Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Nov 05 2023 15:51:39 FOUS30 KWBC 051551 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1050 AM EST Sun Nov 05 2023 Day 1 Valid 16Z Sun Nov 05 2023 - 12Z Mon Nov 06 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR SOUTHWEST OREGON AND NORTHWEST CALIFORNIA... ....16Z Update... Little to no change necessary from the previous D1 forecast issuance. The synoptic scale evolution remains steadfast from model to model with little deviation noted in the dprog/dt of the 500mb and SLP charts. The heaviest rainfall will begin after 20z with a slow shift northward of the main IVT field by the end of the period. The heaviest rain is still forecast to focus over southwestern OR and northwestern CA with the 00-09z time frame as the primary impact portion of the forecast. Latest HREF neighborhood probability of at least 1"/3-hrs reaches upwards of 70% with a maximum of 90% over the northern extent of the Klamath Mtn range. This area has been the focal point of heavy rainfall going back to yesterday morning and afternoon, so the limited recovery period from the previous AR plays into the slightly higher than normal risk of flooding despite a weaker AR signature. Rates will be capped below 1"/hr, but the presence of burn scars within the area on top of moisture rich soils the past few events could lead to localized flash flood concerns, as well as mudslides. Total rainfall between 1-3" is expected across the risk area with much of it falling over a span of 6-9 hrs before dwindling. Kleebauer ....Previous Discussion... No major changes were made with this morning's update to the Marginal Risk area for a small portion of southwest OR and northwest CA. The next AR is expected to being impacting this area late this afternoon on the southern end of a front that will impact northern coastal areas first, then the rain gradually spreads down the coast. This AR will not be as strong as previous ones, with IVT values around 400 kg/ms. As the first front that brings rain to the entire Pacific Northwest coast moves inland, a second front will follow behind it from the southwest, with the rain spreading northward up the coast. The result will be that the most prolonged rains today will be around the OR/CA border, which when added to previous days' rains will result in an isolated flash flooding threat. HREF neighborhood probabilities for seeing 3 inches of rain over the day are between 60-70 percent across the southwest corner of OR. Rainfall rates during the heaviest rains could exceed 1 inch per hour, but like previous days...individual cells should be moving at a fast enough clip that those heaviest rainfall rates will be short-lived. With that said however, there will be numerous rounds of cells capable of producing 1 inch per hour rainfall rates, which will quickly add up over this area. Wegman Day 2 Valid 12Z Mon Nov 06 2023 - 12Z Tue Nov 07 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHWESTERN WASHINGTON STATE AND NORTHWESTERN OREGON... A few more substantive changes were made to the Marginal Risk outlook for the Pacific Northwest with this morning's update. The atmospheric river impacting the Pacific Northwest is likely to be the most persistent across the Marginal Risk area. The result is lesser rainfall totals forecast for points north in northwestern WA, including the Olympic Peninsula, and also further south towards southwestern OR and northwest WA. Due to these lesser amounts of rain now forecast, the Marginal Risk area was trimmed out of these areas, keeping the focus into northwestern OR primarily, as well as far southwestern WA. Rainfall totals in the Marginal Risk area are likely to broadly exceed 2 inches, with local rainfall totals Monday as high as 4 inches. The heaviest rains into the Cascades will likely be confined to Mt. Rainier and points south. A few more urban areas are included in this region, including Portland, which with locally lower FFGs could result in localized urban flooding. The highest threat of flash flooding will be on old burn scars and along the coastal ranges which have been hard hit by multiple consecutive days of rainfall. A surface low centered near the mouth of the Columbia River Monday morning will track northeastward towards Vancouver Island, which will keep strong onshore flow into the Columbia River basin. The heaviest rains will likely be during the daylight hours Monday, with diminishing rates into Monday night. Thus, it appears likely that the Marginal is covering the first half of the period, and could likely be dropped by Monday evening's update. Wegman Day 3 Valid 12Z Tue Nov 07 2023 - 12Z Wed Nov 08 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6NB4gSJwJqwwJTubBp4D8TNIsVBQX1P7ZksedB4YrVSl= f19Q_I-TzJ-dBExSZwp6PQkazbqX4PD_LsCEZfaCPQZphIY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6NB4gSJwJqwwJTubBp4D8TNIsVBQX1P7ZksedB4YrVSl= f19Q_I-TzJ-dBExSZwp6PQkazbqX4PD_LsCEZfaC4phPTDU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6NB4gSJwJqwwJTubBp4D8TNIsVBQX1P7ZksedB4YrVSl= f19Q_I-TzJ-dBExSZwp6PQkazbqX4PD_LsCEZfaC70HGQ30$=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. 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