Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 212238 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 638 PM EDT Mon Aug 21 2017 .SYNOPSIS... High pressure will be centered in the western Atlantic through Tuesday. A warm front resides in eastern Pennsylvania. A cold front will approach from the west late Tuesday and slowly exit off the coast on Wednesday. Canadian high pressure builds into the region on Thursday and should to be the primary influence on our weather into next Monday. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
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A subtle short wave continues to approach from the west and southwest. Some forcing with this feature combined with plenty of instability is resulting in isolated to scattered convection across the western areas. This activity has been slow to advance eastward, however it is now into parts of Berks County with more starting to enter parts of the southern Poconos. The more intense convection has been on the western shore of Chesapeake Bay, however this has been tending to weaken as it tries to slowly shift eastward. There is modest deep-layer shear and combined with the instability, isolated severe thunderstorms are possible through early this evening. The main threat is locally damaging wind gusts especially with storms that sustain a deeper/stronger updraft and more vigorous multi- clustered storms (hail cannot be ruled out as well). Slower storm motion will result in locally heavy rainfall. The PoPs were trimmed back westward for awhile given the slow eastward movement to the convection thus far. The hourly temperature, dew point and wind grids were adjusted with the latest observations then the LAMP guidance was blended in for the next few hours. After sunset and with the loss of diurnal heating, any ongoing thunderstorms will gradually begin to decrease in intensity with only perhaps isolated showers and thunderstorms continuing overnight. Subsidence in the wake of the wave combined with increasing moisture in low levels could support the development of some fog and stratus overnight, particularly outside of the Philadelphia metro.
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&& .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/...
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Some changes were made with the 630 PM update. This was to slow down the PoP increase quite a bit eastward during the day. While a few storms may initiate inland from the coast in the afternoon, the majority of the guidance indicates we need to wait for more organized convection incoming from the west later in the afternoon. This is when the main forcing starts to arrive. Otherwise, initially potentially more cloudiness and some fog may start the day which would then scatter out as the low levels begin to mix. Southerly flow will strengthen ahead of an approaching upper trough and its attendant cold front, leading to warm/moist advection and heat indices to around 100 degrees for the urban corridor from Wilmington to Philadelphia and Trend then up to the NYC metro. A heat advisory is valid from 1 PM through 8 PM Tuesday. The other areas look to fall short of the advisory criteria. Apart from the heat, there will be increasing chances of thunderstorms toward evening as the upper trough approaches the area, though the primary forcing for ascent and best chance for any development will largely be to the north and west. At the moment, some of the deterministic models seem a bit overdone with overall convective coverage. Have sided more with higher resolution data showing the best chance of thunderstorms over higher terrain in the afternoon. Modest flow with veering profiles will once again support the potential for strong to severe storms capable of damaging winds and heavy rainfall toward late day.
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&& .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
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500 MB: Troughing in the Great Lakes region during midweek...will shift into the northeast Friday through this weekend. Temperatures: The month of August so far has averaged within a degree of normal except Mount Pocono where the monthly average was 2.5 degrees below. So... calendar day averages should still be a few degrees above normal Wednesday, then from from Thursday through next Monday should average 2 to 6 degrees below normal on a daily basis. Forecast basis: a 50 50 blend of 12z/21 GFS/NAM MOS was used Tuesday night-Wednesday night, then, unless otherwise noted, the 12z/21 GFS MEXMOS was applied Thursday and thereafter the 15z/21 WPC D4-8 gridded elements of max/min T, 12 hr POP 6 hrly dew/wind/sky. Just tucking this in here... since I didnt think this warranted a climate section on its own... ABE 7.94" of rain so far in August ranks #12 when compared to complete month of August totals at Allentown, 13.47 in 2011 is the record. Records date back to 1912 except we are missing 1920 and 1921. The dailies... Tuesday night...SVR potential early in decent MLC of over 1000J/more than adequate 0-6KM bulk shear (near 45KT), PWAT air of 1.8 inches with residual showers overnight. Low temps about 9 to 12F above normal. The eastward sweep of SVR should be primarily I-78 north. Strong westerly flow at mid levels (40-45kt 700-500MB) should drive a decent lines of showers and tstms across the area as per the 15z HRRRX. Least likely area for any tstms is probably southeast of PHL across far s NJ and DE. The 1703z D2 SPC outlook didn`t change from the 06z vsn. Wednesday...The cold front will likely slow down when it reaches Delmarva and perhaps eventually stall just south of the area across southern VA Wednesday afternoon. Accordingly, it is looking more probable that the majority of the forecast area dries out on Wednesday, especially north of the Mason-Dixon line. The UK is pretty wet on Wednesday in the se 1/2 of our fcst area and...the EC looks pretty unstable. so...opted for a SREF wetter blend to the 12z/21 ops pops...which means continuity with the mid shift fcst. Wednesday night-Monday...High pressure over Canada and the Great Lakes region then starts to build southeastward toward the area on Thursday. This expansive high should remain in control through at least this weekend. The end result will be an extended stretch of pleasant weather Thursday through Monday. Highs in the upper 70s/lower 80s and and lows ranging from the 50s to lower 60s are forecast each day and night (possibly some upper 40s), respectively. While the forecast remains dry for these four days, there looks to be increasing cloud cover heading into the weekend and potentially a risk for an isolated shower with 1) the approach of the upper trough and 2) light onshore flow helping to moisten low-levels over time.
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&& .AVIATION /00Z TUESDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...Any showers/thunderstorms should end later this evening, mostly impacting the KABE and KRDG areas, otherwise VFR. Some areas of MVFR/IFR conditions due to fog and/or stratus may occur overnight and toward daybreak. The extent of the low clouds and fog is of lower confidence. Southerly winds 10 knots or less, becoming light and variable at most terminals. Tuesday...Sub-VFR conditions possible early for some areas due to fog and/or stratus, otherwise VFR. Showers and thunderstorms are expected to develop mainly to our west in the afternoon, however these may approach KRDG and KABE toward early evening. Light winds, becoming southwest and increasing to 10-15 knots. Outlook... Tuesday night...Potential for MVFR or IFR restrictions with showers and isolated storms. This activity should progress to the east- southeast through the night. Wednesday...Scattered showers and isolated thunderstorms mainly for terminals S/E of PHL and earlier in the day. May start off MVFR with lower CIGs but improvement to VFR is likely from NW to SE during the day. A wind shift from SW to NW can be expected early in the day w/ fropa. Wednesday night through Saturday...VFR and generally light winds. Cannot rule out an isolated sprinkle each afternoon or evening.
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&& .MARINE...
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Winds and seas remain tranquil across the waters tonight in weak southerly flow. Patchy fog development will be possible overnight, but should dissipate shortly after sunrise. An isolated shower or thunderstorm is also possible overnight. Tuesday...SCA was issued for the Atlantic coastal waters of NJ and DE for late Tuesday afternoon and night. S-SW winds are expected to strengthen late in the day to 15-25 kt. Some gusts to 30 kt are possible mainly late in the day when the winds will be strongest. Seas will build to 4-6 feet in response to the wind field. Outlook... Tuesday night...SCA continues for the Atlantic coastal waters of NJ and DE. S-SW winds with isolated gusts to 30 kt possible mainly during the evening when the winds will be strongest. Seas build to 4-6 feet in response to the wind field. Wednesday...A wind shift from SW to W-NW should occur on Wednesday behind a cold front. Winds will also decrease as well. There is still a possibility that the SCA may need to be extended into Wednesday morning if seas take a bit longer to subside below 5 ft. Thursday through Saturday...Winds and seas below SCA criteria. RIP CURRENTS... A south to southwest wind will increase during the day Tuesday, which will also allow the surf to increase some. Given a more parallel wind to the coast overall, this should keep the rip current risk reduced some. However, if a longer period swell becomes more dominant then this would elevate the rip current risk more. Based on this and in-house guidance, we will go with a moderate risk of rip currents for Tuesday. This will be re-evaluated early Tuesday morning.
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&& .TIDES/COASTAL FLOODING... Tides are expected to generally remain below coastal flooding thresholds, but there may be a few localized areas of brief minor flooding at times of high tide. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Heat Advisory from 1 PM to 8 PM EDT Tuesday for PAZ070-071-102- 104-106. NJ...Heat Advisory from 1 PM to 8 PM EDT Tuesday for NJZ010-012-015- 017>019. DE...Heat Advisory from 1 PM to 8 PM EDT Tuesday for DEZ001. MD...None. MARINE...Small Craft Advisory from 3 PM Tuesday to 6 AM EDT Wednesday for ANZ450>455. && $$ Synopsis...Drag Near Term...Di Spigna/Gorse Short Term...Di Spigna/Gorse Long Term...Drag Aviation...Drag/Gorse Marine...Drag/Di Spigna Tides/Coastal Flooding...

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