Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Nov 03 2023 15:55:52 FOUS30 KWBC 031555 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1154 AM EDT Fri Nov 03 2023 Day 1 Valid 16Z Fri Nov 03 2023 - 12Z Sat Nov 04 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR THE OLYMPIC PENINSULA OF WASHINGTON STATE... ....16Z Update... There are no changes to the overall synoptic pattern involved with the expected atmospheric river across the PAC Northwest. The forecast remains on track with the heaviest precip footprint lying from a latitude of Eugene, OR up through the Olympic Peninsula. CIPS analog guidance from last night indicated a textbook AR signature for the aforementioned areas with the avg precip for the top 5 analogs producing between 1.5-2.5" across northwest OR and 2.5-4" over the Olympic Peninsula with the highest impact area over the terrain within the Olympic range. Overall rates for the system will run between 0.25-0.5" with max around 0.75" during the peak of the event with best chances of the heaviest rainfall within the immediate coast and into the Olympic mountains. The previous MRGL remains relatively unchanged with the primary focus over the burn scar areas across northwest WA and along the Skokomish river basin where the last event primed the river valley for a slightly better flood potential during the coming event. Time frame of interest will reside between 00-12z within the period with 3"/12-hr neighborhood probabilities running over 80% within the Olympic Peninsula and 5"/12-hr probabilities around 30-35% within the same area. This coincides well within the forecast MRGL domain, so will keep continuity from previous forecast issuance. Kleebauer ....Previous Discussion... A strong shortwave trough at the leading edge of a 150 kt jet streak stretching across the north Pacific will make its way into the coast of the Pacific Northwest today. A plume of moisture with PWATs to 1.75 inches will move into the coast with an occluded front of a strong low, which will be supported by the lift in the left exit region of the jet streak. Peak IVT values will range between 800 and 900 kg/ms, which is on the stronger side of most atmospheric rivers. The result will be a plume of heavy rain that moves into the coast of Washington and Oregon, and is subsequently uplifted by the coastal ranges primarily, with a secondary maximum of heavy rain into the Cascades. Flooding concerns up to this point have primarily focused in the Olympic Mountains, with far fewer impacts noted along the coast further south into Oregon. Thus, the Marginal risk remains largely unchanged except for a few minor tweaks to the area of concern. By the end of the period...early Saturday morning...the rain will have largely pressed inland, resulting in a break in the rainfall...so for most areas this will be a 9 to 12 hour rainfall. Also, the atmospheric river will be rapidly waning in intensity, with maximum PWATs only to 1.25 inches and shifting south by Saturday morning. Expect a widespread 1 to 2 inches of rain along the Pacific Northwest coast, with locally higher totals in the Olympics and portions of the Cascades. While this is the second in the series of atmospheric rivers/fronts, the area has generally had time to recover from the previous rainfall, so flooding concerns remain isolated even on the Olympic Peninsula. As far as timing goes, the rain will start late this afternoon along the coast and quickly spread inland, with heavy rain ongoing through much of the overnight. Wegman Day 2 Valid 12Z Sat Nov 04 2023 - 12Z Sun Nov 05 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR THE OLYMPIC PENINSULA OF WASHINGTON STATE AND FOR THE BURN SCARS OF FAR SOUTHWEST OREGON AND NORTHWEST CALIFORNIA... Residual heavy rainfall from the AR event of the Day 1/Friday period will continue through the day on Saturday, as the main atmospheric river plume sinks south towards the OR/CA border. Additional forcing will result in a renewed round of heavy rainfall as the plume moves over the coastal ranges. Recent burn scars in the area will enhance the flash flooding threat. Thus, a small Marginal risk covering several of the burn scars from recent years along the OR/CA border was introduced with this update. For the most part the area is remote enough that impacts are likely to be nuisance level, though with debris flows from the burn scars possible, it could still pose a hazard to anyone in the wrong place at the wrong time. Meanwhile further north into the Olympic Peninsula, for most of the day the rainfall will likely be scattered and light, though with convective elements involved in any showers. Localized flash flooding can't be ruled out with very little time between the showers and the previous AR from Friday night, but the threat is certainly on the lower side. Wegman Day 3 Valid 12Z Sun Nov 05 2023 - 12Z Mon Nov 06 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR THE ENTIRE PACIFIC NORTHWEST COAST FROM NORTHWEST CALIFORNIA THROUGH THE OLYMPIC PENINSULA OF WASHINGTON STATE... On Sunday, a series of upper level shortwaves will continue barrelling into the coast as the strong jet streak sets up perpendicular to the coast along the OR/CA border. This will result in multiple rounds of heavy rain that will quickly move inland, only to be replaced by the next. Due to there being multiple days of this kind of rain by Sunday, the Marginal Risk was expanded to include all of the coastal ranges from northwest California north. The area may need to be adjusted with future updates depending on how much rain falls and is forecast for the Friday and Saturday time periods, with less rain resulting in a smaller Marginal Risk area. The area is about the same as was inherited, with only minor tweaks to include more burn scar areas along the OR/WA border. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!937oQpJv0dJZgFlL9-xN6AEeq9ArDXM5AqV7QpNRCemR= qVNUAt-I4BOgQWJ8_Ebmi2c8-UAW0e7Y9_EKs-UgghMy4pg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!937oQpJv0dJZgFlL9-xN6AEeq9ArDXM5AqV7QpNRCemR= qVNUAt-I4BOgQWJ8_Ebmi2c8-UAW0e7Y9_EKs-UgDdienFw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!937oQpJv0dJZgFlL9-xN6AEeq9ArDXM5AqV7QpNRCemR= qVNUAt-I4BOgQWJ8_Ebmi2c8-UAW0e7Y9_EKs-UgojmkKTQ$=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) .