Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 09 2023 20:03:17 AWUS01 KWNH 092003 FFGMPD MAZ000-NHZ000-RIZ000-VTZ000-CTZ000-NYZ000-NJZ000-PAZ000-DEZ000-MDZ000-10020= 0- Mesoscale Precipitation Discussion 1047 NWS Weather Prediction Center College Park MD 402 PM EDT Sat Sep 09 2023 Areas affected...I-95 corridor from Baltimore, MD to Boston, MA and west to Upstate NY Concerning...Heavy rainfall...Flash flooding possible Valid 092001Z - 100200Z Summary...Thunderstorms developing along a stationary front will persist into this evening. Rainfall rates of 1-2"/hr are likely, which through training could produce 2-3" of rain with locally higher amounts. Flash flooding is possible. Discussion...The regional radar mosaic this afternoon shows increasing coverage of showers and thunderstorms aligned SW to NE from near Philadelphia, PA through Boston, MA. These storms are firing along a stationary front analyzed by WPC, with the low-level convergence along this boundary driving the most pronounced ascent. At the same time, S/SE low-level flow out of the Atlantic Ocean is surging higher PWs of +1 to +1.5 sigma and SBCAPE of 2000-3000 J/kg into the Northeast, while also lifting isentropically atop the boundary to enhance ascent. A shortwave noted moving across upstate NY is also causing subtle PVA and weak height falls to impinge along the front, within otherwise nearly unidirectional S/SW flow aloft. The overlap of this forcing and thermodynamics has produced radar-estimated rainfall rates as high as 1.5"/hr, and a subsequent FLASH response of QPE-FFG ratio of 125% and CREST unit streamflow above 600 cfs/smi. The high-res guidance is struggling to capture the current activity, so confidence in evolution is slightly below normal. However, the placement of the stationary front overlapping the intense thermodynamics should continue to be the impetus for additional convective development the next several hours. Storms should generally develop along this boundary and then lift slowly N/NE on 0-6km mean winds of 10-15 kts. In some places, the mean flow is parallel to the front suggesting enhanced training potential, which is also reflected by parallel Corfidi vectors, especially from NJ northeastward, and an impressive CAPE gradient. Even in locations where storms do not move parallel to the front, the persistent moist isentropic upglide should enable storms to persist and intensify, with the HRRR indicating 15-min rainfall of around 0.5" in some places. Although the ensemble members are struggling with current activity, the HREF still indicates a 15-25% probability of 3" in the next 6 hours in some areas, which could result in locally higher amounts since current activity is more widespread than models suggest, and pronounced training is likely in some areas. 14-day rainfall according to AHPS has been only 10-50% of normal across much of the I-95 corridor, which has allowed FFG recovery to generally 2-2.5"/3hrs. The HREF exceedance probabilities for this FFG peak at only 15%, but again it should be noted that coverage may end up more widespread and more intense than the HREF suggests. While these probabilities suggest a limited flash flood risk, it is possible that where the most intense rates can train, especially over the more urban corridor, instances of flash flooding could result. Weiss ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!5o9yVY-j1A_Hrn_2exp8ufDnli88EXyfOC0Nw-17tTeZZyN-m776QRjLmkqSM1RxZHkP= 9j6PKq0GOoI97ZKOYoghuvk$ for graphic product... ATTN...WFO...ALY...BGM...BOX...CTP...GYX...LWX...OKX...PHI... ATTN...RFC...MARFC...NERFC...NWC... LAT...LON 43167170 43047085 42527064 42167064 41757163=20 41527271 41197340 40857373 40537391 40127420=20 39857453 39427562 39327629 39317659 39527672=20 39827632 40337578 40757560 41127544 42097413=20 42847298=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) .