Mesoscale Precipitation Discussion 0051
NWS Weather Prediction Center College Park MD
324 PM EST Mon Feb 24 2025

Areas affected...WA/OR Coastal Ranges into Cascades

Concerning...Heavy rainfall

Valid 242022Z - 250815Z

SUMMARY...Areas of heavy rain will return to the WA and OR Coastal
Ranges and downstream into the Cascades later today/tonight. While
peak rainfall rates may reach/exceed 0.5 in/hr on an isolated
basis, they are much more likely to remain in the 0.2 to 0.4 in/hr
range. 12 hour rainfall of 1-2 inches, locally 3+ inches, is
expected through 08Z.

DISCUSSION...GOES West water vapor imagery at 20Z showed a
well-defined closed low centered just west of 130W near 45N. This
upper low had been strengthening this morning but appears to be
nearing maturity. At the surface, an occluded cyclone was depicted
through visible satellite imagery with the WPC 18Z surface
analysis showing a central pressure of 978 mb and this intensity
is likely close to peak strength. The triple point was located
just near the mouth of the Columbia River and what is left of an
earlier axis of warm advection precipitation was lifting north
through central WA. Meanwhile, radar imagery showed a band of
rain, associated with the approaching cold front, moving just past
the Willamette Valley into the Cascades. The diffluent left exit
region of a mid-upper level jet streak was also located over the
lower Columbia River Valley, likely enhancing lift and enhancing
precipitation intensity.

Short term forecasts from the RAP suggest the closed low will
maintain a similar intensity through about 03Z prior to filling as
it moves toward southern Vancouver Island. The cold frontal band
will continue to steadily move inland through 00Z, followed by a
lull in precipitation intensity outside of instability driven
showers due to steepening mid-level lapse rates tied to the
approach of the closed low aloft. RAP forecasts show MLCAPE up to
~500 J/kg along the OR coast through the early overnight.

As the deep low level low nears the coast, a period of strong 850
mb onshore flow is expected to be directed into the coast of
northern OR into southern WA, with 850 mb wind speeds of 70-80 kt
in the 03-09Z time frame from the SW to WSW. While snow levels
will be dropping to ~3500 ft behind the cold front across the
Pacific Northwest, the u-component of the 850 mb wind will be
60-70 kt into the Coastal Ranges and 30-45 kt into the Cascades
with a strong orographic component and steady rainfall of 0.2 to
0.4 in/hr, locally up to 0.5 in/hr possible.

While rainfall rates will not be exceptionally high, the addition
of 1 to 2 more inches, perhaps as high as 3 inches through 08Z, is
expected. Due to widespread 3 to 6 inch rainfall totals across the
region over the past 48 hours, additional rainfall may bring
additional localized concerns for runoff problems including small
stream and urban flooding.

Otto

ATTN...WFO...PQR...SEW...

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

LAT...LON   47912425 47842399 47612365 47572333 47602310
            47032293 46942263 47022225 46962195 46652177
            46332188 45982181 45652168 45362178 44972186
            44622201 44442224 44442248 44492262 44712279
            44852288 44902310 44862327 44742334 44652339
            44492349 44402379 44472410 44582428 44962430
            45732423 46472429 47062450 47622455 47802452