Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 210113 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 913 PM EDT Fri Apr 20 2018 .SYNOPSIS... High pressure builds over our area later tonight and remains in place through the weekend, then shifts offshore on Monday. Low pressure moves east across the Gulf Coast states Sunday and Monday, then tracks up the Mid-Atlantic coast later Tuesday through late Wednesday. High pressure then builds well to our west and south later Thursday and Friday. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... Low pressure is positioned near the Canadian Maritimes this evening while high pressure was centered over the Midwest U.S. The pressure gradient resulted in a breezy day earlier, but now the gusts have subsided for the most part with the loss of daytime mixing. The sustained wind speeds show a more gradual decrease overnight. Overall, not much change from earlier. The winds, and how fast they diminish will be an important part of the forecast for tonight and a key link to the frost/freeze potential. The current thinking is winds should become light late tonight with high pressure building in from the west. Once this happens, temperatures will drop rather quickly overnight, bottoming out in the lower to mid 30s before daybreak (except upper 30s in the cities and along the coast). A Freeze Warning was issued for Cecil County, MD and for Berks, western Montgomery and western Chester counties in PA with lows expected to fall in the 30-32F range and the growing season declared active. A Frost Advisory was issued for northeastern MD, DE, the northern and western suburbs of Philadelphia and central and southern NJ where (1) the growing season is already underway, (2) temperatures are forecast to dip into the mid 30s and (3) calm winds will provide favorable conditions for frost formation. High clouds will start to stream in from the west late but should not have much of an impact on radiational cooling. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/... High pressure moves overhead tomorrow. A jet streak at the base of the upper low to our northeast will be associated with mid and high clouds over the region. There is some uncertainty regarding the extent of the cloud cover tomorrow, but do think there will be enough filtered sunshine or breaks in the clouds to allow for temperatures to reach their full mixing potential during the afternoon. Forecast highs generally range from 55-60F with slightly cooler spots in the southern Poconos and warmer spots in Philadelphia and toward southern Delaware. NW winds 4-8 mph in the morning, increasing to 8-12 mph in the afternoon. && .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/... Summary...Turning warmer, however a coastal low looks to affect our weather Tuesday night through Wednesday. Synoptic Overview...An upper-level trough across the Northeast is forecast to shift offshore later in the weekend. As this occurs, a ridge slides eastward from the Great Lakes driving surface high pressure over our area. However, strong energy underneath this ridge moves across the Gulf Coast states over the weekend into early next week with surface low pressure associated with it. Meanwhile, northern stream short wave over the Great Lakes region is forecast to begin phasing with the southern stream closed low over the Southeast U.S. The model spread increases during this time frame due to the timing of the phasing and the speed of the resulting surface low near the East Coast. The surface low is forecast to basically slide northward by the Mid-Atlantic coast during mid-week before shifting to our northeast to start Thursday. An upper-level trough looks to then settle into the East for the end of next week while weak surface high pressure slides to our south. This trough may eventually lead to another coastal low development with time. For Saturday night through Monday...Surface high pressure builds over the region as an upper-level ridge slides eastward. The surface high though will shift offshore Monday as low pressure tracks across the Southeastern U.S. The presence of surface high pressure over the area will result in subsidence and a lack of clouds overall with less wind, with the airmass modifying through the weekend. This leads to each afternoon getting warmer. The exception will be the coastal locales as light winds Sunday and Monday especially should result in a sea breeze each afternoon and early evening. Low temperatures though will still be chilly due to good radiational cooling conditions, with some places potentially having frost or freeze conditions especially Saturday night. For Tuesday through Thursday...While a coastal low is anticipated to affect our weather, the details are less certain. This is due to the timing of phasing energy toward the East Coast, with a closed low with the southern stream and a short wave arriving from the Great Lakes region within the northern stream. The southern stream energy will drive surface low pressure off the Southeast U.S. coast Tuesday morning, then energy interaction between the northern and southern streams will help turn the low northward by later Tuesday and continue through Wednesday. The latest thinking is to bring the low just off of our coast later Wednesday then into New England Thursday morning. The combination of onshore flow (increasing moisture) and increasing synoptic ascent will bring widespread rain mainly Tuesday night and Wednesday. Some of the rain could be heavy at times, however the system may be moving quick enough where the window of opportunity for heavy rain is shortened. A period of stronger onshore flow could result in some coastal flooding, although we are between the new/full moon cycles. There is still plenty of time to iron out the details, however given enough agreement for widespread rain for a time we continued with likely to low-end categorical PoPs for portions of this time frame. Initial overunning rain should develop later Tuesday, with rain shifting north Wednesday and then ending at night or early Thursday as low pressure shifts to our northeast. For Friday...An amplified upper-level trough may settle into the East with some weak surface features moving through. Depending on the evolution aloft, the airmass looks mild though and therefore some showers cannot be ruled out if there is enough instability during the day. As of now, we followed the WPC guidance rather closely. && .AVIATION /01Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR. High clouds move in toward early morning with CIGs at or above 15 kft AGL. NW winds with sustained speeds decreasing more gradually from 8-12 kt early this evening to 3-6 kt overnight. Saturday...VFR with mid to high clouds (CIGs above 10 kft AGL). Light NW winds 5-10 kt. OUTLOOK... Saturday night...VFR. Northwest winds 5-10 knots, becoming mostly light and variable. Sunday...VFR. Light and variable winds, becoming south to southwest around 5 knots (becoming southeast in the afternoon at ACY, MIV and possibly ILG). Monday...VFR. East to southeast winds around 5 knots. Tuesday...Mainly VFR with increasing and gradually lower clouds, then some light rain should develop at night. East-southeast winds around 10 knots. Moderate confidence. Wednesday...MVFR/IFR conditions probable with a period of rain. A period of gusty east to southeast winds possible, then lighter and turning west to northwest at night. Low confidence with the details. && .MARINE... Tonight...NW winds will continue to be in the 10-20 kt range tonight. Short-period, wind-driven NWly swells will be in the 3-4 ft range in our coastal waters and 2 ft or less in the Delaware Bay. Saturday...NW winds 10-15 kt in the morning will become light/variable toward midday and then S by mid afternoon in association with the development of a sea-breeze near the coast. Mesoscale models show the S winds strengthening to near 15 kt with gusts to 20 kt late in the day in the coastal waters of NJ. OUTLOOK... Saturday night through Monday....The conditions are expected to be below Small Craft Advisory criteria is over our area. Tuesday and Wednesday...Small Craft Advisory conditions looking likely. Low pressure is forecast to move up the East Coast Tuesday night through late Wednesday with onshore flow in advance of it. Winds and seas therefore increase, with wind gusts anticipated as of now to remain below gale force. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Frost Advisory from 2 AM to 8 AM EDT Saturday for PAZ102-104- 106. Freeze Warning from 2 AM to 8 AM EDT Saturday for PAZ060-101- 103. NJ...Frost Advisory from 2 AM to 8 AM EDT Saturday for NJZ012-013- 015>022-027. DE...Frost Advisory from 2 AM to 8 AM EDT Saturday for DEZ001>003. MD...Frost Advisory from 2 AM to 8 AM EDT Saturday for MDZ012-015- 020. Freeze Warning from 2 AM to 8 AM EDT Saturday for MDZ008. MARINE...None. && $$ Synopsis...Gorse Near Term...Klein/PO Short Term...Klein Long Term...Gorse Aviation...Gorse/Klein/PO Marine...Gorse/Klein/PO

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