Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 220326 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1126 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/...
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Amended the forecast overnight to increase PoPs south of the Mason-Dixon Line, where an isolated storm has tracked eastward from near the Baltimore metro to Chesapeake Bay. The latest HRRR and NAM Nest (00Z) indicate that more storms may initiate through the night in advance of a weak perturbation moving through the southern Mid-Atlantic. Radar rainfall rates are fairly excessive with this storm, and given the slow storm motion and very high PWs in place downstream of the storm, some localized heavy rainfall is possible. Should more storms develop and/or the isolated storm intensify, the grids may be updated to include more enhanced wording. 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.
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Some updated thoughts on this period... The first is that there is large spread in forecast temperatures tomorrow, likely owing to some uncertainty regarding the speed with which convection moves into the area (and downstream clouds hindering temperatures from maxing out during the afternoon). However, observations the past two days suggest that heat indices have tended to reach their maxima in the late morning (when dew points have not mixed out substantially). Though clouds may be present in the morning (in association with a perturbation moving through the southern Mid-Atlantic), I am somewhat worried the combination of temperatures and dew points will be quite uncomfortable once again in the urban corridor. My feeling is some heat product will be necessary again, particularly since this will be the third day with heat indices flirting with or exceeding the century mark. The 00Z NAM MOS numbers are quite cool (90 at KPHL), owing to more clouds. However, the GFS MOS is much warmer (95 at KPHL) with dew points likely to be higher than today. MOS has overdone temperatures the past couple days (though not by much), so although there is quite a bit of uncertainty regarding meteorological criteria being reached...the factors of it being a weekend day, the prolonged nature of the ongoing heat, and the potential for heat issues to be reached relatively early in the day all point to the need for some sort of heat-related advisory or warning. Will make the final decision with the 330 am update. The second regards severe/flood potential. There remains large uncertainty with the evolution of upstream convective clusters/complexes as they move along a quasi-zonally oriented boundary from the Ohio Valley to the southern Mid-Atlantic. High-resolution output has large variability in timing/location of the convection, but there are consistent indications of strong convection (combination of substantial instability and fast midlevel flow) and multiple rounds of storms (as upscale growth of initiating convection allows for downstream propagation along the aforementioned boundary) beginning late in the day. This leads to some concern regarding both severe storms and hydrologic issues. More details to come with the 330 am package, but some enhanced wording in the grids will probably be necessary at minimum.
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&& .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 /03Z SATURDAY THROUGH WEDNESDAY/... VFR conditions are forecast for much of the overnight and 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.This timing will be fined out as new information comes in during the overnight hours. A light and variable wind overnight becomes 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 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. && .EQUIPMENT... Dewpoint values at KDOV continue to run a few degrees over actual. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...AMC Near Term...CMS/Gaines/Iovino Short Term...CMS Long Term...AMC Aviation...AMC/Gaines/Iovino Marine...AMC/Gaines/Iovino Equipment...Staff is the U.S. government's official web portal to all federal, state and local government web resources and services.