Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Sun Nov 26 2023 10:40:00 FOUS30 KWBC 260816 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 315 AM EST Sun Nov 26 2023 Day 1 Valid 12Z Sun Nov 26 2023 - 12Z Mon Nov 27 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. As a shortwave trough swings through the base of a longwave trough becoming established along the East Coast today, it will become negatively tilted with a 150 kt jet streak at the trough base near 250 mb moving across the Mid-Atlantic states. A surface low is expected to deepen as it tracks through southeastern New England Sunday night at the same time that the left exit region of the Mid-Atlanitc upper jet max sets up over coastal New England, supporting increased divergence aloft. While strong low level convergence following the surface low track will likely allow for a stripe of heavy rain from southeastern New England to the southern coast of Maine Sunday night, instability is expected to be negligible for inland locations which should limit peak rainfall rates to near 1 in/hr. While the more aggressive members of the 00Z HREF and the 06Z HRRR begin to approach the 2 inches in 3 hour FFG for southeastern New England, the heaviest rain should be only of a 2-3 hour duration for any given location with localized totals of 2+ inches possible for the event, limiting the threat for flash flooding. Given the recent lack of rain for this portion of the Northeast and coordinating with the affected WFOs, no risk area was introduced for southeastern New England. Otto Day 2 Valid 12Z Mon Nov 27 2023 - 12Z Tue Nov 28 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Locally heavy rain is expected to be ongoing within the Gulf of Maine and near the coast of New England at the start of the period (Monday morning). A deepening surface low is forecast to quickly track NNE from eastern MA at 12Z into eastern Maine through 18Z Monday with peak rainfall rates of 0.5 to near 1.0 in/hr to the immediate northwest of the surface low track. The low track and axis of heavy rain should be fast moving within a region lacking appreciable instability, limiting potential for higher rainfall totals along the coast. Therefore, while some of the more aggressive members of the 00Z HREF show 3-hr QPF near or locally in excess of 3-hr FFG for coastal portions of Maine, the probability of exceedance was considered too low for a risk area at this time. In addition, the speed of the surface low should limit maximum rainfall totals into the 1.5 to 2.5 inch range, not considered significant for flooding impacts given recent dry weather across the region and after coordination with the affected WFOs. However, newer model guidance for coastal Maine will have to be monitored for increases and possible changes to the Monday to Tuesday morning Excessive Rainfall Outlook. Otto Day 3 Valid 12Z Tue Nov 28 2023 - 12Z Wed Nov 29 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Otto $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .