Graphic for MPD #0037

Mesoscale Precipitation Discussion 0037
NWS Weather Prediction Center College Park MD
306 AM EST Sat Jan 27 2024

Areas affected...far northwestern CA into western OR and
southwestern WA

Concerning...Heavy rainfall

Valid 270802Z - 271910Z

SUMMARY...Periods of moderate to heavy rain are likely to impact
portions of the Pacific Northwest through 18Z today. 12 hour
rainfall totals of 2-4 inches are expected for the favored
southwest facing slopes of the Coastal Ranges into portions of the
Cascades.

DISCUSSION...Rainfall has been ongoing across the Pacific
Northwest for much of the past 12-24 hour with gauges reporting 1
to 2+ inches since 06Z Friday. Hourly rainfall totals over the
past few hours along the Oregon coast have averaged 0.1 to 0.2
inches with localized hourly totals above 0.25 inches. Layered PW
imagery showed a stream of higher moisture content air extending
from the Hawaiian Islands to the Pacific Northwest, and a second
stream stretching from the central Pacific to a potent shortwave
near 41N 129W. GPS measured precipitable water values along the
Oregon coast were 1.1 to 1.4 inches, being advected on a
south-southwesterly fetch of 40-50 kt in the 850-700 mb layer into
the coast of northern California and southern Oregon. The result
has been a stream of 600-700 kg/m/s IVT (via 00Z GFS) into the
Pacific Northwest coast, but the primarily southerly component of
the low level wind has kept rainfall rates fairly subdued so far.

GOES West water vapor imagery showed the mid to upper level
shortwave near 41N 129W (07Z) tracking toward the northeast at the
front edge of a longwave trough located over the eastern Pacific.
Infrared imagery has shown an expanding region of colder cloud
tops ~150 miles west of the California/Oregon border, just ahead
of the shortwave through 07Z. As the shortwave continues to
approach the West Coast, it is expected to make landfall in
northern Oregon near 15Z, via the 06Z RAP and the 00Z global model
consensus. IVT is expected to gradually increase through 12Z as
the shortwave approaches the coast, with values possibly peaking
above 800 kg/m/s into southwestern Oregon. Some veering of the low
level wind is also expected as the shortwave trough passes the
coast, supporting an increased orthogonal component of the low
level wind with the axis of the Coastal Ranges and Cascades,
providing a boost to rainfall intensity. The 00Z HREF showed a 6
hour window across southwestern Oregon with probabilities of 0.5+
in/hr in the 40-70 percent range. 12 hour rainfall totals of 2-4
inches are expected through 19Z or 20Z across the favored
southwest facing terrain of the Coastal Ranges, extending from Del
Norte County, California to Pacific County< Washington...and
eastward to the southern Washington and northern Oregon Cascades.

Otto

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

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

LAT...LON   46472303 46392216 46282172 46062142 45832143
            45602147 45182154 44762159 44262162 44042173
            43962211 44052236 44212265 44272294 44192315
            43932321 43532337 43222343 42732344 42302333
            41722347 41312378 41062421 41302457 42552488
            43912458 45272444 46012440 46342372
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Last Updated: 306 AM EST Sat Jan 27 2024