Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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795 FXUS61 KPHI 221546 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1146 AM EDT Mon May 22 2017 .SYNOPSIS... Weak low pressure is developing on a warm front from the Delmarva Peninsula into southern New Jersey today. A cold front approaching from the west will merge with the warm front just off the mid Atlantic coast tonight. Low pressure in North Carolina Tuesday will pass east of the mid Atlantic coast Tuesday night. Weak high pressure is left behind over our area Wednesday. Stronger low pressure organizes in the Ohio Valley Wednesday night sending its associated fronts through the mid Atlantic states late Thursday or early Friday as the low moves into New England. High pressure should follow for the weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... High pressure offshore of Cape Cod will slowly drift eastward through today, which will keep a steady onshore flow through the afternoon. A warm front to our south will slowly drift northward through the day as a cold front approaches from the west. Meanwhile, southwest flow aloft will continue to keep moisture over running the warm front. Periods of showers will continue to move across the area through the day as a couple of short wave/vorticity impulses traverse the area. Instability still remains forecast as little to none, but we will maintain the isolated chance of thunderstorms as there is some very weak instability aloft, and it is possible for a few isolated lightning strikes. The main threat today will continue to be the possibility for periods of moderate to heavy rainfall across the area. PW values are forecast around 1.5-1.75 inches across the southwestern two-thirds of the area, especially Delmarva and southern New Jersey. So there is the potential for localized flooding, especially in poor drainage areas and especially if any thunderstorms develop. It still looks like there will be a sharp drop off in rain totals north and west of I95. Expect rain to taper off rather quickly late this afternoon as the cold front approaches the region. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/... Low clouds are expected to persist through at least the first half of the night until dry air advection dominates in the wake of the cold front. Depending on how much rain areas see this morning, some fog may develop as well, but the low clouds should inhibit widespread fog development. Models have trended slightly later with the arrival of the cold front, as a result expect low clouds to linger a bit longer, and thus there will be a shorter opportunity for efficient radiational cooling, especially for the Coastal Plains. Therefore, temperatures from the I95 corridor to the east may not drop off that much overnight. && .LONG TERM /TUESDAY THROUGH SUNDAY/... DRAFT DRAFT DRAFT for the 330 PM discussion: **Flood potential may exist where its rained more than 2 inches today in SNJ and the Delmarva** 12Z NAM not applied for this discussion. Its not off to a good start for today. 500mb: 500mb: A sharpening and unusually strong high amplitude trough in the nations midsection will closed off and weaken as it moves to new England Friday, with ridging to follow this weekend. The next trough will be organizing in the Great Lakes region early next week. Temperatures: Calendar day averages Wednesday-Saturday near normal warming Sunday and Monday to nearly 5 above normal, ahead of the next trough. Forecast basis: a 50 50 blend of the 12z/22 GFS/NAM MOS for Tuesday night through Wednesday night, then the 12z/22 GFS MEX MOS for Thursday with the 15z WPC guidance for Thursday night-next Monday. Any substantial deviations from the basis above, will be noted: The dailies... Forecaster confidence is below average on details through Friday morning. Tuesday night through Wednesday...Depending on the path of the next coastal...a risk of rain south and probably dry north. Thursday...Looks wet in warm air advection. Friday...After any early morning showery rains depart with the cold frontal passage...a breezy west wind should develop with afternoon warming. Memorial Day weekend...Warming will continue in the lower levels with a strengthening west to southwest flow as high pressure starts to build in from the southwest. Showers may approach from the west late Monday but for now...we`ll consider that possibility as a low probability of occurrence prior to sundown Monday evening the 29th. && .AVIATION /16Z MONDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. IFR conditions will continue across the area through the day today. KABE and KRDG remain MVFR, but they are also expected drop to IFR during the next couple of hours. The heaviest showers are expected through 18Z, but chances for rain exist through 00Z. There is a slight chance for thunderstorms but it is too uncertain at this time to include in the TAFs. Even as the precipitation begins to taper off after 18Z, expect the low clouds to linger through the rest of the day time hours. If there are breaks in the clouds late this afternoon into the evening hours, patchy fog may develop. Conditions should begin to improve after 06Z behind a cold front which will bring in drier air. OUTLOOK... DRAFT for the 330 PM issuance. Forecaster confidence on details is below average Tuesday night through Friday morning. Tuesday night...VFR conditions north and possibly the entire area but a risk of MVFR/IFR in rain vicinity KMIV/KACY. Light northeast wind. Wednesday...VFR except possible MVFR/IFR conditions during the morning KMIV and KACY. East to northeast wind gusts to 15 mph. Thursday...VFR CIGS with periods of MVFR/IFR in showers and isolated thunderstorms. easterly winds probably becoming southeast to south at night. Friday...Improving conditions after any early morning showers end with mainly VFR expected. Westerly wind gusts to 20 kt possible in the afternoon. Saturday...VFR. Westerly wind gusts to 20 kt. && .MARINE... Although some gusts above 20 kt are possible this evening, we are expecting conditions to remain below SCA criteria today and tonight. By this evening, fog may develop on both the Delaware Bay and the coastal waters. However, it is uncertain at this time how widespread or dense any fog will be. Any fog that does develop is expected to dissipate in the pre-dawn hours as northerly winds increase. OUTLOOK... Draft for the 330 PM issuance. Forecaster confidence on this outlook section is below average Tuesday night through Friday morning. Tuesday night and Wednesday...Leftover SCA for hazardous seas possible...mainly the DE Atlantic coastal waters. Otherwise northeast to east wind with gusts generally under 20 kt. Thursday...SCA probable for the well organized Ohio Valley low. East to southeast winds build the seas to b between 5 and 8 feet on the Atlantic waters. Friday...SCA may continue for leftover Hazardous seas on the Atlantic portion of our coastal waters. Westerly wind develops with gusts to 20 kt. Saturday...Small chance an SCA for westerly nearshore gusts of 20 to 25 kt. && .HYDROLOGY...
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1137AM: DOX and DIX STA (DP)/STP (LEGACY) are underestimating rainfall. Ditto MRMS. We are using FFMP HPE. PNS for 1.75 inch or greater amts will post soon as we are seeing numerous reports of 1.5 to 2.5 inch rainfall since 230 this morning. At this time...DIX STP (Legacy is performing best vs its DP). DOX STP/STA are almost identical and far too low. The Flood Advisory updated statement (FLS) will post soon. At 1PM we will consider extending the NJ portion and cancel the MD DE portion. The Flood Advisory for generally nuisance potentially travel slowing/altering poor drainage street flooding continues. This advisory is because of overrunning along and just north of the warm front that shows light southerly flow in the warm sector and light northeast flow just north of the boundary. PWAT is near 1.75 inches. 12z NAM is off to a poor start.
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&& .TIDES/COASTAL FLOODING...
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DRAFT for 330 PM discussion: Astronomical tides are at some of their highest of the year this Wednesday and Thursday evenings. Some onshore flow will contribute positive departures and its probable that a few locations along the Atlantic coasts of DE and NJ will exceed the minor coastal flood advisory threshold. It only takes about .6 to .7 feet above high tide to reach the threshold. It`s all in the details, which are yet to be confidently determined. This mornings 12z/22 NAM does not loom to be off to a good start. We will eight its guidance less than other traditionally used models. for now the SIT and ETSS modeling forecasts minor with a small chance of a moderate episode if prior to high tide onshore is strong enough for enough duration.
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&& .CLIMATE... Record daily rainfall at ACY has a good chance of exceedance. 1.19 inches is the daily record set in 1909. as of 1115AM 1.01 inches...appears well on its way to a record for this date. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Meola Near Term...Johnson/Robertson Short Term...Johnson Long Term...Drag/Meola Aviation...Drag/Johnson/Meola Marine...Drag/Johnson/Meola Hydrology...1147A Tides/Coastal Flooding... Climate...

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