503
AWUS01 KWNH 222021
FFGMPD
CAZ000-ORZ000-WAZ000-230820-

Mesoscale Precipitation Discussion 0048
NWS Weather Prediction Center College Park MD
320 PM EST Sat Feb 22 2025

Areas affected...Pacific Northwest

Concerning...Heavy rainfall

Valid 222020Z - 230820Z

SUMMARY...Atmospheric river conditions arriving across the Pacific
Northwest will bring heavy rainfall in across the coastal ranges
and into the windward slopes of the Cascades later today and
tonight.

DISCUSSION...A series of offshore shortwave impulses lifting
northeastward toward British Columbia over the next 6 to 12 hours
will be bringing an offshore frontal system toward the coastal
ranges of western WA and western OR along with strong atmospheric
river conditions.

The latest GOES-W WV suite along with CIRA-ALPW and experimental
LVT data shows a very well-defined trans-Pacific fetch of deep
moisture extending from just offshore of the Pacific Northwest
southwestward all the way down to northwest of Hawaii with notable
tropical origins of some of the moisture transport. Multiple
low-amplitude shortwave impulses are embedded within this
atmospheric river channel, and this will help to facilitate strong
warm air advection and onshore flow into especially the coastal
ranges of western WA and western OR along with the windward slopes
of the Cascades by later today and continuing into the overnight
period that will drive locally enhanced rainfall amounts.

PW anomalies are forecast to rise to 2 to 3+ standard deviations
above the climatological mean by 00Z and continue at this level
through tonight as a strong southwest low to mid-level jet sets up
and persists just offshore and overruns the higher terrain of the
Pacific Northwest. In fact, the 850/700mb moisture flux anomalies
are forecast to rise to as much as 3 to 5+ standard deviations
above normal, with one peak this evening by around 00Z and then
another later tonight as the arrival of each shortwave impulse
corresponds to a stronger low to mid-level wind field.

Given the enhanced deep layer warm air advection and moisture
transport along with some of its tropical origin, the rainfall
rates are expected to be efficiently high. The 12Z HREF guidance
shows high probabilities of seeing 0.50" to 0.75"/hour rainfall
rates across the Olympic Peninsula in the 21Z to 00Z time frame,
with portions of the southwest WA coastal range also seeing
elevated probabilities of these rates toward early this evening.
Areas farther east into the windward slopes of the Cascades and
farther south across coastal OR and gradually coastal northwest CA
should see rates easily reach into the 0.25" to 0.50"/hour range
with rates occasionally over a 0.50"/hour.

Over the next 12 hours, some rainfall totals are expected to reach
as high as 3 to 5+ inches over the higher terrain and especially
the favored coastal ranges and upslope areas of the Cascades with
lesser amounts over the adjacent interior valleys/terrain-shadowed
locations. Generally these initial rains are not likely to cause
much in the way of any flooding threat aside from potentially some
localized urban and small stream runoff concerns where the heavier
and more persistent rates set up. However, additional heavy
rainfall beyond this period going into Sunday will gradually raise
the flooding threat.

Orrison

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...EKA...MFR...OTX...PDT...PQR...SEW...

ATTN...RFC...CNRFC...NWRFC...NWC...

LAT...LON   49162120 48902067 48122052 47472075 47002120
            46402119 45762157 45192153 44712176 44052168
            43342208 42402211 42282262 42932320 42792371
            42262360 41472341 40742345 40022393 40282454
            41532441 43062470 44952423 46622433 47972464
            48332411 48092348 47562304 47682238 48692234