Subj : HVYRAIN: Excessive Rainfa To : ALL From : Mike Powell Date : Sat Sep 16 2023 21:05:00 FOUS30 KWBC 170058 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 858 PM EDT Sat Sep 16 2023 Day 1 Valid 01Z Sun Sep 17 2023 - 12Z Sun Sep 17 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER EASTERN MAINE... ....Eastern Maine... Post Tropical Cyclone Lee continued to track northeastward across the Bay of Fundy. Overall...rainfall rates in eastern Maine have been under half an inch per hour through the day...but several rain bands set up with enhanced rainfall rates and localized amounts in excess of 5 inches in southwest Washington county. Latest radar imagery showed the one area along the coast was pushing off shore while there was an uptick in the coverage of rainfall rates along the Maine and New Brunswick. Maintained the Slight with only minor modifications based on trends in the radar and satellite imagery and fact that additional rainfall on top of increasingly saturated soils could lead to additional flooding concerns despite weakening rates and decreasing coverage. Thinking is that the decrease should occur late this evening given the dry air already in place to the west and the amount of dry air pushing westward over Nova Scotia and eastern New Brunswick. ....Central Gulf Coast to the Tennessee Valley... Needed few changes here with the latest high resolution guidance still showing some widely scattered convection from north of the Florida panhandle northward into the Tennessee Valley that grows in coverage. The day-time run of the HREF guidance showed spotty coverage of 10 to 15 percent probabilities in this area overnight that gradually spreads northward with time. Bann Day 2 Valid 12Z Sun Sep 17 2023 - 12Z Mon Sep 18 2023 ....THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN MID-ATLANTIC & THE SOUTHWEST... ....Southern Appalachians & Mid-Atlantic Coastal Plains... An amplifying upper trough traversing the Ohio Valley will foster strong vertical ascent aloft from the central Appalachians to the Mid-Atlantic on Sunday. By Sunday evening, a 110-knot 250mb jet streak will place its divergent right-entrance region over the Carolina Coast. This allows for deepening low pressure to ensue off the coast and favored upscale growth for widespread thunderstorm activity. Plenty of atmospheric moisture will stream over the Mid-Atlantic with 1.5" PWs near the Southern Appalachians and 2-2.25" PWs along the Southeast coast. The greatest available instability will be found along the southern Mid-Atlantic coastal plain where MLCAPE will range between 500-1000 J/kg. CAPE will not be quite as high in the southern Appalachians (generally <500 J/kg), but the strong upslope component can also aid in bolstering excessive rainfall rates. 12Z HREF depicted 3-hr QPF > 3-hr FFGs probabilities up to 20-30% along the Georgia and Carolina coast Sunday afternoon. Storm motions should be fairly progressive, but highly saturated profiles and deep warm cloud layers support efficient rainfall rates, and thus flash flooding is possible in parts of the Marginal Risk area through Sunday evening. ....Southeast Arizona, southern New Mexico, & West Texas... A weak 500mb disturbance at the nose of a 250mb jet streak off the coast of Baja California will provide the necessary lift aloft, combined with PWs up to 1", to spawn scattered showers and storms in parts of the Southwest. Sufficient daytime heating will also lead to MLCAPE values between 500-1,000 J/kg. The subtropical jet is responsible for generating 15 knot westerly steering winds that not only foster upslope enhancement along orographically favored terrain, but also plays a role in sufficient vertical wind shear to sustain thunderstorm activity. Antecedent soil conditions remain overly saturated given portions of the at-risk region has received as much as 1-3" of above normal rainfall over the past 7-days. The combination of heavy thunderstorms atop sensitive soils has led to the issuance of a Marginal Risk given the flash flood potential in areas with saturated soils, along complex terrain, and dry washes. Mullinax Day 3 Valid 12Z Mon Sep 18 2023 - 12Z Tue Sep 19 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS SOUTHERN NEW ENGLAND... The upper trough responsible for Sunday's excessive rainfall threat in the Southern Mid-Atlantic will only continue to deepen as it tracks over the Mid-Atlantic Monday morning with NAEFS showing 500mb heights below the 10th climatological percentile. Strong jet streak dynamics and PVA out ahead of the trough will encourage the surface low along along the East Coast to continue strengthening throughout the day. The heaviest rainfall will occur to the northwest of the low where strong 700mb Q-vector convergence takes shape. There is no shortage of moisture aloft with 1.5" PWs along the southern New England coast. Meanwhile, the 850mb low that forms will direct a conveyor belt of 850mb moisture flux into New England. One factor that is in question that plays a key role in excessive rainfall rates is instability. At this time, meager instability is expected, although maybe 250-500 J/kg of MUCAPE could manifest itself over southern New England Monday afternoon. Still, the region as a whole has dealt with as much as 4-8" above normal rainfall over the past 7 days from western CT to northern MA. In addition, parts of Maine are likely to still have overly saturated soils just 24-48 hour removed from the rainfall associated with Post Tropical Cyclone Lee. Given the sensitive soils in place, hourly rainfall rates of 1-1.5"/hr occurring over the span of just a couple hours could cause flash flooding. Have posted a Marginal Risk given the region's antecedent soil moisture saturation makes it particularly vulnerable to flash flooding. Mullinax $$ --- * SLMR 2.1a * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .