Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 26 2022 06:22:50 AWUS01 KWNH 260622 FFGMPD ORZ000-WAZ000-261620- Mesoscale Precipitation Discussion 1170 NWS Weather Prediction Center College Park MD 120 AM EST Mon Dec 26 2022 Areas affected...Western WA and Northwest OR Concerning...Heavy rainfall Valid 260620Z - 261620Z SUMMARY...Increasing atmospheric river activity overnight and into early Monday will bring heavy rainfall totals to the Olympic Peninsula and adjacent coastal ranges of the Pacific Northwest. DISCUSSION...The latest GOES-17 satellite imagery along with CIRA-ALPW data show a series of shortwave troughs offshore of the Pacific Northwest with a rather substantial upstream axis of deeper layer moisture extending across the central to northeast Pacific Ocean in association with strong west-southwest trans-Pacific flow. Already there has been one batch of locally heavy rain that has impacted the Olympic Peninsula this evening as one shortwave trough crossed near Vancouver Island and facilitated a belt of stronger moisture transport/onshore flow into the coastal ranges out ahead of an offshore front. Some brief weakening of the low-level jet is expected over the next couple of hours in the wake of this initial surge of energy, but the 00Z HREF guidance and satellite trends show a vigorous upstream shortwave trough near 45N and 135W that is gradually amplifying, and this energy along with a deepening surface low is forecast to advance northeast to just west of Vancouver Island in the 12Z to 15Z time frame Monday morning. This will drive an offshore cold front toward the western WA coastal ranges and across the Olympic Peninsula early Monday and will be accompanied by another strong surge of pre-frontal southwest flow and enhanced moisture transport for heavy rainfall. In fact, a consensus of the 00Z NAM and 00Z GFS solutions show a southwest low-level of 50 to 60 kts impacting the coastal ranges by 12Z and supporting integrated vapor transport (IVT) values of 800 to 1000 kg/m/s. The origins of this atmospheric river activity includes some subtropical moisture tap, and the PW anomalies do reach 2 to 3 standard deviations above the mean with a nose of 1.25 inch PWs likely reaching at least southwest WA and far northwest OR. Strong orographic ascent over the terrain coupled with warm air advection and frontal convergence should yield increasing rainfall rates later in the night that will be capable of reaching at least a 0.25" to 0.50"/hour as far south as the coastal ranges of southwest WA and northwest OR. However, the 00Z HREF guidance suggests the southwest facing slopes of the Olympic Peninsula should see these rates at least occasionally reach 0.50" to 0.75"/hour. These rates will then weaken later Monday morning as the aforementioned shortwave trough pulls away and inland across British Columbia, but expect additional rainfall totals to reach 2 to 4 inches across parts of the Olympic Peninsula, with 1 to 2 inches farther south down into the coastal terrain of southwest WA and northwest OR by mid-morning Monday. Orrison ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!4322AIr0X2vtdKeg-AkU3Lze7JdTZUxZ7BKmpAHZMwGGDB6nUb3EBBfaOF_4Ulfo90AS= RVNoUanfTC8f5hzvPhqnENg$ for graphic product... ATTN...WFO...PQR...SEW... ATTN...RFC...NWRFC...NWC... LAT...LON 48222399 47942328 47482304 46902338 46422303=20 45862314 44892335 44702383 45082403 46662420=20 47852456=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) .