Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 25 2023 00:58:15 FOUS30 KWBC 250058 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 857 PM EDT Sun Sep 24 2023 Day 1 Valid 01Z Mon Sep 25 2023 - 12Z Mon Sep 25 2023 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHEAST AND ARKLATEX... 01Z Update: For the Northeast, the continued north/northeast motion of the remnant circulation of Ophelia will narrow the rain focus from near Trenton, NJ into Southern New England overnight. Recent CAM runs (HRRR and 18Z NAM nest) have shown a small area of >3" totals through 12Z over northern NJ/NEPA/Catskills where FFG is lower due to rainfall the past two days. Area remains sensitive to heavier rates though they have mostly failed to materialize. Nevertheless, will retain the Slight Risk through 12Z focused around NYC. Removed the northern half of the elongated Marginal Risk over the central CONUS with lower rates and any additional rainfall mostly over areas of higher FFG values. Remain focused on the Arklatex where late afternoon/early evening convection has fired and will be ongoing tonight. Slight extension back into the Hill Country per current obs and some guidance. Fracasso 16z Update: The inherited risk areas were generally in good shape, so only made some minor adjustments based on the latest observational/model trends. Over the Northeast the 12z HREF guidance continues to support areas of heavy rainfall today into tonight. Neighborhood probabilities of exceeding 3" are now over 70%, with even some low end 5" probabilities. Instability remains a limiting factor for deep convection and intense rainfall rates. However strong low level convergence and very weak instability should help drive some shallow convective elements embedded within the stratiform rain shield. The shallow nature of activity will mean its mainly warm cloud processes in play, and thus efficient rainfall production is probable. So hourly rainfall around 0.5"/hr on a localized basis seems reasonable (and is supported by the 12z HREF)...but anything 1"/hr or higher seems much less likely. Overall, generally thinking most areas will see between 1 and 3 inches of rain...but localized areas of 3-5" seem probable. Where these higher totals fall some flash flood/flood risk seems possible today into tonight, mainly from northeast PA into northern NJ, southeast NY and portions of CT...especially if there is overlap with any more sensitive urban areas. Over the Southern Plains into the Lower MS valley the Slight risk still look good, with another round of convection developing later today into tonight. There may be some overlap with areas that saw heavy rainfall over the past 24 hours...but the heaviest of the additional rainfall should end up southwest of the hardest hit areas. Convection later today and tonight should train/backbuild to some extent...but should also exhibit a progression off to the south and east as well. Thus while at least isolated flash flooding appears likely into tonight...think the coverage/organization of this risk remains below MDT risk thresholds. We'll continue to monitor, but for now a Slight to higher end Slight risk seems to best portray the threat. Chenard ....Previous Discussion... ....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 IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEAST MINNESOTA AND SOUTHWEST WISCONSIN... 20z Update: We did go ahead and introduce a relatively small Slight risk near the southern MN and WI border. Convection is expected along this axis by Monday afternoon into the evening hours with areas of heavy rainfall likely. The axis of convergence and instability is pretty narrow, with the expectation that storms which develop will tend to remain tied close to this axis of enhanced convergence and instability...resulting in some training potential. The 12z HREF neighborhood probabilities of exceeding 3" are upwards of 80%, with some low end 5" probabilities as well. Thus it seems probable that some areas within this narrow corridor will exceed 3" of rainfall. HREF probabilities of exceeding 3hr FFG have also increased to around 30% across this region. While still in a drought, recent rainfall has at least begun to moisten the top soil levels across the corridor...thus the isolated/scattered FFG exceedance seems reasonable. Currently not expecting any high end impacts, but a few areas of flash flooding seem possible, especially if any more sensitive basins/urban areas are impacted. Overall there is pretty good agreement in the axis amongst the HREF members and global models...adding some confidence to the introduction of a narrow Slight risk. Over the Northeast we did add a small Marginal risk from northeast NJ, into southeast NY, southern CT and Long Island. This is a continuation of the locally heavy rainfall threat that will be possible Sunday night. Expect showers, with some embedded shallow convective cells, to be ongoing Monday morning, and a localized flood risk could thus continue through the morning hours. The Marginal risk from TX into MS was shifted a bit south and east to follow the quicker trend seen in the HREF and global guidance. Given the dry antecedent conditions and high FFG over much of this area, this risk is primarily a localized urban flash flood threat driven by high rainfall rates. Chenard ....Previous Discussion... ....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... 20z Update: Only minor changes to the inherited Marginal risk, which overall remains in good shape. Chenard ....Previous Discussion... ....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!41nICe2qmQQ-DN317mil3hZOH4Iv6jGlWgHjje4rMTJJ= zyhgARlY1hwPwchb2ytkGNWsMNpQNMu1jpLn3OBga926R8c$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!41nICe2qmQQ-DN317mil3hZOH4Iv6jGlWgHjje4rMTJJ= zyhgARlY1hwPwchb2ytkGNWsMNpQNMu1jpLn3OBgRKE6SZw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!41nICe2qmQQ-DN317mil3hZOH4Iv6jGlWgHjje4rMTJJ= zyhgARlY1hwPwchb2ytkGNWsMNpQNMu1jpLn3OBgRG4Bl0k$=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) .