Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Wed Mar 27 2024 08:13:00 FOUS30 KWBC 270840 AAA QPFERD Excessive Rainfall Discussion...UPDATED NWS Weather Prediction Center College Park MD 440 AM EDT Wed Mar 27 2024 Day 1 Valid 12Z Wed Mar 27 2024 - 12Z Thu Mar 28 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE FLORIDA PANHANDLE INTO SOUTHERN GEORGIA AND OVER PORTIONS OF EAST CENTRAL NORTH CAROLINA... ....Southeast... A swath of heavy rainfall is likely today into tonight across portions of the FL Panhandle, into southeast GA and northward into the central/eastern Carolinas. A Slight risk will be maintained across portions of the the FL Panhandle into southern GA. Convection should be ongoing across this area at 12z this morning, with 2"/hr rainfall possible given the instability and moisture in place. At the moment, it looks like activity will have enough eastward motion to limit the duration of the highest rainfall rates and keep any flash flood risk localized. By this afternoon we see an uptick in mid/upper forcing and strengthening low level convergence over the area. This should result in an uptick in convective coverage by this afternoon into this evening. It is unclear whether this activity will be organized enough to pose a flash flood risk. Flash flood guidance exceedance probabilities from both the 00z HREF and 18z REFS ensembles are very low, and not at the level we would expect to see for a Slight risk level threat. However we will have a slow moving boundary overhead, and multiple convective rounds appear probable into this evening. Given the ingredients, there is some chance this convection ends up a bit more robust than currently forecast in the models...and so while this is probably a lower end Slight risk, not confident enough for a downgrade. Morning convection over southeast GA into the Carolinas should be fairly weak. However by this afternoon we are expecting ~1500 j/kg of CAPE to develop near the stalled out front. Convergence near the front along with this increased instability should result in more robust convective development by or just after 18z. Based on HREF probabilities, expect 1"/hr to 1.5"/hr rainfall with this activity, but not currently anticipating anything over 2"/hr. Most indications are that this convective development will aid in pushing the stalled front a bit eastward with time. Thus while initial convection is expected more over east central SC/NC, this activity should push east with eroding instability from west to east. By later in the afternoon into the evening hours the focus for any more intense convection should be gradually shifting east over the Carolinas. This rainfall over the eastern Carolinas will likely be reinforced overnight by a developing low pressure along the front. Overall, neighborhood probabilities from both the 00z HREF and 18z REFS indicate a greater than 50% chance of 3"+ rainfall amounts through 12z Thursday over portions of the central and eastern Carolinas into southeast GA, however the probabilities of 5"+ amounts drop significantly to only around 5%. Both soil saturation and streamflows are near to below average over most of this area, resulting in high FFG values. FFG exceedance probabilities from the HREF and REFS are low. Overall, a tricky call on whether to stick with a Marginal or upgrade to a Slight risk. Decided to go ahead with a targeted Slight risk area over portions of east central NC. This risk was placed where there is overlap between more coverage of 3-5" total rainfall, and 1"/hr or greater rainfall rates. The thinking is that with time FFG will lower as conditions saturate, which may eventually result in a more concentrated flash flood risk over the Slight risk area. This Slight risk area has the potential to see one round of convection this afternoon, and another overnight as the forcing increases and low intensifies. Localized flooding is possible with round one, but the Slight risk come comes into play during the overnight hours when the second round of possible 1"/hr rainfall could briefly train along the trough axis extending northeast from the low. Areas south of the Slight risk over SC and GA have high rate potential, but shorter durations and less coverage of higher totals justify keeping the risk at Marginal. Areas north of the Slight over northeast NC into southeast VA have high total rainfall, but much lower probabilities of higher rainfall rates...justifying keeping this area in the Marginal. ....Northwest CA into southwest OR... We will maintain a Marginal risk across these areas. A cold front will push south across the region today, with some mid level shortwave energy and upper jet support moving over the front as well. This should provide enough forcing to generate rainfall up to 0.5"/hr, especially given some weak instability working into the back side of the front. High res guidance is in good agreement on widespread 1-2" amounts, with pockets of 3"+ rainfall within the more favored upslope areas. With soil saturation and streamflows running above normal over the area, it seems plausible that any embedded higher rates could cause some localized flooding concerns. Chenard 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... 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... 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 = = = --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .