Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Sep 17 2023 20:10:38 FOUS30 KWBC 172010 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 409 PM EDT Sun Sep 17 2023 Day 1 Valid 16Z Sun Sep 17 2023 - 12Z Mon Sep 18 2023 ....THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL IN EFFECT ACROSS PORTIONS OF THE MID-ATLANTIC, THE SOUTHWEST, AND NORTHEAST IL... ....Southern Appalachians & Mid-Atlantic Coastal Plain... An amplifying upper trough traversing the Ohio Valley is fostering 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 provides a favorable environment for deepening low pressure 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 up to 1000 J/kg is expected. The 12Z JAX sounding, which will be more indicative of the expected environment along the GA and SC coast, already had 2" PWs, 1,000 J/kg of MLCAPE, and a warm cloud layer around 14,000ft deep. MLCAPE will not be quite as high in the northern Mid-Atlantic (<500 J/kg) which should cap maximum hourly rainfall rates at 1.5". Rainfall should be by and large more beneficial than problematic from the Shenandoah Valley and Blue Ridge of western VA on north into the southern and eastern PA. Still, given the 12Z IAD sounding had a warm cloud layer close to 10,000ft aloft, some showers could be efficient rainfall producers and nuisance ponding in poor drainage areas cannot be ruled out. A little farther south, however, the strong upslope component may aid in bolstering excessive rainfall rates in southwest VA. Latest 12Z HREF continues to hone in on SC with the best odds for flash flooding today, highlighted by 1-hr QPF > 2" probabilities reaching as high as 30-40% along the southern SC coast. Parts of eastern SC will see heavy storms form north of the warm front around midday, followed by another round of thunderstorms approaching from the west this afternoon. Storms could produce >2"/hr rates along the coast of NC tonight, but the storms progressive movement and sandy soils should handle those rainfall amounts a little better to limit the flash flood potential. The cold front may lead to strong storms over the NC Piedmont and on north and east into southeast VA tonight. The Marginal Risk remains in place, but opted to scale back its northern extent given the lack of available instability from northern VA to the Delaware Valley. ....Southeast Arizona, southern New Mexico, & Far 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. The moisture aloft is also being supplied by southeasterly flow originating out of the East Pacific, where it is injecting fresh 700mb moisture flux into eastern AZ and western NM. Sufficient daytime heating will result in MLCAPE rising up to 500 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 sensitives 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. ....Northeast Illinois into Northwest Indiana... The Marginal Risk remains in place due to the nearby surface low just south of Lake Michigan still producing locally excessive rainfall rates, particuarly south of Chicago where considerable flash flooding is ongoing. RAP mesoanalysis showed 250-500 J/kg of MUCAPE and 1" PWs still available for the deformation axis to utilize and continue producing excessive rainfall rates for a few more hours. WPC's Metwatch desk recently issued an MPD (#1080) for the region, so please read the MPD for more information. Storms should gradually dissipate this afternoon and the Marginal Risk in place will likely be dropped by the 01Z update, if not earlier. Mullinax Day 2 Valid 12Z Mon Sep 18 2023 - 12Z Tue Sep 19 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NEW ENGLAND... ....New England... A deepening upper level trough over the eastern U.S. is setting the stage for a strengthening low pressure system along the Northeast coast on Monday. NAEFS shows there are several unusually strong variables at play that include: 500mb heights that are 2-3 SDs below normal in the Mid-Atlantic, PWs near the 90th percentile along the southern New England coast, and an IVT topping the 90th percentile as far north as the Maine coastline. As the the 250-500mb trough begins to take on a sharper and more neutral tilt, exceptional divergence atop the atmosphere will result in a deepening 850mb low tracking northeast along the coast. 850mb moisture flux will wrap around the low, courtesy of the aforementioned IVT, and help to create a robust deformation zone of moderate-to-heavy rainfall. The southern New England coast has the best odds for hourly rainfall rates approaching 1"/hr with MUCAPE as high as 250-500 J/kg, highlighted by the 12Z HREF showing 40-50% chances for 1-hr QPF > 1" over southeast MA Monday afternoon. Farther inland, instability will be hard to come by, but much of the Northeast (especially ME on south through southern NH and into northern MA) have received as much as 2-4" rainfall above normal over the last 7 days. While hourly rates will struggle to reach 1"/hr within the deformation axis, the region's overly saturated soils make it susceptible to flash flooding. The Marginal Risk remains in place as of this forecast update. ....Southern Florida Atlantic Coast... A cold front approaching South Florida will be a catalyst in sparking strong thunderstorms Monday afternoon. Area averaged soundings within the Marginal Risk show PWs between 1.9-2.1", MLCAPE up to 1,000 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 12Z HREF shows 40-60% probabilities for 1-hr QPF > 2" within the Marginal Risk, as well as 20-30% probabilities for 3-hr QPF > 3-hr FFGs. 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 Monday afternoon. Storms should dissipate by Monday evening with the gradual loss of daytime heating. Mullinax Day 3 Valid 12Z Tue Sep 19 2023 - 12Z Wed Sep 20 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS... ....Central/Southern Plains... 2030Z Update... The forecast from the previous forecast discussion remains on track. Only adjustments to the inherited ERO were to expand the Marginal Risk farther south into North Texas. NAEFS shows a brisk low level jet at 850mb with wind speeds at/above the 90th climatological percentile that will supply rich 850mb moisture flux at the entrance region of the LLJ. Strong storms may form as far south as the Dallas/Fort Worth metro area Tuesday night. There was some consideration to a separate Marginal Risk in the Central Rockies, but there was not enough confidence in whether there would be sufficient moisture and instability at this time. Mullinax ---Previous Discussion--- ....Central/Southern Plains... A sharp increase in moisture will occur across the central and southern plains thanks to Gulf moisture funneling northward on the western periphery of a surface ridge over the east and surface low over the west. LLJ development over the plains will aid in significant low-level moisture transport as indicated by both standardized PWAT anomaly forecast, as well as forecast theta-E indices located over the areas of interest. Aloft, a progressive shortwave will traverse the northern periphery of a ridge in place over Mexico extending into the southern-central Rockies. The timing of the shortwave will place the best ascent from the disturbance within the best diurnal instability maximum over OK/KS on Tuesday afternoon and evening creating a threat for widespread convective coverage, including multi-cell generation. Elevated moisture field in place will create an environment favorable for heavy rainfall within any thunderstorm, especially congealing storm fields where global and ensemble outputs have targeted the area from Topeka down to the Red River for heavy rain concerns stemming from organized convection. A fairly robust signal for >1" of precipitation is being forecast by both the GEFS/EPS with probabilities ~20-25% for both at extended range. Bias-corrected ensemble forecast has a bullseye of over 2" of precipitation between OKC and Wichita which is a great indication of model guidance keying on the area mentioned above. A Marginal Risk has been continued from previous forecast, but it would not surprise if this period is escalated to a higher risk category as we gather more data and trends from hi-res ensemble guidance, as well as deterministic CAMs.=20 Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5vR6NbXJz4vh8X6yEKsZvTdwz8oezY9oaaHXIBemsL0Q= sg4VMIlYgMZkzZo_wFYCr1piFhuWy1Y7fo_Hksb1y63FuZY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5vR6NbXJz4vh8X6yEKsZvTdwz8oezY9oaaHXIBemsL0Q= sg4VMIlYgMZkzZo_wFYCr1piFhuWy1Y7fo_Hksb1HyTOEas$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5vR6NbXJz4vh8X6yEKsZvTdwz8oezY9oaaHXIBemsL0Q= sg4VMIlYgMZkzZo_wFYCr1piFhuWy1Y7fo_Hksb1myk2ZWQ$=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. 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