Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 190641 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 141 AM EST Sun Nov 19 2017 .SYNOPSIS... Low pressure over the Ohio Valley will lift to the north and east and into southeast Canada tonight, and then a strong cold front will move across the region Sunday morning. High pressure then builds in from the west and moves off the Mid-Atlantic on Tuesday. A cold front moves through the region Wednesday night. Low pressure develops along that front and may impact the region on Thursday. High pressure returns on Friday, followed by low pressure passing north of the area on Saturday. && .NEAR TERM /THROUGH TODAY/...
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130 am update: Wind advisory beginning time was pushed up to now as gusts 40 to 50 mph are already occurring in portions of Delmarva and along the immediate coast of southern New Jersey. In addition, convective showers (with even a couple lightning strikes) are developing in the pre-frontal warm sector along and just east of the Delaware River. These showers have the capability of generating strong wind gusts as they race northeastward at speeds of 55 to 70 mph. Also made more adjustments to hourly temperatures, as there remains a huge variation between the rural valleys northwest of the urban corridor and the Philadelphia metropolitan area (on the order of 20 degrees), and this range is increasing. Previous discussion below... Complex forecast through Sunday as a strong cold front moves through the area in the next few hours. Strong/deepening surface low in western NY/PA will progress east-northeast along/just south of the Saint Lawrence River during the next few hours, with an equatorward-extending cold front racing eastward through the Mid-Atlantic, aided by strong pressure rises upstream. Showers continue to develop in the pre-frontal warm sector in the northern Mid-Atlantic, with widespread slab-like lift along the cold front itself contributing to low-topped convection in far western PA southward through the Appalachians of WV/western VA at this time. Given the very strong winds off the surface (850-mb winds 50-75 kts downstream of the front), these showers will not have a difficult time transporting these winds to the surface, particularly with the added effects of water loading and evaporative cooling below the cloud bases. The HRRR is strongly indicative of two waves of stronger wind gusts during the next few hours in the CWA. The first is associated with the frontal band of showers moving through the area in the 08Z to 12z time frame (and perhaps with showers in advance of this band), and the second is after cold frontal passage, tied strongly to the 2-3 hour duration of strongest pressure rises immediately upstream of the front. Wind advisory looks OK for the area, though it may start a little late. Will keep an eye on observations through the night, and may jump the start time closer to 2 or 3 am if radar/surface obs confirm. The strongest winds will ease only slowly during the late morning and afternoon hours, as strong boundary-layer mixing will continue, aided by cold-air advection and increasing insolation with time. Meanwhile, the highest QPF looks to occur mainly north of the Mason-Dixon Line during the next few hours, in closer proximity to the strongest synoptic-scale ascent. There is a noticeable downward trend in precipitation along/east of the Blue Ridge to our southwest, which lends confidence in the general distribution of precipitation accumulations expected. Temperatures will remain steady in the pre-frontal warm sector, though there is sharp cooling in the rural valley locations, with high clouds unable to re-emit longwave radiation back to the surface effectively. Guidance is handling this poorly, to say the least, and have had to make several edits to hourly temperatures already (with more likely to come before frontal passage). Thereafter, temperatures will likely begin a slow fall through the day, though increasing insolation and strong mixing may curtail this to some degree. General forecast is strongly weighted to hi-res 2-m temperatures, especially in the first 12 hours, where effects of clouds/precipitation and urban/rural processes are handled more accurately.
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&& .SHORT TERM /TONIGHT/... Main question tonight is the potential for some snow showers in the Poconos. With a strongly mixed boundary layer and saturation around 900-850 mb, there are indications that snow showers will develop periodically to the northwest, occasionally affecting the southern Poconos. Though the threat exists even this afternoon, the best time window looks to be this evening and tonight as the deep-layer flow becomes more northwesterly (with potential influence from lake-effect processes). Additionally, the main vort max associated with today`s system will be moving through Sunday night, which will supply additional lift and cold air aloft to aid in the development of a shallow (nearly) dry- adiabatic boundary layer in the post-frontal environment. Have kept and even prolonged slight-chances or chances of snow showers in this region, with potential for snow squalls given the strong low-level wind field. Meanwhile, temperatures will turn colder across the area, though the bottom will not fall out given the continued elevated winds (though much lighter than during the day). Current forecast is a mix of MAV/MET/ECS MOS, which are in reasonable agreement, and features temperatures generally a few degrees below seasonal averages. && .LONG TERM /MONDAY THROUGH SATURDAY/... High pressure just north of the Gulf Coast will build east on Monday, and will gradually move off the Mid-Atlantic coast on Tuesday. Chilly conditions on Monday with highs in the 30s up north and in the 40s elsewhere will moderate back into the 50s to near 60 on Tuesday as return flow sets up. From there, low pressure develops off the Southeast U.S. coast and lifts to the north along the coast as a cold front approaches from the west. That front passes through the region Tuesday night or Wednesday. Confidence is low for this timeframe, as there is uncertainty with how far west that low will track and how much precip will fall. For now, will follow a blend of guidance and carry chance PoPs Tuesday night- Wednesday. High pressure builds in from the west on Thursday, but models are indicating another coastal low passing south and east of the area during this time, and depending on how close it tracks, more precip is possible for the area. Will carry slight chance PoPs for southeast portions of the forecast area, but it will take a couple of days or so before the details are ironed out with this potential low. Dry conditions on tap for Friday. Low pressure passes north of the area Saturday and drags a cold front through the region. Some snow is possible up north Friday night, and then precip changes to rain on Saturday. && .AVIATION /07Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of tonight...Generally VFR, but potential for sub-VFR conditions in periodic showers, especially between 06Z and 12Z, in immediate advance of a strong cold front moving through the area. Southwest winds 15 to 25 kts with gusts 30+ kts will rapidly switch to west or west-northwest 20 to 30 kts with gusts to 40+ kts after frontal passage. Showers may also contain strong/erratic/variable wind gusts and convective turbulence. Confidence in CIGs is well below average and in winds is somewhat below average, especially regarding the timing of the frontal passage. Sunday...Rapid improvement to VFR with showers quickly exiting the area early in the morning. Very strong west or west- northwest winds 20 to 30 kts with gusts to 40 kts or so are possible through the day, with the strongest winds generally in the morning. Confidence is well above average. Sunday night...VFR with west winds slowly diminishing to around 10 kts after midnight. Gusts to 25 kts or so possible through the evening hours. Confidence above average. Outlook... Monday through Tuesday...VFR. West winds 10-15 KT on Monday become S 5-10 KT Monday night, and then 10-15 KT with gusts up to 20 KT on Tuesday. High confidence. Tuesday night through Thursday...Potential for rain Tuesday night/Wednesday, then again on Thursday with sub-VFR conditions. Low confidence forecast. && .MARINE... Gale conditions are generally occurring across the waters at this time, with the strongest winds/gusts expected to occur just after frontal passage from about 6 am to noon. Winds 40-45 kts are likely during this time window. Very choppy/erratic seas are likely through the day. Showers are expected to move in from the west late tonight but should move out quickly by mid to late morning. OUTLOOK... Sunday night...NW Gales continue Sunday night, but may end just after midnight on DE Bay. Monday through Monday night...NW winds gusting to 25 KT on the ocean waters, along with 4-6 ft seas. Tuesday through Tuesday night...SW winds 15-20 KT with 25-30 KT gusts on the ocean; SW winds gusting to 25 KT on DE Bay. Winds diminish after midnight Tuesday night. Wednesday...Winds and seas subside to sub-SCA conditions. Thursday...Low confidence forecast with the potential for SCA conditions on the ocean. && .TIDES/COASTAL FLOODING... There is a potential for blow out tides on Sunday, especially in the Delaware Bay and the tidal portion of the Delaware River, where strong W-NW winds will effectively drain water out of the bay. The low tide Sunday afternoon/evening seems to be the most susceptible for low water concerns that could impact navigation in the area. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Wind Advisory until 4 PM EST this afternoon for PAZ054-055- 060>062-070-071-101>106. NJ...Wind Advisory until 4 PM EST this afternoon for NJZ001-007>010- 012>027. DE...Wind Advisory until 4 PM EST this afternoon for DEZ001>004. MD...Wind Advisory until 4 PM EST this afternoon for MDZ008-012-015- 019-020. MARINE...Gale Warning until 5 AM EST Monday for ANZ430-431-450>455.
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&& $$ Synopsis...MPS Near Term...CMS Short Term...CMS Long Term...MPS Aviation...CMS/MPS Marine...CMS/MPS Tides/Coastal Flooding...Robertson

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