Subj : HVYRAIN: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Dec 03 2023 20:09:14 FOUS30 KWBC 032009 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 307 PM EST Sun Dec 03 2023 Day 1 Valid 16Z 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... 16z update... Timing and magnitude of moisture flux orientation into the Pacific Northwest Ranges appears to be on target and aligned with previous discussion(s). The cold front is about to progress through the far NW portion of the PacNW in the next hour or so; and this will likely diminish the lower-end risk for much of the remainder of the forecast time period across portions. However, since it has not fully arrived at 16z and there will be a narrow window prior to 04.12z for additional .10-.25"/hr rates across W WA into the Olympic Range...the thought is to retain much of the Marginal Risk areas in WA and NW Oregon.=20 Further south, the central Oregon coastal Range and downstream Oregon Cascades will remain within the core of the stronger southwesterly onshore flow for greater than six more hours with similar 400 to 600 kg/m/s IVT values. One-quarter to one-third inch per hour will remain on exposed southwestern facing orography with highest peaks perhaps reaching near .5" occasionally. Given strong warm advection, freezing levels rise above all but the most extreme peaks, so there may be some rain on snow melt to help with increased run-off and rapid rise responses on some creeks/streams in the vicinity. So no changes were made to the Slight Risk areas across the coastal Ranges and Oregon Cascades. Gallina 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 MODERATE RISK OF EXCESSIVE RAINFALL FOR THE WEST/SOUTHWEST FACING SLOPES OF THE OLYMPIC MOUNTAINS IN WESTERN WASHINGTON STATE... 21z update:=20 Many of the elements continue to remain on track/in place as prolonged but strong southwesterly warm advection be ongoing at the start of the day 2 period across much of the Pacific Northwest, in particular western Washington into northwest Oregon...expanding south and eastward with time throughout the day. Given the strong flow (over 35-50kts at 850mb and 50+ in 700-500mb layers) over much of the northeastern Pacific with 1.25 to 1.5+ Total PWat values, IVT values will be increasing from 500 kg/m/s nearing 1000 kg/m/s by 03z into the Olympic Range within the core of the greatest moisture flux convergence along/ahead of synoptic height-falls that are slow to slightly below average in eastward progression throughout the day. Given this moist flow is generally orthogonal to the ridge lines of the Olympics, this should support enhanced rain-rates which will start to increase at or above .5"/hr about 04.22-00z with occasional embedded areas in favorable orographic ticking up to ..75"/hr with some hints of 1"/hr with HREF probability up to 25% through the 06z time period. A full suite of Hi-Res CAM guidance provides that little bit higher confidence in higher values of 7"+ totals and 60-75% chance of neighborhood exceedance of 8" through 12z in the Olympic Range. HREF does denote that rates may not rise to meteorological rates necessary for flash/rapid inundation flooding, given 2"/3hr probability only reach 50% and less than 10% for 3"/3hrs; however, given prolonged warm advection over snow pack in the Olympic Range (which is warming through depth), rain on snow will contribute to runoff problems.=20 In coordination with the National Water Center (National Water Model), there are hints of 2-2.5" of snow water equivalent to be utilized about the same time, likely resulting in up to .25"/hr further rates added to the streams and rivers in the area to further enhanced the considerable potential flooding impacts noted in their Flood Hazard Outlook (FHO). So, these combination of factors and agreement from Seattle/Tacoma local forecast office and NWC, a Moderate Risk of Excessive Rainfall was added to much of the Olympic Range in support of hydrologically significant (rivers to Moderate/Major flooding) event to unfold Monday into Tuesday. Gallina ....Prior discussion... 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 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!6C_fP2W4uwW_g0QGHaOE1x1_LRN2B5EBuc-kFpz7La6g= TIQcqhFR6sNMfzTleduM-VL7qfcw935-P4UxKpWDzVjZw4M$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6C_fP2W4uwW_g0QGHaOE1x1_LRN2B5EBuc-kFpz7La6g= TIQcqhFR6sNMfzTleduM-VL7qfcw935-P4UxKpWDyBGwhL0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6C_fP2W4uwW_g0QGHaOE1x1_LRN2B5EBuc-kFpz7La6g= TIQcqhFR6sNMfzTleduM-VL7qfcw935-P4UxKpWDVHrf5Tw$=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) .