Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 210130 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 930 PM EDT Thu Apr 20 2017 .SYNOPSIS... A backdoor cool front is expected to stall across the Mid Atlantic tonight into Friday. As an area of low pressure moves out of the Great Lakes region and into southeast Canada, it will pull a cold front across the area late Friday into Friday night. This front will stall to our south over the weekend. An area of low pressure is forecast to strengthen as it moves off the southeast coast late Sunday, then move north offshore of the east coast through early next week. A cold front is expected either next Wednesday or Thursday as an area of low pressure moves out of the Great Lakes region into southeast Canada. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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A line of showers and thunderstorms is starting to move into Delmarva and extreme southeastern PA. This should continue east and could bring showers, and maybe a thunderstorm to areas from Philly southward. However, a southeasterly and easterly surface flow has been persistent for the last few hours leading to a stable boundary layer for much of the region. Even though this initial line will not reach locations north of Philly, I kept the highest PoPs across the north for late tonight when a second round of showers and isolated thunderstorms will be possible as the surface low and a mid level shortwave trough approaches the region. We are not anticipating any severe weather with tonight`s system. However, there could be localized brief downpours and perhaps some small hail. The southeast flow will likely result in the development of areas of fog in the elevated terrain of eastern Pennsylvania and northern New Jersey due to upsloping. Tonight`s minimum temperatures will favor the 50s in our region.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/... Low clouds are forecast to linger into Friday morning, along with a chance of showers and thunderstorms. The fog may persist in the elevated terrain, as well. Some brief improvement is possible during the early afternoon. However, there is another chance for showers and thunderstorms as a cold front approaches from the northwest. The wind may settle into the southwest around 8 to 10 MPH during the late morning and early afternoon hours bringing warm air into our region. High temperatures are anticipated to be in the 60s in our far northern counties, in the 70s in the middle part of our forecast area, and in the lower 80s in much of northeastern Maryland and Delaware. && .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... There is the potential for some unsettled periods during the long term, but not the entire time expected to be active. Some uncertainty with some model differences at times during the forecast. On Friday night, the frontal boundary that affects our area during the day pushes south of the area. With the boundary to our south, there is also the possibility for some vorticity impulses to move across the area withing the southwest flow aloft. The NAM is the most aggressive bringing in precipitation overnight, while the GFS, ECMWF, and Canadian aren`t as aggressive. We will keep a slight chance of showers overnight due to the proximity of the frontal boundary to the south and potential short wave interaction. As we move into Saturday and continuing into Sunday, the front is expected to stall to the south, while an area of low pressure moves along the boundary well to our south. With stronger short wave/vorticity impulses expected to move across the area this weekend north of the frontal boundary and low pressure system, there will be a better chance of showers across the area Saturday through Sunday. The most likely areas for showers is expected to be from around the Philadelphia area southward. There will remain a chance of showers Sunday night across southern New Jersey and southern Delmarva, but chances decrease through the night as the short wave/vorticity impulses begin to move past the area. By Sunday night into Monday the low pressure to our south is expected to move offshore of the southeast coast, then begin lifting northward offshore of the east coast into the middle of the week. There is some timing differences in how fast the model guidance lifts this low offshore of our area, but there is a general consensus for a enhanced chance of showers around Tuesday for much of the area. Depending on how fast this system lifts north of the area, there could also be a small chance of showers into Wednesday. However, if it`s far enough offshore, most places should be precipitation free. There is also the possibility of a cold front approaching the area during the middle of next week as well. The ECMWF has it approaching Wednesday into Wednesday night, while the GFS has it approaching Thursday into Thursday night. This frontal boundary could bring another chance of showers to the area Wednesday or Thursday. && .AVIATION /02Z FRIDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Ceilings should lower through the evening hours. After 06Z, widespread MVFR conditions are expected. Several sources of guidance show IFR and even LIFR conditions. However, have chosen to include ceilings only as low as 1000 ft AGL in the TAFs for now (guidance was also depicting IFR ceilings last night which never developed, so there may be a low level moisture bias in the guidance now). In addition, two rounds of shra are possible, the first between 02 and 06Z, and the second between 06 and 12Z. Isolated tsra is possible as well, but confidence and coverage are too low to include in the TAFs at this time. Ceilings should slowly improve after 12Z. However, another round of showers and scattered thunderstorms is possible Friday afternoon. We have low confidence in the specific timing and coverage of the precipitation after 18Z so it is not mentioned in the TAFs at this time. The wind is expected to settle into the southeast and east for tonight around 6 to 10 knots. The wind should become light and variable for a time on Friday morning before becoming southwest around 5 to 10 knots for late Friday morning and Friday afternoon. OUTLOOK... Friday night...VFR conditions are currently forecast for Friday night, but there is a chance for lower conditions to develop with a chance of showers. Saturday-Sunday...Better chance for showers and lower conditions over the weekend. The highest chances for showers and lower conditions are expected from the Philadelphia area southward. Sunday night...A chance of showers remains for coastal New Jersey and central/southern Delmarva with possible lower conditions there. Elsewhere, improving conditions overnight. Monday-Monday night...Mostly VFR, lowering clouds overnight. Tuesday...MVFR conditions possible with showers, especially eastern New Jersey. && .MARINE...
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The wind should favor the east and southeast tonight and the south and southwest on Friday at speeds of 15 knots or less. Gusts near or above 20 kt may be possible in the vicinity of any thunderstorms late tonight. Wave heights on our ocean waters are forecast to favor the 3 to 4 foot range. Waves on Delaware Bay are expected to be 1 to 3 feet. OUTLOOK... Friday night-Saturday...Winds may gust around 20 knots at times Friday night into Saturday, but overall should remain below advisory levels. Saturday night-Sunday...Conditions expected to remain below advisory levels. Winds shift from a west-northwest direction to a northeast direction overnight Saturday. Sunday night...Winds and waves begin to increase to advisory levels late Sunday night into early Monday morning. Monday-Tuesday...Small Craft Advisory conditions likely.
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&& .CLIMATE... Overall projections today were identical or slightly warmer than yesterdays. Confidence is above average on the info below. April will probably be a top 10 warmest in the period of record for multiple long term climate stations in our forecast area. Depending on the warmth of this afternoon and Friday and how cold it is on Sunday or Monday when a chilly rain may occur, we appear to be on our way for a top 2 warmest April on record. The usual analysis process first 19 days done, the 20th-26th per our 330 am fcst, the 27-30th from FTPRHA. PHL projects #2, 4.9 above normal and 0.5degs from record. POR 1874 ABE projects #2, 6.1 above normal and 0.4degs from record. POR 1922 ACY projects rer 5.0 above normal,0.4F above prev 2010 56.3 POR 1874 So, there could be some slippage but its unlikely that any of these locations can slip below #10. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Robertson Near Term...Iovino/Johnson Short Term...Iovino Long Term...Robertson Aviation...Iovino/Johnson/Robertson Marine...Iovino/Robertson Climate...

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