Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Dec 05 2023 00:53:35 FOUS30 KWBC 050053 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 752 PM EST Mon Dec 04 2023 Day 1 Valid 01Z Tue Dec 05 2023 - 12Z Tue Dec 05 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE OLYMPIC PENINSULA OF WESTERN WASHINGTON... No changes were made to the ERO with this update as the forecast remains on track. The core of highest IVT will impact the Olympics now through ~06z, and thus also expecting to see the highest rainfall rates during this period. HREF LPMM shows hourly rainfall near to over 0.5" across the west and southwest slopes of the Olympics through 06z, with 2" per 3hr probabilities over 60% in both the 00z-03z and 03z-06z periods. This stretch of more intense rainfall should result in an uptick in flood related impacts. Hourly rainfall of 0.25"-0.5" will impact the western slopes of the WA Cascades as well, gradually shifting south as the night progresses. Through 12z Tuesday, additional rainfall of 4-7" is expected over the western and southwestern slopes of the Olympics, with 2-4" (locally higher) over the western slopes of the WA Cascades, and 1-3" over the coastal ranges of WA (south of the Olympics) and northwest OR. The magnitude of IVT begins to weaken by later tonight, however the axis does begin to slow its southward progression, resulting in a more prolonged period of steady rain over portions of western OR and WA, albeit south of the areas hardest hit tonight. This is covered by the Slight risk that carries into Tuesday across this region.=20 Some of the 2-2.5" of snow water equivalent is expected to be utilized as runoff due to increased freezing levels, likely resulting in up to .25"/hr added to the streams and rivers in the area to further enhance the considerable potential flooding impacts. These combination of factors align with a Moderate Risk of Excessive Rainfall over much of the Olympic Peninsula in support of hydrologically significant (rivers to Moderate/Major flooding) event to unfold tonight into Tuesday. Chenard Day 2 Valid 12Z Tue Dec 05 2023 - 12Z Wed Dec 06 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE OREGON COASTAL RANGES AND PORTIONS OF THE WASHINGTON AND NORTHERN OREGON CASCADES... 2030Z Update... Overall, there is very little change to the previous thinking with the strong atmospheric river activity that will be impacting the Pacific Northwest and gradually settling south down the West Coast for this period. The axis of stronger IVT values will continue to lose some latitude, but also strength as the overall deeper layer onshore Pacific fetch begins to weaken. However, there will be a level of persistence to the atmospheric river axis that will allow for pretty heavy rainfall totals, and this will be on top of some areas that get very heavy rains in the Day 1 period. Still expecting as much as 3 to 5 inches of additional rain, with isolated heavier amounts, for portions of the northwest Oregon coastal ranges and the upslope areas of the southern Washington Cascades. Somewhat lesser amounts will focus for this period down into the northern Oregon Cascades and the central/southwest Oregon coastal ranges. The changes for the outlook for this period include extending the Slight Risk area into the Willamette Valley of northwest Oregon where at least localized 1 to 3 inch amounts are expected for the period which may locally enhance some runoff concerns here as well. Otherwise, very minor adjustments have been made to the Marginal and Slight Risk areas given good overall model agreement on the placement of the heavier rainfall amounts and the general placement of the atmospheric river activity. Orrison Previous discussion... ....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 THE SOUTHERN WASHINGTON AND OREGON CASCADES INTO THE COASTAL RANGES OF WESTERN OREGON AND FAR NORTHWEST CALIFORNIA... 2030Z Update... Generally there are no major changes to the previous outlook other than to expand the Marginal Risk area locally across some of the interior valleys of western Oregon where a combination of the rainfall from day 2 in addition to what occurs for this period may result in somewhat greater concerns for some runoff problems. Also, some of the rains may linger a bit this period back up into the southern Washington Cascades, and given the multi-day sensitivities here from very heavy rainfall totals, any additional rains may aggravate ongoing runoff/flooding concerns. Thus, the Marginal Risk area was locally expanded north across these areas. As mentioned previously, the multi-day atmospheric river activity will be weakening through the period with lowering IVT/moisture transport values and thus a slackening of the rainfall rates with time. Orrison Previous discussion... 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 for those areas (particularly for the larger stem rivers). Churchill/Hamrick/Schichtel Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9ETO3MC748TXz_u5J-Lbog9aAB6oGKe7KB4KCGmkVMwq= AdCuWf3E6P9Na0gmJCNWS_C9Q9fmkMz8JH-aGsadvE5AYFw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9ETO3MC748TXz_u5J-Lbog9aAB6oGKe7KB4KCGmkVMwq= AdCuWf3E6P9Na0gmJCNWS_C9Q9fmkMz8JH-aGsadkYM2ihA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9ETO3MC748TXz_u5J-Lbog9aAB6oGKe7KB4KCGmkVMwq= AdCuWf3E6P9Na0gmJCNWS_C9Q9fmkMz8JH-aGsadp4RBWrg$=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) .