Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 240832 AAA AFDPHI Area Forecast Discussion...UPDATED National Weather Service Mount Holly NJ 432 AM EDT Sat Sep 24 2016 .SYNOPSIS... A cold front will move southward through the Delmarva region early this morning. High pressure over eastern Canada will build southward toward the mid-Atlantic states through the weekend. The center of the high will retreat off the East Coast Monday afternoon. A cold front is expected to move in from the west Monday night before pushing offshore sometime on Tuesday. An area of low pressure may cutoff to our west midweek. High pressure looks to eventually build back into the region late in the week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... A cold frontal boundary will continue to push southward through the region this morning. As a result, a thick overcast will continue to build southward ahead of the front. The overcast should slowly lift and thin out this afternoon with the front moving well south of the region. Due to the overcast, temperatures will struggle to rise into the low 70`s today. The NAM and HRRR indicate enough lift for a brief light shower or two across southeastern PA, central and southern New Jersey during the morning hours. We have some slight chance pops across these regions. Any rainfall will be light. Winds will increase out of the north and northeast around 10-15 mph with slightly higher gusts today centered in the late morning and early afternoon hours. High pressure building toward the region later today will actually allow winds to decrease some by days end. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... High pressure will continue to build southeastward into the region tonight eroding away any remaining clouds. Conditions will be favorable for radiational cooling across the region. More rural locations should fall a few degrees below MET and MAV statistical guidance with most of the region in the 40`s for lows. Still to warm for any frost concerns. && .LONG TERM /SUNDAY THROUGH FRIDAY/... High pressure centered over Hudson Bay and along the Ontario-Quebec border will continue to build southward into the mid-Atlantic region on Sunday. The CAA pattern that develops behind the cold front today will persist over the area on Sunday but start to weaken. Even with abundant sunshine, high temperatures in the lower 70s (60s in the higher elevations) will likely fall shy of climo but only by a couple of degrees. Nonetheless, it will certainly feel like autumn after the summer-like warmth that has occurred across the region through much of September. On Sunday night, there will be a very favorable setup for radiational cooling with the surface ridge axis in the area. With clear skies, calm winds, a very dry airmass in place and increasingly longer nights this late in the year, Sunday night has the potential to be the coolest night of the month and possibly since late May in spots. Under this type of pattern, sided with the colder MOS guidance, if not below it, for low temperatures. Patchy frost may form in the sheltered valleys of northeastern PA and northwestern NJ before sunrise early Monday. High pressure and H8 ridge axis will shift offshore on Monday. Southerly return flow around the high will establish a WAA pattern. However, the WAA will be relatively weak and brief in duration, precluding a significant warm up early next week. This is because a progressive cold front will move through the region Monday night. PoPs were increased to likely for Monday night into Tuesday morning with the 00Z model runs coming into much better agreement with the timing of the rainfall along and ahead of the front. While any rainfall would be much welcomed, it won`t be a drought buster for portions of eastern PA and NJ that are under a D1 drought with QPF amounts generally around one-half inch. There is a significant degree of forecast uncertainty for the second half of the work week with the potential for the upper low over the Great Lakes to become cutoff. Models are having a difficult time handling this system, exhibiting a considerable amount of run to run spread. For example, the latest 00Z ECMWF showed a cutoff low forming over the Ohio Valley Wednesday night and stalls it near the central Appalachians for several days (into next weekend). This run gives us an extended period of wet weather. Meanwhile, the 12Z ECMWF from yesterday showed a much more progressive solution as the low harmlessly kicks out to sea midweek, followed by high pressure in control late in the week and next weekend. The GFS followed the opposite trend. Both the GEFS and ECMWF ensembles exhibit large spread. For now, introduced slight chance PoPs Wednesday afternoon through Thursday but did not go any higher due to the low predictability. Temperatures should be near normal mid to late week. && .AVIATION /08Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. A MVFR stratus deck will continue building southward through the morning hours. This deck has already reached KABE and is encroaching upon KTTN and KRDG. Timing uncertainties of a couple of hours are present with the arrival of MVFR ceilings, so TEMPO groups were included to represent this in the 06Z TAFS. This deck will slowly thin out and lift from north to south during the late morning and afternoon hours. North-northeasterly winds from 7-15 knots are possibile today with any gusts in the late morning or early afternoon hours. Winds will decrease tonight. OUTLOOK... Sunday through Monday...VFR. Light winds Sunday. Southerly flow strengthens Monday with gusts to 20 kt possible during the afternoon. Monday night and Tuesday...SHRA likely as a cold front moves in from the west. MVFR conditions probable with the potential for IFR in heavier/steadier rain. Gradual improvement from west to east during the day Tuesday. Tuesday night and Wednesday....Predominately VFR. && .MARINE... Wave heights will gradually build to around five feet on the ocean waters by this afternoon after the passage of a cold front. Winds will also increase from the north and northeast around 15 knots with some gusts to 25 knots possibile. A SCA is in effect through tonight as the five foot seas may linger. OUTLOOK... Sunday...SCA extending into Sunday morning for the coastal Atlantic zones. Long-period southeasterly swells from Tropical Storm Karl is expected to continue along the coast during the morning. Seas around 5 ft are expecting in the morning before subsiding to 3-4 ft in the afternoon. Monday through Wednesday...No marine headlines expected. Winds may gust out of the south to around 20 kt Monday afternoon and evening ahead of a cold front. Rip Currents...Both long period swells and building seas suggest conditions may be more representative for a moderate risk of rip currents. A moderate risk for the formation of dangerous rip currents will likely continue into Sunday with long-period southeasterly swells from Tropical Storm Karl expected to impact the eastern seaboard. && .CLIMATE... The high temperature yesterday, September 23rd, at Allentown, PA (KABE) was 90 degrees. While this did not set a daily high temperature record, it marked the second latest 90-degree day in the calendar year at KABE since 1970. During this span, the only other date that reached 90 degrees this late in the year was October 8, 2007. Note: A high temperature of 90F on October 8, 2007 was also the latest occurrence of a 90-degree day in a calendar year on record for KABE. Allentown historical climate records go back to 1922. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until noon EDT Sunday for ANZ452>455. Small Craft Advisory until noon EDT Sunday for ANZ450-451. && $$ Synopsis...Klein Near Term...Gaines Short Term...Gaines Long Term...Klein Aviation...Gaines/Klein Marine...Gaines/Klein Climate...Klein is the U.S. government's official web portal to all federal, state and local government web resources and services.