Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Dec 26 2024 03:02:40 AWUS01 KWNH 260301 FFGMPD CAZ000-ORZ000-WAZ000-261500- Mesoscale Precipitation Discussion 1190 NWS Weather Prediction Center College Park MD 1000 PM EST Wed Dec 25 2024 Areas affected...Western WA...Western OR...Far Northwest CA Concerning...Heavy rainfall Valid 260300Z - 261500Z SUMMARY...Atmospheric river activity continues across the Pacific Northwest with areas of heavy rain expected to persist into early Thursday morning before weakening. DISCUSSION...GOES-W Airmass RGB satellite imagery shows a strong shortwave trough amplifying offshore of the Pacific Northwest, and this is driving a rapidly deepening area of low pressure near 45N 131W. This approaching low center along with a strong upper-level jet on the order of 130 to 150+ kts at 250 mb will continue to drive a well-defined atmospheric river into the Pacific Northwest overnight with a gradual southward shift of the higher PW axis/IVT core by early Thursday morning as a cold front gradually arrives and begins crossing the region. IVT magnitudes are expected to peak over the next 3 to 6 hours across the coastal ranges of far southwest WA, western OR and far northwest CA with values impressively reaching as high as 800 to 1200 kg/m/s. However, the offshore energy and attendant cold front is quite progressive and thus these enhanced IVT parameters are not expected to persist for too long. The IVT values are forecast by the 18Z GFS and 12Z ECMWF to steadily weaken by 12Z as the cold front advances inland along with the corridor of stronger low to mid-level flow. Rainfall rates overnight are forecast to generally be on the order of 0.25" to 0.50"/hour with occasional 0.50"+ rates expected for especially the southwest facing slopes of the coastal terrain. The most recent HREF guidance suggests portions of southwest OR and far northwest CA will generally have the highest probabilities of seeing these rates, with peak rates potentially here reaching as high as 0.75"/hour as the cold front arrives later in the night. By early Thursday morning, the rates are forecast to be weakening overall, but there will be some post-frontal instability and persistence of onshore flow to keep shower activity going that may foster some additional brief heavy rainfall rates. A consensus of the latest HREF guidance supports additional rainfall totals on the order of 2 to 4 inches going through early Thursday morning (15Z/7AM PST) across the orographically favored higher terrain, and especially the coastal ranges. Orrison ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!8M49YNk1dZh0T4I6mLeXxA08cPYpvQHh3Td323rWZpzqOpGY-HOXXE4-RDpq1HIAuSTx= yjBfKQ0ENUv6OEEPMdRPsBE$ for graphic product... ATTN...WFO...EKA...MFR...PDT...PQR...SEW... ATTN...RFC...CNRFC...NWRFC...NWC... LAT...LON 47872325 47542211 46972182 46442176 45442171=20 44512185 43832202 43142230 42212222 41632232=20 41052297 40082311 39822346 40102428 41142453=20 42522470 44752434 46162436 47322425=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .