Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Feb 13 2025 00:48:15 FOUS30 KWBC 130048 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 748 PM EST Wed Feb 12 2025 Day 1 Valid 01Z Thu Feb 13 2025 - 12Z Thu Feb 13 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE DEEP SOUTH AND COASTAL AREAS OF CALIFORNIA... 01Z Update... The western edge of the Marginal and Slight Risk areas have removed across the Lower MS Valley to account for where the heavy rainfall threat has ended. The latest radar and satellite imagery shows well-organized convection including supercell thunderstorms and some early hints of a QLCS evolution across areas of southern and eastern MS into western AL. This is occurring as a 40 to 50+ kt southwest low-level jet focuses strong moisture transport along=20 with a moderately buoyant airmass characterized by MUCAPE values=20 of 1000 to 2000 J/kg across the Gulf Coast states. This coupled with a strongly favorable kinematic environment with 50 to 60+ kts of effective bulk shear is expected to maintain organized convection with supercell potential well into the night which aside from the severe weather threat will also be capable of producing high rainfall rates capable of reaching 1 to 3 inches/hour with=20 the stronger storms. The afternoon CAM guidance, including the experimental WoFS, strongly favors a continued threat for cell-=20 training going into the overnight hours with additional rainfall=20 totals of as much as 2 to 4 inches with isolated heavier amounts=20 possible. Areas of southern and eastern MS through central AL will=20 see this threat in particular over just the next few hours, but=20 areas downstream into northwest GA will see a similar concern for=20 these heavy totals closer to midnight and beyond. The threat for=20 flash flooding will continue across all of these areas, with the=20 more urbanized locations seeing the greatest threat for runoff=20 problems/impacts. Elsewhere, no changes were made to the Marginal and Slight Risk areas for the West Coast involving California as the strong deep=20 layer cyclone and associated atmospheric river offshore continues=20 to approach. This will bring heavy rainfall into the coastal=20 ranges overnight, and especially around the Bay Area after midnight which will increase the concerns for flooding going into early Thursday morning. Orrison Previous Discussion... ....Southeast... Large Marginal and Slight Risk areas extend from Louisiana to the=20 central Appalachians and portions of Virginia. The Marginal and=20 Slight Risk areas cover multiple regimes for excessive rainfall=20 potential: 1) Long-duration, moderate to heavy rainfall along an axis extending from east Texas east-northeastward into northern Georgia and the southern Appalachians. Much of this axis has experienced appreciable antecedent rainfall amounts (in the 2-3 inch range on February 11) and models are consistent in depicting at least one more round of heavy rain across that same axis as a thunderstorm complex organizes and moves east- northeastward from in Texas today. Another 1-3 inches of is expected (heaviest from western in to central Mississippi). Given the wet soils from prior rainfall, several instances of flash flooding are expected from central Louisiana through the southern Appalachians and northern Georgia. 2) Somewhat lighter rainfall should occur across portions of Virginia and West Virginia through the day. Not only are soils water-logged and streamflows above average across these areas, but recent snow/ice could melt under persistent warm advection and temperatures slowly rising above freezing through the forecast period. Light to moderate rainfall could enhance excessive runoff/ponding especially in sensitive areas and low spots. 3) CAMs are in very loose agreement in depiction of east-west oriented outflow boundary left behind by convective complex(es) across portions of southern MS/AL through early afternoon. Strong south-southwesterly 850mb flow and abundant moisture should result in reignition of deep convection focused along boundaries for much of the afternoon and early evening. The orientation of these boundaries should focus convective development and may allow for training and intense rainfall rates at times. This is supported in particular by the 12Z HRRR, which depicts 4-8 inch rain amounts across central Alabama and north-central Georgia through 12Z Thursday. There remains some uncertainty in the exact placement of the heaviest axis, but initial 12Z guidance is clustering on this area for the potential of higher-end flash flooding (locally significant), particularly for the urbanized areas like Birmingham and Atlanta metros. ....California... The initial wave of the AR is set to arrive after 00Z this evening, with the best IVT values reaching the coast around 06Z. As a result of this slightly faster timing and consensus of the models for rain rates to approach or exceed 0.5"/hr after 09Z, a Slight Risk was introduced. Through 12Z Thursday, rainfall totals of 1-1.5" will be possible along the favored coastal terrain areas with some isolated amounts nearing 2". Given this, isolated to scattered instances of flooding will be possible. Cook/Taylor Day 2 Valid 12Z Thu Feb 13 2025 - 12Z Fri Feb 14 2025 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL PARTS OF THE SOUTHERN CALIFORNIA COAST... ....California... The peak of the atmospheric river is expected during this period with the initial focus on coastal central California at the start of the period, then the best IVT shifts southward through the period, reaching southern California by the end of the period Thursday night. IVT values from the GFS peak at around 600-700 kg/m/s initially along the central coast then settle around 400-500 kg/m/s as the AR reaches SoCal. The greatest QPF amounts will be confined to the coastal terrain areas as well as for the Sierra. Total amounts for the 24-hour period look to be between 1-3" with the favored terrain areas and peaks topping out between 4- 6"+. The primary concern is the potential for damaging and potentially life-threatening burn scar flash flooding and debris flows, particularly for southern California where the Moderate Risk (Level 3/4) is in place. Even outside the burn scar regions, the intense rain rates and rainfall totals over the highly urbanized area could bring scattered to numerous instances of flash flooding. Elsewhere, a broad Slight Risk (Level 2/4) exists for much of the coastal ranges as well as the Central Valley and lower elevations of the Sierra Nevada. ....Florida Panhandle and Southern Georgia... Lingering heavy rainfall will continue Thursday across portions of the Florida Panhandle and southern Georgia, ahead of the approaching cold front. Rainfall amounts look to be generally in the 1-3" range, with some potential for localized rain rates sufficiently high enough to cause some urban/localized flooding concerns. Hurley/Taylor Day 3 Valid 12Z Fri Feb 14 2025 - 12Z Sat Feb 15 2025 ....Southern California... IVT plume weakens while pushing southward across SoCal to begin the Day 3 Period. While the bulk of the event (Slight-Moderate excessive rainfall risk) is expected on Day 2 or through Thursday night, will maintain a Marginal Risk into the early part of Day 3 (Friday morning), especially if the southward AR progression slows down a bit. ....Mid South... Amplifying longwave trough across the Southwest-4 Corners Region Fri-Fri night will lead to backing mid/upper layer flow and deepening WAA/moistening downstream within a maturing Warm Conveyor Belt (WCB). Thermodynamic profiles will respond appreciably as a result, especially late in the period (Fri night) as the low- level flow backs SSW and increases (to 50+ kts at 850 mb). This leads to robust low-level moisture transport late Friday night into early Saturday morning, with 850-700 mb moisture flux anomalies ramping up to 4-5 standard deviations above normal per the ensembles. There will also be a corresponding uptick in elevated instability as well, to the tune of 500-1000 J/Kg towards 12Z Sat. While the main event will be on Saturday (see Day 4 ERO), will maintain the Marginal Risk given the anticipation of the ingredients ramping up fairly quickly Friday night, which given the probabilistic QPF along with the latest CSU UFVS-verified ERO first-guess fields, could lead to localized runoff issues across portions of the Mid South. by the pre- dawn hours Saturday. Hurley/Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_wOKmmtNVLtT3LcxcpE_GMIUfLVWRmKMXIlo04OOgMoW= yZIbzMZTtGi5U9nHmL5DOTQQ0dalUUnemFfjUzfxucjjfjw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_wOKmmtNVLtT3LcxcpE_GMIUfLVWRmKMXIlo04OOgMoW= yZIbzMZTtGi5U9nHmL5DOTQQ0dalUUnemFfjUzfxTjAVzjM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_wOKmmtNVLtT3LcxcpE_GMIUfLVWRmKMXIlo04OOgMoW= yZIbzMZTtGi5U9nHmL5DOTQQ0dalUUnemFfjUzfxKfoQf9I$=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) .