191
FOUS30 KWBC 211855
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
155 PM EST Fri Feb 21 2025

Day 1
Valid 16Z Fri Feb 21 2025 - 12Z Sat Feb 22 2025

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

Cook/Oravec

Day 2
Valid 12Z Sat Feb 22 2025 - 12Z Sun Feb 23 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF
WESTERN WASHINGTON STATE AND COASTAL OREGON...

19z Update: Only minor changes to the inherited Marginal risk, with
the reasoning described below still on track.

Chenard

...Previous Discussion...

No significant changes made to the previous marginal risk across
coastal sections of Oregon, Washington and in the upslope of the
Washington Cascades and northern Oregon Cascades. The beginning of
a two day atmospheric river event on tap as broad west
southwesterly mid to upper level flow is established across the
northeast Pacific toward the Pacific Northwest. PW anomalies
expected to rise to 2 to 3+ standard deviations above the mean by
Saturday afternoon with the lead shortwave moving toward the Pac NW
coast and continue at that level into Sunday morning. Strong
850-700 mb moisture flux, anomalies 2 to 4+ standard deviations
above the mean expected in the anomalous PW axis, max IVT values
600-800 kg/m/s. This will support hourly rainfall rates of .50"+
and widespread totals of 1-2"+ and max totals of 3-5"+ in favored
terrain regions. Snow levels will be very high day 2 across the
Pac NW, with much of the precipitation falling as rain. This heavy
rain, combined with snow melt will support isolated runoff issues.
Precipitation has been below average over the past month over the
Pac NW, which should mitigate any widespread runoff issues day 1.


Oravec

Day 3
Valid 12Z Sun Feb 23 2025 - 12Z Mon Feb 24 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF
WESTERN WASHINGTON STATE AND OREGON AND NORTHERN IDAHO INTO
NORTHWEST MONTANA...

19z Update: Made a modest expansion of both Marginal risk areas.
With 48 hour rainfall amounts of 3-6" (locally higher) in the
terrain, and 1-3" in valley locations, opted to broaden the
Marginal risk over western WA and OR. When combining the duration
and strength of the IVT this is approaching strong atmospheric
river levels...which verify at Slight risk levels about half the
time. However, given the broad and prolonged nature of the event,
hard to pin down a targeted Slight risk at this point, and the
drier antecedent conditions going into the event may help keep
impacts at the Marginal level. Nonetheless will continue to
monitor, and either way at least some flooding issues are expected.

Rainfall totals have trended up along the central Gulf Coast.
Strong dynamics will likely result in a swath of moderate to
locally heavy rainfall, although even the stronger and further
north GFS fails to bring notable instability inland. Thus still
thinking rainfall rates will not be high enough to justify adding a
risk area in the ERO...so will continue without an area depicted.

Chenard

...Previous Discussion...

The atmospheric river event that started day 2 into the Pacific
Northwest will continue into day 3 as additional height falls
embedded in the broad west southwest mid to upper level flow across
the northeast Pacific move toward the Pacific Northwest.
No significant changes to the plume of anomalous PW values and
anomalous 850-700 mb moisture flux anomalies impacting the Pac NW
day 3, with values 2-4+ standard deviations above the mean
persisting along with max IVT values of 600-800 kg/m/s. This will
support another day of widespread heavy rainfall totals of 1-2"+
and max totals in the 3-5"+ range in the favored terrain regions.
Snow levels again will remain high across much of the Pac NW day 3
with much of the precip again falling as rain. With widespread two
day totals in the 3-5"+ range and max totals 6-10" in favored
terrain regions, increasing soil moisture and stream flow values
will increase the risk of flooding.

A marginal risk area was added inland across portions of northern
Idaho into northwest Montana. The anomalous PW values pushing into
the Pac NW day 2 will move into the Northern Rockies day 3, with
values 2-3+ standard deviations above the mean.  Similar to areas
across the Pac NW, snow levels will be very high day 3 with much of
the precip falling as rain . Two day precip totals of 1-3"
possible across northern Idaho into northwest Montana, combined
with snow melt will support isolated runoff conditions developing.

Oravec


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