Subj : MESO: NE Heavy Rain/Flood To : All From : Mike Powell Date : Tue Aug 15 2023 07:35:00 AWUS01 KWNH 150859 FFGMPD MAZ000-RIZ000-CTZ000-NYZ000-NJZ000-151400- Mesoscale Precipitation Discussion 0929 NWS Weather Prediction Center College Park MD 459 AM EDT Tue Aug 15 2023 Areas affected...Southern New England...Southeast NY & Long Island...Northeastern NJ... Concerning...Heavy rainfall...Flash flooding possible Valid 150900Z - 151400Z SUMMARY...Upstream redevelopment in very high moisture flux environment. Efficient rainfall production with 2-2.5"/hr rates, should continue potential for flash flooding near lifting warm front. DISCUSSION...GOES-E WV suite depicts shortwave at the leading edge of the larger scale dry slot jet continues to rapidly advance eastward into the Hudson valley and central New England. CIRA LPW continues to denote the confluence of two deeper moisture streams through southern New England, with a narrowing stream originally from the Gulf and Eastern Pacific coming through the central Appalachians into the Lower Hudson Valley while strong southerly LLJ surges western Atlantic low level moisture northward. 08z surface analysis shows the associated wave near Trenton, moving quickly; while the warm front continues to lift northward. Strong sfc to 850mb speed and directional convergence (upstream near surface wave) along instability gradient supports updrafts and given proximity to deep layer moisture up to 2.25 through SE NY, across Long Island; moisture flux convergence is supporting highly efficient rainfall production with 2-2.5"/hr rates likely. Deep layer steering is angled just north of parallel to the frontal boundary so some training remains possible; however, it is the directional veering upstream that should support some back-building cells through NYC into central Long Island over the next few hours, maintaining flash flooding risk. North of the strong boundary layer convergence, instability rapidly diminishes and moderate to heavy stratiform should dominate. Duration across urban corridors may cause flooding concerns, but rises should be more gentle in nature and occur in typical poor drainage areas. Of note: GOES 3.9um SWIR shows a north-south convergence band with even higher moisture/theta-E air along/just south of the tip of Long Island, this seems to mark sfc-850mb PW over 1.2" and total values of 2.5". Stronger updrafts, possible supercells appear to have developed on the line. While trends are for continued ENE track, as the base of the shortwave advances, steering may back just enough that a cell could near Block Island or even SE MA and the Islands after day-break. While this is less likely and remains uncertain; extreme rates of 3"/hr are probable given localized isallobaric influences to low level moisture convergence for the stronger rotating updrafts. So even though these islands are less prone to flash flooding due to sandy soil types, if they were to track through rapid inundation flooding becomes possible. Gallina ATTN...WFO...ALY...BOX...OKX...PHI... ATTN...RFC...MARFC...NERFC...NWC... LAT...LON 42277010 41266991 41217038 41197100 40897215 40677286 40457380 40527444 40917454 41307400 41727302 41967216 42147126 = = = --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .