Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 191346 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 946 AM EDT Sat Aug 19 2017 .SYNOPSIS... A cold front has moved offshore this morning. High pressure will build in from the Midwest tonight. This high will move overhead Sunday and then progress offshore Monday into Tuesday. An area of low pressure is expected to pass well to our north for midweek but not before its cold front moves through the region on Wednesday. Canadian high pressure building into the Mid Atlantic on Thursday will likely influence our weather through early next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... The cold front has moved offshore this morning, and the remainder of today will remain dry across the area. The low clouds and fog have lifted but there will remain some clouds through the day, especially across southern New Jersey and the Delmarva this morning. Later today, clouds will likely begin to increase again across the western areas. Dewpoints today will be lower for most places than Friday, but still in the mid to upper 60s with some 70s mixed in. Temperatures will be well above normal again today, but with the expected dewpoints, heat index values will remain in the 90s so not expecting to reach heat advisory levels. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... The axis of the upper trough will cross the region tonight with a shortwave rotating through the area with it. The models continue to show some moisture crossing the region with the shortwave/upper trough. It appears that the moisture peters out before reaching the majority of our area and only a few showers, possibly an isolated thunderstorm, may reach the Lehigh Valley, Poconos and northwest New Jersey. Overnight lows will generally be in the 60s across the region, closer to 70 in Philadelphia and along the coast. && .LONG TERM /SUNDAY THROUGH FRIDAY/... High pressure centered over the Ohio Valley Sunday morning will move overhead by Sunday night. The high will gradually shift off the coast on Monday, but not before it provides us with consecutive days of dry weather and seasonably warm temperatures (highs in the mid to upper 80s both days). An uptick in humidity will occur on Monday in response to the light southerly return flow that develops on the western periphery of the surface high. Due to popular demand, spent a little extra time than usual analyzing moisture fields from various models before constructing cloud cover forecast for Monday afternoon. We are advertising a mostly sunny sky during the 1-4 PM timeframe when the solar eclipse will be viewable in our area. However, there are a couple of things to watch out for that could provide a source of clouds: 1) There will likely be high-level cirrus streaming in from the west with a subtle upper-level jet streak passing by but it should generally be scattered and/or thin enough to not impede viewing of the eclipse. 2) Daytime heating will result in the development of cumulus (CU). Based on the newest 00Z guidance, expect these lower-based clouds to be scattered across a majority of the area, allowing most sky gazers to have a unobstructed view of the eclipse. However, with increasing moisture in boundary layer, it is still too soon to rule out a broken CU deck that could impact viewing, especially south and west of Philadelphia where mixing ratios and RH values are higher at the top of the boundary layer between at 18Z and 21Z. It will also be interesting to see how the temporary lull in insolation during the solar eclipse will affect cloud cover- one would expect the diurnally-driven CU to decrease around and just after totality in a similar manner to what happens near sunset on a normal day. Our next chance for precip looks to be Monday night when a weak shortwave trough embedded within a corridor of stronger westerlies residing to the north of the southern CONUS upper ridge is predicted to pass through the region. This disturbance could provide enough lift to support elevated convection into the night, especially across Delmarva and southern NJ where the airmass will be more moist and less stable compared to more northern locations. Low pressure is expected to deepen midweek as it tracks northeastward across the Great Lakes and into the St. Lawrence Valley. Meanwhile, shortwave energy digging around the backside of the synoptic trough will lead to amplification of this upstream trough. In this pattern, chances for showers and storms will increase going into Tuesday and Wednesday. The focus for convective development Tuesday afternoon will be along a pre- frontal trough that is forecast to sharpen to the lee of the mountains. Tuesday looks to be the hottest day of the period (lower 90s along and south of the urban I-95 corridor) with the warm sector becoming established over the area in advance of the cold front. The best opportunity for rain looks to be Tuesday night and Wednesday in concert with the arrival of the cold front. Note, models are in remarkably good agreement (considering its still five days out) with the position and timing of the surface low and attendant cold front. Expansive high pressure builds in from the Great Lakes late in the week and into the start of next weekend. The Thursday-Saturday period is currently forecast to be dry with slightly below normal temperatures,. However, we will have to monitor the potential for the front that moves through midweek to stall nearby, which frequently occurs in summer. && .AVIATION /14Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of today and tonight...VFR conditions are expected. West to northwest winds around 5 to 10 knots with a few gusts around 15 knots this afternoon. Winds will become light and variable overnight tonight. An isolated shower or thunderstorm possible this afternoon near KRDG/KABE. OUTLOOK... Sunday through Monday night...VFR. NW 5-10 kt on Sunday, becoming calm after sunset, then S-SW 5-10 kt on Monday. Tuesday...Mainly VFR. Low chance for showers and storms with the potential for convective development being slightly more favorable over western-most terminals. S-SW winds 5-10 kt with gusts 15-20 kt during the afternoon. Tuesday night and Wednesday...Increasing chances for showers and storms with the approach of a cold front. This activity will bring a risk for temporary/localized restrictions below VFR. Expect a wind shift from S-SW to NW to occur on Wednesday with fropa. Wednesday night...Improving conditions from NW to SE behind the front. && .MARINE... Tranquil conditions will continue across the area waters through tonight. Northwest winds becoming southwest later today around 10 to 15 knots with a few gusts around 20 knots through tonight. OUTLOOK... Sunday through Tuesday...Winds and seas will be well below SCA criteria. Tuesday night...SCA possible with Sly flow strengthening ahead of cold front. Seas may also build to near 5 ft in our coastal waters. Wednesday...Winds veer from S-SW to W with the arrival of a cold front. Wind speeds should be below 20 kt but it may take time for seas in the coastal waters to drop below 5 ft during the morning. Wednesday night...NW winds could briefly approach 25 kt in post- frontal pressure surge. RIP CURRENTS... The rip current risk for today is low at the New Jersey shore and Delaware beaches. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Klein Near Term...Robertson/Meola Short Term...Meola Long Term...Klein Aviation...Klein/Robertson/Meola Marine...Klein/Meola

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