Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Aug 13 2023 08:41:47 FOUS30 KWBC 130841 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 441 AM EDT Sun Aug 13 2023 Day 1 Valid 12Z Sun Aug 13 2023 - 12Z Mon Aug 14 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OZARKS OF SOUTHERN MISSOURI... ....Mid-Mississippi Valley and the Ozarks... A 30 to 40 kt LLJ will advect abundant Gulf moisture northeastward across portions of the Plains, Ozarks, and into the Lower Ohio Valley today into tonight. An ongoing MCS moving across OK and KS this morning will continue slowly moving eastward, and will likely produce scattered showers and more isolated thunderstorms across MO through this afternoon. Any rainfall this morning will help prime the soils for the main event for the area expected tonight. A front pushing southward across the Plains today into tonight behind a developing low over the northern Plains and upper Midwest will contribute additional forcing to the atmosphere as it runs into the anomalously moist air mass being advected out of the Gulf. The clash of the front and associated moisture will occur along the KS/OK border and extend eastward across southern MO and into the lower Ohio Valley. As the moisture hits both the front and the topography of the Ozarks, expect numerous strong showers and thunderstorms to develop and track eastward along the front. With the moisture being resupplied along the front to the west, training thunderstorms are likely all along the front, but the greatest likelihood of training showers and storms will be across southern MO, where the Ozarks may allow a topographic component to the lift. Further, the developing low to the north will support upper level frontogenesis, resulting in a strengthening southwesterly jet streak. Southern MO will be in the lift-favored right entrance region of the jet, which will allow the convection to persist longer over the area. Southwestern MO eastward through western KY has been very wet over the past several weeks, getting 200-400% of their normal rainfall based on AHPS output. Thus, the area is still considered sensitive to flooding from heavy rainfall. With the abundant moisture available, thunderstorms will be capable of producing 2 to 3 inch per hour rainfall rates, easily exceeding FFGs of 1.5 to 2 inches per hour across southern MO. In coordination with SGF/Springfield, MO forecast office, a Moderate Risk upgrade was introduced with this update for much of southwestern and south-central MO. Collaboration was also conducted with LSX/St. Louis, MO and PAH/Paducah, KY forecast offices, which were kept in a Slight risk for this forecast update due to less certainty as to how well the strongest convection holds up as it moves east into the mid-Mississippi Valley. CAMs guidance suggests the convection should reach the MS River, so an expansion of the Moderate Risk eastward is probable. There is also some inherent uncertainty as to where the heaviest convection sets up, with some guidance suggesting it occurs as far north as St. Louis, to as far southwest as Tulsa and northern AR. Thus, some north-south shifting of the risk areas are also possible, though the guidance has been reasonably consistent highlighting the current area over the past few nights. 00Z HREF 40 km neighborhood probabilities are up to 80% for 3 inches of rain in south-central MO and a 10-15% chance of 8 inches of rain in southwest MO, with a 10-15% chance of having a 100 year ARI event in the same area. These heightened probabilities for heavy rainfall totals contributed to the confidence to upgrade to a Moderate Risk with this update. Elsewhere, the surrounding Slight Risk area was adjusted a bit to account for the latest guidance trends, including expanding the Slight northwest through Kansas City and St. Joseph, and westward along the KS/OK border. For northwestern MO, the added convection will be residual moisture from the storms further south being forced by the developing low over the Upper Midwest, while the Slight along the KS/OK border covers likely widely scattered storms that will congeal into the MCS and flooding rain event further east. ....Upper Midwest... As mentioned above, a developing surface low pushing east across NE/IA will advect moisture northwestward ahead of the low center into the northern Plains and upper Midwest. The comma-head region of the circulation north of the low center will feature a prolonged period of light to moderate rain, with embedded convective elements drawn north as residual moisture and instability from the much stronger storms likely to be ongoing Sunday night further south. While a widespread 1 to 2 inch rainfall event is expected, with added forcing from a strengthening and digging shortwave trough, the combination of separation from the primary moisture source and instability source, and antecedent very dry conditions over MN/WI should greatly limit flash flooding potential, despite the expected widespread 1-2 inch rainfall amounts. The Marginal Risk over this area was maintained, with the greatest flash flooding potential over urbanized areas such as the Twin Cities and any other poor drainage areas. ....Maine... A negatively tilted trough tracking north of the Canadian border will support a renewed round of rather fast-moving but still potentially strong storms across the Pine Tree State this afternoon and evening. This will follow the convection currently ongoing over the area. While rainfall rates of 1 to 2 inches per hour will be possible, this is likely to only exceed FFGs in isolated instances, so the Marginal Risk inherited over the area was maintained. ....Central Sierra Nevada... No changes were made to the inherited Marginal Risk area. Weak signals for showers and storms continue with the latest set of guidance. Any storms that form over the slopes of the Sierras won't take much to develop into localized flash flooding due to the steep topography of the area. There is considerable uncertainty as to how widespread that convection will be, so the Marginal Risk for isolated flash flooding appears reasonable. Wegman Otto/Wegman Day 2 Valid 12Z Mon Aug 14 2023 - 12Z Tue Aug 15 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE GREAT LAKES, AS WELL AS FOR THE CENTRAL AND SOUTHERN APPALACHIANS THROUGH THE NEW YORK CITY METRO AND LONG ISLAND... ....Great Lakes Region... The low that develops over the northern Plains and upper Midwest Day 1/Sun will continue slowly tracking eastward as the vigorous upper level shortwave supporting the low gradually weakens through Monday night. The low will siphon off a bit of the abundant Gulf moisture south of it and track it ahead of and north of the low center as it tracks east. The area of heaviest rain from southern WI and the Chicagoland area east across Lake Michigan and much of the southern L.P. of MI is largely from wraparound rain that will persist to the north of the low center. Instability is likely to be limited in this area, with the flash flooding threat largely resulting from the long duration of the light to moderate rainfall, which is likely to persist in some areas for more than 12 hours. The above isn't to say that there will be no instability, as in fact some convective elements are likely to be present as the low advects some instability as well as moisture from the south into the region. However, extreme rainfall rates are not expected. The areas most likely to see flash flooding are the urban centers, which include Milwaukee, Chicago, and Detroit. ....Appalachians to the NYC Metro Region... An MCS tracking up the TN and OH Valleys will move into the Appalachians around midday, with additional lines of convection resulting from daytime heating likely to develop behind the main MCS and initial line of storms through the central and southern Appalachians into Monday night. Fast WSW flow should help individual convective elements to clear the TN and OH Valleys quickly, as the main MCS tracks into the Mid-Atlantic late Monday afternoon and evening. It's likely there will be an upslope component as the 40 kt LLJ runs into the Appalachians, so there's a notable difference in rainfall totals on the windward/west side of the mountains across eastern TN/far southwestern VA and much of WV as compared with areas just east of the mountains, including most of NC and VA, where relatively little rainfall is expected. As the terrain will support more rapid-onset flash flooding, the Slight Risk was maintained from the southern Appalachians of eastern TN northward through WV and southwest PA. The Slight Risk area was expanded westward across east-central OH due to much more favorable antecedent conditions caused by the widespread flash flooding that occurred yesterday. Despite less upslope forcing, the low tracking across the area will enhance atmospheric forcing over that area over to southwest PA/Pittsburgh area.=20 The Slight Risk area was nudged northward largely out of central/northeast MD and southern NJ, but now including more of central and northeastern PA, northern NJ, and the NYC Metro region. This adjustment was largely based on updated guidance that shows a weak warm front slowly pushing northward, but with abundant low-level moisture featuring PWATs exceeding 2.25 inches running into the front. The warm front being weak will ultimately play a big role in keeping impacts down across the area, as storms will likely form further west where there's better forcing, then race eastward along the front and outrun the forcing, resulting in a weakening trend with eastward progress. 06Z HRRR guidance shows the OH Valley MCS that begins the day near Louisville, KY maintaining strength through central PA Monday evening, then strengthening overnight as it moves into NY and New England. Expect strong storms capable of heavy rainfall rates to develop and race eastward across the Tri-State area around or after midnight. The storms will be quite capable of rates above 2 inches per hour, but their fast movement will limit overall rainfall totals. Nonetheless these rates should easily exceed the 1 to 2 inch per hour FFGs, resulting in widely scattered instances of flash flooding. There remains considerable uncertainty as to where these heaviest rainfall amounts will be seen, given changing guidance as to where the warm front guiding the storms will be set up, but the likelihood this will include the NYC metro has increased enough to warrant the Slight Risk upgrade for the area. ....Four Corners south and east... Mid-level ridging over the Rockies over to Texas will continue to direct weak upper level disturbances northward along the western periphery of the ridge along the AZ/NM border and western NM Monday afternoon. Strong easterly flow will advect somewhat anomalous moisture into the region. In coordination with TWC/Tucson, AZ; FGZ/Flagstaff, AZ; and ABQ/Albuquerque, NM forecast offices, the Marginal was maintained with this update. Generally, think storm motion will offset rainfall rates to an extent, which will diminish flash flooding potential. The areas of greatest risk for flash flooding and highest rainfall totals including the far eastern Mogollon Rim of AZ and into west-central NM. The signal has backed off with this set of guidance across the El Paso metro, so the Marginal Risk area remains for the Sacramento Mtns north into southwestern CO. Wegman Day 3 Valid 12Z Tue Aug 15 2023 - 12Z Wed Aug 16 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE HAMPTON ROADS AREA OF SOUTHEAST VIRGINIA AND THE TIDEWATER REGION OF NORTH CAROLINA... ....Coastal VA and NC... In coordination with AKQ/Wakefield, VA forecast office, a Slight risk upgrade for the Day 3/Tue ERO was introduced with this update. A low that will be tracking across the Midwest today and Monday will move northeast into New England on Tuesday and Tuesday night. The trailing cold front's eastward movement will slow to a crawl as the parent low tracks largely parallel to the front. Abundant atmospheric moisture with PWATs above 2.25 inches will remain in place ahead of the front along the Atlantic coast of the Southeast. The front will provide the forcing with additional weaker shortwaves developing and tracking northeast along a strengthening southwesterly jet streak in place across New England. This will put VA and NC in the favorable right entrance region of the jet. The abundant moisture with PWATs as much as 2.5 sigma above normal running into the slow-moving front will support training convection along and ahead of the front. The slow movement of the front will support training over the same areas, namely the newly-introduced Slight Risk area. Expect widespread 2 to 3 inch rainfall totals across the coastal plain, with locally higher amounts all-but-certain. The big question will be how much flooding this amount of rainfall translates into. For the highly urbanized Hampton Roads region, including Norfolk, Virginia Beach, Hampton, Newport News, Portsmouth, Chesapeake, and Suffolk, this could present a much higher flash flooding risk than the much less urbanized areas of coastal NC. Thus, consider Hampton Roads area in a higher-end Slight Risk. The area will continue to be monitored over the coming days, as any additional slowing in the movement of the front will keep the Slight Risk area under threat for training storms for longer, which will further increase the flash flooding risk. ....Northern NY into New England... The inherited Marginal Risk was largely maintained with this update. Residual moisture behind the low, with energetic upper levels due to the presence of the upper level jet streak should result in widely scattered convection across this area, which has been hard-hit with storms the past several weeks. Think rainfall amounts will be relatively low, with 1 inch amounts likely to be isolated. Nonetheless this amount of rain may still cause isolated flash flooding if it occurs in a particularly sensitive area. ....Four Corners Region... A much weaker signal for heavy rain is presented from the guidance for the Day 3/Tue period as compared with Monday afternoon. However, given expected lower FFGs in this area as a result of Monday's rainfall, and the sensitive areas through southern UT, the Mogollon Rim and northwestern NM, the Marginal Risk area was largely maintained, as widely scattered thunderstorms are likely once again Tuesday afternoon and evening. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6qyneNqy1sZlfGJAEoPNDCqM28acf6UDqO0HjtHYCnoi= fl3VP6WEkpmjViBL7Pn2d29aj2T7grVftfH06D-qHIPE_i8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6qyneNqy1sZlfGJAEoPNDCqM28acf6UDqO0HjtHYCnoi= fl3VP6WEkpmjViBL7Pn2d29aj2T7grVftfH06D-qxYfDaNI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6qyneNqy1sZlfGJAEoPNDCqM28acf6UDqO0HjtHYCnoi= fl3VP6WEkpmjViBL7Pn2d29aj2T7grVftfH06D-qhahoPCo$=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) .