Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Dec 11 2022 08:19:18 FOUS30 KWBC 110819 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 316 AM EST Sun Dec 11 2022 Day 1 Valid 12Z 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 late Sunday and going through early Sunday morning which will bring a new surge of relatively moist onshore flow into the coastal ranges south of the Bay Area and extending down across Monterrey 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, a Marginal Risk has been depicted 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 on Sunday, 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. A look at the current CIRA-ALPW data plots show 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 Sunday night 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. ....Southeast Texas...Southern Louisiana...Southwest Mississippi... Ongoing convection across areas of central to southeast TX and adjacent areas of western LA will likely continue for at least several hours this morning across the northwest Gulf Coast region (southeast TX/southwest LA) and extending northeastward possibly into areas of southwest MS as a cold front continues to settle southeast across the region. The dominant driver of this is an upper-level shortwave trough crossing the southern Plains which will be crossing the lower MS Valley Sunday morning. Divergent flow aloft coupled with moist and at least modestly unstable air with as much as 500 to 1000 j/kg of MLCAPE ahead of the front should maintain at least a broken axis of convection in the 12Z to 15Z time frame across the coastal plain areas, with the activity gradually pushing east-southeast and offshore going into the afternoon hours. he exiting of the shortwave energy through the afternoon should also favor a decreasing level of organization with time as well. Given locally heavy rainfall rates of 1 to 2 inches/hour potentially impacting some of the more urbanized locations near the Gulf Coast, including the Houston metropolitan area, a Marginal Risk area has been carried over into at least the first part of the new Day 1 ERO period. Orrison Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5uhVB22W6Ol0LDleA95OuuU69TAlKBEuMvs7PLv3DDjb= 87EAVgP-qoX9DArx_3VJ3r2VAvbtRoFAJLVJLj5rBCE6H8o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5uhVB22W6Ol0LDleA95OuuU69TAlKBEuMvs7PLv3DDjb= 87EAVgP-qoX9DArx_3VJ3r2VAvbtRoFAJLVJLj5rHcibfvA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5uhVB22W6Ol0LDleA95OuuU69TAlKBEuMvs7PLv3DDjb= 87EAVgP-qoX9DArx_3VJ3r2VAvbtRoFAJLVJLj5ruYJKB0M$=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) .