FOUS30 KWBC 120751
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
251 AM EST Tue Nov 12 2024

Day 1
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 previous forecast remains steadfast with the expected totals
and evolution of our next disturbance progged to enter the PAC
Northwest. The next Atmospheric River is currently maneuvering
through the Northeast Pacific with a well-defined IVT pulse located
around the base of a strong, closed upper low and associated
surface cyclone churning over the North Pacific as indicated via
the latest CIMSS MIMIC-TPW satellite derived product. 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, forecast between 21-03z Wednesday (Late-afternoon and early
evening PST). Some modest instability will be focused along the 
immediate coast as defined by the hi- res guidance and mean SBCAPE
output within the ensemble blend (HREF/NBM). 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 beyond Wednesday
morning, but some isolated flash flood concerns are plausible 
within the initial surge of the AR. The previously inherited MRGL 
risk was maintained coinciding with the higher (>50%) 12-hr 
probabilities of exceeding 2". Totals will be generally between 
1-3" with isolated to 5" into the Olympic Peninsula.

Kleebauer


Day 2
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, AS WELL AS PORTIONS OF THE
SOUTHEAST AND OHIO VALLEY ...

...Pacific Northwest...

Minimal changes were necessary from the previous forecast issuance
as guidance maintains relevant continuity. Atmospheric River from 
the D1 period continues through much of the D2 time frame with a 
waning signal towards Wednesday night into Thursday. IVT core will 
come ashore between Astoria down through the King Range of 
Northwest CA 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 3-5" 
likely occurring northwest of Olympia. Multi-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 and Ohio Valley...

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.

There has been a growing consensus on the expansion of convective
activity across the Deep South into the Ohio Valley with a
formidable heavy rain signature located across the Gulf coast where
the best instability footprint is located, and across the Southern
Ohio Valley where ascent will be maximized within the diffluent
area of the upstream shortwave trough in tandem with the
approaching cold front. QPF off the recent 00z HREF blended mean 
in the final 12-hrs signifies some potential for significant totals
>2" with a few CAMs depicting as much as 6" across portions of LA
and MS with more of a 2-4" plateau across the Ohio Valley. Ensemble
means are generally stable at 1-2" across a large area, but might
be smoothing out the higher totals as is the bias of the ensemble
blend. In any case, the pattern is certainly favorable for at least
isolated prospects for flash flooding, especially in South-Central
LA where recent QPE from 48 hrs prior sat at 5-10" with locally
higher, so the threat in this locale is greatly enhanced just due
to the antecedent soil moisture anomalies within the past few days.
Further north is not nearly as robust in the soil moisture
anomalies, however some areas within TN and Western KY do have 
some lingering top-layer soil moisture from the same event that 
occurred in LA. The previous MRGL risk was maintained with a minor 
adjustment north into Southern IN as reflected within the latest 
QPF trends and expected northern edge of the heaviest precip 
Wednesday night into Thursday. Discrepancy in the exact placement 
of heavier convection across the Gulf states deterred an additional
upgrade over the region, however the threat for at least one risk 
category higher is in play as we move closer to the event. For now,
a blanket MRGL risk was sufficient with the potential for an 
upgrade plausible in the next few forecast updates. 

Kleebauer


Day 3
Valid 12Z Thu Nov 14 2024 - 12Z Fri Nov 15 2024

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

Kleebauer

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