Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 142001 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 401 PM EDT Thu Sep 14 2017 .SYNOPSIS... The remnants of Irma will drift across the Mid Atlantic and through New England tonight through Friday. High pressure will briefly build to our northwest Saturday, before weakening on Sunday and retreating northward through Monday. A weakening frontal boundary will approach the area Tuesday, before dissipating to our northwest on Wednesday. Tropical cyclone Jose will be monitored for its northward progress off the East Coast later in the weekend into the middle of next week. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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Our forecast grids were updated at 1230 PM through Friday with updated rain chances, reduction in areal coverage of thunder. Included T with heavy rain e PA and NNJ this aftn. Also added patchy fog late tonight using the UPS crossover tool. This per skies expected to at least partially clear after whatever coverage of showers persists into the evening hours. PWAT is a tad less today (~1.5") than ydy but still allows for brief heavy rain showers. Its a somewhat tropical looking sky here today. The southwest winds gusty 15-20 kt mid and late afternoon will diminish tonight and become nearly calm in the vicinity of a weak wind shift late tonight. Mesoscale guidance indicates it will get active in the PHL-TTN-BLM corridor late today. Have not done much enhancing of the fcst due to my uncertainty. This forecast update this afternoon is a 50 50 blend of the 12z/14 GFS/NAM MOS.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
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Lingering mid level trough may spark afternoon showers....we`re not sure where but favored hilly terrain of ne PA and nw NJ but cant rule out a brief shower anywhere. Otherwise a nice day and warm...about as warm as it was today (Thursday the 14th). This part of the forecast is a 50 50 blend of the 12z/14 GFS/NAM MOS.
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&& .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... The three forecast challenges during this time frame will be the potential for convection Saturday and Sunday, the proximity of Tropical System Jose in the Monday thru Wednesday time frame, and just how warm it gets Wednesday into Thursday. The long wave pattern features the northern stream well to the north, with a short wave trough lifting out of the intermountain west on Saturday and into Hudson`s Bay on Sunday, and a mean long wave trough becoming established over the intermountain west by midweek. In fact, this is expected to become a high amplitude trough, and conversely, a high amplitude ridge takes shape downstream from the Mississippi Valley into the Great Lakes. Friday night through Sunday...A diffuse, summer-like pattern will be in place, with a weak mid-level trough aloft, and a predominantly diurnal surface reflection. Several short-wave disturbances aloft, along with sufficient columnar moisture, and the surface reflection should be enough to initiate scattered showers as convective temps are reached both Saturday and Sunday. While Bulk Shear is very low given the weak flow, Surfaced Based Cape ranges from 0.5 to 1 kJ, mainly northwest of the fall line. In addition, Precipitable Water values will be around 1.5 inches. This all points to the chance for embedded thunderstorms with heavy downpours. With the expected slow storm motions, some urban and poor drainage flooding is possible, and we have mentioned in the HWO. While Saturday is favored for a greater coverage of showers and thunderstorms compared to Sunday, the highest chance on both days will be northwest of the fall line. Activity will be diurnally driven. While there is the potential for patchy fog both Saturday and Sunday mornings, conditions are more favorable for development on Saturday morning. Temperatures will be several degrees above normal over the weekend, but cooler near the coast, with the development of a sea- breeze on Saturday and more synoptic easterly flow on Sunday. In the Monday through Wednesday time frame, all attention turns to Tropical System Jose, and the potential for impacts in the middle Atlantic. First, the center of the storm has been difficult to fix, given the sheared nature of the system, and this could introduce further uncertainty into the forecast. The National Hurricane Track has shifted westward, about 60 NM as of the 15Z advisory. Jose is expected to make its way west-northwest through a weakness in the subropical ridge thru Saturday, then make a gradual northward turn on Sunday, under the increasing influence of a broad shortwave trough ejecting out of the lower Mississippi Valley and into the middle Atlantic. There is still model disagreement regarding the location, amplitude, and tilt of this feature, which will be critical to Jose`s future track. With Jose increasingly bounded by high amplitude ridging to the east and west, the aforementioned short wave may take it on a north to north-northeast track for quite some time. So the question may be: just how far west will Jose get before making the gradual turn to the north and eventually northeast? The 12Z/14 operational model runs and ensemble means generally take Jose along or inside the 70W benchmark through Tuesday night into Wednesday, before turning it to the northeast. The operational Canadian is an eastern outlier (just east of 70W), while the UKMET is a western outlier (Cape Hatteras). It should be noted that the UKMET has been consistently verifying too far west with Jose. Furthermore, ensemble mean sea-level pressure and spread indicates the disagreement amongst the individual members is with respect to where/when Jose makes the northeast turn, which is an indication that very few members turn it westward toward the coast north of Cape Hatteras. The 15Z NHC forecast takes Tropical System Jose to a point roughly east of Cape Hatteras and south of Providence Rhode Island by 12Z Tuesday (day 5). From there, WPC takes it just east of due north to a point around 50 miles south of the 40N/70W benchmark by 12Z Wednesday (day 6). It`s very important to note that the systems wind field may very well expand by this point due to the interaction with the short-wave. Therefore, it`s even more important not to focus on the center of the storm. In fact, for some perspective, NHC track errors average 211 NM by day 5. At this point, indirect impacts, such as elevated seas and the increased risk for the formation of dangerous rip currents are likely to continue. For maritime interests, the current track increases the potential for direct impacts, such as increased winds (see the marine/rip currents section below). However, it remains too early to speculate on any direct impacts to land, such as wind, rain, and coastal flooding. That said, with the moon at syzygy on the 20th, astronomical tides will become increasingly elevated (AOA 5.0 ft at the Atlantic City NOS) late this weekend into the middle of next week. The synoptic flow will be onshore Sunday through at least Tuesday, along with larger long period swells in association with Jose making their way to the coast. Therefore, we will be watching for the potential of coastal flooding, especially Monday into Tuesday. To reiterate the previous forecaster, please stay tuned to the National Hurricane Center for the latest official track and forecast regarding Hurricane Jose. Remember, we are at the peak of hurricane season for the Atlantic Basin, so it is important to have a hurricane plan in place. For tips as to what to have in your hurricane plan, please check http:/www.nws.noaa.gov/om/hurricane/plan.shtml For Wednesday and Thursday, a gradual return to fair weather is expected with above normal temperatures. In fact, given the modeled subsidence behind departing Tropical System Jose, we expect temperatures to be at least several degrees above average during this time frame, and have went above guidance. However, we are still well below daily record highs, which are at least 90F in at our long term urban corridor climate sites (e.g., PHL). && .AVIATION /20Z THURSDAY THROUGH TUESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Remainder of this afternoon...Mostly VFR cigs except brief MVFR or IFR conds in any heavier showers. Small chance of a thunderstorm, especially NNJ but no thunder in the TAF attm due to low probability hitting our TAF sites. South to southwest wind gusting 15-20 kt this afternoon along and east of I95. Tonight...VFR but patchy IFR conds in st/fog may develop late. Tempo group for expected fog is in the 18z TAFS... with support from the METMOS guidance. Leftover evening showers. South to southwest wind becoming nearly calm late at night. Friday...VFR sct-bkn aoa 5000 ft. A small chance of showers in the afternoon but probability too low to mention in the TAF. Light west wind trend south late in the day. OUTLOOK... There is the potential for some IFR in low clouds and fog Friday night into Saturday morning, and again Saturday night into Sunday morning, although confidence is greater during the former period. Scattered showers and isolated thunderstorms may lead to at least MVFR restrictions both Saturday and Sunday afternoons, especially at ABE and RDG, with greater coverage expected on Saturday afternoon. Otherwise, predominantly VFR during this time frame. Looking ahead to Monday and Tuesday, conditions may lower at times to at least MVFR in late night/early morning low clouds and fog, as well as afternoon showers. Otherwise, VFR. Winds will be generally variable in direction on Saturday, then becoming southeast on Sunday, at or below 10 knots on both days. Winds will back to the north-northeast on Monday and Tuesday, with gusts to at least 20 knots possible, especially at ACY.
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&& .MARINE... Winds and seas should stay below small craft advisory criteria through Friday. A few gusts near 20 KT will be possible this evening on the Atlantic Coastal waters. Outlook... The marine forecast in the Monday through Wednesday time frame is highly dependent on the outcome of Tropical System Jose. In addition to elevated seas due to Jose`s swells reaching the coastal waters, the pressure gradient due high pressure to the north will lead to increased winds. Seas may begin to flirt with 5 ft Saturday into Saturday night, increasing to around 9 feet late Monday into Tuesday. An easterly flow will develop on Saturday, becoming northeast Sunday into Monday, and perhaps north on Tuesday. Wind gusts up to around 25 knots are possible Monday into Tuesday. Therefore, at least SCA- level conditions are possible Saturday through Tuesday. There is also a low potential of gales on Tuesday, as Tropical System Jose makes its closest approach, and the wind field expands. The wind/ sea-state potential has been mentioned in the HWO. Rip Currents... For the rest of today, we have forecasted a marginal moderate risk for the formation of dangerous rip currents. Long period (10 second) southeasterly swells from tropical cyclone Jose continue. Looking ahead...a low or moderate risk is expected Friday then slowly building swells from Jose arrive this weekend, reaching their highest values around next Tuesday before subsiding. It Looks like a moderate risk for Saturday and moderate or high risk Sunday, with HIGH risk Monday and Tuesday. && .EQUIPMENT... The Millville (MIV) observation is unavailable due to an FAA outage, and the Somerville (SMQ) observation is unavailable due to a web issue. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Franck/Robertson Near Term...Drag Short Term...Drag Long Term...Franck Aviation...Drag/Franck Marine...Drag/Franck Equipment...

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