FOUS30 KWBC 251907
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
207 PM EST Wed Dec 25 2024

Day 1
Valid 16Z 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
forecasted evolution of the next atmospheric river (AR) surging
off the Pacific and into the Pacific Northwest today and into
tonight. Model guidance is keen on a surge of moisture represented
by fairly elevated IVT signatures on the order of 600-800 kg/ms,
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
totals over 12-24 hours (and perhaps as high as 5" into the
Olympic Peninsula due to the added orographic enhancement). Given
that soil moisture anomalies (per the NASA SPoRT-LIS 0-1 m layer)
generally range from 70-90th percentile and are about 70-80%
saturated, the inherited Marginal Risk was maintained for this
cycle. Rainfall from prior days has resulted in rises in rivers
within the local watersheds of the areas anticipating the heaviest
rainfall, especially within the Skokomish basin south of the
Olympics, as well as watersheds from southwest OR into northwest
CA (per USGS streamflow anomalies indicated above normal to much
above normal). The elevated stream flows and heavy rain will
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
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
Orleans and urban ground conditions that may be overwhelmed a bit
quicker than regional FFG values are.  Given the small areal
coverage, a categorical Marginal Risk is not warranted at this
time, but there will be a non-zero risk of flash flooding fully
constricted to the urban corridors of Southeast Louisiana late this
morning into the afternoon hours.

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...

...20Z Outlook Update...
The overall outlook covers the excessive rainfall threats well and
only minimal changes were needed. Portions of the Marginal Risk
area in northern California were expanded to cover additional
higher-terrain areas near the Sierra as models have trended toward
an increase in rainfall between 00Z-12Z Friday. Areas of 1-3 inch=20
rainfall amounts are possible amid an approaching atmospheric river
and strong upslope flow during that timeframe.

The remainder of the outlook is on track, with areas of excessive
rainfall expected especially in the Slight risk across Arkansas,=20
western Louisiana, and eastern Texas. Refer to the previous
discussion for more information.

Cook

---Previous Discussion---

..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 PORTIONS OF
NORTHERN MISSISSIPPI, NORTHERN ALABAMA, TENNESSEE, AND COASTAL=20
AREAS OF FAR SOUTHWEST WASHINGTON, OREGON, AND NORTHWEST=20
CALIFORNIA...

...20Z Outlook Update...
A Marginal Risk was introduced from northern Mississippi into
southeastern Tennessee for this update. The region will likely
reside in between a couple of strong mid-level shortwave troughs
(one lifting north across the Ohio Valley and another moving=20
toward the region from the Southern Plains). Convection across the
region will likely persist for most of the day before re-=20
intensifying late in the evening/overnight period. Additionally,=20
convection is expected to focus along synoptic=20
boundaries/convective outflow(s) while training under persistent southweste=
rly
flow aloft. The overall scenario supports an axis of 1-3 inch=20
rainfall totals (locally higher where deeper convection can=20
persist). At least isolated instances of flash flooding are=20
possible where convective bands can materialize.

Models remain in decent agreement with appreciable rainfall totals
across coastal areas of Oregon/California. The Marginal risk in
this area has only been modified slightly from the previous
forecast per small northward trends in the heaviest precipitation
noted in 12Z model guidance. Refer to the previous forecast
discussion for more information.

Cook

---Previous Discussion---

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!-jjGIQaEOrnKf72eVi18SHojmb0HyXfOW69V4_ip__d4=
szUsx5bBs4a3v_cnPYsQJUBS_uW6IkePL4iNnxR4UGgq7K8$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!-jjGIQaEOrnKf72eVi18SHojmb0HyXfOW69V4_ip__d4=
szUsx5bBs4a3v_cnPYsQJUBS_uW6IkePL4iNnxR4UYTfOYQ$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!-jjGIQaEOrnKf72eVi18SHojmb0HyXfOW69V4_ip__d4=
szUsx5bBs4a3v_cnPYsQJUBS_uW6IkePL4iNnxR4XAs_vVI$=20

=3D =3D =3D
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