Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 23 2023 08:19:59 FOUS30 KWBC 230819 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 419 AM EDT Sat Sep 23 2023 Day 1 Valid 12Z Sat Sep 23 2023 - 12Z Sun Sep 24 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHEAST VIRGINIA AND EASTERN OKLAHOMA INTO WESTERN ARKANSAS... ....Mid-Atlantic... Tropical Storm Ophelia is close to making landfall somewhere near the Crystal Coast of North Carolina and will continue to track north through eastern North Carolina today. Ophelia will contain an exceptional easterly Integrated Vapor Transport (IVT) ranging between 750-1,000 kg/m/s, which is above the 99th climatological percentile according to NAEFS for 12Z Saturday. As Ophelia weakens throughout the day, so will the IVT, but it will still routinely be above the 90th climatological percentile to go along with a large areal extent of 1.5-1.75" PWs. In fact, 1.5" PWs will likely be observed as far north as southern New England. Between 12Z Saturday and 00Z Sunday, PWs will hover around or slightly above 2" from eastern NC on north along the I-95 corridor into the Richmond metro area. As Ophelia continues undergoing extratropical transition, it will in effect develop an occluded front and a warm sector that will lift north through eastern NC and the VA Tidewater. Most CAMs agree as much as 500-1,000 J/kg of MLCAPE will be available while elevated freezing levels from the NC Piedmont to the DelMarVa Peninsula suggest war cloud layers could be up to 13,000ft deep. This is a recipe for highly efficient rainfall rates within thunderstorm activity. This is especially the case along the northern and western flanks of Ophelia's circulation where the aforementioned strong easterly moisture transport intersects the surface occluded front and ascends rapidly as a warm conveyor belt, leading to vigorous convection to the north and west of Ophelia's core. The 00Z HREF did depict 30-35% probabilities of rainfall totals >5" from south-central VA to central NC, but localized amounts could approach 8". In coordination with the Wakefield, Raleigh, and Morehead City WFOs, have expanded the Moderate Risk into central NC and southern VA. Farther north, hourly rainfall rates will struggle to top 1"/hr, but the relatively deep warm cloud layers will still support efficient rainfall processes. Instability (MLCAPE of 250-500 J/kg) will be highest over from the Eastern Shore and northern DelMarVa to southern NJ Saturday afternoon and into Saturday evening. The northern flanks of the Marginal and Slight Risks were pulled south slightly given the lack of sufficient instability into southern New England through Saturday evening. ....Upper Midwest to the Southern Plains... An approaching 500mb shortwave trough over the Four Corners region is set to eject into the Southern Plains Saturday afternoon. At the surface, a cold front racing south through the Central Plains will encounter another frontal boundary located north of the Red River and stretching south and east through the ArkLaTex. In response to the falling heights from both the upper level disturbance from the Four Corners and a more robust upper trough over the Northern Plains, an 850mb LLJ will introduce added moisture flux into the region. One the primary concerns is 850-300mb mean winds become oriented quasi-parallel to the front over the ArkLaTex, and with a prolonged LLJ present, backbuilding and training storms are becoming increasingly likely. PWs will approach 2" and surpass it as storms develop, plus there is likely to be no shortage of instability thanks to 2,000-3,000 J/kg of MLCAPE. The 00Z HREF probabilities for FFG exceedance have continued to rise, especially across eastern OK. HREF probabilities for 3-hr QPF > 3-hr FFGs are as high as 60-80% between 00-03Z and 03-06Z across eastern OK. Meanwhile, the longer duration into southwest MO and northwest AR is a concern with 6-hr QPF > 6-hr FFG probabilities up to 40-50%. Chose to expand the Moderate Risk a little farther east as a result. The last QPF forecast calls for 3-4" of rainfall, but localized amounts surpassing 6" is well within the realm of possibility as 24-hr QPF > 5" probabilities on the HREF are 40-45% in east central OK. Farther north, an upper trough taking on an increasingly negative tilt will spawn a strengthening low pressure system that looks to track into the Red River of the North by Saturday evening. An excellent fetch of 850mb moisture flux out ahead of the forming colluded front will wrap around the 850mb low and lead to a well developed comma head of storms on the 850mb low's northwest flank. 00Z HREF showed 3-hr QPF > 3-hr FFGs probabilities north of Pierre that were as high as 30-40% between 18-21Z Saturday. For the day as a whole, probabilistic HREF guidance showed up to a 60-80% chance for 24-hr rainfall totals surpassing 3". The 750 kg/m/s IVT over central MN will play a big role in supplying the anomalous moisture aloft and MLCAPE between 500-1,000 J/kg will provide sufficient instability to generate Excessive Rainfall Rates. The inherited Slight Risk remains in good standing, as does the Marginal Risk in southern MT and northern WY near the Big Horns. Adequate upslope flow and a complimentary tandem of available moisture/instability over saturated soils makes this area still prone to flash flooding on Saturday. Mullinax Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4_-b4kY3KdrEsqCDb6J-zzw83-QxtRw8Owmpg2-WK1oW= 1C8lRdGJugHjgjIZnA3m-SmRoLz5LNxwfks9ec2nzM4lwPI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4_-b4kY3KdrEsqCDb6J-zzw83-QxtRw8Owmpg2-WK1oW= 1C8lRdGJugHjgjIZnA3m-SmRoLz5LNxwfks9ec2n07CV5ik$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4_-b4kY3KdrEsqCDb6J-zzw83-QxtRw8Owmpg2-WK1oW= 1C8lRdGJugHjgjIZnA3m-SmRoLz5LNxwfks9ec2n3UtjTac$=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) .