Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 18 2023 15:52:58 FOUS30 KWBC 181552 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1152 AM 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... ....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 initation. 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 potentail. 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 Valid 12Z Wed Sep 20 2023 - 12Z Thu Sep 21 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN PLAINS, MISSISSIPPI VALLEY, AND NORTHERN MIDWEST... ....Southern Plains/Mississippi Valley... Guidance is in fairly good agreement a remnant MCV originating from the previous evening convection will lead to more convective enhancement across the across the mid-Mississippi Valley Wednesday afternoon and evening. The shortwave that will lead to the flooding concerns over OK the prior period is hinting at closing off or sharpening while entering the Ozarks area of MO/AR as seen on the forecast 500mb vorticity reflections. This would produce large scale ascent near and under the parent MCV which will occur within a zone of prime instability on the western flank of a surface ridge over the eastern CONUS. The question becomes where the MCV goes after crossing into the Mississippi Valley which at this lead is tough to discern given the propensity for convective complexes to shift courses at the last minute based on changes in the downstream environment away from the convective impact. As of now, ensemble mean 500mb forecast shows the best chance for flash flooding concerns from convective initiation will be over eastern OK into southwest MO where guidance has the closed mid-level reflection, leading to the center of the MRGL risk located within those bounds and extended on the northern and southern periphery due to other guidance showing convective risks away from the primary area of interest. A stalled front over the Red River into the Lower Mississippi Valley will also be a trigger for diurnal convective risks within a very moist, unstable environment. FFG's are lower across much of the outlined MRGL risk area, but hints of heavier rate potential could pose from flash flooding concerns, especially in small towns and urban corridors. ....Northern Midwest... Prime theta-E advection will occur over the northern Midwest by Wednesday with a stark change in the thermodynamic environment expected by later in the afternoon. A surface low over central Canada will pivot northeast with a trailing cold front dropping into North Dakota by Wednesday afternoon. The surface low will occlude and drift north with the boundary likely stalling over the northern Midwest, bisecting portions of the Dakotas through northern MN. A digging upper trough over the Northern Rockies will pivot eastward with increasing difluent pattern shaping up over the northern plains allowing for large scale ascent to be aligned with the stalled frontal boundary above. There will be a sharp delineation point in the thermodynamic environment near the stalled boundary which should provide some weak frontogenic forcing to ensue during the diurnal instability maximum. Global deterministic is keying on an area of convection within the stalled boundary, well defined when looking at theta-E progs and surface frontogenesis fields. MLCAPE around 1000 J/kg should be sufficient for storms to develop and impact areas across MN and the Dakotas. Despite higher FFG's, PWAT anomalies approaching 1.5-2 standard deviations above normal would present an environment capable for locally heavy rainfall and flash flood concerns within small town and urban corridors where local FFG's are naturally lower. The positioning of the MRGL could shift or be removed pending the synoptic evolution of the surface low to the north, or the positioning of the stalled frontal boundary Wednesday evening. In any case, elected to maintain the previous MRGL risk and adjust the outline based on ensemble QPF and overlayed to encompass the forecast stationary front. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9jWDYlfYEfC0y76m2Lnjafp1SqbuPCleqcPv4Su1ny6P= TQTouE29Orl1XN9ulKrwa65Jqru3Aj_mCgSbrQUGfxchh3A$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9jWDYlfYEfC0y76m2Lnjafp1SqbuPCleqcPv4Su1ny6P= TQTouE29Orl1XN9ulKrwa65Jqru3Aj_mCgSbrQUG1qIx_mE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9jWDYlfYEfC0y76m2Lnjafp1SqbuPCleqcPv4Su1ny6P= TQTouE29Orl1XN9ulKrwa65Jqru3Aj_mCgSbrQUGPyy6d30$=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) .