Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
257 AM EST Mon Dec 04 2023

Day 1
Valid 12Z Mon Dec 04 2023 - 12Z Tue Dec 05 2023

...THERE IS MODERATE RISK OF EXCESSIVE RAINFALL FOR THE
WEST/SOUTHWEST FACING SLOPES OF THE OLYMPIC MOUNTAINS IN WESTERN
WASHINGTON STATE...

A strong cold front associated with a 954 mb low in the Gulf of
Alaska will move into the coast of WA and northern OR, and stall
out there by later tonight. Moisture riding along and ahead of the
front will move northeastward into the coast, allowing heavy rain
to impact the Willamette Valley and Puget Sound regions between
the coastal/Olympic ranges and the Cascades. Many of the
meteorological elements continue to remain on track/in place as
prolonged (but strong) southwesterly warm advection (which is now
underway) across much of the Pacific Northwest, in particular
across western Washington into northwest Oregon. This strong WAA
will expand south and eastward with time throughout the day. Given
the strong flow (over 35-50kts at 850mb and 50+ in 700-500mb
layers) over much of the northeastern Pacific with 1.25 to 1.5+
Total PWat values, IVT values will be increasing from 500 kg/m/s
nearing 1000 kg/m/s by 03z into the Olympic Range within the core
of the greatest moisture flux convergence along/ahead of synoptic
height-falls that are slow to slightly below average in eastward
progression throughout the day.

Given this moist flow is generally orthogonal to the ridge lines
of the Olympics, this should support enhanced rain-rates which
will start to increase at or above 0.50"/hr with occasional
embedded areas in favorable orographic ticking up to 0.75"/hr.
This should result in the most impressive rainfall totals on the
west facing slopes of the Olympics, where there's good agreement
that rainfall totals of 5 to 7 inches (with local amounts up to 9
inches) expected. The new Hi-Res CAM guidance continues to provide
higher confidence in localized higher values of 7"+ totals (and
60-80% chance of neighborhood exceedance of 8" through 12z in the
Olympic Range). The HREF does denote that rates may not rise to
meteorological rates necessary for flash/rapid inundation
flooding, given 2"/3-hr probability only reach 50% and less than
10% for 3"/3-hrs; however, given prolonged warm advection over
snow pack in the Olympic Range (which is warming through depth),
rain on snow will contribute to runoff problems. 

In coordination with the National Water Center (National Water
Model), there are hints of 2-2.5" of snow water equivalent to be
utilized about the same time, likely resulting in up to .25"/hr
further rates added to the streams and rivers in the area to
further enhanced the considerable potential flooding impacts noted
in their Flood Hazard Outlook (FHO).  So, these combination of
factors and agreement from Seattle/Tacoma local forecast office
and NWC, a Moderate Risk of Excessive Rainfall was maintained over
much of the Olympic Range in support of hydrologically significant
(rivers to Moderate/Major flooding) event to unfold today into
Tuesday. The Slight and Marginal Risk areas were maintained as
well, surrounding the aforementioned westward facing slopes of the
coastal/Olympic ranges and Cascades across WA and into
northwestern OR.

Churchill/Gallina/Wegman


Day 2
Valid 12Z Tue Dec 05 2023 - 12Z Wed Dec 06 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
OREGON COAST RANGES AND PORTIONS OF THE WASHINGTON AND NORTHERN &
CENTRAL OREGON CASCADES...

...Eastern Washington to far Northwest California...

The evolution of the atmospheric river (AR) event across western
Washington and Oregon remains on track for the start of the Day
2/Tuesday period. A strong cold front will guide the AR into the
Columbia River area at the start of the period. Continued
prolonged moderate rainfall rates will result in high rainfall
totals and a considerable hydrologic flooding event as the front
pushes inland and south down the coast. Low level winds will
continue to be in th 45-60kt range but will diminish with time,
but for the heavy rainfall event, the broader moisture plume will
be significantly narrowed but also starting to back for less
favorable orographic intersection with the ranges. This should
reduce the capability of rainfall rates (eventually being limited
to around 0.50"/hr), but given the placement the duration is
expected to increase all given the uncertain strength/amplitude of
the surface to mid-level wave to develop off-shore.

Forecast 24-hour rainfall totals also remain on track and are
expected to largely be in the 3 to 5 inch range in most of the
Slight Risk areas. Grounds are (and will continue to be) fairly
saturated (80%+) suggesting reduced infiltration, but there will
about 24 hrs of respite of even moderate rainfall prior to day 2
period, so while the risk for excessive rainfall totals increase
for the 24 hr range, the short-term rates may be too low for
inducing flash flooding/rapid inundation flooding, though the
river flooding is likely to be considerable. At this time, a
Slight Risk remains in place for this cycle.


...Northern Idaho Panhandle...

A Marginal Risk area was maintained for the northern Idaho
Panhandle (and surrounding portions of WA/MT) with this update.
The plume of moisture associated with the AR will continue inland
and into the ID panhandle Tuesday and Tuesday night. Renewed
upslope flow into the area mountains will locally increase
rainfall rates through this area. Snow levels will be quite high
as the moisture advects a very warm air mass for this time of year
into the area. Thus, in addition to locally heavy rainfall,
snowmelt will be a significant component to any flooding.

Churchill/Gallina/Wegman


Day 3
Valid 12Z Wed Dec 06 2023 - 12Z Thu Dec 07 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
OREGON COAST RANGES AND FOR PORTIONS OF THE NORTHERN CALIFORNIA
COAST RANGES AND CENTRAL OREGON CASCADES...

There continues to be some good news to report across the Pacific
Northwest with an abatement in the atmospheric river and lesser
rainfall totals going into Wednesday and Wednesday night. There is
still a Marginal Risk area of excessive
rainfall from extreme northwest California to the Oregon coastal
ranges and the western Oregon Cascades on Day 3 (Wednesday) with
locally 1 to 2 inches of rainfall expected, but this is a
noteworthy improvement compared to the much heavier rainfall
expected during the Days 1-2 period. Even though rainfall totals
are expected to be lighter, the grounds will remain quite
saturated by this time with elevated creeks and rivers, so there
may still be lingering flooding impacts flooding for those areas
(particularly for the larger stem rivers).

Churchill/Hamrick/Schichtel


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt