Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 02 2023 08:27:05 FOUS30 KWBC 020826 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 326 AM EST Sat Dec 02 2023 Day 1 Valid 12Z Sat Dec 02 2023 - 12Z Sun Dec 03 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR THE FLORIDA PANHANDLE INTO LOWER SOUTHEAST ALABAMA... ....Gulf Coast and Southeast... A strong cold front pushing east out of the Plains and into the Southeast will draw deep Gulf moisture northeastward along and ahead of the front. Meanwhile, a strong southwesterly jet streak will continue intensifying from the southern Plains to the southern Appalachians. This will put the portions of the Gulf coast in the favorable right entrance region of the jet. Helping intensify the jet will be a strong shortwave that will also be tracking northeastward. While the shortwave is unlikely to have a direct impact on the storms, it's influence on the upper jet will help support broad scale lift over the area. Showers and thunderstorms are ongoing across LA and MS this morning. Heavy-rain-producing thunderstorms have been embedded within the broader rain shield, which fortunately have been moving enough to limit flooding impacts somewhat. Nonetheless their tendency to produce 3 inch per hour rainfall rates at times has resulted in localized flash flooding in portions of southwestern LA. Storms and the broader rain shield will push off to the east as more storms develop over the Gulf through the morning due to the aforementioned large scale environmental lift. The heaviest rain of the day and the greatest potential for flooding will likely be in the morning hours/first part of the day today. Supporting the storms are CAPE values between 500 and 1,000 J/kg over the Gulf and the immediate Gulf Coast. Instability generally diminishes as one goes further inland, but the increasing southwesterly flow supported by the jet is helping advect the unstable air mass further inland. There's likely to be a break in the significant rainfall for much of the afternoon through the evening hours between systems. A renewed round of storms is expected to develop overnight tonight into the early morning hours of Sunday. While the storms will have more support in the upper levels as the jet streak intensifies to over 150 kt, the time of day, faster storm motion, and lesser forcing across the FL Panhandle and nearby portions of AL/GA should limit both the storms' intensity and coverage tonight/early Sunday morning. The Slight and surrounding Marginal Risk areas are largely unchanged with this update. Given the above reasons, the Slight may be able to be cancelled with the midday update depending on the evolution of the convection at that time. ....Western Oregon into far Northwest California... The next in the long string of fronts associated with a persistent atmospheric river is already moving into the Pacific Northwest (primarily Washington State) this morning. The heaviest rain will shift southward and diminish through the morning. A stronger front will push into the far northern CA and OR coasts tonight, which is when the bulk of the day's rainfall is expected. Expect a soaking 3 to 5 inches of rain into the coastal ranges of OR and the Cascades through tonight, with local amounts approaching 8 inches by Sunday morning. The impulse of moisture will be accompanied by much warmer air typical of many atmospheric rivers. This will rapidly raise snow levels, allowing snowmelt to contribute some to rising river levels. Since the area has been relatively dry recently, much of the rainfall and snowmelt should be contained in the river banks, so only isolated flash flooding in particularly sensitive areas is expected through Sunday morning. The Marginal Risk was maintained with this update. Wegman Day 2 Valid 12Z Sun Dec 03 2023 - 12Z Mon Dec 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE NORTHERN OREGON COAST RANGES AND CASCADES... A moderately-strong atmospheric river, centered on OR, will continue to push into the coast Sunday while slowly shifting north towards WA. Thus, the heaviest rain in this area is likely to fall during the day, before a brief reprieve behind a cold front Sunday night. With the timing of the heaviest rain being a bit shortened as compared with today's event, expect a bit less rainfall as far as 24 hour totals are concerned, with another 2 to 4 inches of rain expected, with local amounts as high as 6 inches for the day. Adding this rainfall on top of the rain expected today should result in more widely scattered flash flooding impacts due to so much rain falling in a short amount of time. The Slight Risk for much of the northwestern OR mountains was maintained, with a bit of northward expansion into far southern WA. There should be lesser amounts of rain into the Willamette Valley, so lower elevations remain at a Marginal Risk for this update. Further north into west central and northwestern WA, guidance continues to keep the area in relatively lighter rainfall, so the northern extent of the Marginal Risk was unchanged with this update, but may be able to be shifted further south with better CAMs guidance and agreement with future updates. Wegman Day 3 Valid 12Z Mon Dec 04 2023 - 12Z Tue Dec 05 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE WASHINGTON STATE AND NORTHERN OREGON COAST RANGES AND CASCADES... A strong cold front associated with a 964 mb low in the Gulf of Alaska will move into the coast of WA and northern OR, and stall out there by Monday night. Moisture riding along and ahead of the front will move northeastward into the coast, allowing heavy rain to impact the Willamette Valley and Puget Sound regions between the coastal/Olympic ranges and the Cascades. For the Valleys, expect the front to produce 1 to 3 inches of rain with locally higher amounts. For the Cascades and coastal ranges, expect 2 to 4 inches of rain with locally higher amounts to 6 inches. The northeastward movement of the moisture plume will allow for some of the moisture to cross the Cascades crest and onto the lee side of the mountains. Much warmer air moving in with the moisture will also raise snow levels to the summits, so in addition to the heavy rain, some snowmelt will also contribute to rising river levels and potential flash flooding. The most impressive rainfall totals are likely to be on the west facing slopes of the Olympics, where there's good agreement that rainfall totals of 4 to 8 inches can be expected. While some lighter rainfall is expected during the day Monday, the bulk of the heaviest rainfall into the area is likely after 06Z/10pm PST Monday night when the front both stalls out and rainfall rates are boosted by the approach of a strong shortwave trough. Thus, many of the day's expected flooding impacts are expected overnight when they're more dangerous. Changes from the inherited ERO include the Slight Risk area being expanded southward along the coastal ranges into northwestern OR, as areas further south will be impacted more by the favorable antecedent conditions from Sunday's rainfall. For the Olympics specifically, the Slight risk is considered a high-end one due to the possibility of rainfall rates of 1 to 1.5 inches per hour being realized. More of the southern WA Cascades were added to the Slight risk, and the Marginal Risk was expanded east to include more of the east-facing/lee of the Cascades, as more rainfall makes it over the crest. Cities from Portland, Seattle and surrounding suburbs, and Olympia may need to be included in an expanded Slight with future updates if rainfall amounts continue to increase due to urbanization locally lowering FFGs. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7zP7M8KsxlXuQ4yeHUn5VO1_0TGNw7JiSOcIG9UmZ8u3= cmts2nGssHptsawHSudYy7kgJROaUvWLIbRCbFlLLrcYYGE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7zP7M8KsxlXuQ4yeHUn5VO1_0TGNw7JiSOcIG9UmZ8u3= cmts2nGssHptsawHSudYy7kgJROaUvWLIbRCbFlLwh9DAYM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7zP7M8KsxlXuQ4yeHUn5VO1_0TGNw7JiSOcIG9UmZ8u3= cmts2nGssHptsawHSudYy7kgJROaUvWLIbRCbFlL0Ee2xp0$=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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