Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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686 FXUS61 KPHI 250116 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 916 PM EDT Mon Jul 24 2017 .SYNOPSIS... A cold front will move slowly south across our region this evening and offshore tonight. A broad high pressure system extending from the Great Lakes into eastern Canada will gradually build southeast into our area through Wednesday. As this high slides offshore late Wednesday, a warm front will move through our region on Thursday, followed by another slow moving cold front on Friday. High pressure is expected to make a return this weekend into the beginning of the work week. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
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An EF2 tornado has been confirmed near Stevensville, Maryland. A preliminary Public Information Statement (PNS) has been issued. The survey team has returned to the office and updated the PNS. 916 PM Update...Another not-so quiet evening across the area for the third night in a row. As of writing a line of showers and thunderstorms extended from Raritan Bay and NYC southwest along I-95 toward the Delaware River and into northeast Maryland. There have been some potent storm embedded within the line and some weak rotation at times noted in velocity earlier this evening, but as of now it appears things may be starting to stabilize a bit as convection moves into more stable air that is sitting across the Garden State especially north of a Trenton to Freehold line courtesy of an onshore flow. This activity should work all the way to the Jersey Shore and eventually the Delaware Beaches tonight over the next few hours. PoPs were adjusted accordingly based on radar trends and the HRRR. MPO is reporting fog with visibility at 1/2 mile. I added patchy fog to the grids for the hill country of NW New Jersey, the Poconos and the Lehigh Valley for the next few hours as moist, low-level air via a soupy air mass on wet ground and light winds go to work. We may need to expand fog later tonight, but further south visibility should not get as low. Temps and dewpoints were tweaked based on the latest hourly trends. We will need to watch areas that got drenched last night by heavy rain as it will not take much in spots to aggravate the ground or rivers/creeks and get new flooding. Allentown at ABE is now up to 7.91 inches of precipitation for July which ties this for the 11th wettest July on record with 1935 based on a period of record dating back to 1922. The wettest July ever here was 10.42 inches in 1969. Clouds will fill in tonight behind the frontal boundary and some lingering showers may remain into the morning, mainly across the northern parts of the forecast area. With clouds over the air, it will again be mild but should feel cooler as some drier air starts to make its way into our area. Lows will be in the mid to upper 60s from I-78 and north and in the low to mid 70s to the south of I-78.
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&& .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/... Another cloudy day is expected across the forecast area. Easterly flow will keep us cool and highs are expected to remain below normal. Upper 60s to mid 70s across the Poconos and NW New Jersey with upper 70s to mid 80s from about the I-78 corridor and points south. With the boundary exiting the region, much needed drier air will filter in and we should be free of the extra humid conditions we have had lately. Showers may linger through the morning hours but as high pressure build into the region, expect dry conditions through the afternoon. && .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/... One mid-level trough exits the east coast Tuesday night, with a brief zonal flow and some short wave ridging for Wednesday, followed by continued troughiness through the end of the long term period. A frontal system in the Thursday thru Friday time frame features the greatest chance of sensible weather, in terms of increased humidity, and the chance for showers and thunderstorms. This front will slow as it moves offshore, and stall to our south through the weekend into early next week, as waves of low pressure traverse it. All of the available guidance keeps our region dry in the Saturday thru Monday period, but lingering cloudiness is possible, especially south Jersey into Delmarva. In addition, if the guidance slows the southward progress of this feature, this could result in a more pessimistic forecast for the aforementioned area. Showers and thunderstorms with heavy downpours and gusty winds are possible Thursday into Friday. It`s still too early to nail down the spatial and temporal details, as the GFS is much more progressive with the front compared to the Euro, so there is low confidence due to the uncertainty. In fact, the GFS event window would be Thursday into Thursday night, with the Euro window is centered on Friday. Nevertheless, effective bulk shear values rise to near 40 knots, while surface-based cape is in excess of 2.0 kJ across portions of the area. In addition, the front slows as it moves trough or area, while Precipitable Water values rise to around 2.0 inches. These indicators point to some potential for strong thunderstorms and heavy rainfall in the Thursday and Friday period, and we have mentioned this in the Hazardous Weather Outlook. Temperature-wise, slightly below normal temperatures on Wednesday rebound to at or slightly above average from Thursday thru Friday. Average to slightly below average temperatures are expected in the post-frontal air mass on Saturday, with a moderation to near normal in the Sunday thru Monday time frame. In fact, Thursday and Friday are shaping up to be the warmest days, with temperatures well into the 80s to near 90. The only exception may be closer to the coast, where a prolonged onshore flow may develop over the weekend into early next week, keeping temperatures cooler. Finally, depending on the extent of the aforementioned onshore flow, astronomical tides are much lower over the weekend into early next week as we move away from the new moon. This should limit the potential for coastal flooding. && .AVIATION /01Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. A line of showers and thunderstorms will pass across KTTN, KPNE, KPHL and KILG within the next 45 minutes to an hour dropping ceilings to MVFR levels with visibilities dropping to 2-4 SM on average in moderate rain. Winds should shift to more of an easterly direction and drop closer to 5-7 kts once the line of showers and thunderstorms passes. Local erratic higher winds gusts may occur near thunderstorms. KMIV and KACY should see activity pass through after 03Z Tuesday within 1-3 hours with similar drops in ceilings and visibility. Ceilings may rise behind the showers and thunderstorms for a few hours. Conditions will deteriorate further tonight once the rain has moved out. Expect ceilings to start dropping to around 1500-2500 feet, with some locations falling below 1000ft by late tonight/early Tuesday morning. Additionally, some fog may form overnight and visibilities will drop, mainly after midnight. Winds will become light and variable through the overnight period. Tuesday...MVFR/IFR conditions will prevail through much of the TAF period. Guidance shows that some clearing to VFR may occur by early afternoon but confidence is low that things will clear out that quickly. Easterly flow will remain at the terminals with winds around 10 knots or less. OUTLOOK.... There is some potential for MVFR in low clouds associated with a frontal system from Thursday into Friday. This potential may linger into Saturday in ACY and MIV as the southward progression of the front slows. In addition, any showers or thunderstorms during this time frame may briefly reduce ceilings and visibility`s to IFR. Otherwise, VFR for the remainder of the period. && .MARINE...
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A boundary across the region has the winds blowing in different directions across our areas waters. To the north of the boundary, winds are northerly around 10 to 15 knots. To the south of the boundary, winds are west to southwest around 10 knots. A cold front will cross the region tonight and winds should become westerly briefly before turning more north and then east on Tuesday. There is another chance for some showers and thunderstorms later this evening over the area waters, with locally higher seas and gusty winds. OUTLOOK... Wednesday through Saturday...A Small Craft advisory may be needed on Thursday and Friday. RIP CURRENTS... A moderate risk for rip currents remains in effect through this evening for the Delaware and New Jersey beaches. A low risk of rip currents is currently expected for the Delaware and New Jersey beaches on Tuesday. However, an underlying longer period swell may raise the rip current risk to moderate along the New Jersey coast.
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&& .TIDES/COASTAL FLOODING... There is Coastal Flood Advisory in effect for this evening`s high tide along the New Jersey and Delaware coasts. We do not anticipate minor flooding in Delaware Bay. While surface wind turned offshore during on Monday, it is expected to become onshore again this evening, especially along the New Jersey coast. Conditions may be a bit marginal for Delaware. && .EQUIPMENT... Dew point readings at KDOV continue to measure too high compared to surrounding locations and should be treated as unrepresentative of the area. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...Coastal Flood Advisory until 2 AM EDT Tuesday for NJZ012>014- 020-022>027. DE...Coastal Flood Advisory until 2 AM EDT Tuesday for DEZ003-004. MD...None. MARINE...None. && $$ Synopsis...Franck Near Term...Meola/99 Short Term...Meola Long Term...Franck Aviation...Franck/Meola/99 Marine...Franck/Meola Tides/Coastal Flooding... Equipment...

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