Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Feb 24 2025 19:16:44 FOUS30 KWBC 241916 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 216 PM EST Mon Feb 24 2025 Day 1 Valid 16Z Mon Feb 24 2025 - 12Z Tue Feb 25 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE NORTHERN OREGON COAST RANGE INTO THE OLYMPIC RANGE AND OVER THE SOUTHERN WASHINGTON CASCADES... ....16Z update... ....Pacific Northwest... No significant changes for this update across the Pacific Northwest with the 12Z HREF guidance showing a similar output to the previous cycle. The greatest reason for maintaining the Slight Risk areas across the Coastal Ranges and southern Washington Cascades is due to antecedent rainfall over the past 48 hours and above average soil moisture/elevated rivers and creeks. There will be a low-end chance for hourly rainfall near 0.5 inches to briefly impact the Cascades ahead of and with the cold frontal passage later today before snow levels fall. After 00Z, robust low level onshore flow with 850 mb winds of 70-80 kt are expected into the coast from far northern Oregon into southern/central Washington with localized rates in excess of 0.5 in/hr. Local 24 hour precipitation totals for the Coastal Ranges into the Cascades are expected to be 3 to 5 inches but as mentioned in the previous discussion, snow levels will be dropping through 12Z with some of that falling in the form of snow. ....Southeast Florida... Based on the 12Z HREF, a minor northward extension of the Marginal Risk was made to include urban locations as far north as Indian River County where low-end potential for 3 to 5 inches will exist ahead of an approaching surface low and related low level onshore flow coupled with increasing moisture and sufficient instability to support localized rates of 2-3 in/hr. Otto ....previous discussion follows... ....Pacific Northwest... A vigorous shortwave trough supporting a 985 mb surface low will race into the coast of the Pacific Northwest tonight. Its leading cold front will move into the Oregon and Washington coasts today and spread inland through tonight. Upslope flow into the Olympics, coastal ranges, and Cascades will all locally enhance the rainfall rates...which at times may exceed an inch per hour. NASA Sport soil moisture imagery shows the soils across nearly all of Washington, Oregon, and Idaho are at or near saturation in the surface layer. This should allow the rainfall from the front moving into the region today to mostly convert to runoff. Integrated Vapor Transport values will peak around 500 kg/ms across coastal Washington. These values are far from impressive, so those very favorable antecedent soil moisture will play a major role in the potential for flooding. Part of the reason for the lower IVT values is the predominant south to southwesterly flow, which is at an oblique The typical urban areas, low- lying and flood- prone areas, and narrow valleys will be at greatest risk for flash flooding, especially south and west facing slopes. Snow levels will rise to as high as 7,000 ft. ahead of and through frontal passage for southwest Washington and all of western Oregon through the day. As colder air moves in post-frontal passage, the snow levels will crash to around 3,500 ft area-wide. The lower snow levels will allow much of the heaviest precipitation falling into the mountains to remain as snow, and thus reduce the flooding risk through the overnight. Dynamic cooling on the windward side of the mountains may further reduce the amount of precipitation that converts to runoff due to locally reduced snow levels. No significant changes were made to the inherited Slight and Marginal risk areas across the Pacific Northwest. The low is already well-formed and will continue intensifying as it approaches the Pacific Northwest tonight. A few very minor tweaks were made to exclude some of the higher elevations from the flood risk areas due to falling snow levels tonight. ....Southeast Florida... A positively tilted trough traversing the Gulf this morning will intensify as it becomes more neutrally tilted, thus increasing the divergence ahead of the trough. A few persistent thunderstorms are making their way in the general direction of the Florida Keys this morning. The latest HRRR guidance suggests the heaviest thunderstorms will pass south of the Marginal Risk area that covers from West Palm Beach to Miami. Nonetheless heavy rain and potential for training are still possible, prompting the continuance of the Marginal Risk in this area with no changes. This remains a very low-end Marginal Risk threat with much of South Florida in a dry period, and should any flooding occur, it will likely be in poor- drainage and urban areas. Wegman Day 2 Valid 12Z Tue Feb 25 2025 - 12Z Wed Feb 26 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley Day 3 Valid 12Z Wed Feb 26 2025 - 12Z Thu Feb 27 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4WQHmM_7HdXTrVlJcB51-K_yx5dkeK20FAEBMhAnLZL_= MyFhL-sMqLX7IqfeBIKEAIsrLXaGfXvzIPhBms499Hedhgw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4WQHmM_7HdXTrVlJcB51-K_yx5dkeK20FAEBMhAnLZL_= MyFhL-sMqLX7IqfeBIKEAIsrLXaGfXvzIPhBms49jSN2lX0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4WQHmM_7HdXTrVlJcB51-K_yx5dkeK20FAEBMhAnLZL_= MyFhL-sMqLX7IqfeBIKEAIsrLXaGfXvzIPhBms491pTnuRA$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .