408
FXUS63 KDVN 251745
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1245 PM CDT Sat May 25 2024

.KEY MESSAGES...

- A pleasant day is on tap to begin our Memorial Day weekend, with
  lots of sunshine

- Yet another storm system moves in late tonight through the day
  Sunday, which will support more chances for strong to severe
  thunderstorms

- Chances of showers and a few storms will remain with us Sunday
  night and Monday

- Drier conditions return for Tuesday through Friday

&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 251 AM CDT Sat May 25 2024

As we kick-off our Memorial Day holiday weekend, the weather
conditions this weekend will be a mixed bag, with very pleasant
conditions today, but a more active day for Sunday. For today, an
area of high pressure will build in, which will bring plenty of
sunshine with it. If you have any outdoor plans today, it looks like
a great day for them! Light and variable winds this morning will
turn more southerly as the high pressure system moves east, with
southerly winds between 5 to 15 mph expected this afternoon. High
temperatures should warm to the middle to upper 70s.

This respite from storms will be short-lived, though, as our next
storm system moves in late tonight, lasting through the day Sunday.
A broad upper-level trough will develop over the Intermountain West
region today, with a sub-tropical upper-level jet nosing into the
Corn Belt region tonight into Sunday. An area of low pressure will
undergo lee cyclogenesis over eastern Colorado today, and both the
GEFS and ENS ensembles are progging the low to translate towards our
region tonight. Ahead of the surface low, we will contend with a 35
to 45 kt nocturnal low-level jet, which will help enhance the
effective shear magnitudes. As a mid-level shortwave impulse ejects
from the central Plains, convective initiation will follow and
approach our area. Storms appear to move into our region after
midnight tonight, but the CAMs show some disagreement on the onset
timing of the storms. Still, with the LLJ in place, steep mid-level
lapse rates of around 7 to 8 C/km above a low-level inversion, and
most-unstable CAPE values around 500 to 1500 J/kg, a large hail
threat will be in place. There may also be a lower end tornado
threat, mainly due to the orientation of the southwest to northeast
0-3 km bulk shear vectors. CAMs appear to indicate that the
convection tonight into Sunday morning will be more disorganized
compared to the MCS that moved through Friday morning, but if
anything can organize and generate an east to northeast propagating
surge or bow, this could tap into this shear to generate some
mesovortex-genesis tornadoes. SPC continues to carry a Marginal Risk
(level 1 of 5) for severe weather from Independence, IA to
Charlotte, IA to Galva, IL and points southwest of this line.

Eventually, the Sunday AM convection will end by around noon Sunday,
but we aren`t out of the woods yet. There is some uncertainty on how
the Sunday afternoon storm potential will play out, but eventually,
the aforementioned surface low will translate through our region
Sunday PM. If the surface low moves overhead, the more favorable
zone for severe convection will shift south of our area, where a
more pronounced baroclinic zone should establish. As the surface low
moves through, winds will turn more northerly in our area, per the
latest suite of CAMs. We may still get some scattered showers and
storms, but the focus for severe weather should largely be south of
us. Surface-based CAPE Sunday afternoon will increase to between
1000 to 2000 J/kg, but the low-level shear won`t be as strong due to
a lack of LLJ. The mid-level jet will remain strong to our south,
with upper-level divergence overhead, so modeled hodographs are
oriented more straight with very little curvature. At this time, SPC
has kept mainly a Marginal to Slight Risk for severe weather in our
region, but they have trimmed down the Enhanced Risk, which matches
better with my thoughts. The Enhanced is still in place for Hancock
and McDonough counties in west-central Illinois. I would be
interested to see what SPC does with their new Day 2 outlook that
will be issued early this afternoon. The CSU machine-learning severe
probs continue to paint decent probabilities for severe weather
across our south and east, with all hazards possible, with the most
favorable probabilities being for damaging winds and large hail.

Finally, some more heavy rainfall is possible with the Sunday
activity, given Pwat values around 1.3 to 1.7. IVT per the ENS
ensembles are over 97 percent of climatology, so water vapor
transport will be supportive of heavy rainfall. NBM probabilities of
1" of rain with the tonight/Sunday precipitation are around 50 to 70
percent for a large portion of the region, so ongoing river flooding
will only be exacerbated with this rainfall.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Issued at 251 AM CDT Sat May 25 2024

The long-term period will start off with the remnants of the Sunday
convection lingering as the mid-level trough translates through the
region. There will be a brief break in showers and storms early
Monday before yet another chance of showers moves in ahead of an
approaching mid-level shortwave that will dive southeast from the
Dakotas. Mixed-layer CAPE appears to be pretty meager, with values
progged around 500 J/kg, so we might be contending with showers than
thunderstorms.

For Tuesday through Friday, we are looking at something we haven`t
had in a while: an extended stretch of dry and quiet conditions!
Guidance is progging a long-wave ridge over the western CONUS and a
surface high pressure settling across the western Great Lakes region
for much of next week. Temperatures look to be seasonal, with highs
in the 70s for most locations, and lows in the 50s.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1241 PM CDT Sat May 25 2024

VFR conditions will be seen through 06z/26 as high pressure
moves into the Great Lakes. After 06z/26 a band of SHRA/TSRA
will develop across eastern Iowa with further development along
the Mississippi River toward 11z/26. Outside of this band VFR
conditions will be seen with MVFR/IFR in SHRA/TSRA. After 12z/26
SHRA/TSRA will gradually end across eastern Iowa but more
widespread MVFR conditions are expected.

&&

.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...Schultz
LONG TERM...Schultz
AVIATION...08
HYDROLOGY...McClure