Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 220543 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 143 AM EDT Tue Aug 22 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 THIS MORNING/...
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Concerns last night regarding the low-amplitude vort max moving through the region tonight seem to have been realized, as isolated showers and even a couple of storms continue to develop in an arc from northern to eastern Pennsylvania, in close proximity to the perturbation in the primarily zonal midlevel flow. High-resolution guidance simply is of little value with these perturbations, it seems, at least for this warm season. With last night`s simulations basically precip-free as the perturbation moves through, each successive run tonight either seeks to diminish whatever convection develops after an hour or two with little similarity to reality or depicts a completely alternative precip evolution to the previous simulation, either of which basically provides zero confidence in the subsequent evolution of the convection. Fortunately, the showers are sparse and weak, but unfortunately, they are strong enough to tip the buckets at most sites they pass, which means PoPs are not negligible, despite what the guidance would have a forecaster think. In this manner, coarser guidance has been more insightful in at least suggesting the occurrence of these showers (though overdoing coverage/intensity, a common problem with parameterizations and coarse grid spacing). With this in mind, the latest NAM/GFS suggest the possibility of these sparse/weak showers the rest of the night, particularly north of the Mason-Dixon Line. As a result, boosted PoPs to mentionable the rest of the night across the area, with subtly higher values in eastern PA and NJ. Other big question mark is fog/stratus development. So far, stratus has been slow to materialize, though patchy fog is beginning to occur in the absence of low clouds in the usual spots. Crunch time is in the 09Z-12Z time frame, where statistical/operational model guidance both suggest a period of low stratus/fog may occur in a larger area. With isentropic ascent increasing late tonight, cannot ignore the guidance, though it has been overdone so far. Did not change inherited forecast much in this regard, as confidence is low. However, made adjustments to sky cover and winds based on latest trends.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TODAY/...
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Made some additional changes to the grids for this period. Biggest change was to fine-tune the progression of PoPs from northwest to southeast through the area starting mid-afternoon and to lower PoPs a little bit. The reasoning is there remains some discrepancy on convective coverage this afternoon with the latest high-resolution guidance. Strongest larger-scale ascent associated with a pre-frontal trough should remain generally west of the region through most of the afternoon, and smaller- scale ascent in advance of this trough appears rather weak/nebulous. The latest WRF simulations offer little in the way of terrain-induced convection in advance of the pre-frontal trough. Environment remains favorable for severe storms in northwest portions of the CWA tomorrow afternoon, so kept or included wording indicating the potential for frequent lightning, heavy rain, and gusty winds with storms during the afternoon in areas generally northwest of Philadelphia. 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 /TONIGHT 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 didn`t 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 /06Z TUESDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. For the 06Z TAFs...Big question marks tonight with VSBYs/CIGs, as guidance is still strongly suggestive of low stratus and/or fog developing at several of the terminals through 12Z. So far, this has been overdone, but latest surface obs have trended in the direction of at least temporary VSBY reductions at KRDG and KABE as well as temporary CIGs at KPHL and KILG. Will continue and/or add TEMPO groups to indicate this possibility at most sites through 12Z. Thereafter, VFR is likely through the day, with chances for storms gradually increasing north/west of KPHL during the afternoon. Not at all clear if storms will affect the urban corridor tomorrow night, but the chances are high enough for at least a PROB30 mention after 00Z. 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... Winds and seas remain tranquil across the waters tonight in weak southerly flow. Patchy fog development is possible overnight. An isolated shower or thunderstorm is also possible overnight. Tuesday...SCA for the Atlantic coastal waters of NJ and DE starting mid afternoon. 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 may elevate the rip current risk more. Based on this and in-house guidance, we went with a moderate risk of rip currents for Tuesday. This will be re-evaluated early Tuesday morning. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Heat Advisory from 1 PM this afternoon to 8 PM EDT this evening for PAZ070-071-102-104-106. NJ...Heat Advisory from 1 PM this afternoon to 8 PM EDT this evening for NJZ010-012-015-017>019. DE...Heat Advisory from 1 PM this afternoon to 8 PM EDT this evening for DEZ001. MD...None. MARINE...Small Craft Advisory from 3 PM this afternoon to 6 AM EDT Wednesday for ANZ450>455.
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&& $$ Synopsis...Drag Near Term...CMS/Gorse/MMD Short Term...CMS/Gorse/MMD Long Term...Drag Aviation...CMS/Drag Marine...Drag/Gorse/MMD

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