Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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231 FXUS61 KPHI 221408 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1008 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 for the 330 PM discussion: **Flood potential may exist where its rained more than 2 inches today in SNJ and the Delmarva** 500mb: 500mb: A sharpening and unsually 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 nex trough will be organizing in the Great Lakes region early nexzt week. From the mid shift below: Tuesday through Wednesday...Low pressure moves through the Carolinas and is slated to cross Delmarva on Wednesday. The models have trended a little weaker and closer to the coast with the low and have been more Thursday through Friday... progressive, as well. With a more progressive solution, the low should clear the area during the day Wednesday. Rain will start to move in during Tuesday and continue into Wednesday. The best chances for rain remains closer to the coast, mainly south and east of the I-95 corridor. Some isolated or scattered showers are possible across the northwestern areas. Instability is limited and best chances for any thunder are across the southern half of our forecast area. Thursday...A cold front will cross the region early with more showers. Daytime heating will contribute to some additional instability and we could some storms pop up toward Thursday afternoon and evening. Any lingering showers will clear late Thursday into early Friday. Friday through Sunday...Warming will continue in the lower levels with a strengthening southwest flow as high pressure starts to build in from the southwest. Temperatures on Friday will generally be in the low to mid 70s but expect a better response to the increasing warmth with highs over the weekend rising into the mid to upper 70s on both Saturday and Sunday. Some showers are possible but a general clearing trend is expected as we head through the weekend. The models are hinting that the next low pressure to affect our area will arrive late in the weekend. && .AVIATION /14Z 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... Tuesday and Tuesday night...VFR conditions early will lower to MVFR/IFR in rain. North wind early will become more easterly through the day, around 10 knots or less. Wednesday...MVFR/IFR conditions possible with showers through much of the day. Thunder possible late. Light east to southeast winds. Thursday...VFR conditions expected with periods of MVFR/IFR in showers/thunderstorms. South to southwest winds. Friday...Improving conditions with mainly VFR expected. West winds around 10 to 15 knots, with higher gusts possible. && .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... Tuesday through Thursday...Both winds and waves are expected to stay below SCA criteria. Thursday night and Friday...Seas are expected to build and near 5 feet late Thursday into Friday. && .HYDROLOGY... 816A: Please realize that the DOX and DIX STA (DP)/STP (LEGACY) are considerably underestimating rainfall. Ditto MRMS. We are using FFMP HPE. PNS for 1 inch or greater amts will post soon. As of 8A: DEOS Kenton 1.4 and Smyrna 1.34 in Kent County DE. NJ: Upper Deerfield 1.58, Greenwich 1.14, Clayton 1.26, Hammonton 1.14. 732AM: A Flood Advisory for generally nuisance potentially travel slowing/altering poor drainage street flooding has been issued. Rainfall through 7 am has exceeded 1 inch in parts of southwest NJ and central DE and another 1 to 2 inches is expected between 7AM and 1 PM in parts of the advisory area. This 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. The NAM has been fairly consistent on showing 1.75 to 2.6 inch storm totals in parts of this area and it looks like it will come to pass in the PWAT of 1.7 inches. && .CLIMATE... Record daily rainfall at ACY has a good chance of exceedance. 1.19 inches set in 1909. will update once we get access to the data. && .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...Meola 1008 Aviation...Johnson/Meola Marine...Johnson/Meola Hydrology... Climate...1008 is the U.S. government's official web portal to all federal, state and local government web resources and services.