Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jul 17 2024 21:16:52 AWUS01 KWNH 172116 FFGMPD NJZ000-PAZ000-DEZ000-MDZ000-180300- Mesoscale Precipitation Discussion 0643 NWS Weather Prediction Center College Park MD 515 PM EDT Wed Jul 17 2024 Areas affected...Northern MD...Northern DelMarVa...Southeast PA...Central NJ Concerning...Heavy rainfall...Flash flooding possible Valid 172115Z - 180300Z SUMMARY...Thunderstorms along a pair of surface convergence zones will pose a flash flood threat this evening, as will approaching storms from the west. Urbanized communities and areas hit hardest by thunderstorms yesterday are most at-risk to potential flash flooding. DISCUSSION...GOES-16 satellite imagery and 20Z surface observations show two distinct convergence lines over parts of northern MD and eastern PA this afternoon. One convergence area lies along Parr's Ridge in northern MD that then extends northeast into the Lower Susquehanna Valley, while the other is nearly parallel to the I-95 corridor between Baltimore and Philadelphia. As the upper trough to the west approaches, synoptic scale lift will increase while atmospheric moisture content also rises. RAP guidance shows 2-2.2" PWs are forecast to engulf the highlighted region, while at the same time, dew points in the low 70s and MLCAPE >1,000 J/kg will be at these storms disposal for at least several more hours. With these boundaries in place, the concern becomes storms containing rainfall rates >2"/hr tracking over a more densely populated region of northeast MD, southeast PA, and central NJ. While storm motions are progressive with 850-300mb steering winds as strong as 30 knots, the orientation of the mean 850-300mb wind pattern is parallel to these boundaries. This is depicted on the 20Z WoFS as clusters of storms originating near Baltimore that propagate northeast toward the Philadelphia metro area and central NJ along these boundaries. The 20Z WoFS does show low chances (20-30%) for rainfall totals >3" in and around the Philadelphia suburbs, indicative of the potential for totals that would pose a flash flood threat. Portions of northeast MD and southeast PA have also received 1-2" of rain from last night's storms, allowing for 1-hr FFGs to drop to as low as 1.5"/hr in some areas. Farther north, one of the convergence lines extends as far north as west-central NJ, where these storms may link up with additional storms approaching from central PA later this evening. Flash flooding is possible this evening in these areas, particularly in the more heavily urbanized areas where a greater concentration of hydrophobic surfaces exist. The other areas at risk are in portions of southeast PA and northeast MD where soils are a little more saturated following yesterday's heavy rainfall. Note the flash flood threat may continue into this evening as storms along the cold front approach from southwest PA. Mullinax ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!7Smgrj45LhAUHjjcky62pfHJOpfn09LxjgeEzW9qTHm0J2rsejm81GvuOAVBttVeFgNC= Eem56PkxkgC_7HIQ6BVkOTY$ for graphic product... ATTN...WFO...CTP...LWX...PHI... ATTN...RFC...MARFC...NWC... LAT...LON 41047494 40897460 40487454 39887499 39407573=20 39137653 39487709 39827702 40427620 40967530=20 =20=20=20=20=20=20=20=20=20=20=20=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .