Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Sep 24 2023 08:06:56 FOUS30 KWBC 240806 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 406 AM EDT Sun Sep 24 2023 Day 1 Valid 12Z Sun Sep 24 2023 - 12Z Mon Sep 25 2023 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHEAST AND ARKLATEX... ....Northeast... Post-T.C. Ophelia's remnant 850mb circulation will continue to gradually weaken but remain intact as it tracks over the DC/Baltimore metro areas Sunday afternoon and slowly tracks east towards the northern DelMarVa Peninsula Sunday night. There remains no shortage of moisture with PWs of 1.75" likely to be measured from Northern MD and the Lower Susquehanna Valley of PA on east across the Delaware Valley and into southern NJ. These areas in particular reside within a narrow ribbon of easterly 925-850mb theta-e advection with MUCAPE up to 500 J/kg and upslope enhancement into higher terrain (particularly eastern PA and northern NJ). Sampled soundings already showed warm cloud layers up to 13,000ft deep, but given the >90% RH values and lingering buoyancy from both available instability and forced-topographic ascent, hourly rainfall rates could top out as high as 0.75"/hr. The 850mb low will be in no rush to exit off the coast, so a prolonged period of moderate-to-heavy rainfall will occur for much of the day. Given these factors, as well as soils becoming a little more saturated following Saturday's rainfall, have maintained the Slight Risk from the Mason-Dixon line on east to the southern coast of Long Island. There are still likely to be efficient warm rain processes into southern New England, but given the lack of instability north of the coast, have trimmed down the northern extent of the Slight while maintaining most of the inherited Marginal Risk area. ....Southern Plains to the Lower Mississippi Valley... A similar setup that resulted in widespread thunderstorms on Saturday will continue to make its way south into central TX and as far east as the Lower MS Valley. Latest guidance still shows a good signal for Excessive Rainfall rates across the ArkLaTex, but more recent CAMs have put the axis a little farther south into eastern Texas. An 850mb low bordering the Red River will direct a continuous fetch of 850mb moisture flux at a stalled front positioned somewhere near the ArkLaTex, resulting in PWs topping 2" and as much as 2,000-3,000 J/kg of MLCAPE. Similar to Saturday, the steady stream of low level moisture becomes oriented quasi-parallel to the stationary front, providing a suitable setup for potential backbuilding and training storms. Storms will be more than capable of producing 2-3"/hr rainfall rates, and given the potential training of thunderstorms, localized amounts could reach 4-6". This is supported by the 00Z HREF showing 20-25% probabilities of 24-hr QPF > 5". Did increase the footprint of the Slight Risk area to include more of eastern TX and northern LA where recent CAMs showed the potential for storms to inch more to the south. Have held off on a Moderate Risk upgrade given the bulk of the Slight Risk area features higher FFGs compared to their neighbors farther north across eastern OK and southwest MO. Should confidence grow in a swath of 5+" amounts in new guidance over the next 12 hours, there is a chance a Moderate Risk may be needed for parts of the ArkLaTex or east Texas in future forecast cycles. ....Southwest Oregon... The leading edge of one of the upcoming cold season's first atmospheric rivers arrives Sunday afternoon and continues to deliver periods of rain to the Pacific Northwest into Sunday night. A narrow swath of heavy rainfall will be directed at the Oregon coast Sunday night thanks to a robust 1000 kg/m/s IVT that peaks at just over 8 standard deviations above normal for late September. Rainfall totals could range between 1-2" with locally higher amounts closer to 3", but the bulk of this is falling within a 6-12 hour span. Due to these possible rainfall rates, have kept the Marginal Risk that was in place with any nearby burn scars most at-risk for possible flash flooding. Mullinax Day 2 Valid 12Z Mon Sep 25 2023 - 12Z Tue Sep 26 2023 ....THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE PACIFIC NORTHWEST, SOUTHERN PLAINS, LOWER AND UPPER MISSISSIPPI VALLEY... ....Pacific Northwest... A powerful upper low approaching the Pacific Northwest will direct an atmospheric river at the West Coast on Monday, delivering a soaking rain to residents from northern California to western Oregon and Washington. The upper low off the coast is very impressive for late September with heights within the 500-850mb layer 3-5 standard deviations below normal according to NAEFS. This powerful upper trough will direct a robust 500-750 kg/ms/s IVT (peaking around 8 standard deviations above normal) at northern CA and western OR where PWs will be as high as 2-3 standard deviations above normal. This is effectively the first bonafide atmospheric river of the season for the Pacific Northwest. Fortunately, parts of far northern California and southern Oregon could use the rain as parts of the region are in D0-D2 drought. The primary concerns are residual burn scars where soils will be more susceptible to possible debris flows. The Marginal Risk remains in place as rates do not appear to pose a considerable flood threat, but detrimental impacts could be felt in burn scars. ....Southern Plains & Lower Mississippi Valley... A frontal boundary stretching from West Texas to the Ozarks is forecast to weaken and push south throughout the day while at the same time, southerly 850mb moisture flux is expected to wain. These features will still be present at a time when a weak 500mb disturbance approaches from the north, providing some modest vertical ascent atop the atmosphere. PWs are currently forecast to reach as high as 1.75" (1-2 standard deviations above normal according to NAEFS) and MLCAPE between 1,000-2,000 J/kg from central TX to the Lower Mississippi Valley. The available PWs and instability should support >2"/hr rainfall rates within the more intense cells, and soils should be a little more saturated following Sunday's rainfall. The Marginal Risk remains in place with any consideration of a Slight Risk likely to be discussed in future forecast cycles when more CAM guidance becomes available to key in on the more at-risk areas. ....Upper Mississippi Valley... A meandering closed low over the Upper Midwest provides a suitable environment for divergent flow atop the atmospheric column on Monday. PWs will be as high as 1.25" and MLCAPE is likely to range between 250-500 J/kg. A conveyor belt of 850mb moisture flux ahead of an occluded front should help to supply developing storms with enough moisture and lift to generate storms containing rainfall rates >1.5"/hr. This round of storms is also coming on the heels of a 2-day stretch where as much as 2-3" or rainfall is forecast through Sunday night, so soils will be a little more saturated compared to earlier in the week. A Marginal Risk remains in place given that flash flooding has a chance to occur in poor drainage areas, urbanized communities, and where soils are most sensitive. ....Northeast... While it still looks to be a wet day along the southern New England coast, there will be little in the way of instability to work with. Held off on introducing a Marginal Risk for the time being, but should QPF increase and the potential for heavier rainfall rates arise, a Marginal Risk may be needed in future forecast cycles. Mullinax Day 3 Valid 12Z Tue Sep 26 2023 - 12Z Wed Sep 27 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF FLORIDA... ....Florida... There is a large reservoir of 2-2.25" PWs across the Sunshine State with a stationary front draped over the region. At upper levels, a large trough over the Gulf of Mexico is providing some sufficient divergence aloft while a large dome of high pressure in the Caribbean will direct a upper level disturbance to approach Florida from the South. NAEFS shows these PWs a little over 1 standard deviation above normal and MLCAPE values could reach up to 2,000 J/kg. These parameters should be able to support hourly rainfall rates up to 3"/hr in the most intense storms. The other factor worth noting is vertical wind shear, winds withing the 1000-850mb level will be southeasterly, but veer and strengthen with height up to 200mb, allowing for surface-6km shear values to range between 20-30 knots. This could help keep some storms around a little longer than the usual summer-time pulse storms. Given these factors, chose to introduce a Marginal Risk for Florida. This area will likely be tweaked depending upon QPF trends in guidance over the next couple of days. Mullinax Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!75hTPAIUpWvWgGOH_WUMbGAhqCShbb2hYhuipYltRfYJ= XEy779SwDZxYuf_wG6WrM9Hvfq1LBnvCv06Ibw7mDKwDm00$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!75hTPAIUpWvWgGOH_WUMbGAhqCShbb2hYhuipYltRfYJ= XEy779SwDZxYuf_wG6WrM9Hvfq1LBnvCv06Ibw7m-tpHNeM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!75hTPAIUpWvWgGOH_WUMbGAhqCShbb2hYhuipYltRfYJ= XEy779SwDZxYuf_wG6WrM9Hvfq1LBnvCv06Ibw7mTAECA6U$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .