Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 16 2023 20:10:44 FOUS30 KWBC 162010 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 410 PM EDT Sat Sep 16 2023 Day 1 Valid 16Z Sat Sep 16 2023 - 12Z Sun Sep 17 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER EASTERN MAINE... 16Z Update... ....Eastern Maine... Lee is now a Post Tropical cyclone and continues to track north towards the Bay of Fundy. Rainfall rates so far this morning have been on the lighter side, but as Lee's circulation inches closer, a convergence of northerly 850mb flow out of the St. Lawrence Valley and robust easterly flow on the northern flank of Lee will prompt the more intense band of moderate to heavy rainfall to track over eastern Maine. There is no denying the impressive moisture associated with Lee in an area not accustomed to tropical moisture. The 12Z sounding out of Yarmouth, NS measured a PW of 2.21", showcasing the dearth of moisture associated with Lee that will make its way into Downeast Maine. Latest NAEFS also showed IVT values (surpassing 1000 kg/m/s) over Downeast Maine by 18Z that will top the 99th climatological percentile. The big limiting factor remains the lack of instability which will result in capping maximum hourly rainfall rates to around 1"/hr. Additional rainfall amounts from Lee across eastern Maine is likely to range between 1-3" by the time Lee departs Sunday morning. Otherwise, the combination of steady, moderate rainfall atop overly saturated soils will be the primary cause for flooding across eastern Maine today. The 06Z HREF had Washington County with 60-80% odds of seeing >3" of rainfall through 12Z Sunday with a sharp drop off in those probabilities towards central Maine. Have trimmed back a little more of the western flank of the Slight and Marginal Risks given the lack of sufficient instability over central Maine, but otherwise, the Slight Risk remains in place over Downeast Maine. ....Central Gulf Coast... The forecast rationale for the other regions mentioned from overnight (Texas and the Florida Atlantic Coast) continue to remain in good shape. The only adjustment was to include more of the central Gulf Coast and western Georgia in the Marginal Risk area that also encompasses the Tennessee Valley. The FL Panhandle has a stationary front nearby that will act as a trigger for storms later today. PWs are forecast to over around 2" and MLCAPE up to 1,000 J/kg is expected, which is more than enough to generate 2-3"/hr rainfall rates. Southern AL and the FL Panhandle have been on the drier side of normal, which should keep the flash flood threat to highly localized spots. The most at-risk locations would be in the more urbanized settings and in poor drainage areas. In western GA, the approaching cold front should direct storms over central Alabama towards "The Peach State" tonight where rainfall rates of 1.5-2.0"/hr will be possible. I also expanded the Marginal Risk to include more of central Tennessee where recently issued MPD #1078 references the potential for flash flooding through midday. Mullinax ---Previous Discussion--- ....Texas... A progressively moving disturbance originating from convective initiation over eastern NM and the TX Panhandle will continue on a southeast trajectory riding along the theta-E tongue situated from central TX up through the Caprock. Heavy rainfall within the core of the developing mesoscale complex will be in play through the disturbance's life cycle with 00z HREF neighborhood probabilities for 1-2"/hr ranging from 30-40% over the Permian Basin to 20-30% over Central TX by later this morning. The forward speed of the complex will limit the extent of flash flooding over west-central TX, but the axis of heaviest QPF within the hi-res deterministic and HREF mean bisect an area of lower FFG's thanks to previous days of precipitation. Local maximum around 3" will be possible where the rainfall is heaviest within the path of the impending MCS with the best location for higher totals likely oriented over the eastern Permian Basin and western rolling plains in both MAF/SJT CWA's.=20 Areas on the southeast side of the risk will have a bit less to do with the complex and more to do with convective development along the frontal boundary draped over south-central TX. There's a non-zero potential for local totals exceeding 3" over portions of the southeast TX coastal plain and areas just inland according to the latest HREF neighborhood probabilities where signals of ~30% for 2-3"/hr rates are forecast during the afternoon hrs when diurnal instability is at its maximum within a core of elevated PWATs running 1.5 standard deviations above normal. Despite higher FFG's located within the confines of Corpus Christi and surrounding locales, rainfall priming from this evening and potential rates add a slighter better than climo opportunity for localized flooding concerns within the synoptic and environmental setup across southeast TX. A Marginal Risk is in place across the areas mentioned above beginning in the Permian Basin down through central TX into the southeast TX coastal plain near CRP.=20=20=20 ....Tennessee Valley... A digging mid-level trough will enhance a difluent axis present over portions of the Tennessee and Ohio Valleys later today into early Sunday morning. Guidance continues to pinpoint locally heavy rainfall across much of Alabama up through eastern TN/KY before spreading into WV by the end of the D1 period. A prominent jet streak will be intensifying over Midwest, spreading eastward over time leading to large scale ascent downstream from the main trough axis. 00z hi-res deterministic and ensemble blend paints a stripe of heavy precipitation within the bounds of 0.75-1.5" with local maximum up to 4" within two zones; central and southern AL and eastern TN/KY mainly located where areal instability will be at its greatest. The first half of the day will be characterized by increased convective coverage due to diurnal destabilization within the zone of increasing difluence on the lead side of the shortwave trough. The second round occurring later tonight into tomorrow morning will originate from increasing large scale ascent and positive vorticity advection as strong mid-level vort max pivots over the Ohio Valley into WV overnight. A deep flux of moisture will stream northward ahead of the mean trough intersecting with the vorticity maxima generating a rapidly developing QPF field moving northeast out of the Tennessee Valley into the southern half of WV. Rainfall rates exceeding 1"/hr are increasingly likely for areas along and south of the I-64 prior to 12z Sunday as per the 00z HREF neighborhood probabilities which would spell a better signature for localized flooding given the low FFG values thanks to terrain influences and a primed ground from a busy stretch of convection within the past week to 10 days. The previous MRGL risk placement remains with only a few adjustments on the northern and western edges of the outlined risk area. ....Florida Atlantic Coast... A stationary front coupled with sea breeze interactions over eastern FL will create a localized flash flood risk later this afternoon extending from Daytona Beach down through Miami. Best chance for flooding will likely be over the coastal areas where urbanization provides lower FFG's compared to the inland locations, so the western extent of the MRGL risk in place will be fairly close to the coastal plain. The best indicator for the potential is the robust signature for enhanced rainfall rates in excess of 2-3"/hr located over the corridor above. In fact, 00z HREF neighborhood probability for 3"/hr or greater are breaching 50% for two areas in particular; Daytona down to Cocoa beach and West Palm to Fort Lauderdale. These two areas lie within a zone of elevated PWATs running 1.5-2 standard deviations above climo norm. Forecast bufr soundings at KPBI/KMLB indicated weak storm motion potential and prolific low-mid level moisture creating a favorable environment for heavy rains within a confined area. The MRGL risk remains in place with a slight northward extension based on HREF blended mean and probabilistic trends. Kleebauer 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 Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9ICjaRldEXMNmrEZg8CMlmdUUbDWpWsnzsjTpB-FuRgw= fzD3L6yT6UrmpkAx0T5QKEj3ep__qRPt_8T4abs1q369Eqs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9ICjaRldEXMNmrEZg8CMlmdUUbDWpWsnzsjTpB-FuRgw= fzD3L6yT6UrmpkAx0T5QKEj3ep__qRPt_8T4abs19NmjkD0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9ICjaRldEXMNmrEZg8CMlmdUUbDWpWsnzsjTpB-FuRgw= fzD3L6yT6UrmpkAx0T5QKEj3ep__qRPt_8T4abs1sC9tUKY$=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) .