Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Apr 15 2023 16:32:29 AWUS01 KWNH 151632 FFGMPD NYZ000-NJZ000-PAZ000-DEZ000-MDZ000-152230- Mesoscale Precipitation Discussion 0187 NWS Weather Prediction Center College Park MD 1231 PM EDT Sat Apr 15 2023 Areas affected...Mid-Atlantic states from far northern Maryland through southeast Upstate New York Concerning...Heavy rainfall...Flash flooding possible Valid 151630Z - 152230Z Summary...Slow moving showers and thunderstorms developing in the vicinity of an upper low will expand across the northern Mid-Atlantic states this afternoon. Rainfall rates of 1-2"/hr are likely at times, which combined with storm motions that may be as low as 5 kts, could result in rainfall exceeding 3 inches in some areas. Flash flooding is possible. Discussion...An upper level low clearly evident on GOES-E satellite imagery centered near the WV Panhandle will shift northward through this evening while gradually weakening. In the vicinity of this upper low, relatively steeper mid-level lapse rates will help enhance instability. This is already being realized where clearing is occurring noted by SPC RAP analyzed SBCAPE surging to 1500 J/kg. This instability is combining with PWs that were measured at 12Z on the KIAD sounding of 1.22 inches, above the 90th percentile for the date, to provide impressive thermodynamics for convection across the area. Thunderstorms already developing E/SE of the upper low have produced radar estimated rain rates of 0.5-1"/hr according to KCCX WSR-88D, leading to around 1 inch of rainfall at some local mesonet observations. As the upper low continues to pivot northward through the evening, it is likely that showers and thunderstorms will expand in coverage and move slowly around the low. This is reflected by forecast 0-6km mean winds that will back and weaken to just 5-10 kts, while Corfidi vectors collapse to around 5 kts and become anti-parallel to the mean flow despite modest 850mb inflow. This indicates that showers and thunderstorms will slow this aftn/eve, with some training or backbuilding possible. As SBCape climbs above 2000 J/kg, this will help produce rain rates that could frequently exceed 1"/hr as noted by HREF probabilities and the HRRR sub-hourly rainfall forecasts. The most likely area for these rain rates will be east of the upper low where S/SW low/mid level winds drive better thermodynamic advection, and where there will exist just enough 0-6km bulk shear, around 20 kts, to persist convective cells. Where storms train or backbuild into the better instability, these 1+"/hr rain rates could result in locally more than 3 inches of rainfall. This area has been dry as noted by a lack of any rainfall in the past 7-days, leading to 40cm soil moisture that is in the bottom 5th-percentile and 1-hr FFG that is generally 1-2 inches. These dry antecedent conditions should limit the overall flash flood risk, which is reflected by very low HREF probabilities for exceeding this FFG. However, where any of these slow moving storms can train, especially if they occur over any urban areas or more sensitive terrain, flash flooding could occur. Weiss ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!7j_X3MSj7Eec1PoLgxmn170QOuVRLlFsK665XQZIGB7nsw0xP6qhicJIr2a2tKj6C6Qx= 4UAjS1hZBtJxGE8hd0mHaaw$ for graphic product... ATTN...WFO...ALY...BGM...CTP...LWX...OKX...PHI... ATTN...RFC...MARFC...NERFC...NWC... LAT...LON 42227554 42197475 41907417 41357401 40997412=20 40557434 40077467 39857512 39667577 39587624=20 39547667 39567707 39787755 40257750 40867711=20 41447668 42057599=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) .