Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 261318 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 918 AM EDT Sat May 26 2018 .SYNOPSIS... High pressure will be centered well east of the Mid-Atlantic coast this holiday weekend. A cold front moves southward across our area late tonight and Sunday, then stalls just south and west of our area on Memorial Day. A secondary cold front moves through the area on Tuesday. High pressure builds in from the north Tuesday night before moving offshore Wednesday. Moisture from the remnants of Subtropical Storm Alberto may affect the area late in the week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... The 12Z surface analysis shows a trough across the Applachians with the flow ahead of this feature out of the southwest. This continues to transport a warm and increasingly moist air mass into the region. As convective temperatures are reached, expect showers and thunderstorms to develop along and ahead of this feature early this afternoon across the Lehigh Valley and southern Poconos, and move slowly east across the area into this evening. While coverage may be limited at first due to drier air at the mid-levels, this layer will moisten as well, leading to greater coverage by this evening. With storm motion generally east around 20 kts and Precipitable Water values approaching 2.0 inches south of the I-80 corridor, heavy rainfall may lead to urban and poor drainage flooding across this area. While shear is limited AOB 30 kts, DCAPE values approach 1000 J/Kg, leading to the potential for gusty winds, especially south of the PA Turnpike where 0-3 Km lapse rates are steepest. The 12Z sounding from Washington-Dulles indicates temperatures around 15C at 800 hPa, and nearly dry adiabatic lapse rates to this level support high temperatures in the upper 80s to near 90, especially the urban corridor. With a southerly component to the wind developing down the shore as the day progresses, the barrier islands will be the cool spot, with some locations only around 70 degrees. Previous Discussion... High pressure over the western Atlantic is maintaining a SW flow bringing in warmer and more humid air with time. This could also result in a bit of patchy fog early this morning. Heading into the day Saturday, a hot and humid day on tap as the southwest flow really allows for a tropical airmass to build into the region throughout the day. Meanwhile, an upper trough with several shortwaves will approach during the day and move across the region in the afternoon into Saturday night. Highs will generally top off in the mid and upper 80s, with highs around 90 along the I-95 corridor from Newark to Philly, and in inland areas of the Delmarva. Surface dewpoints will climb into the 60s in the southern Poconos and northern NJ, and in the upper 60s to around 70 across southern NJ, southeast PA, and the Delmarva. As the upper trough approaches, scattered showers and thunderstorms will develop by early to mid afternoon over our eastern PA zones..aided by bay breezes from the Chesapeake and terrain circulations in the southern Poconos. Forecast models indicate ML CAPE values climbing to 1500-2000+ j/kg with deep layer shear values approaching 30 knots. There also looks to be a dry layer in the mid levels...at least through the afternoon however PWATs will rise past 1.75 inches. So this all said, heavy rain looks to be the biggest threat with storms and this could lead to urban and small stream flooding with even the risk of localized flash flooding. However given the fairly high CAPE and mid level dry layer, isolated damaging winds may be possible as well as precip loading may aid in downdraft accelerations along with DCAPES already progged to be 700-1000+ j/kg. Scattered Storms initially developing over eastern PA will track east and may affect the I-95 corridor E/SE into NJ and the Delmarva by the late afternoon into the early evening. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... Convection will be ongoing across the region in the early evening. Heading through time, the upper trough and associated shortwave energy detailed above will continue advect east toward the region providing upper level support for continuing lift through the night. Meanwhile, the tropical moisture feed from the south will continue to feed into the area. These factors along with just enough elevated instability look to keep showers and storms going across the region through most of the night rather than dying off through the evening. Of concern is the anomalously high PWATs progged to rise over 2 inches. Also, forecast profiles indicate a modest LLJ with slight veering in the profile and light winds aloft...suggesting slow moving and/or backbuilding and training of cells. Thus, though severe threat should diminish through the evening very heavy rainfall with the threat of urban, small stream and even flash flooding will continue with these storms into the overnight. In fact pattern even looks to resemble Maddox Frontal Pattern identified in some flash flood cases. Highest threat area for heavy rainfall and associated hydro issues looks to be Philly to Trenton and points south. Most of the area will see conditions remain quite warm and muggy through the night with lows in the upper 60s to low 70s. However across northern NJ and the southern Poconos, a backdoor cold front will begin to work south into the region overnight so it will turn cooler here with lows by morning in the low to mid 60s. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Sunday... A backdoor cold front will continue to push south across the area as high pressure over southeastern Canada builds down the eastern seaboard. The front will be the divide between warm and humid air (temps 75-80F and dewpoints near 70F) to the south and southwest and notably cooler air (temps 55-65F) to the north and northeast. The steadiest/more widespread showers on Sunday will be focused along and to the south of the boundary, which includes much of eastern PA and NJ for the morning and locations along and south of the Mason-Dixon line for the afternoon. The risk of thunderstorms and locally heavy rain/flooding will increase in the warm sector during the afternoon and evening across far southern NJ and Delmarva. Coverage/intensity of showers should diminish north of the boundary during the afternoon. There is cautious optimism for a rain- free period for part of the afternoon to the north and especially to the northeast of Philadelphia (before light rain returns during the evening) even though skies are expected to remain cloudy. Temps will not follow a typical diurnal curve due to a notable drop in temperatures that occurs from north-northeast to south-south west in wake of fropa. Monday...A relative lull in the unsettled weather pattern is expected for Memorial Day. The holiday may be our best shot over the next week at a precip-free day with support from latest model consensus. However, could still see a scenario where the backdoor front stalls nearby to our south, prolonging rain chances over mid Delmarva on Monday. The cool airmass in place to start the day on Monday looks to moderate a bit as onshore flow is replaced by a light south-southwest wind in the afternoon. Unfortunately for those looking to take advantage of the start of pool/beach season, stubborn low clouds will limit how much warming can occur- forecast highs are only in the 60s at the coast and generally in the 70s inland. Tuesday...Another backdoor cold front looks to approach from the north. Strong heating in the pre-frontal warm sector will allow for temperatures to rise well into the 80s Tuesday afternoon. Scattered convection is also likely to develop during peak heating over northeastern PA and northern NJ as the cold front arrives late in the day. It is still unclear if this activity will propagate southward into southeastern PA, southern NJ and northern Delmarva Tuesday night or if convection will dissipate after dark. Wednesday through Friday...The remnants of Subtropical Storm Alberto looks to get steered northward up the Mississippi Valley by a weak mid-latitude trough early in the week and then northeastward around the periphery of the downstream ridge during the second half of the week. Chances for showers and storms increase initially across far western zones Wednesday afternoon and night and then area wide Thursday and Friday as the remnants of Alberto track into the eastern Great Lakes. There is also a signal in the extended range for the low to stall over the Mid Atlantic if it becomes cutoff by the end of the work week or next weekend. The risk for yet another heavy rainfall event increases late in the week in concert with the arrival of deep tropical moisture but the extent of a flood threat will ultimately depend on the track of the remnant low, including if and where the circulation becomes a cutoff. && .AVIATION /13Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Mainly VFR. Scattered afternoon showers and thunderstorms may briefly reduce the visibility and lower ceilings to MVFR or IFR with the highest threat of this at KRDG and KABE. Southwest to west wind around 5 to 10 knots. Tonight...Showers, heavy at times, with the chance of embedded storms will eventually result in restrictions developing overnight to at least MVFR for most sites with IFR possible by morning. Outlook... Sunday...Low clouds will advect S-SW across the region behind a back door cold front, resulting in at least MVFR restrictions for most of the day. Periods of IFR restrictions possible, especially toward the coast and elsewhere in heavier showers. Thunderstorm activity looks to mainly setup S/SW of Phila terminals. Light and variable winds prior to fropa will become NE and then E behind it. CIGs likely lower to IFR Sunday night as marine air becomes increasingly established. Monday...IFR CIGs to start the day should gradually improve to MVFR late morning-early afternoon and possibly to VFR late in the day. Light E winds become S in the afternoon. Tuesday and Wednesday...Mainly VFR. S-SW winds shift out of the NW on Tuesday as a cold front moves southward. Winds then veer out of the E-NE Tuesday night and then SE Wednesday. Wind speeds generally under 10 kt through Wednesday morning before increasing to 10-15 kt Wednesday afternoon. && .MARINE... Today...Conditions remain below SCA with SW winds generally around 15 knots with a few gusts up to 20 knots. Seas remain 2-4 feet on the ocean and 2 feet or less on DE Bay.Showers and thunderstorms could affect the waters this afternoon. Tonight...The conditions are anticipated to be below Small Craft Advisory conditions with a southwest breeze ahead of a backdoor cold front. Seas should be 4 feet or less. Outlook... Sunday...A sharp wind shift from SWly to NEly is expected with a cold front moving southward. Expect this wind shift to occur across the northern coastal waters of NJ early in the day, then southern coastal waters of NJ and DE Bay either late morning or early afternoon, and possibly not until late in the day for the DE coastal waters. The strongest NE winds look to develop across the northern coastal waters of NJ on Sunday where gusts to 25 kt are most likely to occur. Seas in these northern zones could also build to 5 ft in the northern coastal waters later in the day in response to the strengthening onshore flow. A SCA has been issued for ANZ450-451 for Sunday afternoon and evening. Monday through Wednesday...Winds and seas below SCA levels. Rip Currents... There is a low risk for the development of dangerous rip currents today. The wind is forecast to be southwest around 10 to 15 MPH with waves in the surf zone building to 3 to 4 feet. A cold front sinking southward across the area on Sunday will cause winds to shift from S-SW to NE. The setup may favor a moderate rip current risk by Sunday afternoon, particularly for Monmouth and Ocean County beaches in NJ, where winds will be strongest (15-20 mph), waves in the surf zone will be highest (4-5 ft) and where a NE flow yields more of an onshore component due to the orientation of the coastline (relative to the shoreline farther south in NJ). && .TIDES/COASTAL FLOODING... Spotty minor tidal flooding is possible with the Sunday evening high tide along the northern coastline of New Jersey and late Sunday night along the tidal portion of the lower Delaware River. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from noon Sunday to midnight EDT Sunday night for ANZ450-451. && $$ Synopsis...Klein Near Term...Fitzsimmons/LF Short Term...Fitzsimmons Long Term...Klein Aviation...Fitzsimmons/Klein Marine...Fitzsimmons/Klein Tides/Coastal Flooding...

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