Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 11 2023 21:58:55 AWUS01 KWNH 112158 FFGMPD MEZ000-MAZ000-NHZ000-RIZ000-VTZ000-CTZ000-NYZ000-NJZ000-PAZ000-120400- Mesoscale Precipitation Discussion 1059 NWS Weather Prediction Center College Park MD 558 PM EDT Mon Sep 11 2023 Areas affected...Coastal Mid-Atlantic states through eastern New England Concerning...Heavy rainfall...Flash flooding likely Valid 112200Z - 120400Z Summary...Showers and thunderstorms will continue to expand across the Northeast through early tonight. Rainfall rates may reach 3"/hr at times, which through training could produce 2-4" of rain with locally higher amounts. Flash flooding is likely. Discussion...The regional radar mosaic early this evening shows widespread showers and thunderstorms organizing along a stationary front arced from the Mid-Atlantic into New England. This surface front is providing strong low-level convergence, aiding modest height falls downstream of a mid-level trough axis and LFQ jet level diffluence to produce robust deep layer ascent. This lift is occurring into a region of extremely favorable thermodynamics noted by PWs measured via GPS of 1.7-1.9 inches, well above the 90th percentile at CHH/GYX according to the SPC sounding climatology, and a ribbon of SBCAPE of 1000-1500 J/kg. The special 18Z U/A soundings at OKX and GYX featured deep warm cloud depths with near most-adiabatic lapse rates throughout, suggesting efficient warm rain processes are dominating, and this is reflected by radar-estimated rain rates above 3"/hr according to KBOX leading to impressive FLASH responses. During the past hour or two, the radar signatures have become more organized and intense, likely in response to the building instability within clearing sky conditions. Instability is progged to continue to climb, albeit modestly, for a few more hours, before waning/exhausting, which will combine with even stronger ascent as the upper jet intensifies and the primary trough axis advects east, to produce widespread heavy rain producing convection. This is reflected by most high-res simulated reflectivity, and the HREF hourly-rain rate probabilities for 2"/hr and 3"/hr reach 30-40%, and 5-15%, respectively. Additionally, the HRRR indicates that some areas may receive 0.75-1"/15 min (brief 4"/hr rain rates). Although mean 0-6km winds will remain rather progressive at 20-25 kts to the northeast, aligned Corfidi vectors of 10-15 kts suggest training is likely, especially where regeneration occurs as parcels isentropically ascend the stationary front. Within bands of the most intense training, rainfall could reach 2-4" with HREF exceedance probabilities for 5" even rising to 20% in a few areas. This region has quite wet recently noted by AHPS 7-day rainfall departures that are 200-300% of normal in many areas. This has led to 40cm soil moisture that is at or above the 98th percentile in many locations, compromising FFG to as low as 1.5-2"/3hrs. The HREF FFG exceedance probabilities across eastern New England are progged to reach 30-40%, suggesting this is the region of greatest probability for instances of flash flooding. However, these intense rain rates could cause local runoff and flash flood issues anywhere training can occur, or atop the less permeable urban areas. Weiss ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!_gbuZlQf_ISpOkiOj9LsDYplTQZv36u9zV6g2KVu6YvGEaujgdwVSbDB4WT3E175Edgm= HGpox3E3nDH2vSc2RVqwFp4$ for graphic product... ATTN...WFO...ALY...BGM...BOX...GYX...OKX...PHI... ATTN...RFC...MARFC...NERFC...NWC... LAT...LON 44766955 44496895 44276903 44026931 43666983=20 42857018 42277013 41946992 41667000 41477033=20 41377093 41037179 40677275 40477336 40187380=20 39947401 39637416 39297445 39217489 39277512=20 39427530 39737525 40187497 40527484 40997470=20 41527449 41827398 42097343 42867240 43797156=20 44287075=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) .