Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 220445 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1245 AM EDT Sun Jul 22 2018 .SYNOPSIS... Low pressure will move northward up the Chesapeake Bay this evening, then through Pennsylvania and western New York late tonight into Sunday. Meanwhile, another area of low pressure over the Midwest will drift south into the Southeast U.S. through the middle of next week. A cold front may eventually move into the region late in the week, then stall near the East Coast heading into next weekend. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... With the low moving northward into Delmarva, the heavy rain is lifting into the northern part of our forecast area. The rain is starting to break up some as a dry slot works its way int our area. With the flash flood threat having diminished, we have gone ahead and cancelled the Flash Flood Watch. An easterly low-level jet near 50 knots is working northward, and a mesoanalysis shows some instability creeping onto the coastal areas from off the ocean. It is here where dew points have risen to around 70 degrees. While a few isolated thunderstorms cannot be ruled out with the ongoing bands of heavier rain sliding northward, looks like the main instability is held just offshore. Still kept a slight chance of thunder for the far eastern areas for awhile tonight. The easterly winds have increased especially along the coast with some gusts to 40 mph thus far. Some additional increase should take place over the next several hours as the surface low works north-northwest and the low-level jet strengthens some northward. As a result, no changes to the earlier issued wind advisory. Did speed up the decreasing PoPs across the southern areas especially as a dry slot works in over the next few hours. The hourly temperature, dew point, wind and wind gust grids were adjusted based on the latest observations then these were blended into mostly continuity. Otherwise, as the low tracks across Pennsylvania later tonight, drier air begins to move in on the backside, and precipitation will begin to come to an end front south to north. Areas across northeast Pennsylvania and northern New Jersey may hold on to rain until daybreak Sunday. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM MONDAY/... As the low pressure system continues to lift to our north and northwest toward the Great Lakes in the morning hours, most precipitation will come to an end. However, our area will still be under the influence of south-southwest flow aloft, along with increasing southerly flow at the surface. This will allow for temperatures to warm during the day, especially if any sunshine breaks out. This will lead to increasing instability during the day, and with any short wave/vorticity impulses moving across the area, scattered showers and isolated thunderstorms will be possible through the day Sunday, especially during the afternoon hours. Since storms are expected to be mostly scattered, and there is not overly strong instability or shear expected, we are not expecting widespread severe weather. Nor are we expecting widespread flooding as PW values will be below 2 inches and any storms should be fairly progressive. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... This unsettled weather pattern is expected to continue into the upcoming week and at least the first part of next weekend. This unsettled pattern can be attributed to a high amplitude pattern that consists of a deep upper trough over the eastern CONUS and downstream ridge blocking over the western Atlantic basin. Tropospheric-deep southerly flow in between these two systems will continue to draw tropical moisture up the eastern seaboard, resulting in very muggy conditions (dewpoints in the 70s) and [at least the threat of] multiple rounds of showers and storms each day. Unlike with today`s event, there doesn`t appear to be any organized synoptic disturbance(s) in particular to produce widespread rain and/or result in a washout on most days next week except possibly sometime midweek when the cutoff low to our west becomes an open wave and finally lifts northeastward through the region. Models continue to struggle figuring out how quickly the shortwave trough ejects downstream toward the Mid Atlantic but the spread has narrowed between sometime Wednesday and Thursday. This timing uncertainty prevented us from going much higher with PoPs in any one period. The threat of heavy rainfall and flooding this week will increase as soils become increasingly wet and stages rise in area basins following each successive round of rainfall. Potential hydro impacts will be added to the HWO. With convective coverage much more disorganized than with today`s event, heavy rainfall should be much more localized and thus responses from runoff on the larger river basins next week is highly uncertain. && .AVIATION /06Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...MVFR/IFR conditions overall, although locally VFR conditions will occur at times this evening. The rain will end from south to north mostly in the 05z-08z time frame. East- northeast winds 10-20 knots with gusts up to 30 knots (highest gusts closer to the coast), then becoming southeast and diminishing overnight. An east to southeast low-level jet at 2000 feet of 40-50 knots will result in low-level wind shear even with the gusty surface winds. Sunday...Mainly IFR to start, then the conditions are expected to improve to MVFR during the morning and may improve to VFR during the afternoon for some areas. There should be a lull in the showers for a time in the morning, then some showers and a few thunderstorms develop especially in the afternoon. Southeast to south winds around 10 knots, with some gusts up to 20 knots possible during the afternoon. OUTLOOK... Sunday night through Thursday...Rounds of showers and thunderstorms through much of next week though activity will generally not be widespread and/or steady. Periods of sub-VFR conditions are possible with this activity. SE winds generally 5-15 kt through Tuesday, becoming S Wednesday and Thursday. && .MARINE... Winds are turning more to the south and starting to diminish. We have cancelled the Gale Warning on the Upper Delaware Bay and converted over to a Small Craft Advisory as winds will continue to gust up to 30 knots through early Sunday. A Gale Warning remains in effect for all of the coastal waters, including the lower Delaware Bay. The wind gusts on the lower Delaware Bay remain around 35 to 40 knots at Brandywine and we have extended the Gale Warning through 08Z. The northern portions of the New Jersey coastal waters are expected to have the strongest winds (40-45 knots). There is a brief window for an isolated gust to reach 50 knots, but we are uncertain how much mixing will occur and if those strongest winds mix down to the surface. So for now we have kept with the Gale Warning. OUTLOOK... Sunday night and Monday...SCA will likely be needed. S-SE winds 10- 20 kt with gusts to 25 kt in the coastal Atlantic waters. Waves will continue to remain elevated in the 4-7 ft range. Monday night through Wednesday...S-SE winds will weaken to below SCA criteria. However, a SCA will likely be needed with seas forecast to remain near 5 ft. Wednesday night and Thursday...S winds increase slightly to 15-20 kt with a chance for 25 kt gusts in the coastal waters. Nonetheless, a SCA will likely be needed with seas still around 5 ft. RIP CURRENTS... High Risk of rip currents has been extended through early Sunday evening for the New Jersey and Delaware beaches. Strong onshore flow this evening along with dangerous surf is resulting in a high risk of rip currents. As for Sunday, the surf looks to remain agitated despite low pressure well to our north. The wave period should lengthen some and with a gusty south to southeast wind, the High Risk of rip currents was extended through Sunday. An enhanced rip current risk should linger into the early part of the new week as a stiff southerly flow keeps wave heights around 5 or 6 feet on the waters off our coast. && .EQUIPMENT... The KDOX radar will remain offline until parts and a maintenance kit to replace the azimuth and elevation motors are received. The current estimated return to service is early next week. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...High Rip Current Risk through this evening for NJZ014-024>026. Wind Advisory until 4 AM EDT early this morning for NJZ012>014- 020-026. DE...High Rip Current Risk through this evening for DEZ004. MD...None. MARINE...Gale Warning until 6 AM EDT early this morning for ANZ450>455. Small Craft Advisory until 6 AM EDT early this morning for ANZ430. Gale Warning until 4 AM EDT early this morning for ANZ431. && $$ Synopsis...Klein Near Term...Gorse/Robertson/Meola Short Term...Robertson Long Term...Davis/Klein Aviation...Gorse/Klein Marine...Gorse/Klein/Robertson/Meola Equipment... is the U.S. government's official web portal to all federal, state and local government web resources and services.