Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Mar 29 2024 23:32:12 FOUS30 KWBC 292331 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 731 PM EDT Fri Mar 29 2024 Day 1 Valid 01Z Sat Mar 30 2024 - 12Z Sat Mar 30 2024 ....THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHERN CALIFORNIA AND DOWNEAST MAINE... ....California... A strong system is forecast to bring widespread precipitation to=20 the state and the potential for excessive rainfall amounts,=20 especially along the southern coast. A closed upper low that will=20 drop south along the northern California coast. This will be on the strong side of climatology for late March, with 500 mb heights=20 dropping 1-3 standard deviations below normal across much of the=20 state by early Saturday. Amplifying southwesterly flow ahead of the deepening system will tap a plume of deeper Pacific moisture, that will spread inland along a well- defined frontal band. The Marginal Risk saw some changes, with some reduction in the San Joaquin Valley and some expansion eastward along the Transverse Ranges, based on the 18z HREF probabilities of 0.5"+ in an hour. A targeted Slight Risk remains over the Transverse Ranges. Guidance=20 continues to show this area as the focus for some of the stronger=20 low-to-mid level moisture advection and PWs as the system's front=20 moves into the region later today. HREF neighborhood probabilities=20 show that accumulations of 3 inches or more are likely in this area -- raising runoff concerns for areas below the snow line. ....Eastern Maine.... Upper level forcing interacting with ample moisture along a slow- moving frontal zone will continue to support moderate to heavy rain for several more hours across eastern Maine as a deepening surface low tracks northeast toward Atlantic Canada. The combination of=20 heavy rain and melting snow keeps the 3-hour flash flood guidance=20 values extremely low, so the risk areas were maintained with=20 minimal changes. Roth Day 2 Valid 12Z Sat Mar 30 2024 - 12Z Sun Mar 31 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA... 20Z Excessive Rainfall Discussion... Model mass-fields and QPF remain comparable to earlier runs so no fundamental changes needed to the outlook or to the forecast reasoning. 08Z Excessive Rainfall Discussion... ....Southern California to southern Nevada and western Arizona... A heavy to excessive rainfall threat developing Friday into early Saturday will spread further south as an unseasonably deep low drops south from the central to the southern California coasts. Strong onshore flow and moisture advection along the associated frontal band, will spread precipitation across southern California, with upslope flow accentuating the threat for heavy to excessive accumulations along the coast, where a Slight Risk was maintained along the Transverse and Peninsular ranges below the snow line. Residual moisture spreading further inland is expected to fuel a lesser threat for excessive rainfall into the lower Colorado Basin, including southern Nevada and western Arizona. Pereira Day 3 Valid 12Z Sun Mar 31 2024 - 12Z Mon Apr 01 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA AND SOUTHERN ARIZONA... 20Z Excessive Rainfall Discussion... What had been a deep closed low dropping southward along the California coast on Saturday will be weakening into an open wave on Sunday. As mentioned in the previous discussion, the potential for some locally heavy rainfall will longer (especially in the region of most greatest precipitable water anomalies that eventually reach southern Arizona). Bann 08Z Excessive Rainfall Discussion... ....Southern California to southern Arizona... The upper low impacting the region days 1 and 2 will begin to transition to an open wave by early Sunday. However, unsettled weather with some potentially locally heavy rains will continue as an elongated positively-tilted upper trough remains centered over the region. Deep southwesterly flow along the leading edge of the trough will continue to support anomalous PWs (1-2 standard deviations above normal) across portions of southern Arizona early in the period. Meanwhile, lingering moisture along with upstream energy dropping into the base of the trough will support showers along the coast and an isolated threat for additional flash flooding concerns. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_WdP8s4J_3pgHSmDmnAiB_RwiXZGCosFMCzy3Odo8KOc= UZCTG5foeZmHdRYnkgqn33Qe-biHzdEHyhmbqdetui6o68w$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_WdP8s4J_3pgHSmDmnAiB_RwiXZGCosFMCzy3Odo8KOc= UZCTG5foeZmHdRYnkgqn33Qe-biHzdEHyhmbqdetwiViOmo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_WdP8s4J_3pgHSmDmnAiB_RwiXZGCosFMCzy3Odo8KOc= UZCTG5foeZmHdRYnkgqn33Qe-biHzdEHyhmbqdetEpQuazM$=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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