Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Aug 13 2023 20:23:59 FOUS30 KWBC 132023 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 423 PM EDT Sun Aug 13 2023 Day 1 Valid 1844Z 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... Round one of convection along the Missouri/Arkansas border will be tapering off as convection continues to track eastward this afternoon. Following a relative lull during the day...additional convection capable of producing very heavy rainfall amounts and rainfall rates is expected this evening into the early morning hours of Monday. The HRRR was not doing particularly well with the ongoing MCS...and the signals were initially mixed about later convection. But the 12Z run of the HRRR has bolstered confidence for an eastward expansion of the Moderate risk area...and fits the overall synoptic pattern of a 30 to 40 kt low level jet feeding abundant Gulf moisture northeastward across portions of the Plains, Ozarks, and into the Lower Ohio Valley through the remainder of the day and into tonight. This morning's rainfall will prime the soils for the upscale growth and coverage of convection later this evening. tonight. 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. Furthermore, the developing low to the north will support upper level frontogenesis, resulting in a strengthening southwesterly jet streak. The 12Z HREF probability continued to focus on southern Missouri for the best probabilities of rainfall rates exceeding 2 inches per hour and for 10 to 100 year ARI...with the probabilities for exceedance peaking by 06Z.=20 Individual storms may be progressive enough but the concern is for any backbuilding convection. Consequently...the Moderate Risk area was expanded eastward a bit to the Mississippi River. The 13/12Z run of the HRRR extended the swath of very heavy rainfall amounts as far east as southern Indiana...and while that solution isn't totally unreasonable...it did seem to be extending too far from the instability. Until/unless there is better support for that much rain falling that far east...the Slight Risk in place there.=20 Elsewhere, few adjustments were needed to the surrounding Slight Risk area. Like areas farther to the south...a relative lull is expected by increasing risks of convection capable of additional excessive rainfall later. ....Upper Midwest... Developing surface low pushing east across NE/IA will advect moisture northwestward ahead of the low center into the northern Plains and upper Midwest with the comma-head region of the circulation north of the low center featuring 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 farther south. The combination of separation from the primary moisture source and instability source, and antecedent very dry conditions over Minnesota/Wisconsin should greatly limit flash flooding potential and only minor adjustments were needed. ....Maine... An upper-level trough and associated cold front crossing the Northeast will bring scattered showers and thunderstorms to the region this afternoon, with generally the greatest concentration across Maine. Expanding areas of convection are seen currently in visible satellite and radar reflectivity imagery, with additional expansion in coverage is expected over the next few hours.=20 Facilitating the convective threat is a nose of MLCAPE values exceeding 1500 J/kg up across areas of central and southern Maine ahead of the front, and this coupled with 30 to 40 kts of effective bulk shear should favor plenty of multi-cell convection capable of producing rainfall rates that may reach 1 to 1.5+ inches/hour. The latest HREF guidance does show rather elevated probabilities of seeing 1+ inch/hour rainfall rates and exceedance of the 3-hour FFG values which are rather low given recent rainfall. Some localized storm totals of 2 to 3 inches will be possible going through the late afternoon hours. Streamflows across Maine are also running quite high, and coupled with moist soil conditions, the additional heavy rainfall threat for this afternoon will encourage more efficient runoff and potentially a few instances of flash flooding. ....Southwest/California... For Southern Nevada and the Mogollon Rim, scattered convection has developed amid rather slow deep layer flow (850-300 mb) of 5-15 kts. The mesoscale environment is characterized by modest ML CAPE of 250-750 J/keg, PWATs of 0.9-1.3" (near the 90th percentile, per VEF/FGZ sounding climatology), and minor diffluence aloft on the eastern periphery of an upper-level low. These conditions have supported rather impressive short-term totals (as indicated by MRMS) as high as 2-3" in an hour (with an inch in as little as 15-min). These totals are highly localized with pulse-like convection likely rapidly dying out, but the combination of slow storm motions and weak diffluence aloft (amid anomalous tropospheric moisture) could continue to support localized (and possibly significant) instances of flash flooding through the late afternoon hours. Locations most at risk of flash flooding include normally dry washes, burn scars, and slot canyons. Rapid flash flooding may also occur over dry locations just downstream of where extreme rainfall occurs, so extra caution should be exercised by those outdoors in low-lying areas. The inherited Marginal Risk area was expanded into the Peninsular Ranges of Southern CA as well based on the 12z HREF guidance. Signals are modest for excessive rainfall in this region...although there is at least some potential for localized flash flooding from any storms that form given steep terrain and existant burn scars. Roth/Bann/Churchill/Orrison 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... 2030 UTC Excessive Rainfall Discussion... Not many changes were needed...with the area from the Appalachians/Mid-Atlantic eastward getting the most attention due to guidance which continues to shift each run to run. The ingredients look to be in place--with the EC Ensembles and the GEFS showing precipitable water values running 1.75 to 2.00 inches / nearly 3 standardized anomalies greater than climatology skirting across or at least near to the southern New England coast after 15/00Z. Forecast soundings support a period of heavy downpours...but overall storms look to be progressive. There was a subtle southward shift of the QPF across Pennsylvania...but maintained the Slight Risk without much change to account for the fact that only a subtle shift in the synoptic pattern could bring heavier rain back over a region with suppressed flash flood guidance as well as an urban component.=20 Maintained the Slight Risk near the upper level system over the Great Lakes as well as the surrounding Marginal risk and with the Marginal risk over the Southwest U.S.. Bann Previous Excessive Rainfall Discussion ....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... ....2030Z Excessive Rainfall Discussion Update... With the 12Z suite of guidance generally maintaining good run to run continuity...made few changes to the Day 3 Excessive Rainfall Outlook.=20 Bann ....0830Z Excessive Rainfall Discussion... ....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!-xO7FuVVHoN7cuPe3WSiaezfMigjS2gjE5sn8q4eiQrm= 77-nVxoiITw4_cZR76OniGr_qll9yFik846UXJOrNG5mKCc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-xO7FuVVHoN7cuPe3WSiaezfMigjS2gjE5sn8q4eiQrm= 77-nVxoiITw4_cZR76OniGr_qll9yFik846UXJOrd38pA3A$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-xO7FuVVHoN7cuPe3WSiaezfMigjS2gjE5sn8q4eiQrm= 77-nVxoiITw4_cZR76OniGr_qll9yFik846UXJOr8kwtQWo$=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) .