Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 271531 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1131 AM EDT Tue Sep 27 2016 .SYNOPSIS... A cold front will stall offshore from this afternoon through Wednesday. Low pressure systems over the Great Lakes and Southeast U.S. are expected to drift toward the region for the later half of the week before finally lifting into New England for the weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Wave of light showers over the Mid-Atlantic will drift to the NE into this afternoon, spreading some light rain showers into southern portions of E MD, S DE, and S NJ. Not much additional rainfall is expected. Clouds continue to stream into the region, but the back edge of the clouds should push to the south and east of N NJ and E PA through this afternoon, and clearing conditions expected as some dryer air works its way into the region behind the front and ahead of an upper low digging into the Great Lakes. The warmest temperatures this afternoon (highs in the mid 70s) will likely reside along the I-95 corridor and the warmer valleys farther to the north and west as these areas will clear out early enough in the day to receive strong daytime heating. Farther south and east toward the southern NJ coast and lower Delmarva, clouds will hold on longer into the afternoon, resulting in highs near 70F.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... The closed low will push slowly southward through the upper Midwest tonight. Meanwhile, a wave of low pressure is expected to develop along the stalled front to our south and east. We should be mainly dry tonight, even along the coast as the secondary low tracks far enough offshore. Good radiational cooling conditions may occur this evening across inland areas under clear skies and calm winds. Therefore, temps should drop quickly after sunset. Forecast lows are in the 40s for the valleys in northeast PA and northwest NJ. However, there is some uncertainty with the temperature forecast as high pressure builds southward. A light northeasterly wind may develop overnight, which could prevent temps from bottoming out. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Strong ensemble signal for a heavy rain event somewhere in the Mid- Atlantic region late this week with uncertainty on the location. Current indications are our CWA will be on the eastern edge of it. Wednesday through Friday: An area of low pressure (most noted in the upper levels) is expected to move southeastward toward our region from the Great Lakes and another area of low pressure will be forming in the Carolinas ( main surface low). The exact locations of these systems will determine where the highest risk for an excessive rainfall event will be. The GFS and Canadian operational models are more progressive with the upper low to our northwest which in turn forms the surface low just to our south. In turn a deep but narrow moisture axis ahead of these features tracks from the Eastern Gulf and Atlantic Oceans northwestward into DE and Eastern PA sending PW values from 1.5-2 inches, very high for this time of year. Sufficient lift and dynamics are present coupled with marginal instability to allow for heavy showers within this moisture axis. A few thunderstorms would be possibile as well with a tall skinny CAPE profile normally seen with more aggressive heavy rainfall events. A hindrance of the heavy rainfall threat looks to be fast moving convective elements in the axis though. This outcome would bring several inches of rainfall to areas south and west of Philadelphia. However, the NAM and the ECMWF model have all of these features further westward which would shift the moisture axis further inland to areas like Baltimore and State College. Waffling on the locations of the players and their respective locations is evident in the ensembles. Many members do suggest an axis of heavy rain sets up but are variable on location. The axis looks to be narrow, maybe only 100 miles wide. Overall, model trends have been to the west with the 00z cycle. So the forecast features heavy showers from Chester county PA south into Delmarva with just lighter showers from Philadelphia and points east. Showers will likely begin to develop from south to north across the region during the afternoon Wednesday. Precipitation is then likely to become more widespread along and near the main axis of moisture Wednesday night. This axis then could move little through Friday but gradually become cutoff from the deeper moisture. By Friday, the precipitation should become much lighter regionwide even with both areas of low pressure lingering over the Mid-Atlantic and Ohio Valley. Rainfall amounts from the individual ensembles are as impressive as the operational runs, so some totals over a few inches are likely just west of our region. It should be highlighted that areas outside of the heavy axis Wednesday night and Thursday may not see much more than lighter showers with rainfall under an inch total eventhough it rains off and on for a couple of days. Diurnal temperature swings should be very limited with clouds and rainfall with temperatures staying in the 60`s for most locations Thursday and Friday. Ahead of the rain, temperatures will be in the 70`s Wednesday. Winds will be out of the east from 10-15 mph on average Wednesday through Friday. Friday night through Monday: Lingering low pressure both at the surface and mid-levels could keep a few light showers around. Some breaks in the clouds are likely as well. The clearing process will be gradual. This will allow for high temperatures to get back into the 70`s for most of the region, with lows falling back into the 50`s. Ensemble guidance is likely a few degrees to cool by Sunday, not accounting for the more abundant breaks and warmer boundary layer temperatures. && .AVIATION /16Z TUESDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. CIGs/VSBYs continue to lift to VFR. Few light -SHRA possible at KMIV/KACY through early afternoon, but no restrictions expected. W-NW winds will back to the SW later this afternoon, but wind speeds will be 10 KT or less. VFR this evening. Patchy fog may develop overnight at the fog-prone terminals, resulting in MVFR or locally IFR visibility early Wednesday morning. OUTLOOK... Wednesday through Friday night: Mainly MVFR or IFR with periods of showers. easterly winds around 10 knots. Saturday: Improving to VFR with isolated showers. East winds 5-10 knots.
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&& .MARINE...
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Occasional rain showers will pass through the waters through this afternoon. Most rainfall will be light, but a few moderate showers are possible. W to NW winds will become SW going into this afternoon and evening, and wind speeds will diminish to 5-10 KT. OUTLOOK... Seas building to five feet by Wednesday night with low pressure nearby. Seas will likely be very slow to drop as the low pressure systems linger. Winds will be easterly with SCA gusts by Wednesday night. Some gusts may reach gale force from ACY northward Wednesday night and Thursday. Winds should decrease by Saturday below SCA criteria. Rip Currents... The moderate risk for the formation of dangerous rip currents is forecast at the NJ/DE beaches today. Conditions may improve to yield a low risk by late afternoon and winds turn offshore and decrease.
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&& .TIDES/COASTAL FLOODING... Spotty minor tidal flooding on the oceanfront is possibile with the Thursday and Friday high tidal cycles. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Gaines Near Term...MPS Short Term...Klein Long Term...Gaines/MPS Aviation...Gaines/Klein/MPS Marine...Gaines/Klein/MPS Tides/Coastal Flooding...

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