Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 220006 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 806 PM EDT Fri Jul 21 2017 .SYNOPSIS... A very warm and humid air mass will remain over the area through this weekend and possibly into Monday. Meanwhile, low pressure along a roughly west to east frontal boundary will result in periods of showers and thunderstorms. By Tuesday, high pressure will build over the area bringing somewhat milder and drier conditions through through the middle of next week. A seasonally strong cold front may cross the area later in the week. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... The Excessive Heat Warning will remain in place for the urban corridor until 8:00 PM. We are anticipating an increase in high level clouds this evening, especially over the southern and central parts of our region, as a convective complex slides from West Virginia into Virginia. Most of the shower and thunderstorm activity with that system is expected to slide to our south. However, we will mention a low or slight chance of showers and thunderstorms for tonight in parts of eastern Maryland, central and southern Delaware and far southern New Jersey. The sky is expected to be mostly clear over Berks County, the Lehigh Valley, the Poconos and northern New Jersey for much of the night allowing some fog to develop in those areas. A light and variable wind is forecast for tonight. Low temperatures should range from the middle 60s in the elevated terrain up north to the 70s in the Philadelphia metropolitan area, southern New Jersey and on the upper Delmarva. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/... We are forecasting some sunshine for the morning hours with an increase in high clouds ahead of an approaching convective complex in the afternoon. The convective complex was located over Wisconsin and vicinity this afternoon. It will continue to work its way to the east southeast. It should begin to arrive in eastern Pennsylvania late on Saturday afternoon. As the system moves into very warm and humid air on Saturday it is expected to strengthen. The thunderstorms may become strong to severe as they approach our region. Conditions are anticipated to be slightly less hot on Saturday than they were today. Maximum temperatures are forecast to range from the lower 80s in the elevated terrain of the Poconos and northwestern New Jersey to the lower 90s in our central and southern counties. Maximum heat index values will likely remain below 100 except in interior section of the Delmarva where heat index values may reach 100 to 103. All locations should remain below their excessive heat criterion, so no warnings or advisories should be necessary. A light west to southwest wind is anticipated to develop on Saturday. && .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/... The extended period starting Saturday night features a transition in the flow aloft from roughly zonal flow across the northern US to a somewhat more amplified pattern with troughing over the east US by the latter part of next week. In terms of weather this translates to very warm/hot and humid conditions this weekend transitioning to a milder and somewhat drier period for at least part of next week. A quasi-stationary east-west frontal boundary is expected to persist across the area through the weekend, although it may lift north a bit on Sunday. Low pressure along the front will favor low-level convergence and areas of showers and thunderstorms. PWAT values near 2 inches will support some heavy downpours and possible flooding. Also SPC has a slight risk of severe for Saturday and Sunday. Timing is tricky but there is some indication of more likely precip for Saturday night and possibly again Sunday night. By Monday morning a fairly pronounced shortwave trof is forecast to be over the Great Lakes and move east to New England by Tuesday morning. The GFS is a bit slower than the ECMWF, but either way the trof should force a cold front across the mid- Atlantic and offshore by Tuesday morning. With the frontal passage on Monday there will be a continued good chance for aftn/eve t-storms. Max temps on Monday ahead of the front look to be still a few degrees above normal. Tuesday and Wednesday should be relatively pleasant with shrtwv ridging aloft and surface high pressure moving across the area. The high is associated with a somewhat cooler and drier air mass. Looks like some low-level warm advection develops Wednesday night and the forecast carries a chance of showers at that time. Chance of precip increases on Thursday as another shrtwv trof and associated cold front approach from the NW. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/... VFR conditions are expected through the balance of the daytime hours and into much of tonight. Showers and thunderstorms moving across West Virginia and Virgina this afternoon and forecast to remain to the south of our TAF sites. Patchy low clouds and fog are anticipated during the late night and early morning hours. VFR conditions are forecast for much of Saturday. Showers and strong thunderstorms are expected to approach from the west late in the day. They should begin arriving around KRDG and KABE about 2100 to 2200Z, and they are expected to reach KTTN, KPNE, KPHL and KILG around or shortly after 2300Z. A light west wind today is anticipated to become light and variable for tonight, then west to southwest on Saturday. OUTLOOK... Saturday Night through Monday...Generally VFR conditions expected, but temporarily lower cigs/vsbys in scattered showers and thunderstorms are likely at times. Tuesday and Wednesday...VFR conditions. && .MARINE... The wind is expected to favor the southwest around 10 knots tonight and it should back to the south for Saturday. Wave heights on our ocean waters will remain around 2 to 3 feet and waves on Delaware Bay should be 1 to 2 feet. Scattered showers and thunderstorms may affect the coastal waters of Delaware and far southern New Jersey, as well as Delaware Bay, tonight. OUTLOOK... Generally sub-SCA conditions are expected Saturday night through Wednesday. However winds and seas may be locally higher in scattered thunderstorms over the waters from Saturday night through Monday. RIP CURRENTS... The energy associated with the underlying 10 to 14 second southeasterly swell continues to weaken and the 6 second south southeast swell has become prominent. As a result, we will continue to carry a low risk for the development of dangerous rip currents for the balance of today and the same should be true for Saturday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...AMC Near Term...Iovino Short Term...Iovino Long Term...AMC Aviation...AMC/Iovino Marine...AMC/Iovino is the U.S. government's official web portal to all federal, state and local government web resources and services.