Rainfall

Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 110 PM EDT Sat Apr 29 2023 Day 1 Valid 16Z Sat Apr 29 2023 - 12Z Sun Apr 30 2023 ...THERE IS A SLIGHT RISK FOR PORTIONS OF THE FLORIDA PANHANDLE AND SOUTHERN GEORGIA... 16Z Update... ...Southeastern U.S... Latest 12Z HREF has continued to come in incrementally drier over northern FL, southern AL, and southern GA. One area that still shows potential for flash flooding is in the FL Panhandle where some members of both hi-res CAMS and global deterministic guidance show a scenario where some of the robust convection in the Gulf of Mexico makes its way northeast (thanks to southwesterly mean 850-300mb winds at 40-50 knots) around midday and into the afternoon hours. The good news is the bulk of the convection should remain progressive due to being influenced by such strong steering flow. That said, thunderstorms will still be capable of producing >2"/hr rainfall rates as a result of PWs surging close to 1.75", MUCAPE rising above 1,000 J/kg, RH values within the LCL-LFC that approach 90%, and warm cloud layers that are 11,000 feet in depth. NASA SPoRT-LIS continues to show an area north and west of Tallahassee where 0-40cm soil moisture percentiles are up to 90% due to 7-day rainfall totals that are 400-600% of normal over the past 7 days. The forward propagation of the storms and the bulk of the most intense convection weakening as it approaches the FL Peninsula will help to reduce the Slight Risk area this forecast cycle, but opted to maintain a Slight Risk for portions of the FL Panhandle and southern GA given their more sensitive soils. Portions of central MS and AL were considered for a possible Slight Risk upgrade as the upper low moves overhead, but more marginal instability levels and soils that are not as sensitive were the primary reasons a Marginal Risk was kept in place. The organizing MCS in the Gulf will head for the northern Florida Peninsula, but based on its progressive storm motions, opted to maintain the Marginal Risk. ...Central PA... The other change to the Day 1 ERO was to introduce a Marginal Risk area in central PA. As low pressure deepens over the Great Lakes and Southeast, the pressure gradient will tighten as central PA becomes sandwiched between these two areas of low pressure to the west and south, and a dome of high pressure near Nova Scotia. The atmosphere will quickly saturate between 00-12Z Sunday with LCL-LFC RH values up to 90% after midnight. Surface winds will be light out of the ENE while at the same time, southerly 850mb winds (associated with a surge in 850mb moisture flux) will accelerate from 15-20 knots 00Z Sunday to 35-40 knots by 09-12Z Sunday. A narrow convergence zone at 850mb will be present over the heart of the Commonwealth at the same time the mean 850-300mb flow is uniformly out of the SW. This sets up a scenario for training convection in an environment where warm cloud layers are as deep as 7,000 feet, MUCAPE could reach up to 250 J/kg, and PWs will range between 1.0-1.25". Antecedent soil moisture conditions have grown more saturated from the Laurel Highlands and central PA to the Poconos and Catskills with 0-40cm soil moisture percentiles approaching 90%. Thunderstorms tonight will have the potential to generate 1.0-1.5"/hr rainfall rates, which is in line with current 1-hr FFGs in central PA. A Marginal Risk has been issued for these reasons, with the greatest chances for Excessive Rainfall coming after 00Z this evening. ...Southern New England, NYC Metro Area, Long Island, & Lower Hudson Valley... Not much change from the overnight shift as the region at-large will continue to deal with periods of moderate-to-heavy rainfall through this evening. Latest RAP guidance suggests the atmosphere will continue to further saturate to the point where RH values in the LCL-LFC layer will climb above 90%. That said, the biggest limiting factor in rainfall rates and flooding potential is the lack is instability. Given the long duration nature of this setup, opted to maintain the Marginal Risk as of this update with any instances of flash flooding being highly localized, and likely confined to poor drainage areas and the most urbanized of communities. Mullinax ---Previous Discussion--- ...Southeastern US... No significant changes were made to the Day 1 ERO area. It's very worth noting the latest runs have backed off significantly on the total expected rainfall in the Slight Risk area, favoring most of the convection remaining south of the area into the Gulf. However, there remains several models that at least depict strong storms locally producing multiple inches of rain in a short time as the main round of convection moves through around mid-afternoon and a second separate and weaker round of mostly showers moves through around midnight. There is still some potential for training convection, but that chance is fairly low. Due to recent rainfall, especially over the Florida Panhandle, the soils remain nearly saturated over most of the Slight Risk area, and thus any rain that falls today is likely to quickly run off into the local watersheds. The Slight Risk area therefore remains, but it's certainly a lower confidence and therefore lower end Slight Risk, that with model trends, could be downgraded later today as the convective forecast by then will be a now-cast for many areas. Elsewhere across east central MS and west central AL, a strong and developing upper level wave will race across the area today. Given some modest instability to 500 J/kg and PWAT around 1.25 inches, think the added forcing will be more effective at adding to the strength of the storms expected to traverse that area this evening. Soil moisture is a little bit drier in that area compared to the surrounding area, and the modest instability may be enough to prevent anything more than isolated flash flooding. Thus, that area in particular is in a higher-end Marginal. ...Southern New England, New York City Metro Area, Long Island, and the Hudson Valley... Convective rainfall has already begun across the Marginal Risk area for CT and points south and west. Rainfall rates as of the time of this writing are approaching 1 inch per hour. In more sensitive areas of urbanized northern NJ and NYC, these rates may be enough to cause localized flooding in poor drainage areas, so southwestern portions of the Marginal Risk are already seeing the rain that may cause flash flooding, and the focus for the Day 1 ERO is for the morning hours. Meanwhile further northeast across MA and RI the threat will be more for later this morning into tonight. Some backbuilding along the back edge of the precipitation is likely across southern New England, so the potential for higher rainfall totals will be there. Wegman Day 2 Valid 12Z Sun Apr 30 2023 - 12Z Mon May 01 2023 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF SOUTHERN MAINE, EASTERN NEW HAMPSHIRE, SOUTHERN NEW YORK AND CENTRAL/EASTERN PENNSYLVANIA... ...Northeast... A strong shortwave trough over the Southeastern US Sunday morning will round the eastern periphery of a very energetic longwave trough during the day Sunday, then will be absorbed by the longwave trough Sunday night, which will energize a second wave associated with the longwave trough itself across the Northeast Sunday night. This highly energetic flow pattern combined with 2 separate 120 kt jet streams Sunday afternoon that get mixed out by Sunday night will help to rapidly advect moisture and instability from the Southeast north into the Northeast. PWATs along the Mid-Atlantic coast may approach 2 inches, while much of the Northeast stays in between 1 and 1.75 inches. Meanwhile there will be some instability to 500 J/kg, but the main forcing will be the highly energized upper trough and attendant shortwaves. Little has changed with this pattern compared to 24 hours ago. As all of the storms will be tracking northeastward, the Slight Risk area over PA and NY will be the first to see its convective rain, with the Slight highlighting an area with particularly low FFGs due to both previous rounds of rain and today's. Expect multiple waves tracking southeast to northwest as the low and Midwest upper trough wrap the aforementioned Atlantic moisture westward. Isolated areas may pick up over 3 inches of rain, with widespread 2 inch amounts in the Slight risk area. Meanwhile, the Slight Risk over NH and ME remains for the late overnight hours as the main cold front with a 50 kt easterly LLJ forces convective rain into the steep topography of NH and ME. While the time it will be raining will be shorter than areas south and west, the rainfall combined with upslope will be extra intense along the east facing slopes of the mountains. A lighter southeasterly flow off the Atlantic will allow for light to periods of moderate rain to soak the area even before the main front pushes through Sunday evening. In between the two Slight risk areas, FFGs are a bit higher, and since the Atlantic moisture will have to cross multiple mountain ranges before reaching VT and northeastern NY, expect rainfall rates to not be quite as high. Nonetheless with strong forcing and convective elements, expect the area to still get locally 1-2 inches of rain, heavy at times, with the higher amounts on the east facing slopes of the Berkshires and Catskills. ...Florida... The one significant change to note covering all 3 of the first days of the EROs was across Florida on Sunday. There is good model agreement that the trailing cold front will have made much more progress down the Florida Peninsula by 12Z Sunday morning, and as such the Marginal risk threat has been shifted well to the south to cover more of the central and southern Peninsula, with northern portions likely staying dry for much of the day, save a passing shower or 2. Meanwhile, the trend in the guidance has been for less rainfall since the main forcing for the low will be further away from the southern FL Peninsula than if it were starting the day further north. Either way once again the main flooding threat will be Sunday morning. By Sunday afternoon there may be a few lingering storms along the Atlantic seaboard, but the main event will be over for the day. Low FFGs accounting for recent rains in this area was the main driver for keeping the Marginal Risk area going with this update. Wegman Day 3 Valid 12Z Mon May 01 2023 - 12Z Tue May 02 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt