Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 272024 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 424 PM EDT WED JUL 27 2016 .SYNOPSIS... Periods of showers and thunderstorms are expected later Thursday into Friday with heavy rainfall and potential for localized flash flooding. Unsettled weather may continue at times into the weekend before drier weather moves in sometime early next week. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... dewpoint temperature observations at 300 pm indicate a frontal boundary is still draped over the Delmarva just north of Dover... then further east across southern New Jersey near Millville and Atlantic City. Dewpoints north of this front are in the 60s... south of the front in the 70s. Because of the position of this front...and with little movement expected during the overnight hours...the weather grids have been populated with thunderstorms using chance to slight chance pops across the Delmarva and southern New Jersey. Given the high dewpoint temperatures in this area...patchy fog has been added to the weather grids during the late overnight hours. low temperatures are expected to range in the 60s north to mid 70s south...and mid to upper 70s in the urban corridor...especially Philadelphia. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/... The frontal boundary is expected to remain draped across the Delmarva during the morning hours...but shower activity may be limited until daytime heating kicks in a little later in the day, and as an impulse moves east along the front later in the morning or in the afternoon. the best chances for shower and thunderstorm activity on Thursday will be from Philadelphia south...especially from late morning through the afternoon. SPC has placed a good portion of this area under a Slight risk for severe weather on Thursday, but if any severe weather does develop, it appears the most likely time may be late afternoon and evening. The main threats will be strong winds and heavy rain. Temperatures will once again peak over 90 degrees in most locations (expect in the higher elevations up north),and with dewpoint temperatures in the 70s south and 60s north, the air will once again feel very muggy. However...even though it will feel hot and uncomfortable...threshold values for excessive heat advisories and warnings are not expected to be met. && .LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/... Highlights... * Main Concern: Heavy Rain/Flash Flood Potential Thu night/Fri * Secondary concern: Strong to Severe Thunderstorm Risk Thu night/Fri Details... Thursday night and Friday... 1) Heavy Rain/Flash Flood Potential Thu night and Fri: Interesting setup Thu night into Fri as there appears to be several ingredients coming together that have us concerned for localized flash flooding. A couple of shortwaves will combined with anomalously high Pwats, perhaps near record levels of 2.5+ inches. In addition, a mid level trough axis along with a low level surface boundary will provide a focus for convection. This combination will yield periods of showers and thunderstorms with pockets of heavy rainfall and a localized flash flooding potential. The biggest uncertainty is where the axis of heaviest rain sets up as that will be determined on the mesoscale level. However, given the above ingredients felt it warranted a Flash Flood Watch for the southern 2/3rds of our region Thu night into Fri. Timing also remains uncertain, but highest risk might be late thu night into Friday morning when main shortwave moves across the region. However, more showers and thunderstorms are possible Friday afternoon. 2) Strong Severe Weather Potential Thur night and Fri: The other concern will be the potential for strong to severe thunderstorms, with the greatest concern across the southern half of our region. This is where SPC SREF and other guidance shows the greatest risk of 1000+ J/KG of MLCape along with a period of 40+ knots of 0 to 6 km shear match up. Tough to pin down the timing, but late Thu night/Fri am may be when the greatest risk exists with the main shortwave. Biggest concern would be strong to damaging straight line wind gusts, but will have to watch for the low risk of a spinup near the triple point. Saturday through Wednesday... Unsettled weather will continue at times this weekend, given boundary in the vicinity. Timing uncertain this far out, but some guidance suggesting the greater risk for scattered showers/thunderstorms will be on Sunday with the next shortwave. Given clouds and risk of precip will keep highs mainly in the 80s to near 90 this weekend. Drier and somewhat less humid may finally arrive early next week. However, afternoons will still be quite warm with highs well up into the 80s to near 90. && .AVIATION /20Z WEDNESDAY THROUGH MONDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Thunderstorms over southern NJ at the start of the TAF period will weaken as they move east over the next couple of hours. Otherwise VFR conditions are expected through the TAF period. Scattered afternoon cu will dissipate during the evening. The wind is forecast to be 8 knots or less. The direction may favor the west at inland TAF sites however the sea breeze currently over the coast at TAF issuance time will continue to push westward through the day. The wind will be generally be variable overnight. Outlook... Thursday through Sunday...Mainly VFR conditions are expected Thursday morning. However, the potential for shra/tsra Thursday through Sunday will support tempo MVFR/IFR conditions at times. && .MARINE... Mainly sub-sca conditions are expected through the short-term period. however...gusty winds and rough seas are always possible in the vicinity of convective activity. Outlook... Thursday night through Monday... Relatively weak pressure gradient will keep winds and seas below small craft advisory thresholds. Main concern for mariners is the threat for strong thunderstorms Thursday night and Friday. Rip currents... There is a low risk for the development of dangerous rip currents for today due to the relatively light wind and tranquil seas. The probably risk for the development of dangerous rip currents on Thursday is also low. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Flash Flood Watch from Thursday evening through Friday evening for PAZ070-071-101>106. NJ...Flash Flood Watch from Thursday evening through Friday evening for NJZ009-010-012>024-026-027. DE...Flash Flood Watch from Thursday evening through Friday evening for DEZ001>004. MD...Flash Flood Watch from Thursday evening through Friday evening for MDZ008-012-015-019-020. MARINE...None. && $$ Synopsis...Frank Near Term...Miketta Short Term...Miketta Long Term...Frank Aviation...NRR Marine...Miketta/Frank is the U.S. government's official web portal to all federal, state and local government web resources and services.