Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 222009 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 409 PM EDT Mon May 22 2017 .SYNOPSIS...
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Weak low pressure in southeast Virginia will move east of New Jersey later tonight while a weak cold front in central Pennsylvania crosses our area. 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 on Wednesday. Stronger low pressure organizes in the Ohio Valley Wednesday night sending its associated fronts through the mid Atlantic states early Friday. Then high pressure follows on Saturday. However another storm system should develop over the nations midsection Sunday, threatening to mar a portion of Sunday or Memorial Day with a period or two of showers.
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&& .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... 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. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/... 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. && .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
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12Z NAM not applied for this discussion. It was not off to a good start today.
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Hazards: it seems almost inevitable that some sort of hydro related products will eventually be needed by next Monday in this pattern but whether its more than nuisance flooding FLS`s is debatable. All we know is that widespread 1.5 to 2.5" occurred in less than 12 hours over parts of the Delmarva and southeast NJ today with a fairly large area of 3+ in southeast NJ. Suspect isolated max amt of 4" today with small streams and rivers still rising late today. Legacy DIX STP was best with considerable low bias in DOX STA/STP and the MRMS products.
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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 somewhat Sunday and Monday, 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. We did modify guidance temps downward Thursday and Sunday in response to GGEM and ECMWF cooler pattern. The dailies... Forecaster confidence is below average on details the entire period. Tuesday night through Wednesday...Depending on the path of the next coastal...a pretty good chance of rain far southeast and probably dry north where a nice Wednesday is expected. SREF PWAT is down. Wednesday night and Thursday...Looks wet in warm air advection. Questionable how much warming can occur with what looks to be a neg tilt southern extension of the trough moving toward our area and causing a triple pt low to approach. That also may permit a better chance for isolated thunderstorms. SREF PWAT back up to 1.5". Friday...Partly sunny and a breezy west wind should develop with afternoon warming and only a small chance of a residual shower in the north. Memorial Day weekend...A good start is anticipated Saturday. Sunday and Monday should see a period or two of showers and possible thunderstorms. Still plenty of uncertainty but its not looking to be a picture perfect weekend.
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&& .AVIATION /20Z 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... Forecaster confidence on details is below average through Friday morning. Tuesday night...VFR conditions north and possibly the entire area but a risk of MVFR/IFR in showers vicinity KMIV/KACY. East to 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 probable 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 25 kt possible in the afternoon. Saturday...VFR. Westerly wind.
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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... Forecaster confidence on this outlook section is below average Tuesday night and Wednesday then above average thereafter. Tuesday night and Wednesday...may need an SCA for hazardous seas for 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 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 nearshore gusts to 20-25 kt. Saturday...Marine headlines not likely.
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&& .HYDROLOGY...
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Hazards: it seems almost inevitable that some sort of hydro related products will eventually be needed by next Monday in this pattern but whether its more than nuisance flooding FLS`s is debatable. All we know is that widespread 1.5 to 2.5" occurred in less than 12 hours over parts of the Delmarva and southeast NJ today with a fairly large area of 3+ in southeast NJ. Suspect isolated max amt of 4" today with small streams and rivers still rising late today. Legacy DIX STP was best with considerable low bias in DOX STA/STP and the MRMS products.
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&& .TIDES/COASTAL FLOODING...
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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. 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. Right now... no action needed.
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&& .CLIMATE...
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Record daily rainfall at ACY has a good chance of exceedance. 1.19 inches is the daily record set in 1909. As of 359 PM... 1.85 inches fell.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Drag 408 Near Term...Robertson Short Term...Robertson Long Term...Drag 408 Aviation...Drag/Robertson 408 Marine...Drag/Robertson 408 Hydrology...408 Tides/Coastal Flooding...408 Climate...408

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