Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Sat Apr 29 2023 08:33:00 FOUS30 KWBC 290856 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 455 AM EDT Sat Apr 29 2023 Day 1 Valid 12Z Sat Apr 29 2023 - 12Z Sun Apr 30 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR THE FLORIDA PANHANDLE INTO SOUTHERN GEORGIA... ....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 $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .