Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 170052 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 852 PM EDT Sun May 16 2021 .SYNOPSIS... High pressure will continue to build across the eastern U.S. through the week, leading to quiet but summer-like pattern. There is a chance for a few showers and thunderstorms as we head into the week as the center of the highs gradually shifts to our south and west. && .NEAR TERM /THROUGH MONDAY/... The last of the showers were dissipating quickly this evening with the loss of daytime heating. A short wave trough traveling in the northwesterly mid level flow passed off our coast early this evening. A secondary impulse is expected to follow in its wake overnight. Once it passes, we are anticipating a decrease in cloud cover over much of our region. The wind is forecast to be light and variable overnight, with lows mostly in the 40s and lower 50s. For Monday, our region is forecast to be on the western side of the slowly departing shortwave trough. Some additional shortwave energy moves through within the lingering northwesterly flow aloft during the day. The guidance overall is not all that excited about shower chances, and this is probably due to a bit more subsidence arriving in the wake of the main shortwave trough which would add to at least some mid level capping. In addition, the flow aloft is forecast to start taking on a little more anticyclonic curvature. Given this plus with the moisture still on the low side for this time of the year, carried just some slight PoPs during the afternoon for the inland areas. While a rumble of thunder cannot be ruled out, PoPs are so low and therefore opted to leave a mention out for now. Temperatures are expected to rise into the lower to mid 70s across the area except cooler in the higher terrain, and along the coast (south to southeast wind increases some through the day). && .SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/... A large upper-level ridge of high pressure will begin to build eastward across the Mid-Atlantic Tuesday as a weak shortwave trough shifts further off the New England Coast. At the surface, high pressure will broaden to encompass a large portion of the eastern half of the CONUS. This will lead to a warm May Day with highs climbing into the upper 70s to near 80. With relative humidity values in the 30-40 percent range, this will feel quite comfortable compared to the sultry summer days we know all too well. Light winds and clearing skies will make it even that much more pleasant. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... The upper-level ridge, somewhat reminiscent of an Omega Block, will gradually shift eastward, with a closed 500-mb anticyclone developing across the Ohio and Tennessee Valleys Wednesday into Thursday. This will lead to some very warm, dare we say, hot weather by late week with highs nearing 90 degrees. The paramount question of this forecast is how far east the mid- level ridge will build before eroding to the southwest. If it builds further eastward, then we are more likely to see highs well into the low 90s Thursday and Friday. The Canadian appears to favor this scenario more than the GFS with the EC being somewhat of an average of the two. Being this far out, went with a highly blended approach to the highs, favoring a warm bias of a degree or two, especially Thursday and Friday as my instinct is that the GFS is suppressing the ridge too much. By Friday, the ridge begins to deamplify a touch and shift to our south and west as a series of shortwave troughs ride around the circumference of the ridge. This leads to anticyclolysis across our region and opens the potential for each subsequent shortwave and associated surface low/cold front to push further south across New England. There is still decent disagreement on when this occurs, whether it be early in the weekend or into next week. If the ridge ends up retreating southwest much faster, any one of these fronts could penetrate our region as a backdoor cold front, leading to some forecast bust potential. I`m not fully on board this yet, but there is some potential for this to occur Friday, but moreso into next week outside of this forecast timeframe. The good news is that relative humidity values will stay on the low side with the very dry forecast, so heat indices don`t climb too far above air temperatures. Should we see an expanded coverage of shower and thunderstorm with any of these fronts pushing further southward towards our region, this could elevate RH values and raise heat indices into the weekend. && .AVIATION /01Z MONDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Overnight...VFR. Wind light and variable to calm. High confidence. Monday...VFR. A few isolated showers possible in the afternoon. Light and variable winds becoming south to southwest mainly 5-10 knots, however becoming southeast mainly at MIV and ACY. Moderate confidence. Outlook... Tuesday and Wednesday...VFR with light west-southwesterly winds in the morning increasing to 5 to 10 knots each afternoon. High confidence. Thursday...VFR with a very slight chance for an afternoon shower or thunderstorm, especially at KRDG and KABE. Southwest winds from 5 to 10 knots turning more southerly into the afternoon. Moderate confidence. Friday...VFR with another slight chance for a brief afternoon shower or thunderstorm. Southwest winds from 5 to 10 knots. Moderate confidence. && .MARINE... The conditions are expected to remain below Small Craft Advisory criteria through Monday. Southerly winds will prevail. Our friends at the USCG like to remind everyone enjoying the water in the Spring that the water temperatures are only in the upper 50s to lower 60s. Outlook... Tuesday through Friday...winds and seas are expected to remain below advisory thresholds with high pressure building overhead. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Davis Near Term...Gorse/Iovino Short Term...Davis Long Term...Davis Aviation...Davis/Gorse Marine...Davis/Gorse

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