Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Dec 03 2023 08:29:30 FOUS30 KWBC 030829 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 328 AM EST Sun Dec 03 2023 Day 1 Valid 12Z Sun Dec 03 2023 - 12Z Mon Dec 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR ALL OF THE OREGON COAST RANGES AND MUCH OF THE OREGON CASCADES... A moderately-strong atmospheric river, centered on OR, will continue to push into the coast today through this evening. A brief reprieve from the rain is expected tonight between impulses of energy. On top of the rain that's already fallen, expect another 2 to 4 inches of rain through this evening, with local amounts as high as 6 inches for the day. More widely scattered flash flooding impacts are likely today due to so much rain falling in a short amount of time. NASA Sport imagery shows soil moisture profiles about normal for this time of year across western OR, or about 75% saturated. The additional rainfall today should take less time than yesterday to cause flash flooding impacts since there's rain ongoing and soil moisture levels continue to increase. The ERO risk areas today remain largely the same, with the only change of note to extend the Slight Risk into the northwest corner of CA, from Crescent City northward as the coastal ranges in that area may see 2 to 4 inches of rain through this evening. The Slight Risk remains in effect for much of the OR Cascades, as rising snow levels will allow for snowmelt to contribute to rising river levels on the west side of the range, along with most to all of the precipitation on the foothills of the Cascades falling as rain. Much lighter amounts of rain are expected from central WA north today as the focus says largely over OR, with only 1 to 2 inches of rain expected. Since this too will be added to previous' days rains, an isolated flash flooding instance can't be ruled out, and the Marginal remains unchanged. Wegman Day 2 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. As far as timing goes, light shower activity is expected for much of the day Monday, with the main rainfall expected overnight Monday night...spreading southward down the coast with time. For the valleys, expect the front to produce 1 to 3 inches of rain with locally higher amounts through early Tuesday morning. For the Cascades and coastal ranges, expect 3 to 5 inches of rain with locally higher amounts to 7 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 5 to 7 inches with local amounts to 9 inches expected. Once again the bulk of the heaviest rainfall will be from Monday evening through 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. This amount of rain in a relatively short amount of time is likely to cause flash flooding concerns in the Olympics and surrounding plains, and the Slight Risk remains a higher end concern for the Olympics and south/west facing slopes. No significant changes were made to the ERO risk areas. Despite some small increases in rainfall amounts, particularly in the Olympics, the timing and evolution of the atmospheric river event remains largely unchanged from previous days and forecast confidence remains high. Wegman Day 3 Valid 12Z Tue Dec 05 2023 - 12Z Wed Dec 06 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE OREGON COAST RANGES AND PORTIONS OF THE WASHINGTON AND OREGON CASCADES... The atmospheric river (AR) event across western Washington and Oregon will be ongoing to start the Day 3/Tuesday period. A strong cold front will guide the AR into the Columbia River area at the start of the period. The heaviest rainfall rates of the day will likely be through the morning hours as the front pushes inland and south down the coast. An upper level shortwave disturbance will track eastward into coastal WA/OR through the day, then quickly weaken Tuesday night. This should help to lower rainfall rates (which eventually should lower flash flooding concerns) Nonetheless, 1 inch per hour rates are likely for much of the day Tuesday in much of the Slight Risk area, again with a slow southward motion to the area of heaviest rainfall with time. Also notable is the orientation of the AR. It will start off the day oriented southwest to northeast, which will enhance any upslope component to the heavy rain. By Tuesday night, the occluding front will be oriented more south-southwest to north-northeast, which will diminish the upslope rainfall, at least as concerns the heaviest rainfall rates. One notable exception may be along the CA/OR border, where a mesolow may try to form late Tuesday night, which could briefly turn the flow towards the west in the area, increasing rainfall rates locally. 24-hour rainfall totals will largely be in the 3 to 5 inch range in most of the Slight Risk areas, with the best chance of the highest rainfall totals along the CA/OR border due to the possible development of the mesolow and associated wind direction shift. It's important to emphasize that these rainfall totals are on top of previous' days rains...with soils all throughout western WA and OR at or near saturation by this point. Thus, a large majority of the rainfall expected Tuesday and Tuesday night should convert to runoff (particularly the further north you go), so lesser amounts of rain are necessary for flash flooding impacts. The Slight Risk area along the coastal ranges was extended southward into northwest CA with this update for the reasons stated above, and the Slight was expanded southward to the central OR Cascades since that area should spend a longer time in the core of the AR due to the slow southward drift of the AR. ....Northern Idaho Panhandle... A Marginal Risk area was introduced for the northern Idaho Panhandle with this update. The plume of moisture associated with the AR will continue inland and into the ID panhandle Tuesday and Tuesday night. Renewed upslope flow into the area mountains will locally increase rainfall rates through this area. Snow levels will be quite high as the moisture advects a very warm air mass for this time of year into the area. Thus, in addition to locally heavy rainfall, snowmelt will be a significant component to any flooding. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!811C0eNHkah9rxy0C6km9j3-iVJruThsHwEHrI7T0ScE= HcwN7uqxH2Lp8KkhAwSqdH0PpgOK7UeRKTB_yIp2zutxM9o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!811C0eNHkah9rxy0C6km9j3-iVJruThsHwEHrI7T0ScE= HcwN7uqxH2Lp8KkhAwSqdH0PpgOK7UeRKTB_yIp2m1Gogok$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!811C0eNHkah9rxy0C6km9j3-iVJruThsHwEHrI7T0ScE= HcwN7uqxH2Lp8KkhAwSqdH0PpgOK7UeRKTB_yIp2AG4DB_U$=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) .