Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Dec 11 2022 20:25:56 FOUS30 KWBC 112025 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 325 PM EST Sun Dec 11 2022 Day 1 Valid 1900Z Sun Dec 11 2022 - 12Z Mon Dec 12 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE TRANSVERSE AND PENINSULAR RANGES OF SOUTHERN CALIFORNIA... ....Central California Coast... Additional compact shortwave energy will be dropping south down along the central CA coast later today and and going through early Monday morning which will bring a new surge of relatively moist onshore flow into the coastal ranges south of the Bay Area and extend southward across Monterey and San Luis Obispo counties. The 00Z HREF guidance suggests a wave of low pressure will be settling down across the immediate offshore waters of the terrain with an axis of at least moderate moisture convergence coinciding with strong mid-level forcing reflective of the compact 500/700 mb trough/closed low dropping southward. All of this coupled with steepening mid-level lapse rates should favor some bands of heavy showers that will be capable of producing rainfall rates up to and locally over a 0.50"/hour. For the period ending 12Z/Monday, an additional 1 to 2 inches of rain will be possible locally. Given the heavy rainfall over the last 24 hours, and localized burn scar considerations, these bands of heavy showers may result in some isolated runoff concerns. As a result, maintained the Marginal Risk without modification to address this next round of heavy rainfall potential. ....Transverse and Peninsular Ranges of Southern California... The current atmospheric river settling down across central CA will be arriving across the Transverse and Peninsular Range of southern CA during the day today, and bringing with it a southwest to northeast oriented axis of heavy rainfall that will be aligned orthogonal to the terrain. The heavy rainfall will be driven by robust warm advection ahead of a cold front with rather strong 850/700 mb moisture transport that will be reaching as high as 4+ standard deviations above normal for this time of year. CIRA-ALPW data plots still shows moisture offshore of southern CA already well established in the 850/700 mb layer and this coupled with the onshore/upslope flow into the terrain should favor heavy rainfall rates that reach as high as 0.50" to 0.75"/hour with isolated heavier rates possible where any convective elements focus near the cold front. The rains should taper off later tonight behind the cold front passage, but for the first 12 hours of the period, some 2 to 3 inch rainfall amounts are expected with isolated heavier amounts. Generally the eastern portion of the Transverse Range is favored for the heaviest totals. These rains will support at least localized runoff and flash flooding concerns, with an emphasis on the area burn scars. A Slight Risk is maintained across these coastal ranges as a result. Refer to WPC Mesoscale Convective Discussion 1155 for additional details. ....Southeast Texas...Southern Louisiana...Southwest Mississippi... Ongoing convection has moved off the coast and the threat for additional organized convection capable of producing excessive rainfall. As a result...removed the Marginal Risk along the Gulf coast. Orrison/Bann Day 2 Valid 12Z Mon Dec 12 2022 - 12Z Tue Dec 13 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS... 2030 UTC Update... Made a slight adjustment to the eastern side of the Marginal Risk area due to some uncertainty in the forward speed of the area of showers and thunderstorms that gets ejected eastward. A Marginal Risk remained in place for the update. Bann ....Southern Kansas...Central Oklahoma...Northern Texas... A strong upper-level trough and associated cold front will be ejecting east out toward the central and southern High Plains Monday night and Tuesday morning. A strengthening southerly low-level jet is forecast to reach 50 to 60 kts Monday night ahead of the front which will yield very strong moisture transport/convergence and a return of at least an elevated pool of instability (MUCAPE of 500 to 1000 J/kg). The rapidly improving thermodynamic environment coupled with strong upper-level divergence arriving ahead of the ejecting upper trough will set the stage for a developing axis of rather organized convection at least in a broken fashion across the central and southern Plains with the latest guidance emphasizing an area from southern KS down through northern TX for the heaviest amounts. The convection will tend to be rather progressive, but the improving shear parameters and enhanced moisture transport will favor convective cells capable of producing 1 to 2 inch/hour rainfall rates going into early Tuesday morning. The 06Z to 12Z/Tuesday time frame is generally the window in which convection will be initiating and growing upscale, although some activity may begin Monday evening. A Marginal Risk area is maintained for this update. Orrison Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_tDnBASMyvmsVylJ88BraKfJw2HEjr3v2G-j45WWwuaS= zO6_2eoIMyFvF3Scs1x0sgb94crGTkVsG3AxPDQRH9Qqm9Y$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_tDnBASMyvmsVylJ88BraKfJw2HEjr3v2G-j45WWwuaS= zO6_2eoIMyFvF3Scs1x0sgb94crGTkVsG3AxPDQRKF_AEeU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_tDnBASMyvmsVylJ88BraKfJw2HEjr3v2G-j45WWwuaS= zO6_2eoIMyFvF3Scs1x0sgb94crGTkVsG3AxPDQRzu9Tsdc$=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) .