Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 18 2023 20:15:04 FOUS30 KWBC 182014 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 414 PM EDT Mon Sep 18 2023 Day 1 Valid 16Z Mon Sep 18 2023 - 12Z Tue Sep 19 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN EFFECT ACROSS PORTIONS OF NEW ENGLAND... 16Z Update... ....New England... The forecast reasoning for the Slight Risk that was provided from overnight shift remains on track. The main adjustment this forecast update was to expand the Slight Risk farther south into southern New England. Latest FLASH soil saturation shows parts of western CT, northern RI, and eastern MA sporting 75-95% soil saturation thanks to 300-600% of normal rainfall over the past 7-days. The 12Z OKX sounding seemed fairly representative of what the environment over southern New England will look like this afternoon which include a 1.6" PW, 100-250 J/kg of MUCAPE, and a warm cloud layer depth close to 10,000'. The limited instability should cap rainfall rates north of I-95 to below 1"/hr, but communities along and south of I-95 could see storms exceed 1"/hr within the more intense cells. This is supported by the 12Z HREF which shows 30-40% probabilities over southeast MA. Regardless, the 12Z OKX sounding still shows periods of rain with embedded thunderstorms will likely be efficient rainfall producers, and combined with the sensitive soils in place, this update opted to extend the Slight Risk farther south to include more of southern New England. ....Southern Florida Atlantic Coast... The Marginal Risk in southeast Florida also remains on track with little change to the reasoning behind the need for a Marginal Risk. The 12Z MFL sounding measured a PW of 1.93", roughly 2,000 J/kg of MLCAPE, and a 14,000ft deep warm cloud layer aloft. The urbanized metro areas are most at-risk for flash flooding, or in areas impacted by storms producing rainfall rates close to 3"/hr. The newest 12Z HREF does contain up to 50-60% probabilities for 1-hr QPF > 3", so most CAMs are on board with the potential for torrential downpours within the strongest thunderstorms. The 12Z HREF also depicts 50-70% probabilities for 3-hr QPF > 3-hr FFGs not far from the Miami metro area this afternoon. Mullinax ---Previous Discussion--- ....New England... Surface low across the Carolinas will continue to trek northeast, intensifying under the influence of the right entrance region of a strengthening upper level jet max situated over the Northeast. Upper trough to the west will pivot eastward taking on a slightly negative tilt leading to the surface low being pulled a bit closer to the coast than previously forecast. Increasing meridional flow ahead of the mean trough will advect tropical moisture poleward leading to PWATs running 1-2 standard deviations above normal across much of New England. As the surface reflection reaches the latitude of Long Island and points north, this will be when the low will peak in intensity with a strong east to southeast fetch off the Atlantic. U-vector wind field on both the NAM/GFS deterministic show a anomalous tongue of easterlies across southern New England up through ME and NH with an upslope signature on the eastern slopes of the Whites and Berkshires. 00z HREF blended mean and ensemble bias-corrected QPF indicate a robust area of precip located within the confines of central and northern New England due to both the synoptic scale forcing present as well as the mesoscale components interacting with the terrain across the interior. A general 1.5-2.5" of rainfall is forecast over southeast MA up through northern New England, including areas that are already saturated from Lee. QPF maximum of over 4" are now popping up within the upper quartile of outcomes within the latest hi-res ensemble package. The primary focus is within the terrain and central, Downeast ME where FFG's are already very low after the 2-5" of rainfall from Lee, as well as the priming from the lead wave ahead of the surface low that occurs overnight. In a probabilistic realm, the 00z guidance has also ramped up from previous runs with 3 hr rainfall of greater than 1" and 2" now exceeding 50% and 25% respectively over the targeted area which would create a heightened flash flood risk given the antecedent conditions. Despite the lack of enhanced instability, enough of a signature for ~500 J/kg of MUCAPE over southern MA/ME and Downeast ME would introduce some convective enhancement in banding that occurs within the WCB and deformation axis on the northwest flank of the surface low. Given all of the above and with collaboration agreement from both Gray and Caribou, have introduced a SLGT risk over a large portion of ME and the eastern side of NH with emphasis on the east slopes of the Whites. Kleebauer=20 ....Southern Florida Atlantic Coast... A cold front approaching South Florida will be a catalyst in sparking strong thunderstorms this afternoon. Area averaged soundings within the Marginal Risk show PWs between 1.9-2.2", MLCAPE up to 2,000 J/kg, and falling 500mb heights as a broad upper trough approaches from the northwest. Storms should form through both surface based heating and along the sea breeze front propagating inland, but they could also fire along the cold front to the north as well. This makes for a busy afternoon in South Florida, especially along the more urbanized area of southeast Florida. The 00z HREF shows 50-70% probabilities for 1-hr QPF > 2" within the Marginal Risk, as well as 40-50% probabilities for 3-hr QPF > 3". With such a high concentration of impermeable surfaces along the southeast coast of FL, there is the potential for flash flooding in poor drainage areas. Storms should dissipate by Monday evening with the gradual loss of daytime heating. Mullinax/Kleebauer Day 2 Valid 12Z Tue Sep 19 2023 - 12Z Wed Sep 20 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS OKLAHOMA... 2030Z Update... ....Oklahoma... The synoptic and mesoscale breakdown is broken down well from overnight shift. Primary adjustment today was to extend the Slight Risk farther north into southeast Kansas. The nose of the LLJ (NAEFS shows 850mb winds over north TX/central OK above the 90th climatological percentile) will be placed over southeast Kansas with strong 850mb theta-e advection into the lower Missouri River Valley. The new 12Z HREF does show probabilities up to 20-30% for 24-hr QPF > 5" for Tuesday, the bulk of which comes Thursday evening and into the overnight hours. These factors went into why the Slight Risk was expanded farther norther into southeast Kansas. ....South Florida... Another day with a stalled front nearby, another day for thunderstorms capable of producing excessive rainfall rates of 2-3"/hr. Area averaged guidance soundings over South Florida showed MLCAPE of 1,500 J/kg available, as well as PWs near 2" and averaged low-mid level RH values near 80%. Compared to the Day 1 Outlook, parts of southwest FL are also at risk for torrential downpours. This is also following what will be a busy Monday when it comes to storm coverage, so some soils could be a little more saturated and more prone to flash flooding. In collaboration with MFL, did include much of the CWA under the Marginal Risk. Slightly farther up the coast into MLB's southern zones, the 12Z HREF showed up to 40-60% probabilities for 24-hr QPF > 5", which is a good signal for storms likely containing excessive rainfall rates over a densely populated area. For these reasons, introduced a Marginal Risk for most of South Florida. Mullinax ---Previous Discussion--- ....Oklahoma... Surface pattern will shift abruptly with a deep moisture flux over the southern plains stemming from strong southerly flow brought on by a pressure gradient between low pressure development over West TX and surface ridging east of the Mississippi. A textbook return flow pattern will advect Gulf moisture through the plains with global standardized PWAT anomalies reaching 1-1.5 standard deviations above normal by later Tuesday afternoon. Theta-E ridge positioned from east TX up through central and eastern Oklahoma will be the primary focus for where convective concerns will arise later Tuesday afternoon and evening as a solid instability axis over the above area will work in tandem with a shortwave approaching from the west along the northern periphery of an upper ridge centered over Chihuahua. As the shortwave approaches, large scale ascent will increase and convection will fire along the western edge of the theta-E gradient in place over northwest OK and southern KS. Bulk shear around 35-40 kts present will be enough to maintain decent structures for any storms that do develop leading to multi-cell and discrete supercell modes within the period of initiation. Nocturnal LLJ will ramp up after 00z with storms over OK taking on more upscale growth and propagating through central and eastern OK by nightfall. Forecast bufr soundings over central OK paint an ominous picture of the overall environment, especially the wind field and propagation points. Hodographs from point soundings near and north of OKC show a potential for back-building convective field based on the proposed veering winds within the mean flow, and as SPC stated on their latest risk update, the ability for storms to back-build from outflow boundaries left over from earlier convection could lead to training elements that would exacerbate flooding potential. Ensemble bias-corrected QPF and all global deterministic pin a large area of 1.5-2.5" of rainfall over central OK up towards the KS border. CIPS analogs were also pinpointing this area for heavy rainfall potential with 3 out of the top 5 analogs for Tue-Wed producing widespread 2-3+" totals in their respective evolution(s). With such a robust synoptic and mesoscale signature backed by prior analog based data, felt it was worthy to upgrade central and eastern OK to a Slight Risk with agreement from the Norman WFO.=20=20=20=20 Kleebauer Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-N_bT1IKsa_XxXAz0n_8so4iVXYy2A5UR0m6CDFHIhkA= 76-0_VPirhMrSb7s9SFF0GgsI5taU3_zHAXNpAfRiny-ebY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-N_bT1IKsa_XxXAz0n_8so4iVXYy2A5UR0m6CDFHIhkA= 76-0_VPirhMrSb7s9SFF0GgsI5taU3_zHAXNpAfRPLj_WlQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-N_bT1IKsa_XxXAz0n_8so4iVXYy2A5UR0m6CDFHIhkA= 76-0_VPirhMrSb7s9SFF0GgsI5taU3_zHAXNpAfRHHM3k68$=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) .