Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 261745 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 145 PM EDT Fri May 26 2017 .SYNOPSIS... Low pressure will move northeastward and away from our area today. Meanwhile, a weak surface trough is expected to cross our region this afternoon. A front will then remain to our south and west Saturday and Sunday as weak low pressure tracks along it. Another area of low pressure is forecast to track from the Great Lakes Sunday and across southeastern Canada Sunday night and Monday. An associated weakening cold front will move through our area later Monday, then a secondary cold front arrives during Tuesday. High pressure is forecast to build to our south Wednesday and Thursday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 130 pm update: Showers are approaching far northwest portions of the forecast area, but coverage is spotty and precip is very light. Current temps/sky cover/winds/PoPs are matching expected trends, so few edits were needed with the latest update. 930 am update: Latest water-vapor imagery indicates the upper low is spinning off the coast of New England with drier, zonally- oriented flow to its south and west. A subtle, elongated lobe of vorticity centered near Lake Erie will move eastward through the northern Mid-Atlantic today. Weak lift in advance of this maximum in vorticity will generate showers today in portions of Pennsylvania and New Jersey, though these will be light and generally widely scattered. Current PoPs seem reasonable for this threat and did not change. Main adjustments to the forecast were to hourly temperatures based on latest trends and to sky cover, which more heavily weighted the high-resolution model output. Generally, expecting somewhat increased low- to midlevel clouds during the day as heating continues. Also adjusted winds somewhat based on the somewhat gustier looking BUFKIT soundings. Previous discussion... The mid level low was making its way over southeastern New England early this morning. It should progress northeastward, approaching Nova Scotia late in the day. The surface low will remain fairly well aligned with the mid level feature. Scattered showers were lingering in eastern Pennsylvania, and northern and central New Jersey early this morning. They should pull northeastward along with the low. Another round of showers is expected around midday, mainly in eastern Pennsylvania, and northern and central New Jersey. An impulse rotating around the departing low along with some marginal instability are forecast to set off the shower activity at that time. Low clouds should linger early this morning. While skies may remain mostly cloudy over much of eastern Pennsylvania and northern New Jersey today, we are expecting a substantial decrease in cloud cover over the upper Delmarva and southern New Jersey as drying works its way into our region. A west northwest wind is anticipated to increase to 10 to 15 MPH with gusts around 20 MPH. We have gone on the high end of the temperature guidance for today. Highs are forecast to range from around 70 in the Poconos to near 80 in southern Delaware. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/... Weak high pressure is forecast to build into our region from the west for tonight. We are anticipating dry weather conditions. The sky is expected to become mostly clear during the evening with an increase in high and perhaps some mid level clouds toward morning ahead of our next weather system. There may be some localized fog late tonight as the ground remains quite moist from the recent rainfall. A northwest wind around 10 MPH is expected to diminish. Minimum temperatures should favor the 50s. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Summary...Unsettled at times, however warming temperatures are expected especially next week. The holiday weekend does not look to be a washout, but some showers and thunderstorms cannot be ruled out especially Sunday (cooler temperatures) into Memorial Day. Synoptic Setup...While a weak ridge slides across the Northeast Saturday into Sunday, a short wave trough is forecast to track from the Ohio Valley to off the Mid Atlantic coast over the weekend. A closed low in south-central Canada Saturday is forecast to amplify south and eastward through the middle of next week. It is forecast to remain centered well to our north, however cyclonic flow will be in place along with several short waves revolving around it. This will drive a series of fronts eastward. The exact timing of the embedded features are less certain given an amplified pattern with a closed low involved. We used a model/continuity blend for Saturday through Sunday night, then blended in the 00z WPC Guidance thereafter. Some adjustments were then made following additional collaboration with our neighboring offices. For Saturday and Sunday...Some generally weak ridging is forecast to slide across the Northeast and Mid Atlantic later Saturday into Sunday. However, the guidance continues to show a short wave trough embedded within the faster flow during Saturday. While this occurs, a frontal zone is to our south and west where a weak surface low may track along. This may result in some showers and thunder especially across the southern areas Saturday afternoon and evening. We kept some chance PoPs for this potential with the highest PoPs across the southern zones. It is less certain how much convection may develop with a northern extent due to an onshore flow especially Sunday and the instability axis should be farther south and west. Our attention then turns to a closed low in south-central Canada, which gradually slides south and east and amplifies some. This will drive low pressure eastward and a front to our south will try and lift northward as a warm front through Sunday. It appears the low-level flow turns more onshore Sunday resulting in cooler temperatures. Low pressure across the Great Lakes should be occluding and this adds some uncertainty regarding the details for our area. A weak low may develop along the frontal zone near Delmarva which may focus some afternoon convection Sunday, or we wait for convection to our west with the occlusion to arrive late in the day and at night. For now, we kept PoPS in the chance range Sunday, however these increase some at night with an initial weakening front and the idea of incoming convection. For Monday and Tuesday...A closed low is forecast to gradually expand into the Northeast. This will drive surface low pressure eastward north of the Great Lakes. An initial surface low should be moving away from the Mid Atlantic coast early Monday as a frontal zone sits offshore. The main weakening cold front is forecast to move across our area Tuesday and shift offshore Tuesday night. The afternoon of either Monday and/or Tuesday may become convectively active, however this will depend on short wave timing and potential convective development to our west. The timing overall is less certain given the scope of the closed low to our north-northwest and plenty of short wave energy revolving around it. Kept chance PoPs for Monday (highest in the afternoon and early evening), then slight chance to chance PoPs on Tuesday. We are anticipating warmer temperatures ahead of the weak cold fronts. For Wednesday and Thursday...As the center of the closed low gradually shifts across eastern Canada, the overall trough becomes more established across the Northeast into the Mid Atlantic. The main surface low is forecast to be shifting east of James Bay Canada with its trailing cold front east of our coast to start Wednesday. A cyclonic flow will be in place and a surface trough should be sliding across the Great Lakes region. While less likely at this time, a few showers still cannot be ruled out given the synoptic setup. For now, kept some slight chance PoPs across portions of the northern and western areas Wednesday. Given the core of the closed low well to our north, keeping the coldest air aloft there, daytime temperatures look to be mostly in the 70-80 degree range. High pressure is currently forecast to build to our south during this time frame. && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. For the 18Z TAFs...BKN CIGs are hovering around 3500-6000 feet this afternoon and should continue to rise slowly before scattering out early this evening. Shower coverage is spotty, so any expected impacts this afternoon at the terminal should be minimal at best. CIGs return tomorrow morning, with showers moving in from the west during the afternoon. Decent chance of sub-VFR conditions tomorrow, especially during the afternoon. Kept conditions VFR through 18Z, but models have been trending faster with the deteriorating conditions, so this may be moved up in later TAFs. West winds 10 to 15 kts with gusts 20 to 25 kts this afternoon decreasing below 10 kts tonight. OUTLOOK... Saturday...VFR during the day, then ceilings should lower to MVFR or IFR late at night. A few showers and thunderstorms possible, especially south and west of KPHL. Confidence on the low side regarding timing and coverage of showers/thunder. Sunday...MVFR/IFR ceilings possible with some showers and thunderstorms especially in the afternoon and at night. Monday and Tuesday...Conditions should improve to VFR during Monday morning and continue through Tuesday. However, some showers and thunderstorms cannot be ruled out each day. && .MARINE... 1230 pm update: Extended the small craft advisory for the northern/central New Jersey coastal waters until 10 pm. Seas remain elevated (5 to almost 7 feet) in this area, and latest wave guidance has been around one foot too low through the morning hours. The rest of the advisory remains unchanged at this time. Previous discussion... Low pressure will continue to move to the northeast and away from our region today. The wind is expected to settle into the west northwest at 10 to 20 knots. Wind speeds are forecast to decrease to 5 to 10 knots tonight. We will keep the Small Craft Advisory in effect for our ocean waters until 6:00 PM. Wave heights remained in the 5 to 6 foot range early this morning and it should take the better part of the day for them to subside below 5 feet. Wave heights will continue to decrease tonight. RIP CURRENTS... A west wind is expected to develop today. However, wave heights off the coast will likely remain in the 5 to 6 foot range for much of the day. Also, the tidal range remains amplified due to the new moon. As a result, we will keep the moderate risk for the development of dangerous rip currents for today. It appears as though waves will diminish for Saturday. A westerly wind is forecast to back toward the south and southeast in the afternoon. However, speeds should favor the 5 to 10 MPH range. The rip current risk is expected to be low on Saturday. OUTLOOK... Saturday and Sunday...The conditions are expected to be below Small Craft Advisory criteria. A sea/bay breeze Saturday afternoon should enhance the wind along the coast for a time, then a more pronounced onshore flow Sunday may result in wind gusts to around 20 knots. The seas are currently anticipated to remain below 5 feet. Monday and Tuesday...The conditions are expected to be below Small Craft Advisory criteria. Some showers and thunderstorms are possible each day, however timing and coverage is of lower confidence. && .TIDES/COASTAL FLOODING... Latest guidance is indicating another night of minor coastal flooding, though more marginal than last night`s. We are reviewing the guidance now and closely monitoring observations during the low tide cycle to determine if advisories will be required for tonight. At this point, it looks like they will be needed, at least for the Atlantic coast. Also, we remain concerned about the potential for a round of minor coastal flooding on Saturday night as the wind may become onshore again at that time. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EDT this evening for ANZ452>455. Small Craft Advisory until midnight EDT tonight for ANZ450-451. && $$ Synopsis...Gorse Near Term...CMS/Iovino Short Term...Iovino Long Term...Gorse Aviation...CMS/Gorse/Iovino Marine...CMS/Gorse/Iovino Tides/Coastal Flooding...CMS/Iovino is the U.S. government's official web portal to all federal, state and local government web resources and services.