599
FOUS30 KWBC 302030
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 PM EDT Sun Jun 30 2024

Day 1
Valid 16Z Sun Jun 30 2024 - 12Z Mon Jul 01 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN ARIZONA AND MUCH OF SOUTHWEST COLORADO...

...16Z Update...

...Central Plains...

In coordination with DDC/Dodge City, KS; ICT/Wichita, KS; and
OUN/Norman, OK forecast offices, the Slight Risk for these areas
was downgraded to a Marginal with this update. The ongoing MCS
which caused numerous instances of flash flooding in southwest
Kansas is slowly dissipating as is typical with the diurnal
weakening of the LLJ. Thus, while there are a few pockets of
greater than 1 inch per hour rainfall rates ongoing, the current
and forecast trend over the next few hours is for continued
weakening of the rainfall rates. For the rest of today into
tonight, expect no more meaningful rainfall in this area. The
expected development of a new MCS over northern KS and NE tonight
will be entirely north of the heavy rainfall region from this
morning, and thus not adding to any ongoing flash flooding.

...Southwest US...

Monsoonal southerly flow of deep Pacific moisture continues across
the Four Corners Region today. With maximum diurnal heating this
afternoon, expect a renewed flareup of slow moving convection
capable of rates to 1.5 inches per hour. Portions of southern
Arizona have been hit repeatedly over consecutive afternoons with
this same convection which will remain in roughly the same place.

In similar fashion, the same moisture plume will advect north
across much of Arizona and into southwest Colorado, where its
meeting with the terrain there and some weak upper level support
will cause a renewed round of showers and thunderstorms in this
also hard-hit area this afternoon. Localized totals as high as 2
inches could likely cause additional flash flooding in the terrain
and adjacent valleys of southwest Colorado, so the Slight remains
in place with no changes.

Wegman

...Previous Discussion...

...Southwest U.S...
Moisture will continue to serge northward into parts of Arizona and
New Mexico in broad anticyclonic flow around a mid- and upper-level
ridge over the Southern Plains. Convective chances will be aided by
mid-level height falls from the Pacific Northwest extending into
the central Great Basin by evening. Any convection that develops
should tap into an airmass characterized by precipitable water
values in excess of an inch from eastern Utah and western Colorado
to values approaching 2 in near the international border in
southeast Arizona by evening (2 to 4 standardized anomalies above
climatology and greater than the 95th climatological percentiles).
The resulting instability should be enough to support some
localized intense rainfall rates that pose a flash flooding
risk...especially in complex terrain and over burn scars and
normally dry washes.

...Eastern Seaboard to Florida Peninsula...
A cold front associated with low pressure north of the
international border will be moving into an anomalously moist
airmass from the Mid-Atlantic region southward with precipitable
water values expected to be as high as the 2.0 to 2.3 inch range in
a 50 to 100 mile wide corridor from the Carolinas to the Florida
panhandle by 01/00Z (approximately 3 standardize anomalies greater
than climatology and near the 95th climatological percentile). The
HREF focused its highest probability of 1- and 2-inch per hour
rates and low-end probability of 1- and 3-hour rainfall exceeding
flash flood guidance near the front given the moisture flux
convergence. There was a similar region of higher probabilities
extending along the Florida peninsula...presumably along sea breeze
boundaries and interaction with a weak shortwave seen on water
vapor imagery early this morning.

...Plains of Northeastern Montana...
Convection capable of producing locally heavy rainfall is expected
to develop later today as a shortwave moves from Washington/Oregon
early in the period and eventually emerges east of the Rockies.
Between falling surface pressures approaching from the west and
surface high pressure over southern Canada moving south and
east...surface dewpoints over the plains of northeast Montana
should climb through the 50s in response to increasing moisture
transport once a low level jet forms. Given expected rainfall
amounts from what should be progressive storms...a Marginal is
still sufficient.

Bann

Day 2
Valid 12Z Mon Jul 01 2024 - 12Z Tue Jul 02 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOW COUNTRY OF SOUTH CAROLINA AND THE NORTH GEORGIA COAST...

...2030Z Update...

...Southeast US Atlantic Coast...

In coordination with CHS/Charleston, SC forecast office, a
Moderate Risk upgrade was introduced with this update. A cold front
will become stationary as it stalls along the coast. Abnormally
high PWATs on the order of 2 to 3 sigma above normal and above 2
inches will be in place along the coast on Monday. As this moisture
runs into the stalling front, the slowing much cooler, drier air
mass will hold the front in place from the north as the abundant
moisture and instability moves into the front from the south. For
more northern areas, the front is likely to continue east and
therefore push the heaviest rain off the coast. Meanwhile for
portions of the SC and GA coasts, the front will stall out largely
perpendicular to the orientation of the front. This will allow for
the convection that develops along that front to struggle ot move,
while consistently being fed with moist, unstable air. Thus, the
flash flooding threat has increased as the forecast for rainfall
have also increased. Uncertainty remains with exactly where the
front stalls out, as even a small eastward shift in the guidance
would mean big changes (in the positive/drier direction) as all the
heaviest rainfall occurs offshore. Nonetheless with some
consistency that heavy rain ill fall along the coast, vulnerable
urban centered in the area such as Charleston, SC and Savannah, GA
may be adversely impacted with numerous flash floods, with some of
them significant if that convection forms and stalls over those
cities. Regardless, the heaviest rainfall rates of up to 3 inches
per hour would support potential flash flooding most anywhere rates
of that magnitude can occur for a significant period of time.

...Upper Midwest...

Relatively few changes were made to the overall large Slight risk
area across portions of the Upper Midwest. A low tracking in
between will result in a relative minimum of rainfall over most of
SD and west central MN, but in between 2 areas of greater rainfall,
one over ND and northern MN and the second from northeast NE to
western WI. Both areas are of concern for flash flooding due to
very favorable antecedent soil moisture conditions from recent
heavy rainfall.

The area of greater concern is to the south from NE to WI, as both
greater rainfall is expected here as these storms get "first dibs"
at the flow of Gulf moisture moving in on the LLJ, and the flooding
ongoing in this area is of greater magnitude than areas further
north. Thus, this area is still in a higher-end Slight with a
possibility for a MDT upgrade with future updates if QPF increases.
For now as forecast rainfall remains similar to previous forecasts,
no major changes were needed.

Further north the Slight was extended back to the east to include
the Arrowhead as the corridor across northern MN gets more rainfall
than previous forecasts indicated. Due to also favorable antecedent
conditions, widely scattered instances of flash flooding are
expected.

Wegman

...Previous Discussion...

...Upper Midwest/Northern Plains...
The system that helps foster a risk of excessive rainfall on Day 1
will continue to propagate eastward during the period and a risk of
excessive rainfall on Day 2 over portions of the Upper Midwest.
With upwards of 12 hours of southerly winds drawing deep moisture
into region...some storms will be able to produce locally intense
rainfall rates and locally heavy rainfall amounts. The
amounts...while actually fairly modest...could fall in areas
recently soaked by heavy rainfall which has made some places
hydrologically sensitive. The previously issued Slight Risk still
captures the area and only minor changes were needed.

...SOUTHEAST US...
Introduced a Slight Risk area along and near the coast from South
Carolina into parts of Georgia. With the cold front sagging
southward acting to focus convection and a fetch of post-frontal
on-shore flow persisting into the afternoon...felt some enhanced
chances for flooding/run-off problems existed. Elsewhere in the
Southeast...deep moisture and weak flow south of the front will
open the potential for disorganized convection to form along any
boundary/outflow...so left a unfocused Marginal surrounding the
Slight.

...Southwest US...
Monsoonal moisture will be lingering over the Southwest
(especially Arizona and New Mexico) that is drawn back north and
east around an upper level ridge. Maintained a Marginal Risk at
this point given the uncertainty of cloud-cover lingering from
convection on Day 1 and its ramifications for when and where
convection fires later in the day and how much subsidence there is
in the wake of a wave moving north and east of Arizona...and the
magnitude of dry air in the sub-cloud layer. Will be able to
refine this later.

Bann

Day 3
Valid 12Z Tue Jul 02 2024 - 12Z Wed Jul 03 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MIDWEST TO WESTERN GREAT LAKES...

...2030Z Update...

...Upper Midwest...

No big changes were made with this update. Concerns are increasing
in eastern Iowa and the surrounding areas from northern MO to
southern WI as the same influx of moisture and instability results
in training thunderstorms across this region. Antecedent conditions
are also quite favorable to flash flooding due to recent heavy
rainfall. In coordination with DVN/Davenport, IA forecast office, a
Moderate risk was discussed as potential in future updates, but
there`s still lots of time to review future model data to best
determine impacts. Thus, a Moderate Risk will need to be considered
with future updates.

...Southeast...

A Marginal Risk area was introduced with this update targeting the
Florida Panhandle, but extending into the Hilton Head Island area
of far southern SC. This will be a continuation of the convection
from the Day 2/Monday period for this area as the front gets a
nudge on Tuesday towards the Gulf. Thus, overall rainfall amounts
are expected to be lower than on Monday, but of course any
continued rainfall at 12Z Tuesday may require an upgrade in areas
hard hit from Monday night. Elsewhere, expect slow moving
convection with plentiful Gulf moisture allowing for the potential
for very high rainfall rates in the cores of the strongest storms.
With less organization to the storms as compared with Monday, think
the flash flooding threat is a bit lower. The greatest threat lies
roughly from Hilton Head, SC southwest to Apalachicola, FL.

...Southwest...

No big changes in the Southwest as the typical monsoonal storms
form once again over AZ and NM, though minimal organization should
prevent all but isolated flash flooding.

Wegman

...Previous Discussion...

...Upper Midwest/Northern Plains to Western Lakes...
Numerous showers and thunderstorms should be on-going ahead of an
advancing cold front and associated impulse/wave aloft. There
should be enough instability of support local downpours that may
fall on ground still saturated from heavy to excessive rainfall
recently. In addition to new or re-newed flooding from this batch
of rainfall is the concern for on-going flooding to be aggravated.

...Southwest...
Maintained the Marginal Risk area over the Southwest US on
Tuesday...with some southern suppression of this moisture shown in
line with the global guidance and decreased areal coverage of 1-
and 2-inch contours from the QPF spaghetti plots. Will keep an eye
open for trends in upcoming model cycles.


Bann


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt