FOUS30 KWBC 110751
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
251 AM EST Mon Nov 11 2024

Day 1
Valid 12Z Mon Nov 11 2024 - 12Z Tue Nov 12 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

A non-zero chance for flash flooding exists within the far
Southeast Parishes of LA and along the immediate AL/MS coastal 
plain as lingering moisture on the northern periphery of the 
remnant low from Rafael will persist within the area mentioned 
above. Frontal progression to the northwest will slowly sink 
southeast with some CAMs initiating a period of stronger boundary 
layer convergence that would capable of north-south alignment of 
heavier rainfall situated over the Southeast-most Parishes in LA. 
The factor that will likely limit the threat will be the lower end 
rates between 1-2"/hr max that fall well-below the current FFG 
markers located over the proposed impact area. Totals of 2-4" with 
locally as high as 6" are depicted within a few of the CAMs, but 
it's not a full agreement in the suite neighborhood probabilities 
dropping off precipitously for accumulations >3". Decided to retain
the current nil ERO nationally, but wanted to point out the low-
end probability (<5%) for isolated flood concerns in the zone 
encompassing the above Parishes to the immediate coastline across 
both AL/MS. 

Kleebauer


Day 2
Valid 12Z Tue Nov 12 2024 - 12Z Wed Nov 13 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS WESTERN
WASHINGTON AND NORTHWEST OREGON...

The next Atmospheric River will impact the PAC Northwest with an
IVT pulse between 500-800 kg/ms signaling a modest moisture 
advection regime that historically leads to marginal flash 
flooding concerns along the immediate WA/OR coasts, the terrain of
the Olympics and along the foothills of the Cascades where the 
orographic enhancement pattern is defined. PWAT anomalies between 
1-1.5 deviations above normal will enter the region by the second 
half of the period with the heaviest precip likely coinciding with 
the initial moisture surge into the coastal plain. Some modest 
instability will be focused along the immediate coast as defined by
the some of the hi- res guidance and mean SBCAPE output within the
ensemble blend. Rates will be generally ~0.5"/hr within the 
heavier bands, but some 0.75-1.25"/hr rates are plausible within 
the Olympics in Northwest WA state, as well the immediate coast of 
WA down into Northwest OR. This is the primary axis of the IVT 
pulse with a distinct west to southwest orientation of the mean 
flow. This initial surge will be more of a primer for the following
day as the pattern spills into Wednesday, but some isolated flash 
flood concerns are plausible within the initial surge of the AR. 
The previously inherited MRGL risk was maintained with only a minor
adjustment a touch further south in Oregon to reflect the latest 
trends in the mean QPF depicting >2" along the OR coast. Totals 
will be generally between 1-3" with isolated to 4" into the Olympic
Peninsula, much of the precip falling within the 00-12z window on 
Wednesday. 

Kleebauer


Day 3
Valid 12Z Wed Nov 13 2024 - 12Z Thu Nov 14 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS WESTERN
WASHINGTON AND OREGON, NORTHWEST CALIFORNIA, AND OVER THE
SOUTHEASTERN U.S...

...Pacific Northwest...

Atmospheric River from the D2 period continues through much of the
D3 time frame with a waning signal towards Wednesday night into
Thursday. IVT core will come ashore between Astoria down through
the King Range of Southwestern OR with a signal for an additional
2-4" of rainfall with locally as high as 5.5" within the orographic
enhancement in-of the Klamath Mountains. Rates will maintain a 
steady- state of 0.25-0.5" over a span of several hours, but the 
primary core of the IVT pulse will likely ramp up rates >0.5"/hr 
with some 1-1.5"/hr rates plausible across those elevated coastal 
areas of OR and Northwestern CA. Rainfall will continue up into WA 
state with the heaviest rain confined to the Olympics with an 
additional 2-4" likely occurring northwest of Olympia. Two day 
totals over the coastal areas of WA/OR will settle between 3-6" 
with a max of 8" confined to both the Klamath-Siskiyou and Olympic 
Ranges thanks to some orographic enhancement providing localized 
QPF maxima that is customary from AR orientations extending from 
southwesterly flow. 

Additional flash flood opportunities will exist over portions of
Northwest CA to areas south of Mount Shasta where remnant burn
scars will act as an area of heightened concern with local FFG
indices much lower than surrounding areas. Totals will be between
2-4" in this area with the max of 5" occurring over the Northwest
tip of CA north of Eureka where heavier rainfall longevity will 
enhance totals locally. A MRGL risk was maintained for the threat
with much of Western WA/OR encompassed, as well as Northwest CA
with an extension inland to south of Mount Shasta to cover for the
burn scar areas that could see >1.5" of rainfall on Wednesday. 

...Southeast U.S...

A sprawling surface ridge east of the Mississippi will quickly
migrate eastward allowing for a strong return flow pattern across
the Southeastern U.S along the the western flank of the ridge.
Lingering moisture stemming from the remnants of Rafael will
fester in the Northern Gulf before being ushered poleward around
that clockwise circulation from the high pressure controlling the
pattern across the Mississippi Valley and points east. PWATs will
elevate up close to +2 deviations lending credence to a suitable
airmass for maintaining convective activity. Further west, a
shortwave trough will migrate into the Central Plains with surface
low pressure forming downstream across the Southern Plains/Mid-
Mississippi Valley creating a strengthening convergence pattern
along a cold front that develop on the tail end of the low.
Progressive flow within the setup will alleviate some of the
concerns of lingering precip that will develop from the Lower
Mississippi Valley to points north and east. 

The jury is still out on the expected intensity of the precip that
develops, but considering the convective prospects given the
theta-E advection pattern anticipated along with a source of
surface based convergence along and ahead of the forecasted cold
front, some stronger cores capable of rates >1-2"/hr and totals
breaching 3" will be possible, as noted by some of the current
deterministic. Ensemble bias corrected QPF output indicates some
2-3" amounts across parts of the Deep South signaling the threat.
UFVS First Guess Field has a MRGL risk encompassing parts of LA and
MS where the heavy rainfall from recent days has really dropped the
FFG indices across parts of the area. The antecedent conditions
over Southwest and Central LA will be privy to a heightened flash
flood threat leading a MRGL risk extending back into those areas.
The MRGL encompasses much of the Lower Mississippi Valley over into
AL extending up into TN, coinciding with the forecasted meridional
extent of the theta-E progression which would lead to convective
precipitation modes. Will assess as we get closer, but this is the
proxy for where the expected heavier rainfall threats could occur. 

Kleebauer

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