Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Feb 04 2024 18:37:27 FOUS30 KWBC 041837 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 136 PM EST Sun Feb 04 2024 Day 1 Valid 16Z Sun Feb 04 2024 - 12Z Mon Feb 05 2024 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA... ....Central to Southern California... The deepening low off the Central to Northern California coast will strengthen the Atmospheric River event into large portions of Central to Southern California into Sunday afternoon. The strongest IVT -- up to 1000 kg/m/s -- will be impacting the Central California coast range, south into the Transverse Range and inland into northern portions of the San Joaquin Valley and Sacramento Valley during this time frame. Models remain consistent in showing widespread heavy totals from the northern portions of the Peninsular Range through the Transverse Range and northward along the Central California Coast Range. The High Risk was maintained with minimal change through the Transverse Range where the best upslope component will occur and covers where the moisture plume is expected to stall Sunday night into Monday morning across the eastern Transverse Ranges when the deep layer flow backs somewhat, holding up its forward progression.=20 Significant, life-threatening flash flooding is expected in the High Risk area. Areal average totals of 3-5" likely from the Central CA Coast Range south into the Transverse Range, with isolated 10"+ totals possible. The High Risk area fit well where the HREF neighborhood probabilities are high for 3", 5", and 8"+ totals. Changes to the other risk areas were modest. ....Southeast, eastern Gulf Coast and Florida... The broad and unusually strong upper level low will be sinking slowly southeastward into the northeast Gulf of Mexico, bringing very cold air aloft over warm Gulf of Mexico waters, which has already initiated convection south of Louisiana this morning.=20 Initially, progressive convection with heavy rain is crossing the FL Peninsula and North FL within a warm conveyor belt circulation ahead of a low approaching the FL Panhandle. Comma head heavy rains are expected persist most in and near southern GA long enough to continue to support a Marginal Risk. As the unusually cold air aloft continues to drive convection over the Gulf of Mexico, a new surface low forms, which turns the low-level flow across southern FL back to the southwest with some increase in magnitude Monday morning. Even though precipitable water values only maximize in the 1-1.25" range, within the cold air mass across the FL peninsula, the atmosphere should fully saturate. MU CAPE of 500-1000 J/kg is expected to hang around ahead of the convective low, per RAP forecasts. Left the Marginal Risk areas more or less alone in southern FL, only bridging across the southern Peninsula. Hourly rain totals to 1.5" with local amounts to 3" remain possible. The guidance continues to fluctuate on QPF across the southern FL peninsula, so confidence is low on details, but based on the available ingredients, the potential remains for isolated flash flood risk late Sunday night into early Monday morning. Roth Day 2 Valid 12Z Mon Feb 05 2024 - 12Z Tue Feb 06 2024 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA... ....Southern California... A stationary moisture plume with sufficient moisture and low-level inflow will get stuck across portions of southern California well into the afternoon hours, northeastward into southern Nevada as the front pushing through southern California slows and the moisture band pivots somewhat. This will support widespread heavy precipitation across southern California from the coast into the Peninsula Range and much of the Transverse Range. An increasing amount of instability will also be available, moving into the western portion of the band. With soils expected to be increasingly saturated across eastern portions of the Transverse Range due to the heavy rains Sunday afternoon and night, a high risk remains where additional 2-4" of precipitation is possible, bringing local two day totals over 10". Across the deserts of southeast California, toward the Nevada border, model consensus is for an axis of moderate to heavy precip totals in the above mentioned anomalous PW axis with low- to mid-level frontogenesis and nearly unobstructed flow from the Gulf of California moving into the region. Across the San Joaquin and Sacramento Valleys, some isolated heavy precip totals possible here where slow moving or training of cells may occur in the persistent southwesterly mid to upper flow. The main adjustment to the risk areas was some trimming on their eastern and southern sides due to the slowing seen in the model guidance. ....Portions of the Florida Peninsula... A potentially convective low with instability in advance of the system could focus heavy rainfall across southwest FL during the morning as it makes landfall, and the Gold/Treasure/Space Coasts into the afternoon as instability advects in from the warm Gulf Stream/east while unusually cold air passes aloft due to a uniquely anomalously deep cold low. Precipitable water values of 1-1.25" are expected within the chilly air mass, which supports saturation. Hourly rain totals to 1.5" with local amounts to 4" are possible, but the guidance is unclear whether convection will weaken as it moves ashore or stay just miles off the East Coast.=20 Raised a Marginal Risk as a precaution due to the uncertainty present in the forecast and the potential if heavy rains fall within urban areas.=20 Roth Day 3 Valid 12Z Tue Feb 06 2024 - 12Z Wed Feb 07 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA & THE DESERT SOUTHWEST... The axis of anomalous moisture will begin to push eastward as the longwave trough aloft off the California coast pushes inland.=20 Additional moderate to heavy rainfall totals are possible early on across eastern portions of southern California. While low-level inflow/effective bulk shear is not expected to be sufficient to organize convection within the instability expected to be present, saturation from the previous two days and the risk of 0.5"+ an hour totals led to an expansion of the Slight Risk area westward to Santa Barbara.=20 While the anomalous moisture axis and flux will be moving inland into the Southwest, it should linger across portions of southeast California into southern Nevada. Heavy precip totals of 1-3" are expected in the upslope of the western to central Arizona, with the potential for overlap across the Desert Southwest with heavy rainfall expected during previous days. Changes were minimal to the risk areas across the Desert Southwest. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9vvB6jCFReEPSHlwNbS74v5m7OF8Ne97tjteZImworL-= FPSm9IuQ4IOu1bX4FfyRAfWuaYh-tu8h3shVOCu63Xb96o0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9vvB6jCFReEPSHlwNbS74v5m7OF8Ne97tjteZImworL-= FPSm9IuQ4IOu1bX4FfyRAfWuaYh-tu8h3shVOCu6eA04JZM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9vvB6jCFReEPSHlwNbS74v5m7OF8Ne97tjteZImworL-= FPSm9IuQ4IOu1bX4FfyRAfWuaYh-tu8h3shVOCu6uTTKTaA$=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) .