices. While not directly factored into
heat index, the southerly winds do look to be blowing 15-20 mph
with gusts around 30 mph. This can ease the stress on people a
bit, but counteracting that to some degree is completely full
sun forecast. So all in all, confidence on the higher side for
this 3-5 days out forecast.

Given our confidence in this, we have issued an Extreme Heat
Watch for all of eastern Iowa and a smidge of adjacent Illinois.
A reminder that this headline was previously known as Excessive
Heat Watch, but no change in our criteria with the new name.
Subsequent shifts will evaluate both the areal extent and
duration of this, as well as whether it will need to be upgraded
depending on confidence in some of those subtle details
mentioned in the prior paragraph.

For Tuesday-Wednesday, the pattern becomes more unsettled as
the baroclinic zone oozes southward over/near the Corn Belt
region. At this distance this just means "chance of showers and
thunderstorms" for multiple periods. Also, if this is slower to
do so, Tuesday may continue the upcoming heat stretch.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1236 AM CDT Thu Jun 19 2025

An area of showers and storms is moving southeatward to near DBQ
early in the TAF period. Placed a tempo group there for MVFR
visibilities in storms through 08 UTC. High pressure is
forecast to pass to the south of the area through TAF period.
VFR conditions are eforecast through the period. Given recent
rainfall fog may be a possibility especially at KBRL. West winds
around 5 to 10 knots can be expected through 00 UTC Friday
before winds turn to the south in the wake of high pressure.
Diurnally driven cumulus are possible Thursday with ceilings
around 6 kft.


&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...Extreme Heat Watch from Saturday afternoon through Sunday
     evening for IAZ040>042-051>054-063>068-076>078-087>089-
     098-099.
IL...Extreme Heat Watch from Saturday afternoon through Sunday
     evening for ILZ015-024.
MO...None.

&&

$$

SHORT TERM...Uttech
LONG TERM...12
AVIATION...Cousins