Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Sat Nov 04 2023 08:27:00 FOUS30 KWBC 040821 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 420 AM EDT Sat Nov 04 2023 Day 1 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, THE BURN SCARS OF FAR SOUTHWEST OREGON AND NORTHWEST CALIFORNIA, AND THE URBAN CORRIDOR OF SOUTHEAST FLORIDA... ....Pacific Northwest... The atmospheric river impacting WA and OR this morning will continue pressing inland through the day today. That will lead to quite a bit of dry time along the WA coast for much of the day today. Scattered showers may develop some convective elements along the Olympic Peninsula through the evening hours, which could locally raise chances of flash flooding should any of those stronger showers move over sensitive areas. The Marginal Risk for the Olympic Peninsula remains in effect, though it's a lower end threat today as compared with both previous days and upcoming ones. Expect up to 1 inch per hour rainfall rates with the heavier showers, but since they'll be relatively fast moving, should not result in much additional rainfall across the Olympics once the main area of rain there ends early this morning. Further south, the Marginal Risk area along the OR/CA border associated with the Smith River complex burn scar remains unchanged with this update. The core of the heaviest rain associated with the atmospheric river will move over this area through the day today and will be aligned at a perpendicular enough angle to the terrain to include upslope enhancement on the south/west facing slopes of the mountains. This could lead to nuisance flooding and debris flows downstream of the burn scar. Expect 2 to as much as 3 inches of rain over this area...much of it falling in a 6-10 hour period through the morning into the afternoon. ....Southeast Florida... A low-confidence Marginal Risk area remains through the urban I-95 corridor of southeast Florida today. A tropical wave and stalled out boundary are likely to develop along the coast through the day today, with PWATs as high as 2 inches off the coast over the Bahamas, but dry northerly flow dropping PWATs to under an inch over northern FL, resulting in a boundary. Local influences such as any sea breezes, convergence boundaries, or outflow boundaries from any storms may have an outsized influence on where the boundary sets up. As such the CAMs are in poor agreement on rainfall amounts, placement, and timing. The biggest concern with the Marginal Risk area is that it's quite possible the heaviest rain remains just off the coast, which is quite reasonable given the support of the Gulf Stream adding instability whereas the land mass would not. The front is also separating two air masses characterized by nearly opposite wind directions on either side of the front, and the warmth/moisture side is favoring a southeasterly flow, which would support keeping any convection off the coast. However, should the convection develop over the mainland, it's likely to be over a flood-sensitive portion of either Ft. Lauderdale/Miami or both, so it may not take all that much rainfall to cause problems. Thus, the Marginal remains as inherited, but for those reasons described above, it's very low confidence and may need to be dropped should the convection largely remain offshore. Wegman 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 $$ d --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .