Subj : MESO: Heavy Rain PacNW To : All From : Mike Powell Date : Sat Jan 27 2024 09:46:00 AWUS01 KWNH 270806 FFGMPD CAZ000-ORZ000-WAZ000-271910- 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 = = = --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .