Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 151617 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1217 PM EDT Tue Aug 15 2017 .SYNOPSIS... A weak front will affect the weather across our region today and tonight. High pressure will build in for Wednesday and remain into early Thursday. Another low and its associated fronts will affect our weather Thursday and into Saturday. More high pressure arrives for Sunday and persists into early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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The past couple of hours have seen slow-moving convection developing west of Chesapeake Bay and a couple of convective cells with impressive rainfall rates moving through the Dover area as well as portions of Cumberland and Cape May Counties in NJ. Additionally, even somewhat weaker convection to the northwest of Philadelphia has produced 0.5-1.25 inch rainfall totals in a swath from KMQS to KDYL. As the associated midlevel perturbation moves eastward this afternoon, expect more widespread moderate to heavy rainfall to affect the region, generally along and south of the I-76 corridor where marginal instability will reside. Lightning has begun in central Maryland, so the slight chance of storms in the grids looks good at this time. Added moderate rainfall earlier to the grids, and will update them again shortly to include heavy rain wording in a corridor along and east of I-95 from New Castle County to Mercer County. Would not be surprised to see a few flood advisories be required through mid afternoon, especially near the urban corridor. An update to the HWO is forthcoming, as well. Previous discussion below remains relevant... Perturbations within fairly weak west-southwest flow aloft will move through the southern portions of the region today. Widespread light rain/showers have developed from the Blue Ridge northeastward into southeast Pennsylvania and central/southern New Jersey this morning. Radar trends are indicating convective cells increasing in coverage/intensity in the past couple of hours, with a few lightning strikes off the Delmarva coast (earlier). Instability will be increasing today as warm air advection continues in advance of the perturbations, even with low overcast conditions continuing. A surface front is just south/east of the CWA and will serve as a focus for enhanced (albeit not strong) convergence through the day, and large-scale ascent will be plentiful given differential cyclonic vorticity advection downstream of the perturbations, combined with the aforementioned warm air advection and upper-level divergence associated with a departing jet streak in the Northeast. Additionally, low-level moisture will continue to advect into the region via onshore flow north of the surface boundary (and aided by TC Gert in the western Atlantic). The KWAL 12Z sounding observed a PW near 2.2 inches, and these higher PWs are expected to surge northward to the I-76 corridor late this morning. This sets the stage for a swath of decent QPF this afternoon, with the 06Z NAM indicating this would occur near and south of the I-76 corridor (where surface moisture convergence is projected to maximize during the early afternoon). With large- scale dynamics quite favorable for deep/sustained lift and marginal but sufficient instability advecting northward (enabling convective enhancement), there are ingredients in play for moderate to potentially even locally heavy rainfall totals in southern portions of the area. Once the convective evolution of the precipitation today becomes clearer, may need to add some heavy rain wording to the forecast for relevant portions of the area. For now, broadbrushed QPF in the quarter to half inch range generally south of I-195/I-276, but think local totals up to and possibly exceeding one inch are possible, especially south of the Mason-Dixon Line (as suggested by HRRR and 00Z WRF simulations). Will be interesting to see how this plays out today.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... Few changes made to this period, with precipitation chances rapidly diminishing after 6 pm. With skies expected to clear quickly from west to east and residual moisture near the surface, ingredients are certainly in place for at least patchy and perhaps more widespread fog formation. Made some modifications to sky cover to adjust for projected timing of clearing and made some tweaks to fog coverage/timing as well based on changes to the sky cover grids. Otherwise, going forecast is in good shape. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... High pressure, both at the sfc and aloft, will be building across the Middle Atlantic region Wednesday and Wednesday night. This should bring fair weather for the period, with a warm day expected with highs in the mid to upper 80s over the area. Lows Wednesday night will be a summer-like across the southern areas with readings only dropping into the upper 60s to low 70s. Across the north, lows on Wednesday night will range from the upper 50s to mid 60s. A slow moving low pressure system will begin to affect the water with increasing clouds and higher chc for showers/tstms beginning Thu and persisting into the early part of the weekend. It will not be a total washout thru this period, but mostly just showery conditions. The best chcs for precip will be Thu night and Friday, mostly for the N/W areas. Temperatures for this period will be normal or a little above normal (Friday). A cold front will bring slightly cooler and drier air back to the region Saturday. Some of the models are showing that a weak wave of low pressure will move along the front Sunday and this may bring some clouds and slightly better chc for a few showers across Delmarva early Sunday. The norther parts of the fcst areas will remain dry. High pressure will gradually build in later Sunday and it should remain into early next week. Temperatures for Sunday and Monday will be mostly near normal. && .AVIATION /16Z TUESDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Sub-VFR conditions are stubborn to erode at the terminals, and this will likely remain the case through mid afternoon. Added TS to ILG/PHL/MIV and will likely need to add them at some point to ACY/PNE/TTN (when timing of the storms becomes clearer). Main timing of storms looks to be 17-21Z in the urban corridor and 18Z to 00Z at MIV/ACY. RDG/ABE may see a shower or two, but little more for the rest of the day. CIG/VSBY improvement should occur around sunset, but will be short-lived as areas of fog set in around 04Z-06Z and remain through daybreak. Expect several modifications to the TAFs during the next few hours as the evolution of the precipitation event unfolds. OUTLOOK... Wednesday through Wednesday night...Mainly VFR. MVFR possible in late night and early morning low clouds and fog. Thursday through Friday...Mainly VFR, but MVFR possible in showers and thunderstorms. Late night and early morning low clouds and fog possible. Saturday...Some morning MVFR possible, then VFR behind an exiting cold front.
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&& .MARINE... Few changes to the marine forecast this morning. Main change was to increase northwest winds a little late tonight and to include some patchy fog along the coast of New Jersey and in Delaware Bay via advection processes late tonight. Otherwise, previous discussion below remains relevant. Seas will be slowly building during the next 12 hours as Hurricane Gert makes its closest, yet distant, approach to our area. It will remain well offshore, but longer-period swells should continue to increase during the period. We continue the Small Craft Advisory for hazardous seas on our ocean waters from late this afternoon until noon Wednesday. Scattered showers through today, with a small chance of lightning this afternoon. Winds/seas may be erratic near any thunderstorms that occur. Additionally, there may be some visibility restrictions in showers, though widespread/dense fog is not anticipated. Winds should generally be from the southeast around or under 10 kts today turning light northwest later this coming Tuesday night. OUTLOOK... Wednesday thru Saturday...Low end SCA seas possible Wednesday morning due to offshore Hurricane Gert. Otherwise sub-SCA conditions expected through this week and into the early part of the weekend. Sct tstm may bring locally higher winds and seas. RIP CURRENTS... Gert is forecast to pass well to our southeast later today. Swells from the Hurricane will likely bring at least a moderate risk for the development of dangerous rip currents today into Wednesday. Swim only in the presence of lifeguards where safety prevails. Otherwise, you`re on your own with any rescue potentially critically delayed. There have been numerous drowning fatalities this year along the NJ coast. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory for hazardous seas from 4 PM this afternoon to noon EDT Wednesday for ANZ451>455. Small Craft Advisory for hazardous seas from 10 PM this evening to noon EDT Wednesday for ANZ450. && $$ Synopsis...O`Hara Near Term...CMS/Drag Short Term...CMS/Drag Long Term...O`Hara Aviation...CMS/Drag/O`Hara Marine...CMS/Drag/O`Hara is the U.S. government's official web portal to all federal, state and local government web resources and services.