229
FXUS63 KDVN 231913
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
213 PM CDT Thu May 23 2024

.KEY MESSAGES...

- A line of thunderstorms is expected to move through the area late
tonight, which could bring some strong to severe thunderstorms with
it, with additional chances of showers and storms to redevelop
Friday afternoon.

- A quiet start to the Memorial Day weekend is expected for
Saturday, with more chances of showers and storms on Sunday. Some
storms Sunday could be strong to severe.

- Active conditions will continue into Monday before drying out
  on Tuesday and Wednesday.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 207 PM CDT Thu May 23 2024

Another warm, quiet late Spring afternoon was unfolding across
all of eastern Iowa, northwest Illinois and far northeast
Missouri. 18z surface analysis depicted broad surface high
pressure over the Midwest and Ohio River Valley, with a strong
surface low just north of the Great Lakes. Another surface low
was evident over northeast Wyoming ahead of a potent shortwave
in the north central Rockies. This shortwave will be the driver
of our weather through much of the short term period.

Quiet, dry and warm conditions will prevail through this afternoon
with PM highs approaching the upper 70s to low 80s. As we head into
the evening, attention will turn to our west as a complex of showers
and thunderstorms, some severe, develops across the north central
Plains. This complex will be fueled by the aforementioned surface low
and shortwave, with a cold front developing in tandem. In addition,
a strong 40-55 kt LLJ will provide additional vertical shear and
moisture to this complex, ensuring it continues its track east
tonight.

This complex should survive the trip and work its way into our
western CWA around 09-12z (300a-600a). After this, there is a bit of
uncertainty of what happens with this as it tracks across the
region. So far, there are two scenarios: 1) The complex continues
tracking into the area with additional redevelopment through the
morning as it encounters diurnal heating and a decreasing cap (see
12z HRRR and NAMnest), or 2) the complex tracks more northeast that
east along the CAPE gradient, impacting areas mainly along and north
of Interstate 80 with scattered activity to the south that could
break the cap (see 06z HRRR). As of this forecast discussion, I find
myself leaning more towards Scenario #1 with latest HREF guidance
favoring a decrease in SBCIN over areas along and south of
Interstate 80.

Should scenario #1 play out, we would need to watch for storms
capable of producing damaging wind gusts and large hail thanks to
the presence of high instability (MLCAPE around 2000 J/kg), very
steep mid-level lapse rates around 7-8 C/km, favorable deep layer
shear around 30-35 kts and a more linear storm mode favored. A
tornado cannot be ruled out especially with any storm that
interacts with remnant boundaries or that can become surface based.

This complex should exit the area by late morning to midday, with
potential for another line of showers and thunderstorms to move
through the area during the afternoon/evening with the surface cold
front. This threat will be heavily dictated by which scenario we see
in the morning. If we see scenario #1 play out, much of our PM
threat will be diminished with coverage more isolated to scattered
due to more atmospheric stabilization. Should scenario #2 play out,
storms during the afternoon could be stronger with more instability
to work with.

The Storm Prediction Center continues to highlight areas across
east central and eastern Iowa in a Level 1 (marginal) risk of
severe thunderstorms through 12z Friday, with a Level 2 (slight)
risk along and east of a Manchester IA to Ottumwa IA line after
12z Friday.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Issued at 207 PM CDT Thu May 23 2024

Friday Night Through Daytime Saturday...

High pressure will provide a dry start to the holiday weekend. Look
for near normal highs in the low to mid 70s, and lows Saturday
morning in the upper 40s to mid 50s.

Saturday Night on...

Active weather returns Saturday night as remnants from a complex of
showers and thunderstorms moves into the area. This complex will
dictate how things evolve for daytime Sunday, with potential for
another round of showers and thunderstorms during the afternoon and
evening. There is a chance these storms could be severe, with the
latest SPC Day 4 outlook painting a 15% chance (slight) for areas
along and south of Interstate 80. Higher chances remain to the south
of the area near a upper jet poleward exit region, where there is
enhanced upper level divergence in play. Other severe statistical
guidance including CSU and CIPs are in agreement of severe
potential, so please continue to monitor the latest forecasts if you
have plans Sunday.

Chances of showers and thunderstorms will persist into Memorial Day
as another shortwave moves into the area. Coverage looks to be more
isolated to scattered with this feature with lower levels of
moisture and forcing at play.

We quiet down by the middle of next week with high pressure moving
across the area, though this will be short-lived with zonal flow and
a renewed round of shortwaves moving into the area by late next
week. Temperatures look to average near normal for the period.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1232 PM CDT Thu May 23 2024

VFR conditions are expected through midnight before a
thunderstorm complex moves through the region late tonight and
early Friday morning. PROB30 groups continue to be utilized to
identify the time periods most favorable for storms, and have
been added in this TAF for KMLI/KBRL where confidence is
increasing for storms.

Southerly winds around 5 to 10 knots are expected through this
evening, with a few gusts up to 20 knots.

&&

.HYDROLOGY...
Issued at 1255 PM CDT Thu May 23 2024

Watch/Warnings:

River Flood Warnings have been issued for the Cedar River at Cedar
Rapids and at Conesville, both for minor flooding.

Issued a Flood Watch for the Cedar River at Cedar Bluff for
potential to reach into the minor flood category.

Issued a Flood Watch for the Iowa River at Wapello and Oakville for
the potential for minor flooding.

Issued a Flood Watch for the Mississippi River at Gladstone,
Burlington, and Gregory Landing for the potential for minor flooding.

Flood Watch has been continued for the Wapsipinicon River at De Witt
for the potential for Major flooding.

Flood Warnings continue for the following:

Iowa River at Marengo and North Skunk River at Sigourney for
moderate flooding.

Wapsipinicon River at Anamosa Shaw Rd. for minor flooding.

See Flood Warning, Flood Watch, and Flood Statements for more
information.

Most of the river forecasts are starting to be based on routed flow.
Some attenuation is still possible as the crest wave makes its way
through the river systems over the next two to seven days. Because
of this, confidence in some forecasts, especially those forecast to
reach flood stage or a respective flood stage category beyond day 4,
remain lower.

Additional heavy rainfall is possible through the Memorial holiday
weekend which may cause rivers to rise more than expected so
stay tuned!

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...None.
IL...None.
MO...None.

&&

$$

SHORT TERM...Speck
LONG TERM...Speck
AVIATION...Speck
HYDROLOGY...Haase