Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 221904 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 304 PM EDT Mon May 22 2017 .SYNOPSIS...
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Weak low pressure is developing on a warm front from the Delmarva Peninsula into southern New Jersey today. A cold front approaching from the west will merge with the warm front just off the mid Atlantic coast tonight. Low pressure in North Carolina Tuesday will pass east of the mid Atlantic coast Tuesday night. Weak high pressure is left behind over our area Wednesday. Stronger low pressure organizes in the Ohio Valley Wednesday night sending its associated fronts through the mid Atlantic states late Thursday or early Friday as the low moves into New England. High pressure should follow for the weekend.
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&& .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
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An occluded frontal boundary approaching the area from the west this afternoon and evening is expected to move across the area tonight. The associated warm front is expected to stay to our south as an area of low pressure moves off the Delmarva coast later this evening. North of the warm front and low pressure, there will continue to be showers moving across the area, with the heaviest rainfall across southern New Jersey and Delware late this afternoon. The majority of the showers are expected to dissipate this evening and overnight. There will be plenty of moisture around through tonight, especially before the frontal boundary pushes through the area. As winds become light and variable, it is possible that some fog may develop. How thick it becomes is uncertain as there is some vorticity forecast to move across the area, and positive low level lapse rates, so there could be enough weak lift and turbulence to prevent it from become widespread dense. So no Dense Fog Advisory is planned at this time.
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&& .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/...
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The frontal boundary that moves across the area overnight is expected to stall to our south. An area of low pressure is forecast to move along this boundary from the south, as the first low pressure lifts to the northeast. The low pressure, nor the frontal boundary, is expected to make its way back into our area during the day, however there will be a chance of showers to lift across portions of the area later during the day on the north side of the low. The greatest chance for showers will be across southern Delmarva and souther New Jersey. With no instability forecast, we do not expect any thunderstorms across our area.
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&& .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... DRAFT DRAFT DRAFT for the 330 PM discussion: **Flood potential may exist where its rained more than 2 inches today in SNJ and the Delmarva** 12Z NAM not applied for this discussion. Its not off to a good start for today. 500mb: 500mb: A sharpening and unusually strong high amplitude trough in the nations midsection will closed off and weaken as it moves to new England Friday, with ridging to follow this weekend. The next trough will be organizing in the Great Lakes region early next week. Temperatures: Calendar day averages Wednesday-Saturday near normal warming Sunday and Monday to nearly 5 above normal, ahead of the next trough. Forecast basis: a 50 50 blend of the 12z/22 GFS/NAM MOS for Tuesday night through Wednesday night, then the 12z/22 GFS MEX MOS for Thursday with the 15z WPC guidance for Thursday night-next Monday. Any substantial deviations from the basis above, will be noted: The dailies... Forecaster confidence is below average on details through Friday morning. Tuesday night through Wednesday...Depending on the path of the next coastal...a risk of rain south and probably dry north. Thursday...Looks wet in warm air advection. Friday...After any early morning showery rains depart with the cold frontal passage...a breezy west wind should develop with afternoon warming. Memorial Day weekend...Warming will continue in the lower levels with a strengthening west to southwest flow as high pressure starts to build in from the southwest. Showers may approach from the west late Monday but for now...we`ll consider that possibility as a low probability of occurrence prior to sundown Monday evening the 29th. && .AVIATION /19Z MONDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. IFR conditions remain across the TAF sites this afternoon as low clouds and rain continue across the area. There has been some breaks in the low level clouds, revealing some slightly higher cloud decks across the area, so it is possible that there could be some temporary MVFR conditions develop. But with the amount of moisture around the area, we expect the lower clouds to fill back in. An occluded cold front is forecast to move across the area overnight tonight. As this front moves across the area overnight and toward daybreak Tuesday, it is possible for conditions to improve to MVFR from west to east. It is entirely possible that conditions may improve to VFR during the day Tuesday, especially for ABE and RDG. But we will keep MVFR conditions further south across ILG, PHL, PNE, and TTN. IFR conditions remain in the forecast for ACY and MIV as there is the possibility they may be affected by an area of rainfall that lifts into our southern areas north of an area of low pressure to our south. Generally east-southeast winds will continue across the TAF sites this afternoon north of the warm front to our south. However, winds are expected to become light and variable this evening and overnight. As the occluded frontal boundary moves across the area, winds will become north-northwest to north-northeast. Speeds through the next 24 hours are expected to be 5-10 knots or less. OUTLOOK... DRAFT for the 330 PM issuance. Forecaster confidence on details is below average Tuesday night through Friday morning. Tuesday night...VFR conditions north and possibly the entire area but a risk of MVFR/IFR in rain vicinity KMIV/KACY. Light northeast wind. Wednesday...VFR except possible MVFR/IFR conditions during the morning KMIV and KACY. East to northeast wind gusts to 15 mph. Thursday...VFR CIGS with periods of MVFR/IFR in showers and isolated thunderstorms. easterly winds probably becoming southeast to south at night. Friday...Improving conditions after any early morning showers end with mainly VFR expected. Westerly wind gusts to 20 kt possible in the afternoon. Saturday...VFR. Westerly wind gusts to 20 kt.
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&& .MARINE...
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Winds and waves will remain below Small Craft Advisory levels tonight through Tuesday. Wind directions will begin out of the southeast this evening and overnight, before becoming north- northwest to north-northeast late tonight into Tuesday as an occluded front moves across the area. OUTLOOK... Draft for the 330 PM issuance. Forecaster confidence on this outlook section is below average Tuesday night through Friday morning. Tuesday night and Wednesday...Leftover SCA for hazardous seas possible...mainly the DE Atlantic coastal waters. Otherwise northeast to east wind with gusts generally under 20 kt. Thursday...SCA probable for the well organized Ohio Valley low. East to southeast winds build the seas to b between 5 and 8 feet on the Atlantic waters. Friday...SCA may continue for leftover Hazardous seas on the Atlantic portion of our coastal waters. Westerly wind develops with gusts to 20 kt. Saturday...Small chance an SCA for westerly nearshore gusts of 20 to 25 kt.
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&& .HYDROLOGY... 1137AM: DOX and DIX STA (DP)/STP (LEGACY) are underestimating rainfall. Ditto MRMS. We are using FFMP HPE. PNS for 1.75 inch or greater amts will post soon as we are seeing numerous reports of 1.5 to 2.5 inch rainfall since 230 this morning. At this time...DIX STP (Legacy is performing best vs its DP). DOX STP/STA are almost identical and far too low. The Flood Advisory updated statement (FLS) will post soon. At 1PM we will consider extending the NJ portion and cancel the MD DE portion. The Flood Advisory for generally nuisance potentially travel slowing/altering poor drainage street flooding continues. This advisory is because of overrunning along and just north of the warm front that shows light southerly flow in the warm sector and light northeast flow just north of the boundary. PWAT is near 1.75 inches. 12z NAM is off to a poor start. && .TIDES/COASTAL FLOODING... DRAFT for 330 PM discussion: Astronomical tides are at some of their highest of the year this Wednesday and Thursday evenings. Some onshore flow will contribute positive departures and its probable that a few locations along the Atlantic coasts of DE and NJ will exceed the minor coastal flood advisory threshold. It only takes about .6 to .7 feet above high tide to reach the threshold. It`s all in the details, which are yet to be confidently determined. This mornings 12z/22 NAM does not loom to be off to a good start. We will eight its guidance less than other traditionally used models. for now the SIT and ETSS modeling forecasts minor with a small chance of a moderate episode if prior to high tide onshore is strong enough for enough duration. && .CLIMATE... Record daily rainfall at ACY has a good chance of exceedance. 1.19 inches is the daily record set in 1909. as of 1205 PM 1.13 inches...appears well on its way to a record for this date. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Drag Near Term...Robertson Short Term...Robertson Long Term...Drag Aviation...Drag/Robertson Marine...Drag/Robertson Hydrology... Tides/Coastal Flooding... Climate...

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