165
FXUS63 KDVN 240555
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1255 AM CDT Fri May 24 2024

.KEY MESSAGES...

- A line of thunderstorms is expected to move through the area
  from early Friday morning through midday, which could bring
  some severe weather with it; scattered showers and storms may
  redevelop Friday afternoon and evening.

- 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 SATURDAY/...
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 /SATURDAY NIGHT THROUGH THURSDAY/...
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 /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1255 AM CDT Fri May 24 2024

VFR conditions were observed across the area early this morning,
but this will be short-lived as a period of thunderstorms will
move through the area later on this morning, lasting through the
afternoon hours. The latest high-res models are showing various
solutions for coverage of these thunderstorms, so confidence in
timing is rather low. However, we are confident that the
thunderstorms, along with heavy rainfall, will lead to MVFR/IFR
conditions, and perhaps some isolated LIFR visibilities, along
with gusty winds in excess of 30 knots. We have used TEMPO
groups to highlight the most likely time frame for this
morning`s storms. Then, a cold front will sweep through the area
this afternoon, which will bring yet another round of storms.
We have used PROB30 groups for this activity as timing will be
dependent on how things play out this morning.

&&

.HYDROLOGY...
Issued at 1025 PM CDT Thu May 23 2024

The flood warning for the North Skunk River near Sigourney has
been upgraded to Major flood category. Routed water from a
sparse data area upstream where the heaviest rain fell a few
days prior has led to a rapid rise in the last 6 hours and is
now above Major flood stage. The forecast calls for a crest of
24.5 feet by Friday afternoon, but there is uncertainty in the
amount of water that remains to move through the reach and the
crest may need to be further adjusted.

The Wapsipinicon River near DeWitt was upgraded to a Flood
warning. Major flood stage is being forecast based on routed
flow, and is supported in output from the Hydrologic Ensemble
Forecast Service Model Simulation (HEFS), which places high
confidence on reaching Major Flood stage (12.5 feet). The
forecast has gone up this evening with a crest now over 13 feet
next week. This fits near the most likely range from HEFS of
12.8-13.1 feet. Some attenuation is possible as the routed
flow moves through the river system, but at the same time there
will be additional rounds of rain Friday and Sunday of which
could total over 1 to 2 inches. There will likely be changes to
the forecast in the coming days as the rain lays out and the
extent of the routed flow is better known, so stay tuned!

Routed water and additional rain through the weekend is leading
to rises on most other tributary rivers and the mainstem of
the Mississippi, especially south. Flood warnings or watches
are in effect for portions of the Cedar, Iowa, Skunk and
southern sections of the Mississippi river so please refer to
the latest Flood Statements and Flood Warnings for details.

&&

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

&&

$$

SHORT TERM...Speck
LONG TERM...Speck
AVIATION...Schultz
HYDROLOGY...McClure