Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 291928 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 328 PM EDT Thu Sep 29 2016 .SYNOPSIS...
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Low pressure to the west will gradually move northeast toward the Great Lakes and weaken through Sunday. It will continue to slowly move eastward by Tuesday and bring a cold front through the region. Several other waves of low pressure will move around the larger low and unsettled weather will prevail. High pressure builds to our north for Tuesday into Wednesday.
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&& .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... Noon ESTF: essentially no change to the qpf philosophy. 12z GFS looks way underdone despite considerable instability aloft late today with NW-SE PWAT axis through the Delmarva and Del Bay and increasing to nearly 2 inches. Cold air damming front just off the coast is going to be primary lift mechanism of the unstable air in a continuing mdt waa pattern. looks to me like a 6 hr period of R+ 00z-06z s De and 3 hrs in PHL around 09z-12z Fri. 1045 AM ESTF: Have upgraded to a coastal flooding warning tonight based on modeled increasing ene flow this evening and increasing positive surge departures over the last 15 hours. Standard warning details will be added to the warning text once the advisory portion this morning has been discontinued. 930 AM ESTF: No change in qpf thinking from the 8am below except to reduce pops NNJ and far ne PA, also dropping small chance thunder in a portion of that area. Have lowered temps and dews near and north of I78. Have increased gusts 5 mph throughout the area. 8 AM ESTF: Thinking that guidance is constantly modeling max rainfall far too far west. the increasingly ely inflow at the sfc this evening combined with being closer to the sbli subzero and constant instability aloft through Friday and increasing 850 inflow to -4SD as well as growing pwat axis to 2" by morning suggests to me another 2-6" in much of delmarva between 12z today and 12z Friday. That will include more tstms. RER`s already GED and will update this later today. Dual pol STA looks much better than the legacy STP at DOX and DIX for the Delmarva (DOX returned to service early yesterday afternoon, missing the morning heavy rain that occurred MD e shore). That means adding 70 to 90% to LEGACY output on the DELMARVA should be helpful/useful reality. 8.5" max so far...isolated...more general 4-5 inchers parts of the Delmarva. Thanks to extensive cloud cover, do not expect temperatures through the day to be much higher than what we saw overnight. This will also limit instability, though isolated thunderstorms are still possible thanks to meager elevated instability. Tonight...low level flow should shift more easterly, and mid and upper level flow back to southerly. As a result, another round of rain, heavy at times will be possible for the southern half of our region. The persistent surface high just to our north and associated subsidence may be enough to keep heavy rain out of the northern half of the region, but with the onshore flow, expect at least light rain for almost all of the region. Persistent low level cloud cover will once again limit nocturnal radiational cooling overnight, meaning our lows should be within 10 degrees of Thursday`s highs. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/... A couple of areas of low pressure at the surface will continue to be located to our south and west, while high pressure remains to our north. In between, a stationary boundary will remain in place just to our south. This will keep an onshore flow across our area. Meanwhile, a closed low aloft will be spinning to our west as well. This will keep a persistent southerly flow aloft across the area at the same time. With the southerly flow aloft, cloudy conditions, and a chance of light rain and/or drizzle will continue across the area. It will not likely rain the entire time, but several periods of rain will be associated with several short waves/vorticity impulses rounding the closed low aloft to our west. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
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Unsettled weather continues to be forecast for much of the extended period, especially the end of this week into the weekend. The persistent upper low to the w will gradually will and move n and ewd. Various different disturbances will move around this low, and these individual features are difficult to time at best. So unsettled wx is expected Sat and again on Sun, there shud be more dry than wet periods and Sun will be drier than Sat, there will still be lingering precip and clouds into the weekend. The upr low finally crosses thru New Eng late Mon into Tue and any last vestiges of precip shud go with it. There cud be some precip on Mon, but again, small chcs. Tue cud end up being a decent day then by midweek the global mdls have divided themselves into 2 camps, a faster and slower camp. If the faster camp is correct we could...repeat could (also could not) have impacts from tropical storm Matthew by mid or late week. If the slower camp is correct, then any potential impacts would be much later. There are still a lot of differences in the mdls wrt to both track and speed of the sys. Please see latest NHC advisories.
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&& .AVIATION /19Z THURSDAY THROUGH TUESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Through tonight...MVFR cigs variable IFR conditions in showery rains. e-ne winds gust 20-30 kt at times, with the gusts 30 kt vcnty PHL/ILG and ACY. Iso thunder possible but not in the TAF attm due to low confidence. Friday...MVFR CIGS variable IFR conditions in stratus/periods of rain-drizzle. East to northeast winds are expected to gust 20-30 knots for much of the area Friday, with the strongest winds near the coast. OUTLOOK... Fri night-Sat night...A prolonged period of MVFR or IFR conditions. Some SHRA expected to affect the TAF sites through the period. Moderate confidence. Sun-Mon...Some improvement, possibly to VFR. Low chc SHRA. Moderate confidence.
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&& .MARINE...
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GLW continued with verification of gales already most areas. Only upper DE Bay waters have not yet verified and they may not til late today. May extend GLW in the 330 PM fcst through Friday for southern NJ and extend NNJ into Friday night, in large part dependent on the afternoon collaboration. Waves heights 8 to 13 ft throughout through Friday morning slowly subsiding late Friday. Outlook... Fri night-Sat...Gale warning has been extended into overnight Friday night for the northern waters and could be extended further. Elsewhere, conds will gradually decrease into Saturday, but a backend SCA is likely before conds drop below. Sat night-Mon...Wind and seas expected to drop below SCA criteria Sat night or early Sun mrng and remain there thru Mon. RIP CURRENTS... There is a high risk for the formation of dangerous rip currents along the NJ shore and the DE beaches today. Wave heights along the coasts 5 to 9 feet. Friday...Mount Holly`s last day of our daily 530am surf zone forecast this year...probably high risk due to residual heavy surf DE and ongoing easterly gale NJ.
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&& .HYDROLOGY... Thinking that guidance is constantly modeling max rainfall far too far west. The increasingly ely inflow at the sfc this evening combined with being closer to the sbli subzero and constant instability aloft through Friday and increasing 850 inflow to -4SD as well as growing pwat axis to 2" by Friday morning suggests to me another 2-6" in much of Delmarva between 12z today and 12z Friday. That will include more tstms. note a possible sfc trough just off the mid Atlantic coast. That aids the lift near the Delmarva coast. its usually the first lift area that gets the heaviest convective associatedrainfall. RER`s already GED and will update this RER later today. Dual pol STA looks much better than the legacy STP at DOX and DIX for the Delmarva (recall DOX came back on line early yesterday afternoon, missing the Wednesday morning heavy rain that occurred on parts of MD`s eastern upper shore). that means add 70 to 90% to LEGACY out put on the DELMARVA should be helpful. 8.5" max (isolated in s DE), mostly 4`s and 5`s, so far. 00z/29 ECMWF max rainfall next 48 hours about 5" Md E shore. && .TIDES/COASTAL FLOODING... Have upgraded to a coastal flooding warning tonight ocean county southward, based on modeled increasing ene flow this evening and increasing positive surge departures over the last 15 hours. Standard warning details will be added to the warning text once the advisory portion this morning has been discontinued. Expect a resurgence of inflow wind with gusts near 40 kt at the time of this evenings high tide cycle. Otherwise advisories continue as previously forecast today. Minor tidal inundation flooding has to be occurring this morning southern NJ and De coasts. The constant inflow may be making it a bit worse in the back bays than on the coast. Last evening ACY was a touch into flood at 712PM (6.35 MLLW with threshold 6.3). This morning Atlantic City NOS registered 6.6Ft MLLW or .3 ft over the threshold. This morning Lewes DE registered 6.6` MLLW (threshold 6.3), Cape May 7.15 ft (threshold 7 feet MLLW) Since tonight`s high tide cycle is higher than that of this morning, and inflow should be stronger, flooding will be worse...probably half a foot to a foot deeper, and expect reaching MDT threshold s NJ and De Atlc shores. The following two high tide cycles Friday will probably endure minor tidal inundation flooding. Additionally...waves this morning were at least 7 to 8 feet on the coast (BTHD1) and may grow to 10 feet tonight. This could add splashover wave runup problems to the lowlands. && .CLIMATE... September will be/is a top 10 warmest month through most of our forecast area, for the 3rd consecutive month! Also September will be/is enjoying a second consecutive top 5 warmest month in the period of record for PHL and ABE. We`ve run the actual numbers through the 28th, then added the forecast high/low for 29th and 30th. Philadelphia: Expect a 73.8 degree average or about 5 degrees above the monthly normal of 69.1. This will be a #4 or #5 warmest September on record for Philadelphia. Philadelphia September average temperature rankings 75.4 -1881 74.5 -2015 74.1 -1931 74.1 -1930 73.8 -2005 and 2016 #4 (small chance PHL avg will be 73.7 or #5 ranking) 72.9 -2010 72.9 - 1921 Philly ranked #7 warmest July followed by a warmest ever August in the POR dating back to 1874. Atlantic City: Expect a 71.2 degree monthly average temperature, or 4 degrees above the monthly normal of 67.2. This would equate to an 8th warmest September in the period of record dating back to 1874. Atlantic City recorded a #7 warmest July and then a warmest ever August. 73.3 - 1961 72.8 - 1881 72.3 - 1931 and 1930 71.7 - 1921 71.6 - 2015 71.5 - 1933 71.3 - 2005 71.2 - 2016 #8 71.0 - 2010 70.6 - 2011 and 1906 Allentown: Expect a monthly average of 69.4 degrees, or 5.5 degrees above the monthly normal of 63.9 and a 4th warmest September on record. Allentown ranked 8th warmest July and #2 warmest August in the period of record. 70.8 - 1961 70.3 - 1980 69.7 - 2015 69.4 - 1931 and 2016 (small chance Allentown will rank #5 at 69.3 degrees) 68.7 - 2005 Daily rainfall records for September 29 where there is maybe a small chance of exceeding: PHL 1.87 1963 ILG 1.97 1963 GED 1.35 2015 RDG 3.55 1934 && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Flash Flood Watch through Friday afternoon for PAZ070-071. NJ...Coastal Flood Warning from 6 PM this evening to 1 AM EDT Friday for NJZ020>027. High Rip Current Risk through Friday afternoon for NJZ014- 024>026. Flash Flood Watch through Friday afternoon for NJZ016>027. Wind Advisory until 6 AM EDT Friday for NJZ020-022>027. Coastal Flood Advisory until 2 AM EDT Friday for NJZ016. Wind Advisory from 11 PM this evening to noon EDT Friday for NJZ013-014. Coastal Flood Advisory from 6 PM this evening to 1 AM EDT Friday for NJZ012>014. DE...Coastal Flood Warning from 6 PM this evening to 1 AM EDT Friday for DEZ002>004. High Rip Current Risk through Friday afternoon for DEZ004. Flash Flood Watch through Friday afternoon for DEZ001>004. Wind Advisory until 6 AM EDT Friday for DEZ003-004. Coastal Flood Advisory until 2 AM EDT Friday for DEZ001. MD...Flash Flood Watch through Friday afternoon for MDZ008-012-015- 019-020. MARINE...Gale Warning until noon EDT Friday for ANZ452-453. Gale Warning until 2 AM EDT Saturday for ANZ450-451. Gale Warning until 8 AM EDT Friday for ANZ454-455. Gale Warning until 2 AM EDT Friday for ANZ430-431.
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&& $$ Synopsis...Nierenberg Near Term...Drag Short Term...Drag Long Term...Nierenberg Aviation...Drag/Nierenberg Marine...Drag/Nierenberg Hydrology... Tides/Coastal Flooding... Climate...

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