Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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021 FXUS61 KPHI 211320 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 920 AM EDT SAT MAY 21 2016 .SYNOPSIS... Low pressure will deepen off the Virginia coast today and move offshore tonight and Sunday. An upper air disturbance will remain across the area from Sunday through Tuesday. High pressure will ridge across the area Wednesday and remain for part of Thursday. Another disturbance will arrive for the end of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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An upper-level trough from the Great Lakes to the Ohio Valley will gradually shift eastward through the day. As this occurs, low-level warm air advection will assist in strengthening isentropic lift. An area of pressure falls is forecast to take place off the coast of Virginia, which will allow surface low pressure to develop through the day. The combination of the surface low development, isentropic lift and large scale ascent incoming from the west will result in an area of rain moving across much of the area today. The forecast challenge is the timing and pin-pointing the area where the heavier rain sets up. An initial area of warm advection rain across western Pennsylvania is forecast by much of the guidance to weaken some as it moves east-northeast this morning. As this occurs, surface low development to our south allows more focused lift to shift southeastward. The NAM looks to compact regarding its rain shield despite its surface low closer to the coast. If this occurs, then even less rain would fall across the western zones. The latest thinking is that all areas get into the rain by the end of the day, however did slow down the pop increase for awhile with a northeastern extent. The heaviest rainfall is mainly from near the I- 95 corridor on south and eastward given the coastal surface low development. Given increasing moisture and lift, the rain may become moderate at times mainly across the southern and eastern areas. As the surface low develops and starts to track northeastward, the pressure gradient tightens to its north. This results in breezy conditions developing closer to the coast. At this time, any instability looks rather meager given the onshore flow therefore thunder was not added. As for the high temperatures, we used mostly a continuity/MOS blend. With the strengthening onshore flow, clouds and incoming rain, we are expecting a cool day. The northern areas may get warmer prior to the rain arriving. With the rainy areas taking on a more triangle look leaving the Philly Metro potentially drier for longer today, high temperatures may edge out a few degrees warmer along with northern NJ. With the 9:30 am update, the rain shield coming into western parts of the region has encountered some drier air at the surface. Radar returns are showing moderate to heavy rain falling in Berks county PA southward into the Delmarva. However, only very light is currently reaching the ground. Pops were trimmed back after reviewing recent modeling trends in the HRRR and RAP which show this area of rain decaying late this morning and the area of rainfall with coastal low forming further eastward.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... The upper-level trough is forecast to sharpen across the east tonight as an omega blocking pattern develops. This will drive surface low pressure northeastward and away from our area. As this occurs, the pressure gradient relaxes and therefore the winds that start out a bit gusty along the coast will diminish at least some. As the forcing shifts offshore with the surface low, any steadier rain is expected to also shift offshore early with some showers lingering. The showers will mostly be driven by the amplifying upper- level trough. These showers may be more organized just to our west within the trough axis itself. We therefore lowered pops some through the night but allowed for at least scattered showers through the overnight. With the idea of abundance of clouds around through the night, temperatures should not drop off considerably but it will be cool. As for the low temperatures, we used mostly a MOS blend. && .LONG TERM /SUNDAY THROUGH FRIDAY/...
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The upper air pattern will change considerably this week and by week`s end it will feel more like summer. The pattern begins with a closed low across the Middle Atlantic and northeast U.S. This low will fill and exit off into the Atlantic Tuesday night and Wednesday. The upper low will bring mostly unsettled conditions with scattered showers much of the time and a chc for an afternoon tstm across southern NJ and the Delmarva Monday. Temperatures from Sunday thru Tuesday will be mostly below normal...but cloud be at normal levels on Tuesday...depending on how much influence the upper low still has at this time. On Wednesday...Upper heights will rise and sfc high pressure will settle across the area. Under fair skies...temperatures will climb above normal...something which has not happened much during May. Highs in the low 80s are expected at this time. The warm weather will continue for the rest of the extended period...but rain chcs will also increase for Thursday and into Friday. By this time, several short waves from the Midwest will have traveled across the Ohio Valley and across the Middle Atlantic. Pops will mostly be in the chc range for now, since the details will not be able to be resolved until the week progresses a little more. Our pops are a good fit with our neighboring offices. Thunder will be possible during the period with the added instability with abundant daytime heating.
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&& .AVIATION /13Z SATURDAY THROUGH WEDNESDAY/... THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. Today...VFR ceilings lowering to MVFR this afternoon, then IFR toward late afternoon. KABE and KRDG may remain at VFR ceilings all day. Rain overspreads the area from southwest to northeast from about 14-18z. The heaviest rain should be near and south/east of KPHL. Light southeast winds, becoming east then northeast and increasing to 10-15 knots this afternoon with some gusts to 20 knots. The strongest winds are expected to be closer to the coast. Tonight...MVFR to IFR conditions overall, with perhaps VFR ceilings lingering at KABE. The greatest chance for IFR conditions should be from KILG to KPHL/KPNE to KTTN on south and east. LIFR ceilings cannot be ruled out at KMIV and KACY. Any remaining steady rain tapers early to some showers with some visibility restrictions at times. Northeast winds 10-15 knots with gusts up to 20 knots early (less at KABE and KRDG), diminishing through the overnight. outlook... Sunday thru Tuesday...VFR most of the time. Sct Showers thru the period may temporarily lower CIGS or VSBYS to MVFR/IFR. Tuesday night and Wednesday...VFR expected. Fog possible Tue night. && .MARINE...
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Low pressure will develop off the Virginia coast during the course of today, then move out to sea tonight. The pressure gradient will tighten today as a result with an onshore flow therefore increasing. It appears that the NAM is a bit aggressive with the surface low since it is more consolidated and stronger. We leaned toward a weaker wind field and therefore gales are not anticipated at this time. The strengthening wind field spreading north looks to take longer today, therefore we delayed the start time of the Small Craft Advisory from south to north. Given the surface low track and pressure gradient, we are more confident on the southern waters. The increasing winds will build the seas with these becoming more robust later today and tonight from south to north. The winds should drop below advisory criteria this evening therefore we kept the 02z ending time. Moderate risk for rip currents today as well with the onshore flow and increasing easterly winds. A cool, raw day to be out at the beach. outlook... Sunday and Sunday night...SCA seas expected to continue on the Ocean. Sub-SCA conditions across Delaware Bay. Monday and Monday night...SCA conditions possible on the Ocean with the lingering effects of the coastal storm. Seas 3-5 ft. Tue thru Wed...Mostly Sub SCA conditions expected.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EDT Monday for ANZ452>455. Small Craft Advisory from 4 PM this afternoon to 6 AM EDT Monday for ANZ450-451. Small Craft Advisory until 10 PM EDT this evening for ANZ430- 431.
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&& $$ Synopsis...O`Hara Near Term...Gaines/Gorse Short Term...Gorse Long Term...O`Hara Aviation...Gorse/O`Hara Marine...Gorse/O`Hara Tides/Coastal Flooding...

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