Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Dec 27 2022 03:03:35 AWUS01 KWNH 270303 FFGMPD ORZ000-WAZ000-271500- Mesoscale Precipitation Discussion 1172 NWS Weather Prediction Center College Park MD 1001 PM EST Mon Dec 26 2022 Areas affected...Western OR...Southwest WA Concerning...Heavy rainfall Valid 270300Z - 271500Z SUMMARY...Strong atmospheric river conditions will be occurring overnight with heavy rains overspreading the coastal ranges and higher terrain of the Cascades. Heavy showers will linger into Tuesday morning. DISCUSSION...A strong upper-level trough and associated surface low offshore of the West Coast continues to advance east-northeast and will bring a significant surge of Pacific moisture into many areas of the Pacific Northwest overnight. The latest CIRA-ALPW data analysis shows a well-defined trans-Pacific fetch of deep moisture with subtropical origins spanning large areas of the central and eastern Pacific Ocean, and this moisture axis is aiming into the coastal ranges of many areas along the West Coast. The 18Z GFS and 12Z ECMWF both show IVT values peaking in the 800 to 1200 kg/m/s range across western OR and southwest WA by around 06Z as the offshore surface low continues to deepen and drives a strong southwest low-level jet of 50 to 70+ kts and a nose of PWs near or just over 1.25 inches into the coast. The 850/700 mb moisture flux anomalies are forecast to reach 3 to 5+ standard deviations above the mean, and with the subtropical origins of the atmospheric river, there will be a high degree of rainfall efficiency. The onshore flow overnight will be coming in at a relatively low angle to the coastline, but the southwest facing slopes of the coastal ranges and the higher terrain of the Cascades should still see enhanced rainfall rates that may reach 0.50" to 0.75"/hour over the next 3 to 6 hours. A cold front will be gradually crossing the West Coast between 06Z and 12Z, but there will be plenty of post-frontal onshore flow and moisture transport that will maintain a threat of heavy showers going through Tuesday morning. Facilitating this will be the strong offshore surface low approaching western WA by late Tuesday morning. Thus, while the heaviest rainfall rates will be overnight, there should still be some heavy post-frontal shower activity into Tuesday morning, and the latest HRRR and HREF guidance support these rates still reaching as high as 0.25" to 0.50"/hour. Expect the favored coastal ranges and upslope areas of the Cascades to see additional rainfall amounts of 2 to 3 inches of rain by mid-morning Tuesday with isolated heavier amounts. Orrison ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!8CZ-CDgWLDarjU7izTzAY4brJTg6d4vVQy1gtw_wlta8NBFcp7yQsIri9mFCqCPjYkN1= OFy66-F5Gug49PJPqtLAaL4$ for graphic product... ATTN...WFO...MFR...PDT...PQR...SEW... ATTN...RFC...NWRFC...NWC... LAT...LON 46702349 46272273 46422211 46362176 46072147=20 45792157 45422162 45072175 44732188 44032191=20 43412220 42742280 42662339 42862386 42852437=20 43552444 44712424 45762413 46452416 46652385=20 =20=20=20=20=20=20=20=20=20=20=20=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .