Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 290820 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 420 AM EDT Thu Jun 29 2017 .SYNOPSIS... High pressure will become anchored off the East Coast through Saturday, before moving farther out to sea into early next week. A weak cold front will settle into our region late Sunday, however it will then stall in our area next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Early this morning...broken deck near 15000 feet will somewhat limit radiational cooling. Light southwest wind. Today...Mostly sunny with cu/sc forming 9am to Noon and then dissipating late in the day with patchy cirrus above. a gusty southwest wind to 25 mph this afternoon. Dewpoints 10 to 15F warmer than ydy. Max temp this afternoon 1 to 5f warmer than normal. Fcst basis was generally a 50 50 blend of the 00z/29 GFS/NAM MOS except the GFS MOS temps look too cool today so used a combo of ydys max temp (continuity) with the warmer 00z/29 METMOS. Of interest this NAMMOS shows better cooling along the coasts late this afternoon as an increasingly gusty south wind cools the beaches and increases the risk for the formation of dangerous rip currents, especially heading toward the 7PM low tide (which occurs after beach patrols go home) - a more risky time to be in the water without the immediately swift and sure rescue opportunity provided by on-duty lifeguards. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/... Fair (there may be a deck of sct-bkn clouds near 7000 ft, especially late). Warm air advection with a stirring southwest wind with scattered gusts 15-20 mph. No NSSL WRF or HRRRX for 2 days but the 00z/29 NCAR ens have a fair amt of measurable I-80 north tonight. That plus the 00z/29 EC elevated instability is quite robust with SWI near -3 and TT51 toward dawn in the Poconos. Think the GFS is going be more accurate than the qpf dry 00z/29 NAM/EC. Otherwise, the forecast basis was a 50 50 blend of the 00z/29 GFS/NAM MOS. Low temps tonight 5 to 11 degrees above normal. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Summary...Heat and humidity for awhile, although the humidity may lower for a time next week. Times of some convection, with Saturday afternoon and night looking to currently have the greatest chance. The details though will depend on the timing and location of short waves, and a weak cold front that ends up stalling across our region next week. Synoptic Overview...A strong short wave carves out a trough in the Midwest and Great Lakes Friday and Saturday then gradually shifts eastward later in the weekend and next week. There may be several stronger short waves within this trough, which will impact the sharpness and timing eastward. At the surface, high pressure remains anchored offshore which will result in a very warm to hot and humid southwest flow for awhile. The main feature is a weak cold front that will settle into our area late Sunday, however it looks to then stall in our vicinity next week. The presence of the incoming front and trough nearby will result in some convection. We used a model/continuity blend for Friday through Saturday night, then blended in the 00z WPC Guidance thereafter. Some adjustments were then made following additional collaboration with our neighboring offices. For Friday...High pressure anchored offshore will maintain a southwesterly surface flow across our area. The low-level moisture increases with dew points into the 60s. As the low-level warm air advection continues in addition to perhaps some subsidence on the northwest side of offshore high pressure, daytime highs will peak around 90 degrees for several areas. This combined with higher dew points will result in heat indices into the mid 90s during peak heating. There will be a southwesterly breeze, and as mixing deepens the dew points may locally lower some during peak heating which in this case would keep the heat indices a bit lower. The dew points however are expected to increase at night as mixing lessens and additional moisture advection occurs from the southwest. There does not appear to be a whole lot of forcing to initiate convection, with perhaps some early near the Poconos within warm air advection. A few storms may then develop mostly in the afternoon due to terrain induced circulations/convergence. Therefore, we scaled back the slight chance to low chance PoPs and confined them to the far northwestern areas especially for the afternoon into the early evening. For Saturday and Sunday...Low pressure is forecast to track to the north of our area into Sunday. This will pull a cold front slowly eastward, and it is currently forecast to arrive into our area late Sunday. The synoptic setup would suggest though that a lee-side trough will be in place. The main upper-level trough is forecast to hang back over the Great Lakes for Saturday, then more eastward progress occurs Sunday. The bulk of the convection Saturday may reside just to our west closer to the frontal zone, however some convection is anticipated tied to the lee-side trough as buoyancy and instability should be sufficient to allow for at least scattered convection to develop. Much of the guidance has turned cooler for Saturday, which is primarily due to more cloud cover and a quicker development of convection. The cloud cover initially may be overdone in the guidance, however the airmass will be very warm and humid. Given the uncertainty, did not change the temperatures much. The current forecast has the heat indices peaking into the mid to perhaps upper 90s in the afternoon Saturday. It is still possible a heat advisory may be needed, mainly for the urban corridor from Wilmington to Philadelphia/Camden to Trenton. There is still some potential that upstream organized convection moves across our area Saturday night. Given less certainty with the overall coverage, held PoPs no higher than the chance range. The weak cold front slowly arrives later Sunday, however a lee-side trough in place may still serve focus for some convection. There could be a lull however between short waves, therefore only went with low chance PoPs during the daytime then trended dry at night. For Monday and Tuesday...As an upper-level trough remains in place across the Northeast to the northern Mid Atlantic, Monday may end up dry despite a weak frontal zone stalling in our region. There does not appear to be much in the way of forcing. A weak surface low may track along this boundary though and induce some convection during Tuesday. This potential could be modulated by upstream convection. Overall a very warm to hot airmass should remain in place, although the dew points should lower some especially for about the northern half of the forecast area. For Wednesday...The influence of a west to east frontal zone should eventually shift southward. This should allow a bit more cooling to work in from the northwest, however there still looks to be the chance for some convection. This may be more south than north where deeper moisture resides, however this will depend on the position of the front and timing of short wave energy. && .AVIATION /08Z THURSDAY THROUGH MONDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Through today...VFR sct-bkn aoa 15000 ft through 12z, then a sct deck of 7000 ft develops late this morning with 3000-4000 ft sct developing in NJ this afternoon where a little more low lvl moisture is modeled. Southwest wind becomes gusty 20-25 kt this afternoon. Tonight...VFR sct to possibly bkn clouds near 6000 or 7000 ft. South to southwest wind with scattered gusts to 15 kt. We noted guidance hinting at LLWS late tonight but its not in the fcst attm. OUTLOOK... Friday and Saturday...An isolated thunderstorm possible Friday mainly near and north and west of KABE and KRDG. Some showers and thunderstorms should then increase Saturday from west to east. Significant but brief reduced conditions, especially visibility, will occur during any heavier showers/storms. Southwest winds increase each day to 10-15 knots, with local gusts up to 20 knots possible. Sunday...Mostly VFR, however scattered showers and thunderstorms are possible. West-southwest winds 5-10 knots. Monday...VFR overall. West-northwest winds less than 10 knots. && .MARINE... SCA headlines posted. Winds and seas will begin to build mid day. Small craft advisory conditions are expected to develop by mid afternoon along the Atlantic coastal waters. 30 kt gusts should develop north of Little Egg inlet this afternoon (ACY). On the Delaware Bay, expanded the SCA to the upper Bay and ended it later on DE Bay. OUTLOOK... Friday...A Small Craft Advisory may be needed during the day, especially for the ocean zones. However, held off for now as there is some uncertainty in how much mixing occurs outside of right along the coast. For now, capped the wind gusts at 25 knots. This will also have an impact on the seas. Any increase in the winds should be diminishing at night. Saturday and Sunday...Other than some increase in the winds Saturday afternoon, the conditions are anticipated to be below Small Craft Advisory criteria. Some showers and thunderstorms are expected at times. Monday...Sub-advisory conditions are anticipated with weaker winds. RIP CURRENTS... We start low this morning and remain low on DE this afternoon but will forecast a Moderate risk for the formation of dangerous rip currents on the NJ coast as southerly winds increase with gusts to 25 or 30 mph. Low risk does not mean NO risk and it is always advised for ultimate safety, swim in the presence of the lifeguards. They have the flotation devices that can more easily save a life. Where the surf zone waters are steeper, there may be fewer rip currents, but there, the danger of wave related injury increases. An ongoing DE 6+ year study shows the bulk of the injuries (45%) occur in 10-20 year old age bracket with also another 40 pct of the injuries to 40-60 year old age group. The 10-20 year old surf zone injuries are associated with body boarding and body surfing, while the 40-60 year old waders are knocked down by wave action, especially with back turned to the ocean. Males are statistically far more likely to be injured or lose their life in the surf zone but as June 15 reminds us...ANYONE is vulnerable. && .CLIMATE... Updated as of 418 AM Thursday. Monthly avg temp for June PHL projecting 74.5 or 1.2 degrees above the average of 73.3 ABE projecting 70.7 or 1.6 degrees above the average of 69.1 Seven of our eight long term climate sites will average above normal probably from about 8 tenths of a degree for TTN/ILG to as much as 2 degrees for GED. Mount Pocono is our only below normal average and projecting 1-1.5 below). These projections were based on our 330 am Thursday forecast. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 2 PM this afternoon to 6 AM EDT Friday for ANZ431-450>455. Small Craft Advisory from 2 PM this afternoon to midnight EDT tonight for ANZ430. && $$ Synopsis...Gorse Near Term...Drag Short Term...Drag 418 Long Term...Gorse Aviation...Drag/Gorse 418 Marine...Drag/Gorse Climate...418

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