Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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435 FXUS61 KPHI 281014 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 614 AM EDT Wed Sep 28 2016 .SYNOPSIS... An area of low pressure will move across the Ohio River Valley today and combine with another low across the central and southern Appalachians Thursday into Friday. This low will lift back across the Ohio River Valley and into the eastern Great Lakes region Saturday into Sunday and bring a frontal system across our area. High pressure is then expected to build across the northeast late Monday into Tuesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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550 AM update...Added patchy dense fog wording to the forecast database early this morning for southern NJ, far southeastern PA and eastern MD and interior DE. A Special Weather Statement was issued earlier to cover this hazard. The fog should erode from northeast to southwest between about 7 and 9 AM once a northeast wind develops. We are also closely monitoring strong convection developing upstream across central VA and southern MD. It`s will likely move into the Chesapeake Bay after daybreak and possibly impact the eastern MD shore. Added isolated thunderstorms in the forecast for these areas. These showers should then weaken later this morning as it moves into a more stable airmass residing over the lower Delaware Valley. Previous Discussion... An area of low pressure at the surface over the northern Great Lakes will move southward through the Midwest states today. The surface low continues to weaken with the closed upper low being vertically stacked. High pressure over eastern Canada will continue to build southward across the Northeast states and into the northern mid- Atlantic region today. This will create a cold- air damming setup with a wedge of cooler air near the surface continuing to drain down the eastern side of the Appalachians and into the area. A tightening pressure gradient between the high to our north and low pressure to our south and west will produce gusty E-NE winds across the area today, especially during the afternoon. The boundary-layer flow will prevent a stationary front over central VA from advancing northward into the area today despite the deep southerly flow ahead of the upper low higher up in the atmosphere. We continued to follow the idea that the showers developing upstream of us will struggle to make much progress northeastward through the forecast area today, especially from the Phila metro, north and east as high pressure builds down into the area. Stratocu is anticipated to quickly develop later this morning and afternoon as the marine layer advects inland. Forecast soundings support a deep enough moist layer for at least patchy drizzle to develop this afternoon, especially toward the NJ coast. In our far western zones (e.g., Chester/Berks Co, PA) and southern zones (Delmarva), there will be lesser influence from the high and a strengthening 30-35 kt jet streak at 925 mb will advect moisture off the Atlantic. There should be sufficient isentropic lift to produce showers across these southern/western zones today. Expect showers to increase in coverage and intensity later this afternoon, especially toward the eastern shore of the Chesapeake Bay. Added isolated thunder to the forecast across the Delmarva for this afternoon as most guidance depicts a corridor of higher instability advancing in from the south. Highs temperatures range from the mid/ upper 60s along and north of the I-80 corridor to mid/upper 70s across southeastern PA, southern NJ and Delmarva.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/... The upper low is forecast to slow down over the Midwest states tonight as it becomes cutoff. High pressure will continue to remain anchored to our north while the frontal boundary remains stalled to our south. The easterly low-level jet between these systems over the mid Atlantic will strengthen further to around 40 kt. Expect showers to expand northeastward across the entire area tonight. Expect for the NAM-based guidance, models continue to show a band of heavy rainfall developing very close to the western most zones of our CWA. The trend has been to shift the axis of higher QPF farther south and west, likely as models are more accurately capturing the magnitude of the cold-air damming pattern. Additionally, with the greatest low-level convergence situated at nose of the low-level jet forecast to be positioned to our west, the greatest risk for excessive rainfall may remain on the western side of the Chesapeake Bay. QPF amounts of 1-2 inches are forecast for eastern MD and out toward Chester/Berks Co. in eastern PA. We opted to not issue a Flood Watch at this juncture. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Unsettled weather continues to be forecast through the first half of the extended period, with generally fair weather for the second half of the period. For Thursday into Friday, an area of low pressure at the surface will be located to our southwest, while high pressure remains to our northwest. In between, a stationary boundary will be located just to our south. This will keep an onshore flow across our area. Meanwhile, a closed low aloft will be spinning to our west as well. This will keep a persistent southerly flow aloft across the area at the same time. With the southerly flow aloft, overrunning precipitation is expected to continue area within the onshore flow Thursday and Friday. It will not likely rain the entire time, but several periods of rain will be associated with several short waves/vorticity impulses rounding the closed low aloft to our west. By Saturday, and especially Sunday, the high to our north begins to move eastward, and the low to our west will begin lifting back toward the Great Lakes region. This will eventually pull the frontal boundary to our south across our area on Sunday. There will continue to be a chance of showers Saturday through Sunday as several short wave/vorticity impulses move across the area as the closed low aloft begins to break down and lift to the northeast. By Monday into Tuesday, the surface low and low aloft pass to our east, and drier air begins to move into the area as high pressure tries to move in from the north. The high may remain to our north, Tuesday which could produce an onshore flow which may end up leading to more cloud cover and cooler conditions for Tuesday. && .AVIATION /10Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR and light winds early this morning. E-NE winds will increase after sunrise this morning to 5-10 kt. Marine stratocu is forecast to develop during the late morning and early afternoon. With the 06Z TAFs, we brought in MVFR CIGs earlier. E winds will gust to around 20 kt late this afternoon and evening. DZ may develop across the I- 95 and coastal TAF sites during this time. Then showers will expand northeastward across the area tonight. Expect MVFR CIGs to lower to IFR sometime this evening or overnight. There is still uncertainty with the onset timing of IFR. OUTLOOK... Thursday-Saturday...A prolonged period of MVFR or IFR conditions with periods of rain is expected to affect the TAF sites through the period. East to northeast winds are expected to gust 20-25 knots for much of the area. Saturday night-Sunday...Some improvement, possibly to VFR by Sunday. Showers remain possible. && .MARINE...
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SCA goes into effect this afternoon for ANZ450/451 and the remainder of the waters this evening as NEly winds surge down the coast. A Gale Watch was previously in effect for ANZ450/451 for tonight but we do not anticipate gales to develop before Thursday. Winds of 15- 25 kt are expected with gusts to 30 kt possible tonight off the NJ coast. Seas in the coastal Atlantic waters will subsequently build to 5-7 ft tonight. OUTLOOK... Thursday...Gale Watch in effect for the northern half of the Atlantic coastal waters. Small Craft Advisory in effect elsewhere. Thursday night...Gale Watch in effect for ALL areas. Friday...Gale force winds may remain across the northern half of the Atlantic coastal waters. Small Craft Advisory level winds may remain elsewhere. Friday night-Saturday...Winds expected to drop below Small Craft Advisory levels, however, seas may remain above advisory levels through the period. Saturday night-Sunday...Winds and seas expected to drop below Small Craft Advisory levels. Rip Currents... There is a moderate risk for the formation of dangerous rip currents today. This is due to the combination of an increasing onshore wind and building waves in the surf zone. Conditions may warrant a high risk late in the day if winds and seas increase quicker than currently expected. The probably risk for the formation of dangerous rip currents is at least moderate to perhaps high on Thursday.
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&& .TIDES/COASTAL FLOODING... A persistent onshore flow is expected for the rest of the week into the weekend. The strongest winds are expected Thursday through Friday. Tides levels are forecast to increase through this period, and we could reach minor coastal flood levels. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Watch from Thursday evening through late Thursday night for ANZ430-431-452>455. Small Craft Advisory from 6 PM this evening to 6 PM EDT Thursday for ANZ430-431-452>455. Gale Watch from Thursday morning through late Thursday night for ANZ450-451. Small Craft Advisory from 1 PM this afternoon to 6 AM EDT Thursday for ANZ450-451. && $$ Synopsis...Robertson Near Term...Klein Short Term...Klein Long Term...Robertson Aviation...Klein/Robertson Marine...Klein/Robertson Tides/Coastal Flooding...Robertson

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