Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Mar 29 2024 08:15:23 FOUS30 KWBC 290815 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 415 AM EDT Fri Mar 29 2024 Day 1 Valid 12Z Fri Mar 29 2024 - 12Z Sat Mar 30 2024 ....THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHERN CALIFORNIA AND DOWNEAST MAINE... ....California... Overall, the forecast remains much the same, with the latest guidance showing good run-to-run continuity with an unseasonably strong system that is forecast to bring widespread precipitation=20 to the state and the potential for excessive rainfall amounts,=20 especially along the southern coast.=20 A vigorous shortwave diving southeast through the eastern Pacific this morning is expected to develop a closed upper low that will drop south along the northern California coast later today. This=20 will be an anomalously deep system, with 500 mb heights dropping=20 1-3 standard deviations below normal across much of the state by=20 early Saturday. Amplifying southwesterly flow ahead of the=20 deepening system will tap a plume of deeper Pacific moisture, that=20 will spread inland along a well-defined frontal band. A broad Marginal Risk was maintained across much of central into southern California, extending from the coast to the Sierra foothills. A targeted Slight Risk was also kept over the=20 Transverse Ranges. Guidance continues to show this area as the=20 focus for some of the stronger low-to-mid level moisture advection=20 and PWs as the system's front moves into the region later today.=20 HREF neighborhood probabilities show that accumulations of 3 inches or more are likely in this area -- raising runoff concerns for=20 areas below the snow line. ....Eastern Maine.... Upper forcing interacting with ample moisture along a slow-moving frontal zone will continue to support light to moderate rain, with precipitation becoming more focused across eastern Maine as a deepening surface low tracks northeast off of the New England=20 Coast toward Atlantic Canada. MRMS data indicates amounts of 2=20 inches or more have fallen across portions of DownEast Maine over=20 the past 24 hours, while HREF probabilities show another 1-2 inches likely before precipitation changes over to snow on the backside=20 of the deepening low. With rain continuing to fall, a Slight Risk=20 was added in response to the already saturated soil conditions,=20 which are reflected by 3-hour flash flood guidance values that=20 have dropped below 0.25 across parts of the area. Pereira 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... ....Southern California to southern Nevada and western Arizona... A heavy to excessive rainfall threat developing Friday into early=20 Saturday will spread further south as an unseasonably deep low=20 drops south from the central to the southern California coasts.=20 Strong onshore flow and moisture advection along the associated=20 frontal band, will spread precipitation across southern=20 California, with upslope flow accentuating the threat for heavy to=20 excessive accumulations along the coast, where a Slight Risk was maintained along the Transverse and Peninsular ranges below the=20 snow line. Residual moisture spreading further inland is expected=20 to fuel a lesser threat for excessive rainfall into the lower=20 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... ....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=20 the region. Deep southwesterly flow along the leading edge of the=20 trough will continue to support anomalous PWs (1-2 standard=20 deviations above normal) across portions of southern Arizona early=20 in the period. Meanwhile, lingering moisture along with upstream=20 energy dropping into the base of the trough will support showers=20 along the coast and an isolated threat for additional flash=20 flooding concerns. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4rO09RjDKoz9D5dx9qd5gha3QRsKxA20dR3dZyjVbAuj= y3R3gAxppbcwWIAcLGzfpv-Q5AV1psecjYdXSzu3vmJTUDU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4rO09RjDKoz9D5dx9qd5gha3QRsKxA20dR3dZyjVbAuj= y3R3gAxppbcwWIAcLGzfpv-Q5AV1psecjYdXSzu3kiltPxA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4rO09RjDKoz9D5dx9qd5gha3QRsKxA20dR3dZyjVbAuj= y3R3gAxppbcwWIAcLGzfpv-Q5AV1psecjYdXSzu39_Kk2Cg$=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) .