Subj : HVYRAIN: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Dec 03 2023 20:09:45 FOUS30 KWBC 032009 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 308 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 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 NORTHERN & CENTRAL OREGON CASCADES... ....21z update... ....Eastern Washington to far Northwest California... For the most part, the AR remains on track for evolution and focus for continued prolonged moderate rainfall rates resulting in high rainfall totals and considerable hydrologic flooding event. 12z guidance trends were slightly faster with the initial height-falls core of global scale cyclone across the eastern Gulf of Alaska going into British Columbia. The trailing edge/cold front will be crossing Washington State angling offshore of Oregon by the start of the forecast period (05.12z). As a result, a weak surface inflection will result in the mid-level shortwave ridging. This will delay and perhaps even back the main core of the AR moisture across the coastal range of Oregon into the northern Cascades from just north of Mt. Adams in WA to just south of Mt. Hood in OR.=20 Low level winds will continue to be in th 45-60kt range but will diminish with time, but for the heavy rainfall event, the broader moisture plume will be significantly narrowed but also starting to back for less favorable orographic intersection with the ranges.=20 This should reduce the capability of rainfall rates, but given the placement the duration is expected to increase all given the uncertain strength/amplitude of the surface to mid-level wave to develop off-shore. As a result, nearly all global guidance is increased rainfall totals to 3-5" across the range from N Douglas to Tillamook county, while reducing totals further south and east (into northwest California, southern Oregon Cascades). Grounds are and will continue to be fairly saturated (80%+) suggesting reduced infiltration, but there will about 24hrs of respite of even moderate rainfall prior to day 3 period, so while the risk for excessive rainfall totals increase for the 24hr range, the short-term rates may be too low for inducing flash flooding/rapid inundation flooding, though the river flooding is likely to be considerable. Will have to monitor the trends into the Hi-Res CAM time frame for any potential upgrade in ERO category, which remains plausible. However, at this time, a Slight Risk remains in place for this cycle. Elsewhere... Small cosmetic adjustments to the edges of the Marginal Risk in E WA/N ID/NW MT were made based on trends in 12z global guidance. Gallina ....Prior discussion... 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!4kQ2todyaX9vNmu3mJ4VvDMFllxb0RUNqY85mNjmnoPA= QFXzg5BSsgTzXdg6d6fkNZhi4pLSU8PpFwSDhSfE7msug-M$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4kQ2todyaX9vNmu3mJ4VvDMFllxb0RUNqY85mNjmnoPA= QFXzg5BSsgTzXdg6d6fkNZhi4pLSU8PpFwSDhSfEy7qvVFw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4kQ2todyaX9vNmu3mJ4VvDMFllxb0RUNqY85mNjmnoPA= QFXzg5BSsgTzXdg6d6fkNZhi4pLSU8PpFwSDhSfERMDV7j8$=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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