FOUS30 KWBC 251542
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1042 AM EST Wed Dec 25 2024

Day 1
Valid 12Z Wed Dec 25 2024 - 12Z Thu Dec 26 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL AREAS
OF WASHINGTON, OREGON, AND NORTHWEST CALIFORNIA...

...16z update...
There are no changes in trends/analysis to change current
ERO/Marginal Risk zones across the PacNW at this time...

---Prior Discussion---
There has been little change in the=20
 forecasted evolution of the next atmospheric river (AR) surging=20
 off the Pacific and into the Pacific Northwest today and into=20
 tonight. Model guidance is keen on a surge of moisture represented
 by fairly elevated IVT signatures on the order of 600-800 kg/ms,=20
 bringing in a wave of heavier precipitation entering coastal WA/OR
 by this afternoon, and spreading southward into coastal northwest
 CA later tonight. The latest WPC QPF calls for 2-4" localized=20
 totals over 12-24 hours (and perhaps as high as 5" into the=20
 Olympic Peninsula due to the added orographic enhancement). Given=20
 that soil moisture anomalies (per the NASA SPoRT-LIS 0-1 m layer)=20
 generally range from 70-90th percentile and are about 70-80%=20
 saturated, the inherited Marginal Risk was maintained for this=20
 cycle. Rainfall from prior days has resulted in rises in rivers=20
 within the local watersheds of the areas anticipating the heaviest
 rainfall, especially within the Skokomish basin south of the=20
 Olympics, as well as watersheds from southwest OR into northwest=20
 CA (per USGS streamflow anomalies indicated above normal to much=20
 above normal). The elevated stream flows and heavy rain will=20
 enhance local flash flood prospects for anywhere in proxy to those
 watersheds, so the threat is a bit higher than normal compared to
 what is usually forecast for an AR of this strength.

In addition to longer-fused flood concerns, elevated instability is
forecast to reach 250+ J/Kg (MU CAPE) late in the period, which
will enhance the hourly rainfall rates (increasing the threat for
localized flash flooding in association with burn scars or other
particularly sensitive terrain). The latest (00z) HREF depicts high
probabilities (between 40-70 percent) of 0.25"+/hr rates between
06-12Z Thu (per 10-100 km Ensemble Agreement Scale neighborhood
probabilities, indicating strong spatial agreement between the CAMs
in association with orographic enhancement) and some chance of=20
also realizing 0.50"+/hr rates (40-km neighborhood probabilities as
high as 40-60 percent).

Churchill/Kleebauer/Hurley


...Southeast Louisiana...

GOES-E, regional RADAR and surface observations, note weak low-
level cyclonic convergence lifting northward along the SSE to N
oriented warm/stationary front across south-central LA. This has
locally enhanced surface heating and moisture return to enhanced
some modest instability. Convergence enhanced by the confluent
flow, has broken out a few thunderstorms that are breaking out
across the southern parishes of St. Mary/Assumption and Terrebonne.
Bulk shear values in the vicinity of the warm front support weakly
rotating updrafts that further enhance moisture flux convergences
supporting low-level moisture loading for some efficient rainfall
production. Given .75-.9" in the sfc-850mb CIRA LPW layer and
20-30kt inflow, rates of 1.5"/hr with lower potential for locally
2"/hr rates may occur given the environment/storm mode. As such,
there is potential of locally 2-3" and in the vicinity of New=20
Orleans and urban ground conditions that may be overwhelmed a bit=20
quicker than regional FFG values are.  Given the small areal=20
coverage, a categorical Marginal Risk is not warranted at this=20
time, but there will be a non-zero risk of flash flooding fully=20
constricted to the urban corridors of Southeast Louisiana late this
morning into the afternoon hours.=20

Gallina


Day 2
Valid 12Z Thu Dec 26 2024 - 12Z Fri Dec 27 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE ARK-LA-TEX
REGION AND SURROUNDING PORTIONS OKLAHOMA AND MISSISSIPPI...

..East Texas and Lower Mississippi Valley..

A repeat of the Christmas Eve setup (though displaced a bit north
and eastward) is progged for east TX extending into the Lower
Mississippi Valley as a more potent shortwave trough/cut-off low
digs eastward from the Southern Plains into the Ark-La-Tex. Given
highly favorable ascent aloft (right-entrance region of 90+ kt jet
streak) coupled with surface cyclogenesis and an increasingly
destabilizing environment ahead of the trough, convection is
expected to rapidly organize and progress eastward within the
expanding warm sector. Given the prior days rainfall over East TX,
as well as more sensitive soils from the Ark-La-Tex northeastward
into the Ozarks (per 3-hr FFGs of 1.5-2.5"), a Slight Risk was
introduced (in coordination with local forecast offices) as
available CAMs (00z FV3 and experimental MPAS-NSSL, 06z HRRR)
suggest the potential for localized short-term (3-6 hour) totals
of 2-4". This seems plausible, given the strong dynamics of the
setup and sufficent tropospheric moisture for realizing 1-3"/hr
rates (with precipitable water values of 1.2-1.4", which is not
only well above the 90th percentile for late December, but is near
average values for the peak of the wet season). As more hi-res
CAMs come into range of the full event later over time, expect
adjustments and refinements to the Slight Risk area.


...Pacific Northwest...

A secondary IVT pulse will occur on Thursday during the prolonged
AR event creating a flash flood potential extending into the D2
time frame. The multi-day precipitation forecast is upwards of 3-6"
(and locally higher in the Olympics). The second pulse will
actually protrude farther inland across OR, creating a risk for
flooding inland (compared to the previous period). The AR will
eventually slide southward with more emphasis placed on southwest
OR into northern CA late in the period, leading to more heavy
precip over already saturated soils. The inherited Marginal Risk
was maintained, given the wet soils and fairly consistent QPF.

Churchill/Kleebauer


Day 3
Valid 12Z Fri Dec 27 2024 - 12Z Sat Dec 28 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL AREAS
OF FAR SOUTHWEST WASHINGTON, OREGON, AND NORTHWEST CALIFORNIA...

A tertiary pulse of marginal IVT will push onshore between the
southern WA coast down through northwest CA with another round of
moderate to heavy rainfall signaled over the coast to just inland
encompassing the coastal ranges of southwestern OR and northern CA.
This is likely one of the weaker IVT pulses within the run leading
into the period, however the compounding factor of significant
rainfall anticipated prior to D3 allows for very wet antecedent
conditions with soil moisture anomalies already breaching the 80th
percentile for places up and down the OR and northern CA coasts. An
additional 2-4" will lead to areas approaching 6-8+" over the span
of 5 days in the anticipated impact zones lending credence to a
continued MRGL risk for flash flooding. The previously inherited
forecast was generally maintained with the primary concerns being
increased runoff capabilities and local urban flooding for coastal
towns along the OR and northern CA coasts.

Kleebauer


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!5LjJArXojfy0q0ndELX5fB6GVs9lJu_SX4Aep0CdLYJd=
H59I9za81YyL6pdQ4z94PlsUpFRlUQLcfQ1rIyC0eWzRFCw$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!5LjJArXojfy0q0ndELX5fB6GVs9lJu_SX4Aep0CdLYJd=
H59I9za81YyL6pdQ4z94PlsUpFRlUQLcfQ1rIyC02XR784Y$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!5LjJArXojfy0q0ndELX5fB6GVs9lJu_SX4Aep0CdLYJd=
H59I9za81YyL6pdQ4z94PlsUpFRlUQLcfQ1rIyC0HudcM2o$=20

=3D =3D =3D
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