Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 230146 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 946 PM EDT Mon Jul 22 2019 .SYNOPSIS... A cold front will slowly make its way offshore through Tuesday, with multiple waves of low pressure riding along it. By Wednesday, that front should push further offshore, with high pressure building in behind it. This large area of high pressure will continue to influence our weather from the middle of this week into the weekend and through beginning of next week. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... 930 PM Update...Severe thunderstorms have moved out of the forecast area. A few thunderstorms and heavy rain showers are expected to continue into the overnight hours. Flash flooding still remains a concern as precipitable water values are still high and forcing for assent remains. The greatest threat of flash flooding will be in areas of training of showers/storms, generally from the Lehigh Valley and south. The Flash Flood Watch will continue until 8 AM tomorrow morning. Updated temperatures, dewpoints, and winds to match current observations and the latest hi-res guidance. This resulted in dropping high temps a degree or two tomorrow. 630 PM Update...Damaging storms have been moving through eastern PA, parts of the Delmarva, and NJ but these will be off the coast within the next couple hours. Heat Advisory is cancelled as these storms have cooled temps. Previous discussion... 330 PM Update... Severe weather and flash flooding threat continues into this evening... Storms beginning to pop over eastern PA into northern NJ in unstable environment. At the surface, low pressure beginning to develop over eastern PA along a diffuse frontal boundary while in the upper levels shortwave energy associated with deepening upper level trough is approaching from the west. As we head through the late afternoon into early this evening, Expect convection to become better organized...likely taking the form of a line or line segments moving into our eastern PA zones and affecting the I-95 corridor and points S/E over NJ and the Delmarva during roughly the 5-8 pm time frame. Given ML CAPES of 1500-2000 j/kg with effective bulk shear near 35 knots this should allow for line to sustain itself and produce a good potential for severe weather. Damaging winds look to be the biggest threat given modest wind field aloft and steep low level lapse rates which should help to aid in bringing downdrafts to the surface. Do still have concerns that there could be potential for isolate tornadoes with the most likely spot for this being near and north of the front over portions of northern NJ along the I-78 / I- 80 corridor where winds will be more backed in the low levels. This heightened risk should also extend west into adjacent portion of eastern PA. Finally, flooding/flash flooding remains a threat given very high PWATs though the one mitigating factor is that storms should be fast moving. By the mid evening, first round of convection should be off to the east with a relative lull in activity likely as low pressure moves east across Long Island. However the problem is that the trailing front will get hung up across the area with upper level trough still lagging off to the west over the Ohio Valley. And then by the late evening into the overnight period (11pm - 2am) as more shortwave energy arrives this should result in another wave developing along the front with additional widespread showers and possible embedded storms moving in from the SW. At this time front should be hung up right around the I-95 corridor and it`s this area we are particularly concerned about for potentially heavy showers/storms overnight as PWATs will remain very high with some elevated instability to work with. Thus, flooding/flash flooding still a threat especially if heavy rain hits areas that happen to have also gotten hard hit this evening. Severe weather threat by this time will be lower though. It will still be another fairly warm and muggy night near and south of the front across the urban corridor and points S/E with lows still only getting down to the low 70s with high dew points as well. It will be cooler north of here though with lows in the 60s. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... By Tuesday morning, second wave of low pressure will be slowly moving off the New Jersey coast and this will help to gradually drag the front farther south finally. As such, expect rain/showers continuing into the early morning at least along and south of the I-95 corridor with this activity gradually ending from north to south through the late morning into the afternoon with the frontal passage. Any of this additional rain occuring Tuesday should not be as heavy with diminishing instability but could still exacerbate any flooding issues that could be ongoing from earlier. Not expecting much sun as area remains under influence of deep upper trough to the west and front only lingers just off the coast. Highs will generally be in the mid to upper 70s and with dew points in the 60s it will certainly feel much more comfortable! Forecast becomes a little tricky for late day Tuesday into Tuesday night as yet one more wave may try to develop along front just off the coast and this could bring in another round of rain/showers...mainly for coastal areas. It shouldn`t be particularly heavy though. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Overview... An extended stretch of mainly dry and seasonable weather is expected for most of this week and into the weekend. At upper levels, a lingering trough will remain over the East from Wednesday into Thursday. This trough will weaken and shift southward through the week as a ridge moves out of the Great Lakes and builds into southern Canada and New England. At the surface, a broad region of high pressure will exist downstream the ridge and upstream the trough. This high will start the midweek period over the Midwest, and as the upper level features slowly shift, the high will gradually make its way eastward. It will not crest over our region until the weekend. Even by early next week, we will probably continue to feel the influence of this high. The resulting sensible weather is something we really have not seen much of this year: several consecutive days (at least) of dry and quiet weather. Temperatures also look near normal for this period, as we are between cooler than normal air to the south associated with the lingering trough, and much warmer than normal air that will build in with the ridge well to our north. Dailies... Wednesday... Our early week frontal system will still be departing on Wednesday, and it looks like considerable cloud cover and possibly some light showers may linger towards the coast especially in the morning. As a wave of low pressure moves offshore, some guidance is even hinting at an inverted-trough like feature which could throw some light showers back into coastal NJ. However, the trend for the day will be clearing skies and drier weather, especially to the west. Temperatures only getting into the low 80s in most areas with the cooler air behind the front. Wednesday night-Saturday... Dry and quiet. A very gradual warming trend in 850mb temperatures should yield a slow warming trend at the surface, with each day generally a degree or two warmer than the previous but no warmer than the upper 80s by Saturday. Dew points should remain comfortable also, with those also showing just a slow uptrend heading towards the weekend but remaining well away from the oppressive territory we have seen. Sunday-Monday... Temperatures should continue coming up slowly during this period, with some 90s potentially returning to the picture. Humidity will also continue to creep up, but . A weak frontal system may approach on Sunday, with possibly some showers up north. However, high pressure still looks to be the dominant feature into early next week, and we will likely have to wait until later in the week to see any more robust frontal systems. && .AVIATION /02Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...The strongest storms have weakened and moved offshore this evening, however additional showers with the potential for some embedded storms will affect the TAF sites by later this evening through the overnight. Chances for severe weather will be lower with this second round but generally expect deteriorating cigs/visbys with with widespread MVFR likely by the overnight and periods of IFR likely by morning...especially for the I-95 terminals. Tuesday...Rain/showers should be tapering off from north to south but MVFR restrictions likely to continue for most sites through most of the morning if not into the early afternoon in some places. Outlook... Wednesday... Mainly VFR, except MVFR possible at least through the morning at MIV and ACY. Winds north-northeast at 5 to 10 kt. Wednesday night-Saturday... An extended period of quiet, VFR conditions is expected. Winds mainly light and variable, favoring light easterly flow if anything. && .MARINE... Marginal SCA conditions at times for the coastal waters as low pressure develops over PA then moves off the coast. Bigger Story will likely be strong to severe storms moving over the coastal waters (as well as the Delaware Bay) this evening and these could produce wind gusts of 40-50 knots in the strongest storms. These should be east of the waters by late evening. For Tuesday, SCA level seas likely to continue most of the morning even as winds diminish so SCA runs until noon. Conditions sub SCA by Tuesday night but more rain/showers possible with reduced visibilities as front will be stalled over coastal waters. Outlook... Wednesday-Saturday... Winds and seas are expected to remain below SCA criteria. 3 to 4 foot seas Wednesday diminish to mainly 2 to 3 feet for the remainder of the period. RIP CURRENTS... Winds will be shifting to the NW-N as the cold front passes over the coast tomorrow morning. The resulting risk for the development of dangerous rip currents will be low across the New Jersey Shore and Delaware Beaches. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Flash Flood Watch until 8 AM EDT Tuesday for PAZ054-055- 060>062-070-071-101>106. NJ...Flash Flood Watch until 8 AM EDT Tuesday for NJZ001-007>010- 012>027. DE...Flash Flood Watch until 8 AM EDT Tuesday for DEZ001>004. MD...Flash Flood Watch until 8 AM EDT Tuesday for MDZ008-012-015- 019-020. MARINE...Small Craft Advisory until noon EDT Tuesday for ANZ450>455. && $$ Synopsis...O`Brien Near Term...Fitzsimmons/Staarmann Short Term...Fitzsimmons/Staarmann Long Term...O`Brien Aviation...Fitzsimmons/O`Brien/Staarmann Marine...Fitzsimmons/O`Brien/Staarmann

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