Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Mon Dec 04 2023 09:52:00 FOUS30 KWBC 040757 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 257 AM EST Mon Dec 04 2023 Day 1 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... A strong cold front associated with a 954 mb low in the Gulf of Alaska will move into the coast of WA and northern OR, and stall out there by later tonight. 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. Many of the meteorological elements continue to remain on track/in place as prolonged (but strong) southwesterly warm advection (which is now underway) across much of the Pacific Northwest, in particular across western Washington into northwest Oregon. This strong WAA will expand 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 0.50"/hr with occasional embedded areas in favorable orographic ticking up to 0.75"/hr. This should result in the most impressive rainfall totals on the west facing slopes of the Olympics, where there's good agreement that rainfall totals of 5 to 7 inches (with local amounts up to 9 inches) expected. The new Hi-Res CAM guidance continues to provide higher confidence in localized higher values of 7"+ totals (and 60-80% chance of neighborhood exceedance of 8" through 12z in the Olympic Range). The HREF does denote that rates may not rise to meteorological rates necessary for flash/rapid inundation flooding, given 2"/3-hr probability only reach 50% and less than 10% for 3"/3-hrs; 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. 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 maintained over much of the Olympic Range in support of hydrologically significant (rivers to Moderate/Major flooding) event to unfold today into Tuesday. The Slight and Marginal Risk areas were maintained as well, surrounding the aforementioned westward facing slopes of the coastal/Olympic ranges and Cascades across WA and into northwestern OR. Churchill/Gallina/Wegman Day 2 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... ....Eastern Washington to far Northwest California... The evolution of the atmospheric river (AR) event across western Washington and Oregon remains on track for the start of the Day 2/Tuesday period. A strong cold front will guide the AR into the Columbia River area at the start of the period. Continued prolonged moderate rainfall rates will result in high rainfall totals and a considerable hydrologic flooding event as the front pushes inland and south down the coast. 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. This should reduce the capability of rainfall rates (eventually being limited to around 0.50"/hr), 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. Forecast 24-hour rainfall totals also remain on track and are expected to largely be in the 3 to 5 inch range in most of the Slight Risk areas. Grounds are (and will continue to be) fairly saturated (80%+) suggesting reduced infiltration, but there will about 24 hrs of respite of even moderate rainfall prior to day 2 period, so while the risk for excessive rainfall totals increase for the 24 hr 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. At this time, a Slight Risk remains in place for this cycle. ....Northern Idaho Panhandle... A Marginal Risk area was maintained for the northern Idaho Panhandle (and surrounding portions of WA/MT) 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. Churchill/Gallina/Wegman Day 3 Valid 12Z Wed Dec 06 2023 - 12Z Thu Dec 07 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE OREGON COAST RANGES AND FOR PORTIONS OF THE NORTHERN CALIFORNIA COAST RANGES AND CENTRAL OREGON CASCADES... There continues to be some good news to report across the Pacific Northwest with an abatement in the atmospheric river and lesser rainfall totals going into Wednesday and Wednesday night. There is still a Marginal Risk area of excessive rainfall from extreme northwest California to the Oregon coastal ranges and the western Oregon Cascades on Day 3 (Wednesday) with locally 1 to 2 inches of rainfall expected, but this is a noteworthy improvement compared to the much heavier rainfall expected during the Days 1-2 period. Even though rainfall totals are expected to be lighter, the grounds will remain quite saturated by this time with elevated creeks and rivers, so there may still be lingering flooding impacts flooding for those areas (particularly for the larger stem rivers). Churchill/Hamrick/Schichtel $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .