Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 16 2023 15:56:09 FOUS30 KWBC 161556 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1155 AM 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 IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS EASTERN NORTH CAROLINA INTO THE VIRGINIA TIDEWATER AND SOUTHEAST MARYLAND COASTAL PLAIN... ....Mid-Atlantic Coastal Plain... A robust shortwave trough over the Midwest will pivot eastward while taking on a neutral tilt as it crosses through the Ohio Valley. Surface cyclogenesis along the SC coast will initiate Sunday morning, tracking northeast up the Atlantic sea board leading to increasing precipitation coverage on the western side of the surface low as it works within the right-entrance region of a strengthening upper-level jet streak bisecting the Mid-Atlantic. Large scale ascent downstream of the mean trough will also generate scattered to widespread precipitation over the central Mid-Atlantic states Sunday morning and afternoon before shifting eastward, but the primary focus will be along the coastal plain(s) where general instability is highest and PWAT anomalies run upwards of 1 standard deviation above climatological norm. 00z GEFS ensemble probabilities of 1-2" of total rainfall are highest within a corridor from Charleston, SC up through the Maryland coastal plain with the best risk located over eastern NC where 50-60% probabilities are beginning to show up within the ensemble. This correlates well with the axis of best PWATs and instability which is what we want to see in regards to flash flooding concerns. It is possible that a secondary area over eastern WV could be included in the MRGL risk due to convective concerns on Sunday morning between 12-18z as thunderstorms over southern WV move off to the northeast given the mean flow. There's still some discrepancy in coverage and overall intensity during that time frame, but with low FFG's located along and west of the Blue Ridge, could see an expansion out that direction if later deterministic and HREF probabilistic fields support the addition. Otherwise, the Marginal risk remains over the eastern Mid-Atlantic in conjunction with convective development and eventual surface cyclogenesis.=20 Kleebauer Day 3 Valid 12Z Mon Sep 18 2023 - 12Z Tue Sep 19 2023 The probability of rainfall exceeding flash flood guidance is than 5 percent... Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-qzgtOGzRiJu5EE2cF-xCWu-DaimpEUJPkM387WR3-h9= hk6HSBRvThhttS_IONPweD_MUWHewQ41GOSgj6X2BNZ5UJo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-qzgtOGzRiJu5EE2cF-xCWu-DaimpEUJPkM387WR3-h9= hk6HSBRvThhttS_IONPweD_MUWHewQ41GOSgj6X2Mz9gRc8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-qzgtOGzRiJu5EE2cF-xCWu-DaimpEUJPkM387WR3-h9= hk6HSBRvThhttS_IONPweD_MUWHewQ41GOSgj6X2xFfletg$=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) .