FOUS30 KWBC 260059
QPFERD

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

Day 1
Valid 01Z Thu Dec 26 2024 - 12Z Thu Dec 26 2024

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

Very minimal changes were made for the 01Z update across the
Pacific Northwest. Rainfall over the past 12 hours has generally=20
amounted to 1 to 2 inches across the upslope regions of the=20
northern OR into WA Coastal Ranges and Olympic Mountains, per area=20
observations. A few areas of moderate to heavy rain were ongoing=20
across the coasts of Washington and Oregon but a surge of higher=20
intensity rainfall is likely tonight ahead of an approaching=20
shortwave. Water vapor imagery shows this shortwave located near=20
43N 135W at 00Z, racing eastward with an expected surge in low=20
level winds for the Oregon coastline overnight. Recent RAP=20
forecasts have been fairly consistent with peak 850 mb winds=20
increasing to near 80 kt between 03-06Z for the south-central=20
Oregon coast, although there should be a largely southerly=20
component to the low level flow along the Pacific Northwest coast,=20
until the associated cold front moves through near/after 06Z.=20

In addition to the increasing IVT (800-900 kg/m/s per recent
GFS/RAP forecasts), lift will be aided by right-entrance ascent
tied to a 130-160 kt upper level jet max centered near 250 mb about
400-500 miles west of the Oregon coast (per GOES West DMVs).
Rainfall rates are likely to increase above 0.5 in/hr, perhaps as
high as 1 in/hr for favored SSW facing slopes of the Coastal Ranges
in the 03-06Z time frame. Veering low level winds and increasing
mid-level lapse rates (weak instability) in the post-frontal=20
environment will favor more showery/cellular precipitation echoes
despite the lowering IVT/moisture flux into the terrain between=20
06-12Z.

Through 12Z, an addition 1 to 3 inches of rain (perhaps local=20
maxima near 4 inches) is expected for the favored SSW facing slopes
of the Coastal Ranges from northern California into Oregon and
southwestern Washington. While Oregon has been in a relative low in
anomalous precipitation over the past 2 weeks (higher anomalies in
western Washington and northern California), the potential for
flooding cannot be ruled out for susceptible/low lying areas of the
Pacific Northwest.

Otto


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
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,
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 sufficient 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
AREAS OF FAR SOUTHWEST WASHINGTON, OREGON, AND NORTHWEST
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
toward the region from the Southern Plains). Convection across the
region will likely persist for most of the day before re-
intensifying late in the evening/overnight period. Additionally,
convection is expected to focus along synoptic
boundaries/convective outflow(s) while training under persistent southweste=
rly
flow aloft. The overall scenario supports an axis of 1-3 inch
rainfall totals (locally higher where deeper convection can
persist). At least isolated instances of flash flooding are
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!5HZ5_jc3lVIgmy20aEY_2Mydz6SQHsyVjxMEflljodCO=
E8-_wT495b5ytw6cqON-_oPMLRltjZpjtbeNlZYVXp5znM4$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!5HZ5_jc3lVIgmy20aEY_2Mydz6SQHsyVjxMEflljodCO=
E8-_wT495b5ytw6cqON-_oPMLRltjZpjtbeNlZYV39kPsFw$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!5HZ5_jc3lVIgmy20aEY_2Mydz6SQHsyVjxMEflljodCO=
E8-_wT495b5ytw6cqON-_oPMLRltjZpjtbeNlZYVZYPloTk$=20

=3D =3D =3D
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