Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Nov 04 2023 15:54:48 FOUS30 KWBC 041554 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1150 AM EDT Sat Nov 04 2023 Day 1 Valid 16Z Sat Nov 04 2023 - 12Z Sun Nov 05 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR THE BURN SCARS OF FAR SOUTHWEST OREGON AND NORTHWEST CALIFORNIA... ....16Z Update... The previous MRGL across the Olympic Peninsula was removed with the event coming to an end shortly as the drier air advects in before the cold front. The MRGL risk over FL was also removed as the frontal boundary will stay far enough offshore with the instability gradient far enough off the coast and steering pattern expected to keep primary convection offshore. A few cells may wander onshore within both Miami-Dade and Broward counties, but prospects for heavy rainfall necessary for flash flooding have dropped off precipitously. ....Pacific Northwest... Cold front associated with our low pressure and atmospheric river event is currently positioned just of the coast of WA and northwest OR with low-level WV satellite indicating a tongue of drier air beginning to protrude the immediate coast. This will subsequently diminish the heavy rain potential across WA state where the heaviest rain has fallen overnight through this morning. Further south, seeing the indications of the AR shifting focus into southwest OR with heavier echoes now positioned to come ashore within the next hour. Guidance is very consistent on a period of heavy rainfall between 16-22z across the immediate coast and areas within the Klamath Mtns. 12z HREF blended mean QPF has wavered very little from run-to-run over the past 24 hrs with a general 1.5-2.5" expected over the southwest corner of OR into the very northwest edge of CA over the rest of the morning and afternoon. Smith River complex burn scar just inland of the coastal plain will remain the primary focus for flash flooding as low-topped convection is possible within the area during the peak of the event. With the lower threshold necessary for flash flood risks within the burn scar, there was plenty of merit to continue on with the MRGL risk from prior forecasts. Kleebauer Day 2 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... Rainfall is likely to continue for much of the day Sunday into the coast of the Pacific Northwest on Sunday. As a 150 kt jet streak moves into northern CA, a low that develops in the left exit region will track into Washington State, correlating with the approach of a longwave trough that moves into the coast by Monday morning. The result of this pattern will be continued generally light rainfall into much of the coast in the form of showers, but as several small shortwaves move through, brief impulses of heavier rain will be embedded in the overall fast westerly flow into the coast. Once again the bullseye of heaviest rainfall looks to be along the OR/CA border along the coast, where another 2 to 4 inches of rain appears likely, with amounts more in the 1-3 inch range from the central OR coast north through the Olympic Peninsula of Washington State. The Marginal risk remains unchanged, encompassing all of the coastal ranges from the northwest corner of CA northward. Had this been an isolated day, it very likely would have had no reason for a Marginal risk at all, but since it's the latest in a series of fronts, shortwaves, and atmospheric rivers, the soils all along the coast will be unable to handle much rainfall before anything additional becomes runoff. The Olympic Mountains and the burn scars, especially the Smith River complex, along the OR/CA border definitely have the highest risk of seeing flash flooding through Sunday and Sunday night, but the overall threat remains isolated. Wegman Day 3 Valid 12Z Mon Nov 06 2023 - 12Z Tue Nov 07 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 AND PORTIONS OF THE SOUTHERN WASHINGTON AND NORTHERN OREGON CASCADES... A low starting out the day Monday centered off the coast of the Olympic Peninsula will weaken as it drifts northwestward into Vancouver Island and dissipate by Monday night. The cold front south of the low will move into the coast early in the day, spreading into Idaho by Monday night. While the front will keep the flow westerly/onshore at the coast and into the Cascades through the Day 3/Monday time period, the colder air moving in will result in falling snow levels all across the Pacific Northwest. Thus, much of the moisture moving into the Cascades, particularly the northern Washington Cascades, will likely fall as snow into the higher elevations. The inherited Marginal risk was trimmed a little bit, especially out of the northern Cascades on the assumption that most of the precipitation will be snow. The cold air taking longer to get into southern WA or northern OR should allow enough of the expected precipitation to fall as rain in these areas to justify the Marginal Risk, especially given the many previous days of rain in this area. Overall rainfall amounts broadly should continue decreasing on Monday as compared to previous days, but given the sensitivity of this entire area with almost all of the rainfall likely translating into runoff, the Marginal Risk is justified and largely remains in place. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_axo5CMCM-YOeI1xxZIoC-dXbJTjZ9qgyPhkxRuGVdnh= aCKO75-aXmZLAOim_t0PaGCIpnWWmf2oFhW4kg7-h2uhCaw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_axo5CMCM-YOeI1xxZIoC-dXbJTjZ9qgyPhkxRuGVdnh= aCKO75-aXmZLAOim_t0PaGCIpnWWmf2oFhW4kg7-gtos8iA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_axo5CMCM-YOeI1xxZIoC-dXbJTjZ9qgyPhkxRuGVdnh= aCKO75-aXmZLAOim_t0PaGCIpnWWmf2oFhW4kg7-DK_PIT8$=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) .