Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 190037 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 837 PM EDT Sun Jun 18 2017 .SYNOPSIS... High pressure over the western Atlantic will keep our region in a warm and muggy airmass. A cold front will approach from the west Monday, then move through the area Monday night. A secondary cold front passes through Tuesday night or Wednesday before weak high pressure builds in Wednesday night and Thursday. Bermuda high pressure looks to influence our weather once again late in the week. A cold front should slowly approach the area next weekend. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... Showers have largely diminished this evening with only a few isolated ones popping up across our northern areas. A few more showers may continue to develop through early tonight. A line of thunderstorms is moving eastward through central Pennsylvania and may skirt through the Poconos region if it can hold together that long. Otherwise, very warm and humid conditions will continue tonight. Mild conditions overnight with lows only dropping into the low/mid 70s across the area. A bit of haze or light fog possible late tonight away from the urban areas. && .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... A cold front will cross the area Monday afternoon and evening. Severe thunderstorms will accompany the front with damaging winds the primary threat and large hail secondary. SPC has maintained the enhanced risk for SVR for much of our area. In addition, heavy rains will accompany the activity that could lead to flash flooding. Scattered showers will occur outside of the time window of the frontal passage, with activity possible in the morning prior to the fronts arrival. We have extended the flash flood watch westward to include Berks county, the Lehigh Valley and up into Nrn NJ. More information regarding the flash flooding threat is included in the Hydro section of this AFD. Highs Monday will be in the mid 80s to near 90 and with the humidity, we could get near Heat advisory criteria in a few spots. We will not issue any heat-related headlines attm. The front will slow as it crosses the area Monday night. Pops will decrease back below chc across the N/W areas by morning, while at the shore we will keep likely pops in overnight. Lows Monday night will drop into the low 60s N/W and remain in the low 70s S/E. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Midweek... The cold front is expected to have cleared the forecast area and move off the coast by Tuesday morning. This looks like a typical weak summer-time front with only slightly cooler air moving in behind it (highs about 2-5 degrees cooler on Tuesday than Monday). Nonetheless, it will feel notably more comfortable on Tuesday with dewpoints in the low to mid 60s vs low to mid 70s the day prior. A secondary cold front moves through on Wednesday, but temperatures should be similar to Tuesday as downsloping westerly winds offsets any post-frontal CAA. Backed off on rain chances for both days as forecast soundings show a subsidence inversion that should cap our convective chances and keep just about the entire region dry. Nonetheless, an isolated shower can be ruled out particularly during peak heating Wednesday afternoon when the secondary cold front and weak mid-level shortwave trough approaches from the west. Late week... Southerly return flow reestablishes along the eastern seaboard late in the week as Bermuda high pressure builds poleward. The resultant pattern will lead to a rise in temperatures and humidity levels starting Thursday and continuing into Friday. Thursday is trending drier, but chances for showers and storms increase Friday as we start to get into a more moist and convectively unstable airmass. The potential for some tropical moisture associated with a tropical disturbance that will be entering the Gulf of Mexico next week may get pulled northward toward the area. Showers and storms could be very efficient rain producers if the above scenario were to pan out, so we will have to closely monitor the potential for an additional hydro concerns, especially considering the wet antecedent conditions It`s not the most likely scenario and can only be characterized as a low confidence, highly conditional flooding threat at this point. Next weekend... Unsettled weather can potential continue into the weekend though the considerable model spread for D6 and D7 results in a low confidence forecast. Most of the guidance progs a cold front moving through the region but solutions with respect to fropa timing range between Saturday morning and Sunday night. Some guidance advertise multiple separate fropa this weekend. Regardless of the uncertainty, it still appears that temperatures will be a bit at or slightly above normal this weekend since downstream ridge blocking will prevent the more significant push of cooler/drier post- frontal air from moving into the forecast area next weekend. && .AVIATION /01Z MONDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR conditions continue at the terminals this evening and will continue through much of the early overnight period. With warm, moist conditions remaining across the region, some fog/haze may develop at the more rural locations (away from the urban areas). However, winds will remain a little bit stronger tonight than they have the past couple of nights and this should limit fog formation from becoming widespread. Some isolated showers are popping up across eastern Pennsylvania and nearby areas in New Jersey this evening. Do not anticipate that any showers will impact the terminals. However, the line of showers and thunderstorms to west of KRDG may near the area tonight. The line looks like it will likely break up and stay to the north of the area terminals. Winds continue to gust around 20 to 25 knots this evening from the south to southwest. Winds will start to lighten up late tonight and become around 5 to 10 knots through the overnight period. Monday...Plenty of weather action expected with a cold front expected to cross the area late in the afternoon and evening time. Increasing/lowering CIGS with higher probs of showers and tstms spreading from NW/SE through the day. Restrictions across the north are possible as early as 13-15Z with most of the restrictions occurring after the 18z period for the Del Valley and S/E. Severe weather expected with strong winds and hail possible. OUTLOOK... Monday night...Showers and storms continue. MVFR should be common but locally heavy showers and storms could lead to temporary IFR or LIFR restrictions. Tuesday through Thursday...Low possibilities for sub-VFR restrictions from stratus/fog late night/early morning each day. Otherwise, VFR. && .MARINE...
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DE BAY: we`ve raised-updated the wind gusts about 7 kt higher on DE Bay for Monday between 14z-23z...and issued a small craft advisory. Small chance an hour of gale gusts during the afternoon. Meanwhile, there may be isolated gusts to 25 kt into this Sunday evening on the De Bay but the primary wind event there is on Monday. We`ve added in the possibility of 35-45kt southwest to west wind gusts in any thunderstorm occurrence between 5 PM EDT Monday and midnight Monday night (MWW updated). We will continue with the SCA flag tonight and Monday for the Atlantic coastal waters. Significant rounds of showers and tstms expected with SMW`s likely to be issued late Monday and early Monday night. Sct showers are possible in the morning too but the main activity to be near 8 PM EDT. Seas...5-7 ft on the ocean and 1-4 ft across Delaware Bay. OUTLOOK... Monday night...The SCA continues for the coastal Atlantic waters with S-SW winds gusting to 25 kt at times and 5-6 ft seas. Tuesday...Winds veer slightly out of the W-SW and weaken to below 25 kt. However, the SCA may need to be extended into Tuesday with seas hovering near 5 ft. Held off on the extension for now since confidence for meeting criteria in the fourth period is not very high. Tuesday night through Friday...Winds and seas expected to remain below SCA headline criteria. RIP CURRENTS... The threat for rip currents remains in the high category through Monday evening. We continue to expect a period w/frequent oscillations between a 6-8 second easterly swell and a 15-17 second swell, with the latter containing the majority of the wave energy. The primary wave period is forecast to be around 7 seconds from the south though Wednesday. However, the underlying 13 to 16 second southeasterly swell is expected to linger into the mid week period. As a result, the risk for the development of dangerous rip currents is anticipated to be at least moderate through the period along the coasts of Delaware and New Jersey.
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&& .HYDROLOGY... The Flash Flood Watch that was previously issued for Monday afternoon and evening for the urban I-95 corridor was expanded westward to include Berks County, the Lehigh Valley and far northwestern NJ. Flash Flood Guidance (FFG) is somewhat high in these newly included areas (2.5"/1hr and 3.5"/3hr) but these rates could certainly be realized given how much environmental moisture is available (PWATs exceed 2") and the potential for training storms at least initially until the convection may try to organize into a line and then accelerate eastward as the cold pool strengthens. The highest risk of flash flooding still looks to be the urban I-95 corridor during the late afternoon and evening owing to lower FFG (1.5 to 2.5"/1hr and 2.5 to 3.5"/3hr). There was also consideration to expand the watch to the southeast to include the tier of zones in Delmarva and southeastern NJ, but opted to hold off for now as FFG is higher (3"/1 hr, 4-5"/3 hr) and the greatest risk of heavy convective rains looks to hold off until Monday evening. There is a concern that storms will slow down after dark as the cold pool weakens but the rainfall rates may also decrease as the storm intensity wanes with the loss of heating. Models are in decent agreement with QPF for this event, showing amounts generally range between 1-2" with localized amounts of 3-5" (especially seen on the higher-resolution models). && .EQUIPMENT... DIX 88D should be ots for 2-4 days beginning probably on Tuesday. This is for a significant system upgrade, that will be occurring at other nearby NWS 88D`s, subsequent to DIX. That means reliance during our outage will have to shift to other nearby NWS 88D`s and/or FAA TDWR`s. NWS NWR and phone broadcast of the marine and surf zone forecasts. Problems continue here at NWS PHI and will not be resolved until at least Monday. 22z obs: There was a 3 minute network outage that affected transmission of the hourly observation for many sites in NJ/DE. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Flash Flood Watch from Monday afternoon through Monday evening for PAZ060>062-070-071-101>106. NJ...Flash Flood Watch from Monday afternoon through Monday evening for NJZ001-007>010-012-013-015>019. High Rip Current Risk through Monday evening for NJZ014- 024>026. DE...Flash Flood Watch from Monday afternoon through Monday evening for DEZ001. High Rip Current Risk through Monday evening for DEZ004. MD...Flash Flood Watch from Monday afternoon through Monday evening for MDZ008. MARINE...Small Craft Advisory until 6 AM EDT Tuesday for ANZ450>455. Small Craft Advisory from 10 AM to 11 PM EDT Monday for ANZ430- 431.
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