Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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894 FXUS61 KPHI 261326 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 926 AM EDT Tue Sep 26 2017 .SYNOPSIS... Hurricane Maria will move slowly northward off the coast of the Outer Banks through Wednesday before turning sharply east- northeastward into the open Atlantic through the end of the week. A cold front will move through the Mid-Atlantic on Thursday, and a reinforcing front will progress through the area Friday and Friday night. Cool high pressure will settle into the Northeast this weekend through at least early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... High pressure will remain over the northeastern states today. Hurricane Maria will be located off the North Carolina coast. The circulation around the two systems will keep an onshore flow in our region for today. Low clouds remained along and to the southeast of the Interstate 95 corridor at mid morning. The sky was mostly sunny in areas to the northwest. The low clouds are forecast to erode slowly during the late morning and early afternoon hours. They may linger through the day near the coast. The wind should favor the northeast and east. Speeds are expected to be around 10 MPH on the coastal plain and around 5 MPH inland. Gusts are expected along the coast. Maximum temperatures are anticipated to be in the 70s near the coast due to the onshore flow, and in the 80s inland. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... Low clouds and patchy fog will develop once again across the coastal plain and the Delmarva late tonight. NE winds will diminish to 5-10 MPH along the coasts and to 5 MPH or less inland. Meanwhile, Maria will begin to weaken to a Tropical Storm as it begins to sharply curve out to sea. A few outer rain bands may move into southern portions of the forecast area, but not expecting much, if any, QPF. Lows tonight generally in the low 60s to the north and in the upper 60s to around 70 to the south. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Main forecast concerns for the long-term period are precipitation chances Wednesday and again Friday night and Saturday and a sharp downward trend with temperatures versus what has been observed recently. Analysis of the 00Z operational model suite suggests that the overall large-scale pattern is decently agreed upon, but rather large discrepancies exist with the smaller-scale vorticity maxima that will be progressing through and/or affecting our region through the period. Additionally, poor run-to-run model continuity continues, owing largely to uncertainty with the flow pattern in the northern Pacific and the apparent substantial influences downstream. At 12Z Wednesday, Maria will be off the coast of the Outer Banks. A potent vort max will be progressing through Ontario with southwesterly midlevel flow between the two. Although the main impacts from Maria will be well south/east of the area, the pressure gradient will be tightening in the southern CWA, so expect breezy conditions to develop in southern Delaware and far southeast New Jersey. Northeasterly surface flow should keep the area socked in clouds much of the day, with only gradual improvement from northwest to southeast. Some showers are possible, especially near the coast, as Maria makes its closest approach to the area. However, a considerable portion of the model output is dry through Wednesday night. The exceptions are the 00Z CMC/ECMWF, which indicate some QPF close to the coast. The totals from the CMC look overdone, but the geographic agreement with the ECMWF suggests that precipitation chances should remain. I did lower them considerably northwest of I-95, where there is little remaining evidence of sufficient lift for showers. One noteworthy development: the 00Z ECMWF is considerably slower and continues on the west side of the guidance with the track of Maria. By Thursday, it becomes an outlier with the position of Maria, so I neglected to give the new simulation strong weighting in the forecast. As the Ontario trough moves rapidly east-northeast into the Canadian Maritimes on Thursday, Maria should begin the eastward turn. The ECMWF is weakest with this trough and considerably more sluggish in advancing Maria farther offshore - this does lower confidence in Thursday`s forecast a little bit. A surface low will develop and progress rapidly eastward through Quebec and New Brunswick, with a cold front sweeping across the Eastern Seaboard Thursday. Cooler and noticeably drier air will seep into the Mid-Atlantic upstream of the front, with forecast highs Thursday around 10 degrees colder in the urban corridor and up to 20 degrees colder in the southern Poconos. The change should be less severe near the coast (which will be cooler on Wednesday given the onshore flow). Kept most of Thursday dry given the subsidence upstream of the front. An upstream shortwave trough digs southeastward into the Great Lakes Thursday night, and this is where much of the forecast uncertainty lies. The GFS is much more progressive with this trough than the CMC and especially the ECMWF. The GFS has trended stronger with the first trough in eastern Canada and weaker with the second, which does not agree with the recent trends of the NAM or ECMWF. Additionally, there is very large variation in the upstream flow run-to-run with the GFS. The stronger agreement between the CMC/ECMWF made me lower the weighting of GFS-based guidance Friday onward, and I may not have done so enough. The main effects on the forecast are with precipitation chances Friday night and Saturday, with the GFS about 6-12 hours faster in general (more Friday night versus early Saturday for the CMC/ECMWF). For now, kept the highest PoPs late Friday night (a compromise of the two camps), but my suspicion is that PoPs need to linger into a portion of Saturday, especially for the eastern zones. Trended this direction with the forecast, but expect further (perhaps substantial) changes with PoPs during this period. Cool/dry high pressure moves in thereafter, with a prolonged period of near to slightly below seasonal temperatures Saturday onward. Models have large phase/amplitude variations with the midlevel ridge developing in the East during this period, so the duration of the cool/dry spell is a little uncertain at this point. && .AVIATION /13Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Stratus producing IFR conditions at KACY/KMIV/KILG/KPHL/ KPNE/KTTN will be slow to dissipate, and conditions will probably not lift to VFR until midday or so. VFR west of I-95 through the TAF period. Fog/stratus looks to redevelop late tonight with sub-VFR conditions likely once again. E-NE winds will increase to 5-10 KT, except at KACY, where E-NE winds will increase to 10-20 KT. LGT/VRB winds tonight. Outlook... Wednesday: MVFR/IFR conditions possible, especially from the urban corridor eastward, with gradual improvement to VFR from northwest to southeast during the day. Some showers possible at KMIV/KACY. Northeast winds 5 to 15 kts with stronger gusts near the coast (up to 25 kts or so). Confidence below average. Wednesday night and Thursday: Winds should become north/northwest as a cold front sweeps through the region. May see some lingering showers/low clouds at KMIV/KACY Wednesday night. Speeds should reach 10 to 20 kts on Thursday, with higher gusts possible. Confidence average. Thursday night and Friday: Predominantly VFR. Northwest winds 5 to 15 kts. Confidence above average. Friday night and Saturday: Showers and localized sub-VFR CIGs/VSBYs possible. Winds generally north or northwest around 10 kts. Confidence well below average. && .MARINE... Small Craft Advisory continues today and tonight for the ocean waters. Seas on the ocean will build to 8-9 feet. NE winds will range from 15-20 KT with gusts up to 25 KT for southern NJ and DE ocean waters. Otherwise, NE winds range from 10-15 KT. With these increasing offshore seas, can expect breaking waves at the shores of around 7 feet at southern NJ and DE beaches. A high surf advisory has been issued for tonight through Wednesday for southern NJ and DE coastal areas. Outlook... Wednesday: Northeast winds with gusts above 25 kts likely in the southern NJ/DE coastal Atlantic, trending lighter in Delaware Bay and the northern/central marine zones. Seas near 10 feet in the Atlantic, with rough surf likely. A high surf advisory continues for the southern NJ/Delaware beaches. Showers possible. Wednesday night: Gradual decrease in winds, though advisory- level gusts still possible south of Atlantic City. Seas will be slow to decline. A chance of showers. Thursday and Thursday night: Cold frontal passage will increase winds to advisory level in the Atlantic waters and potentially Delaware Bay. Seas well above advisory criteria. Friday: Seas nearing but likely still above 5 feet, but winds should diminish below advisory criteria. Friday night and Saturday: Sub-advisory conditions expected, but a chance of showers. Rip Currents... Due to swells emanating from Hurricane Maria, there continues to be a HIGH risk for the development of dangerous and life threatening rip currents for the ocean beaches today and tonight. The rip current risk will likely be in the high category Wednesday and Thursday as long-period southeasterly swells from Maria continue. Swell heights around or above 10 feet on Wednesday will likely generate high surf/beach erosion issues for much of the surf zone but especially for southern New Jersey and Delaware. Overall, not a good time to spend at the beach the next few days. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...High Rip Current Risk through Wednesday afternoon for NJZ014- 024>026. High Surf Advisory from 6 PM this evening to 6 PM EDT Wednesday for NJZ024-025. DE...High Rip Current Risk through Wednesday afternoon for DEZ004. High Surf Advisory from 6 PM this evening to 6 PM EDT Wednesday for DEZ004. MD...None. MARINE...Small Craft Advisory until 6 AM EDT Thursday for ANZ452>455. Small Craft Advisory for hazardous seas until 6 AM EDT Thursday for ANZ450-451. && $$ Synopsis...CMS Near Term...Iovino Short Term...MPS Long Term...CMS Aviation...CMS/MPS Marine...CMS/MPS is the U.S. government's official web portal to all federal, state and local government web resources and services.