Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Mar 27 2024 23:43:01 FOUS30 KWBC 272342 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 742 PM EDT Wed Mar 27 2024 Day 1 Valid 01Z Thu Mar 28 2024 - 12Z Thu Mar 28 2024 ....Southeast... A swath of heavy rainfall is likely today into tonight across=20 portions of the Carolinas into southeast VA, which will likely occur in a warm air advection pattern near a front ahead a=20 developing low pressure. MU CAPE is 500-1000 J/kg in this region. Effective bulk shear of close to 40 kts could lead to cell training near the boundary and potentially mesocyclones. Precipitable water values of 1.3-1.5" could lead to hourly rain totals to 2", which in a training scenario could support local amounts to 4". Both soil=20 saturation and streamflows are near to below average over most of=20 this area, resulting in high FFG values. This situation remains on=20 the border of a Marginal and a Slight Risk, so left the risk level=20 be, merely reorienting the areas based on the 18z HREF probabilities of 0.5"+ an hour and 2"+ and 3"+ in 12 hours. The thinking is that with time FFG will lower as conditions saturate, which could=20 result in a more concentrated flash flood risk in or near the=20 Slight risk area. Localized flooding is possible. Areas south of the Slight risk over SC and GA have high rate potential, but shorter durations and less coverage of higher totals justify=20 keeping the risk at Marginal. Areas north of the Slight over=20 northeast NC into southeast VA have high total rainfall, but much=20 lower probabilities of higher rainfall rates due to lesser=20 instability which would be easier to erode, justifying keeping that area in the Marginal. Roth Day 2 Valid 12Z Thu Mar 28 2024 - 12Z Fri Mar 29 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF EASTERN NORTH CAROLINA... 20Z Excessive Rainfall Discussion... Latest trends in QPF allowed for a slight reduction in areal coverage of the Slight Risk and Marginal Risk areas located over the eastern portion of North Carolina. Isolated maximum rainfall amounts approaching 3 inches remains possible with downpours delivering upwards of an inch per hour before the system translates off-shore. Also maintained the Marginal over portions of New England as the system propagates northward and spreads rain northward. Bann 08Z Excessive Rainfall Discussion... An area of low pressure moving up the coast will result in a heavy rain threat to portions of the eastern Mid-Atlantic Thursday into Thursday night. Highest rainfall totals are expected to be across eastern NC into southeast VA, where 1-3" of additional rainfall is probable. By this time instability should be lower across this area, although strong convergence and weak instability near the low track should still support rainfall totals upwards of 1"/hr, especially over eastern NC. Just like described in the day 1 discussion, a Slight risk upgrade was issued over eastern NC where there is overlap in potential 3"+ total rainfall and 1"+/hr rainfall rates Thursday morning. Areas north of the Slight risk should have lower rainfall rates, warranting just a Marginal risk. A swath of 1-3" of rain is also expected across eastern New England. Given this region will be on the cool side of the developing low, not thinking we'll see hourly rainfall any higher than 0.25". However recent rainfall has resulted in above average soil saturation and streamflows across this area...so while flash flooding appears unlikely given the low rates...some areal flood concerns are a possibility given the saturated conditions. Rain on snow over eastern ME is also likely, although with temperatures only in the 30s and 40s with this rain, unlikely to see a rapid snow melt. Nonetheless, given the wet and or snowy conditions of late across this region...the additional 1-3" of rain warrants the continuation of the Marginal risk. Chenard Day 3 Valid 12Z Fri Mar 29 2024 - 12Z Sat Mar 30 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHERN CALIFORNIA... 20Z Excessive Rainfall Discussion... Based on the 12Z guidance, there should be increasing rainfall rates late Friday night/early Saturday morning as a strong closed low makes its way southward along the California coast. The magnitude of moisture flux still looks strong enough to warrant the Slight Risk area...especially in/near the Transverse Ranges. Bann 08Z Excessive Rainfall Discussion... A strong closed low dropping south off the CA coast will bring a heavy rainfall threat to the state Friday into Friday night. This is a pretty strong system for the time of year and latitude, with NAEFS anomalies indicating 850-700mb height forecasts near climatological record lows. Not the strongest IVT magnitudes with this system, but still over the 90th percentile, and ~30 kts of southerly flow at 850mb is aligned well to provide upslope enhancement into the Transverse Range. Still some timing uncertainty given we are still 3 days out, but the current model consensus indicates an uptick in rainfall as low level convergence increases over the Transverse Range between 06z-12z Saturday. Likely enough forcing, low level convergence and weak instability to support hourly rainfall over 0.5" across portions of Santa Barbara and Ventura counties. The 00z downscaled GFS and ECMWF indicate upwards of 2-4" of rain across these counties by 12z Saturday...and while this is higher than the official WPC forecast (given some lingering timing/strength uncertainties), these amounts are certainly possible. Thus think the Slight risk over these counties remains warranted. This Slight risk is surrounded by a broad Marginal risk over much of central CA. Less of a focus for heavy rainfall here, and the cold front will be progressive. Nonetheless enough forcing is seemingly present to drive a localized heavy rainfall threat, especially if some instability can work into the system. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9c4qWdXNvbJObzZWWSAzRlb_XPyxvTTs2MaXBEfP6fXA= Halt8C7mJEGLvmAVKJa71r0UKXC2g32eU02ECIMwJe29Jns$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9c4qWdXNvbJObzZWWSAzRlb_XPyxvTTs2MaXBEfP6fXA= Halt8C7mJEGLvmAVKJa71r0UKXC2g32eU02ECIMwMJdlkME$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9c4qWdXNvbJObzZWWSAzRlb_XPyxvTTs2MaXBEfP6fXA= Halt8C7mJEGLvmAVKJa71r0UKXC2g32eU02ECIMwm8VCdrM$=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) .