120
FXUS63 KDVN 012028
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
328 PM CDT Mon Jul 1 2024

.KEY MESSAGES...

- Generally two rounds of convection are expected from later
  tonight into Wednesday morning accompanied by a threat for
  heavy rain. There is potential for flash flooding, initially
  this threat is low but may increase Tuesday night. Severe
  weather potential also exists, mainly late Tuesday PM through
  evening.

- Unfortunately, Independence Day is looking to be a wet day, as
  a warm front brings the potential for widespread showers and
  storms once again. Too soon to determine the severe risk.

- Moderate to Major River Flooding continues, with further
  changes to river forecasts expected as we transition from
  predicted rainfall to observed rainfall over the next 36 hrs.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 320 PM CDT Mon Jul 1 2024

A dry easterly fetch off high pressure over the Great Lakes and
Ohio Valley is providing another fabulous day with low
humidity and below normal temperatures around 70 to the mid 70s.
A mid level ridge extends from the Upper Midwest to the Lower
Mississippi Valley. A low amplitude wave is shifting east from
the Missouri River Valley early this afternoon, but rain has
been dissipating as it moves toward E Iowa and encounters the
mid level ridging and low level dry air. Showers and even a few
storms were noted further back into W Iowa and W Missouri
within the moisture gradient and main PWAT reservoir of 1.75 to
2 inches.

Initially late this afternoon/evening we`ll get some weak lift
to brush especially north/west portions of the service area.
The dry air sub-cloud will continue to chew away, but eventually
enough saturation may occur to where some sprinkles would be
possible with a very low potential (<20%) for measurable.

Otherwise, the focus will revert back to our west tonight where
additional showers and storms are anticipated to develop over
portions of the Northern and Central Plains ahead of an ejecting
shortwave from the Rockies. This approaching wave and
a veering low level jet with strong moisture advection (PWATs
ramping to 1.5 to 2 inches) will bring increasing chances for
showers and storms later tonight into Tuesday AM, with the
greatest chances (60-80%) north/west of the Quad Cities.
Severe threat appears very low with the primary instability
axis residing further west. Heavy rain is possible with this
activity, but the threat for excessive rainfall appears low
with the convection anticipated to be more scattered in nature.
Nonetheless, will have to keep an eye out given the burgeoning
tropospheric moisture content, and a favorable SW component to
the LLJ for possibly repeated convection. Can`t totally rule out
some localized excessive rain/isolated flash flood risk.
Lows tonight look to be notably warmer with aid of a SE breeze,
clouds and gradual increasing moisture.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Issued at 320 PM CDT Mon Jul 1 2024

Another round of showers and storms is expected Tuesday late
afternoon into Wednesday, as stronger synoptic forcing aided by
a shortwave aloft and the right entrance region of a 100-120 kt
250 hPa jet steak overspread the Upper Midwest, while at the
surface a cold front moves through. This will be especially true
through the nighttime hours, which appear to have the best
chance for widespread heavy rainfall. The primary concern for
this period remains heavy rain and flash flooding. NAEFS and ENS
continue to support PWATs climbing to 1.75 to around 2.00
inches by 00z Wednesday, which is 2 to 3 standard deviations
above climatology and approaches the 99.5 percentile or is near
max of their respective climatologies. NBM guidance shows high
probabilities of at least 1.00 inch from Tuesday evening through
Wednesday AM over the west 2/3rds of the service area, and an
area of modest chances (40-50%) for at least 2 inches of rain
focusing over northwest portions of the service area. Given the
convective nature and magnitude of the synoptic ascent doubling
the PWAT would give a general idea of the higher limit, which
in this case with PWATs around 2.00 inches would yield
potentially higher amounts of around 4.00 inches. 12z HREF 24 hr
probability matched mean QPF depicts this potential as well
with swaths of 3-5 inches by 12z Wednesday. With rivers rising,
we will be watching this heavy rain potential closely, as this
will lead to fluctuations in the river forecast. In addition,
the significant moisture advection (IVT) and a southwesterly LLJ
of 40 kt could contribute to backbuilding and an elongated MCS,
resulting in a risk of flash flooding. This flash flood threat
may be highest especially across east-central and northeast
Iowa, into northwest Illinois, where rainfall in the past week
has accumulated 125-200+ percent of normal and soils are
saturated, and where the signal has been pretty consistent in
the deterministic QPF and ensemble probabilistic QPF guidance.
The potential though extends also into southeast Iowa, northeast
Missouri and west central Illinois. Once the convection
organizes with low level flow parallel and nearly equal to the
deep layer flow the Corfidi Vectors are slow, so some
training/backbuilding potential seemingly exists. However, the
uncertainty lies with the potential for cold pool development
which if occurs could push the convection along and limit the
flash flood potential. As convective trends become better
defined over the next 12-18 hrs we may need to consider Flash
Flood headlines.

Severe weather potential will exist by Tuesday PM/evening, as
deep layer shear 0-6km increases to 35-45+ kts aided by the
approaching shortwave trough. Models try to build in the
instability during this time, especially across SE Iowa and
NE Missouri with MLCAPE increasing to 1000-2000+ j/kg. Initial
supercell potential will exist before upscale growth into a
line of storms, transitioning to a wind threat. SPC continues
with an Enhanced Risk (level 3/5) for parts of SE Iowa and
NE Missouri where this better instability is progged.
A Slight Risk (level 2/5) extends across much of the rest of
E Iowa and a portion of WC Illinois. Main threat again looks
to be damaging wind, and a secondary lower threat for a tornado
with favorably low LCLs. Warm cloud depths over 12-14kft AGL
should limit the hail threat.

A brief break in the wet weather is expected Wednesday, but the
emphasis is on brief, as a surface warm front passes north into
the area as early as late Wednesday night/early Thursday AM
with convection potential increasing (40-70%) south of I-80.
Then, the shower and storm potential increases over the rest of
the area on the 4th of July into Thursday evening, as a vigorous
shortwave trough evolves into a closed low while traversing
near the international border to the north. Ensembles point to
PWAT values of 2.00+ inches again pooling near and south of the
boundary supporting a heavy rain threat. Where this warm front
sets up will be critical to where this heavy rain threat
and potential severe weather threat eventually sets up, and
will bear watching in the coming days.

Next weekend the pattern looks to transition to a mean trough
over the north central U.S. Thus, the active pattern looks
to persist with additional shower and storm chances with
lower predictability on timing given the lower amplitude
shortwaves. Many periods have PoPs Saturday-Monday, but there
will likely be plenty of dry hours. Temperatures look to be
near normal.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1157 AM CDT Mon Jul 1 2024

VFR conditions expected to persist through this evening. There
is a slight chance (10-20%) for a brief shower late this
afternoon and early evening at KCID and KDBQ, however there is a
copious wedge of dry air sub-cloud and so it may be more
sprinkles than anything with no impacts. By later tonight and
Tuesday morning (after 06z through 15z), the shower and storm
chances will be increasing with a veering SW low level jet and
abundant moisture advection with the greatest chances at KCID and
KDBQ requiring prevailing SHRA/TSRA mention, with lower chances
PROB30 at KMLI while potential appears too low for mention at KBRL
right now. MVFR to local IFR conditions (mainly for visibility) will
be found in the storms and heavier showers. Winds will be predominantly
from the E/SE around 10-15 kt and then occasionally gust 20-25 kt.
LLWS is possible at KCID and KDBQ late tonight/Tuesday morning, but
too much uncertainty as winds may not decouple completely tonight and
marginal criteria precludes mention with the 18z TAFs.

&&


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

&&

$$

SHORT TERM...McClure
LONG TERM...Gunkel
AVIATION...McClure