Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 02 2023 20:15:16 FOUS30 KWBC 022015 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 314 PM EST Sat Dec 02 2023 Day 1 Valid 16Z 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... 16Z update... The convection along the Gulf Coast in proximity to the warm front is producing very efficient rainfall this morning. Radar trends/observations along with the latest hi-res guidance continue to depict several inches of rainfall focusing over the Florida Panhandle, southern Alabama and southern Georgia, some of which is to far outside of the inherited risk areas. Given these trends, the Slight Risk area was expected 2-4 counties further eastern across Florida and Georgia as well as the Marginal Risk expanding eastward further into Georgia. Campbell 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... 16Z update... The latest guidance and radar trends support a small southward/eastward expansion of the Marginal Risk area over northwest California. This surge of the Atmospheric River is still expected to bring 3 to 5 inches with up to local 8 inches to the region. Campbell 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... 21Z update... The latest guidance continues to depict areal averages of 2 to 6 inches with local maximums of 8 to 12 inches across the Pacific Northwest. There was a notable increase in QPF pushing onshore across the central and southern coast of Oregon. The Slight Risk area was expanded toward the Oregon/California border to account for this uptick. Additionally, there was a very minor adjustment of the Marginal Risk area near the southern boundary over the Oregon Cascades.=20=20 Campbell 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 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5bTFJFADdM_J-V11nzlOUc-3vLoLVxIZEhRc8vjPrAAV= iQjkGEMRWvBrJuedUgmixMAtDM3fwS6jFIr5ye-gpyGruo4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5bTFJFADdM_J-V11nzlOUc-3vLoLVxIZEhRc8vjPrAAV= iQjkGEMRWvBrJuedUgmixMAtDM3fwS6jFIr5ye-ggH9g7Ow$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5bTFJFADdM_J-V11nzlOUc-3vLoLVxIZEhRc8vjPrAAV= iQjkGEMRWvBrJuedUgmixMAtDM3fwS6jFIr5ye-gpyPlVN8$=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) .