Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Mar 01 2023 07:32:35 FOUS30 KWBC 010732 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 231 AM EST Wed Mar 01 2023 Day 1 Valid 12Z Wed Mar 01 2023 - 12Z Thu Mar 02 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR ARKANSAS AND TENNESSEE... Mid-South/TN Valley... A low-level boundary wavers around TN this period as one cyclone ejects eastward from the Great Lakes and another moves into the Southern Plains from the West. Precipitable water values are expected to rise into the 1.25-1.75" range, which up to 3 sigmas above the mean for early March per 00z GFS guidance. Inflow from the Gulf is expected to range from 25-45 kts out of the west-southwest at 850 hPa into the region -- with effective bulk shear to match -- which should organize convection when combined with the 500-1500 J/kg of MU CAPE moving in from the south. The flow is generally unidirectional with height, indicating that training bands are possible with hourly totals within them up to 2". The amount of effective bulk shear allows for mesocyclones, which can be efficient rain producers, especially if two or more line up which could lead to hourly rain totals up to 2.5".=20 Temperatures at 700 hPa are generally 3-6C, implying little to no mid-level capping inversion exists. There is a signal in the current guidance for local amounts in the 2-4" range, with the ingredients available. Flash Flood Guidance values are particularly low in the southern Appalachians and modest across the remainder of the area -- roughly 2"/3 hours. The top 10 cm of soils are between the 55-70% saturated across the region per NASA SPORT imagery. Widely scattered flash flood issues are anticipated. Continuity was adjusted westward in the western edges of the Marginal and Slight Risk areas and southward across the northern side of the Marginal Risk area to account for changes in the recent model QPF guidance. Southern California... There are enough ingredients in play to hoist a Marginal Risk of excessive rainfall over portions of Southern California for the morning hours of Wednesday, which was coordinated with SGX/the San Diego CA forecast office. GFS-based IVT rises into the 500-600 kg/ms range during the morning hours as a cold front moves through the region. Roughly 200 J/kg of CAPE along with initially stagnant 850 hPa inflow towards 50 kts early on to maintain precipitable water values towards 0.75". The available ingredients support 0.5" an hour totals, and the 18z/00z HREF show probabilities of such above 50%, particularly before 16z. Snow levels should start as high as 4500-5000 feet before descending quickly during the afternoon hours as the upper low swoops in from the north and precipitable water values descend quickly. There could be a brief period in the early afternoon where enough moisture is around while instability increases due to the cooling aloft for heavy rain-related issues, but with snow/freezing levels dropping, the excessive rainfall threat should be easing during the afternoon hours. Roth Day 2 Valid 12Z Thu Mar 02 2023 - 12Z Fri Mar 03 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS IN AND AROUND NORTHERN ARKANSAS...=20 =20 ....ArkLaTex to Mid-South and Lower Ohio Valley...=20 A strong closed low moves from the Southwest towards the MO/AR border this period, forcing a surface cyclone to bomb in the Southern Plains/Midwest. It will also allow for significant divergence/difluence aloft to develop downstream as the upper level troughing becomes negatively tilted. Ahead of the system, much of AR remains within 1.25-1.75" precipitable water value range for about 18 hours, which is 3 sigmas above the mean for early March. MU CAPE=20 rises above 2000 J/kg in spots nearby and upstream across northeast TX and southeast OK. Convergent inflow at 850 hPa as well as the mean 850-400 hPa flow backs from west-southwest to southwest and eventually south-southwest and its strength lies within the 25-60 kt range, with effective bulk shear to match. In addition, there is significant overlap in the low- to mid-level frontogenesis, which will also aid rainfall efficiency which is expected to the primary source of heavy rain production in the northernmost portions of the Slight and Marginal Risk areas. The available ingredients allow for both training convective segments and mesocyclones, which can be efficient rain producers when two or more line up, potentially leading to hourly rain totals to 2.5". The heavy rainfall pattern should be hemmed in from the south by 6-9C 700 hPa temperatures, implying a mid-level capping inversion exists across portions of LA/closer to the Gulf coast.=20 The guidance advertises local amounts in the 3-6" range, but more is possible as the low-level low backs as convection initially develops -- there's a potentially higher ceiling closer to 9" on a localized basis before forward propagation occurs later Thursday night/early Friday. There is some overlap of the heavy rainfall within the days 2-3 forecast across southwest TN and AR where flash flood guidance values are already modest. There's a broad area of 55-70% saturation in the top 10 cm of soils from the ArkLaTex through AR, TN, northern MS, and southern KY per NASA SPORT imagery, which has been trending downward lately since it hasn't rained significantly in a while, but rainfall on Wednesday is bound to lead to a higher degree of saturation before Thursday morning dawns. Along with the heavy rainfall forecast for Thursday and Thursday night, the stripe of heavy rain that fell across Little Rock AR about 5 days ago is of concern. Considering all of the above, the threat level remains Moderate Risk, though guidance trends have led to some northwest shift in the Risk areas. Some eastward elongation across northern KY was coordinated with LMK/the Louisville KY forecast office. =20 Roth=20 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!_LPUafpgcRXaJvMvW02uRnC5CUrAwdXlzvuNQD98aFao= jAy04zW7jn4ZyoPgAglY2DPwsNkP1f50vTDGh8b24gTIkKM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_LPUafpgcRXaJvMvW02uRnC5CUrAwdXlzvuNQD98aFao= jAy04zW7jn4ZyoPgAglY2DPwsNkP1f50vTDGh8b2g2qaCv0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_LPUafpgcRXaJvMvW02uRnC5CUrAwdXlzvuNQD98aFao= jAy04zW7jn4ZyoPgAglY2DPwsNkP1f50vTDGh8b2v5QaxR0$=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) .