474
FOUS30 KWBC 012027
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 PM EDT Sat Jun 1 2024

Day 1
Valid 16Z Sat Jun 01 2024 - 12Z Sun Jun 02 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
FAR SOUTHERN ALABAMA AND THE WESTERN FLORIDA PANHANDLE...

...16Z Update...

The Moderate risk area remains unchanged with this update. While
the main area of rain associated with the MCS has both moved east
of the area and weakened, backbuilding storms over coastal Alabama
and a line of additional convection over southeastern Mississippi
tracking towards the Moderate risk area support its continuance.
Rain totals close to 10 inches last night in some areas have led to
extremely saturated soils across the Moderate Risk area. Thus,
while the storms going forward are likely to be more scattered and
may not quite produce the extreme rainfall rates seen last night,
their ability to produce additional flash flooding given the
favorable antecedent soil conditions will still be enhanced. The
Moderate Risk was coordinated with MOB/Mobile, AL forecast office.

...Upper Texas Gulf Coast and Southwest Louisiana...

Good agreement in the CAMs, despite their overall poor handling of
convection, for additional storms to develop from Houston east
across southern Louisiana prompted the expansion of the inherited
Slight Risk along the Gulf Coast. The area has seen repeated rounds
of storms over the last several weeks, so soils remain very
saturated as well as the rivers remain in Minor to Moderate Flood
stage. The storms have increased potential to organize into
clusters this afternoon as compared with previous days in this
area. Thus, while the CAMs consensus is for the storms to only
persist for anywhere from 1 to 3 hours over this area, think any
clusters will produce widely scattered instances of flash flooding
indicative of a Slight Risk, rather than the inherited Marginal
risk.

...Portions of the Central Plains...

Guidance remains in relatively good agreement for this afternoon`s
round of dry line convection to focus in southwest Kansas and the
OK & TX Panhandles. Thus, while isolated storms are certainly
possible further south, and especially along the Rio Grande, the
isolated nature of the convection will limit the flash flooding.
Areas along the immediate Rio Grande may have a higher risk as
storms that form over the mountains of Mexico my drift across the
river near Del Rio south to Eagle Pass.

...Ohio and Tennessee River Valleys...

The inherited Slight was downgraded to a Marginal with this update
in coordination with the impacted forecast offices. CAMs guidance
trending downward and lack of instability has decreased the
potential for flash flooding.

Wegman

...Previous Discussion...

A potentially high-impact event along the Gulf coast across
southern Alabama and the western Florida will have already started
by the time the Day 1 period begins at 01/12Z. Slow moving showers
and thunderstorms will be capable of training over the same area
with extreme rainfall rates leading to flash flooding, with some
concerns for locally significant impacts. Storms that formed prior
to 01/12Z taped a moisture rich atmosphere along a ribbon of
instability to produce a 1 hour rainfall rate in excess of 3 inches
according to MRMS radar estimates. Between rainfall rates that are
expected to exceed 2 or 3 inches...confluent flow of deep moisture
off the Gulf of Mexico and am environment supportive of back-
building and training...increasing areal coverage of impactful
flash flooding has prompted an upgrade to Moderate Risk,

Elsewhere...there is a Slight risk of excessive rainfall extending
north and east of the activity along the Gulf as a shortwave lift
northward from the the Lower Mississippi Valley over a region made
more hydrologically sensitive by heavy rainfall earlier in the past
3 to 7 days. Rainfall amounts and rates should be tapering off
with time as low level flow becomes less supportive with time so
maintained a broad apron of Marginal.

The risk for excessive rainfall will redevelop across portions of
the western high plains as another round of late day convection
develops near the dry line that propagates out towards the central
plains. Introduced a Slight Risk over parts of Kansas southward to
the Texas Panhandle where there is the best overlap for areal
average rainfall amounts of an inch or so...with some places having
had convection traverse the region over the past couple of days.

Bann

Day 2
Valid 12Z Sun Jun 02 2024 - 12Z Mon Jun 03 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL AND NORTHERN PLAINS AS WELL AS THE WASHINGTON CASCADES
AND SEATTLE METRO AND CENTRAL IDAHO MOUNTAINS...

...2030Z Update...

...Washington Cascades and Seattle Metro...

A Slight Risk upgrade was introduced for the Washington Cascades
and Seattle Metro Area with this update in coordination with
SEA/Seattle, WA and PQR/Portland, OR forecast offices. An
atmospheric river event is forecast to begin early Sunday morning,
and continue into Monday. A plume of moisture associated with the
atmospheric river will slam into the Cascades and coastal ranges,
resulting in rainfall rates up to 1/2 inch per hour. Snowmelt will
future add water to the streams and rivers draining the Cascades to
Puget Sound. The area has been somewhat drier than normal lately,
which will mitigate the flooding concerns a bit, but increase the
"shock factor". The greatest flooding concerns are in areas with
steep valleys/canyons, urban areas, burn scars, and other poor
drainage areas.

By Monday the rain should shift north largely into Canada, but
there will be some lingering rainfall into the Olympics and
Washington Cascades.

...Bitterroots and Salmon River Mountains north of the Salmon
River...

A Slight Risk upgrade was introduced in coordination with
MSO/Missoula, MT forecast office. The same atmospheric river
impacting the Washington Coast will continue inland into the
mountains of eastern Washington and Idaho. Much of the moisture in
this area will be beneficial and not result in many flooding
concerns for eastern Washington and northern Idaho, but as the
plume of moisture drifts south into the Bitterroots and Salmon
River Mountains, well above average snowpack at the higher
elevations with almost 9 inches of snow-water equivalent will be
subject to the 1-3 inches of rain forecast for this area. This will
result in snowmelt being a significant contributor to river and
small stream rises draining those mountain ranges. Given the
steepness of the terrain, the rises may be very fast, resulting
in flash flooding.

The rain will continue moving southward through late Sunday night
into Monday, so the threat for flooding will shift south as time
goes on. Given the drier antecedent conditions in the valleys, the
lesser amounts of rain forecast for the Salmon River Mountains
south of the Salmon River, Sawtooth, and southern Bitterroot
Ranges should keep the flooding threat at Marginal until Day
3/Monday.

...Northern Plains...

No significant changes were made to the ongoing Slight risk across
the area. With the poor predictability of convection and relative
similarity in the guidance in this area from run-to-run, the Slight
looked good. See the previous discussion below for more.

Wegman

...Previous Discussion...

The focus for an excessive rainfall threat shifts into the northern
and central plains on Sunday and Sunday night as a low amplitude
trough makes its way out of the northern Rockies and tracks towards
the western Great Lakes. Moisture transport into the region will
be aided by strengthening low level flow over the plains which taps
deeper moisture...with enough pooling ahead of an approaching cold
front to boost precipitable water values into the 1.6 to 1.8 inch
range by 03/00Z. The lift resulting from the mid level wave and the
formation of 80 to 100 kt jet streak near the international border
supporting initial development that spreads south into an
increasingly unstable airmass with higher precipitable water values
by Sunday evening.

Maintained the Marginal risk area across the coastal regions of
Washington and Oregon. 00Z numerical guidance shows a period of 12
hours or so with prolonged and sustained moisture transport from
the eastern North Pacific Ocean directed normal to the coastal and
Cascade ranges. IVT values peak in the 700 to 900 kg per meter per
second range around 03/00Z and then weakens in the early morning
hours on Monday. While atmospheric rivers are normally much of a
consideration in summer...the upper level jet is quite anomalous
with the u-component of the 500 mb flow being greater than 5
standardized anomalies greater than climatology helping boost
precipitable water values to greater than an inch over the western
half of Oregon and southwest Washington by 03/06Z (at or above the
90th percentile for this time of year) with 1.5 inch precipitable
water values along the immediate Oregon coast by Sunday evening.
That all gives support to the NBM and WPC QPF guidance of 1 to 3
inches...with the highest amounts in the west aspect of the Coastal
Ranges and the Cascade Range...by the time that Day 2 ends at
03/12Z.

Bann

Day 3
Valid 12Z Mon Jun 03 2024 - 12Z Tue Jun 04 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL
IDAHO MOUNTAINS...

...2030Z Update...

...Central Idaho Mountains...

A Slight Risk upgrade was introduced in coordination with
MSO/Missoula, MT; BOI/Boise, ID; and PIH/Pocatello, ID forecast
offices. The atmospheric river ongoing from Day 2/Sunday will
continue pressing slowly southward into the day on Monday. Rainfall
amounts are not super impressive, generally between 1 and 2 inches
in the area. However the rain will be falling on a well above
average snowpack at the higher elevations of the Salmon River,
Bitterroots, and Sawtooth ranges, and well as on the higher peaks
in the area. The combination of rainfall and snowmelt will cause
large rises on local area streams and creeks, as well as
significant rises on area rivers. Any rainfall from Day 2 will
begin raising river levels, and with most of the rain expected for
the Slight Risk area on Day 3, the more significant flooding will
occur into central Idaho on Monday.

...Washington Cascades and Olympics...

There is expected to be a significant decrease in the amount of
rainfall across this area on Monday as compared with Sunday. So
while flooding may be ongoing at the start of the period, think the
lesser amounts of rain closer to 1 inch should not add too much to
any existing flooding, so a Marginal Risk remains in place in
coordination with SEW/Seattle, WA forecast office.

...Mid-Mississippi Valley and the Ozarks...

No major changes were made to the Marginal Risk in place across the
region. The signals for heavy rain remain diffuse, and certainly
lower than previous days, so the Marginal Risk, while over some
saturated soils continues to look good. It`s quite possible that
targeted Slights may be needed in this region with better model
agreement and hopefully helpful CAMs guidance.

Wegman

...Previous Discussion...

Central U.S....
There is a Marginal Risk of excessive rainfall over a broad and
rather unfocused area across the central U.S owing to weak and
difficult to time shortwave energy ejecting out over the Plains
ahead of largely zonal flow that gradually backs as broader height
falls over the Rockies. The atmosphere over the plains will
generally be moist enough to support locally heavy rainfall from
any storms that form...but confidence was below average as to the
placement and timing of the forcing.

In the West...
There is a Marginal Risk of excessive rainfall over
portions of the northern Intermountain region as zonal flow draws
deeper moisture inland that interacts with the terrain in the
northern Intermountain region (especially parts of the Bitterroots
and Sawtooth ranges). The resulting QPF is generally expected to be
in the 1 to 1.5 inch range where the flow impacts the
terrain...with spaghetti plots suggesting a low end chance for 2
inch totals. At the same time...weak shortwave ridging along the
Washington/Oregon coast will lead to a decreasing risk of excessive
rainfall as rainfall rates decrease and areal coverage diminishes
with time.

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