Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 201320 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 920 AM EDT Thu Jul 20 2017 .SYNOPSIS... High pressure will remain offshore through early next week. A weak front will cross the area tonight and Friday. This front will remain nearby over the weekend and keep conditions unsettled through the period. Cooler temperatures will arrive for the first part of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Main concerns today are heat and storm chances, with heat taking much of the forecast priority. Guidance has come in considerably warmer (i.e., about 2-5 degrees) for highs today versus what they were calling for at this time yesterday...yet another case of statistical models underforecasting temperatures within strong ridging until the near-term period. Using yesterday`s temperatures and dew points as a guide and making adjustments for expected mixed-layer evolution (in areas not affected by diurnal convection yesterday), temperatures will likely reach the mid to locally upper 90s with dew points near or just above 70 during the afternoon. This places much of the region in heat advisory criteria. Excessive heat warning continues for the I-95 urban corridor, with heat indices generally well above 100 this afternoon for several hours. Heat advisory was issued for the urban corridor of the Lehigh Valley (Reading and Allentown) eastward to Middlesex and Monmouth Counties and southward to the Delmarva Peninsula, generally surrounding the excessive heat warning. There are some uncertainties with advisory/warning criteria being met, including cloud cover this morning associated with some showers that developed near Chesapeake Bay. Moreover, the HRRR is showing the potential for more convection late this morning into this afternoon generally along and south of I-76. There is not much support from other high-resolution guidance, but the rest of the guidance did fairly poorly with the showers that popped up in northeast Maryland overnight (at least, more so than the HRRR). Another complication is mixing this afternoon, with higher- resolution guidance showing a well-mixed boundary layer this afternoon in much of the area, particularly along and west of a surface trough that will set up in vicinity of I-95 today. Similarly, there are indications of increased mixing in southern Delaware and adjacent eastern Maryland, which may keep the Georgetown and Easton areas generally below advisory criteria. Despite these concerns, the intensity of this particular period of hot temperatures (which will also be fairly long duration, given the past couple of days and continuing through at least Friday) warrants advisory issuance. Regarding convective potential, the presence of a surface trough with very strong diabatic heating today suggests potential for an isolated storm or two in much of the area. Thought it wise to include slight chances of convection across the CWA late this morning and this afternoon. Given a well-mixed boundary layer, strong wind gusts may occur with any storm that develops. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/... Main concern tonight is storm chances. Latest high-resolution guidance is suggestive of a potential cluster or clusters of storms approaching/entering the CWA this evening and/or overnight. Perturbations in the flow are convectively driven and poorly simulated, even at such short time ranges, so confidence is not particularly high. The strongest signal is for reasonably organized convection to move through the northern CWA at some point this evening/overnight. The 06Z NAM is quite aggressive in producing a swath of fairly high QPF generally along/north of I-78 in the 00Z to 06Z time period. The 00Z GFS provides some support, though with somewhat lighter/broader QPF (per its coarser grid spacing, at least partially). The 06Z HRRR also provides a similar depiction, though perhaps shifted somewhat northward (toward the New York border) in this general time frame. The 00Z NAM Nest, meanwhile, suggests weaker/weakening convection, but perhaps in multiple rounds. This solution seems like an outlier, especially compared to the WRF-ARW/NMM simulations. Analysis of subsequent simulations will be critically important to hone the timing and evolution of convection during this period. With a decent, though small-scale, perturbation moving through the region during this period, large-scale ascent will be available for increased organization of convection. Given the hot/humid air along and south of the stronger midlevel flow, cold-pool development/maintenance may not be problematic, despite nocturnal stabilization processes. As such, strong to severe storms with straightline winds the primary threat cannot be ruled out, especially this evening. Capped PoPs at slight to low-chance categories for now, given uncertainties with upstream convection and model variability with timing and location of the more organized storms, with decreasing chances with southward extent. Temperatures tonight will be warm, and conditions will be muggy. Not much relief from the heat expected today. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Hot and humid conditions will continue through the weekend with some relief beginning Tuesday. An excessive heat warning for the Philadelphia metro areas extends into Friday with temps and dew points near what they will be today. Heat index values of Friday may be a degree or two lower than Thursday, but still in the upper 90s to low 100s across most areas. Clouds will be more frequent Saturday and Sunday, so while it will still be hot, a bit less so than Thu and Fri. By Tuesday of next week, cooler air from N/W will arrive over the area behind a cold front. Showers will be frequent during the long term but it`s difficult to find a period when then chances are too low to put in the grids, or to pinpoint which locations are more favored than others to get them. Operational models are not great at predicting convection in general, so a broad brush with help from the ensembles usually works well in these kind of patterns. Thunderstorm activity associated with a flattening short wave will begin to diminish as it arrives over our area Thursday evening. The associated surface front returns northward for the weekend and chc or low likely pops for tstms are in for these periods and also Monday. It will not rain all of the time, but certainly consider showers when making weekend plans. Another cold (or cool) front passes the area late Monday. It appears for now, that this will finally bring drier and cooler air to the region. We should experience a few rain-free days early next week as high pressure builds in from the west. && .AVIATION /13Z THURSDAY THROUGH MONDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Generally VFR through the period. A cumulus deck around 5000 feet is anticipated through much of the day. Storms may reach areas north/west of KPHL near/after sunset tonight, with restrictions possible, especially at KABE. Winds generally light west or southwest through the period. OUTLOOK... Fri thru Monday...Mostly VFR. Patchy haze and Sct tstms. Areas of fog possible in the pre-dawn hours. && .MARINE... Sub-advisory conditions are expected the next 24 hours, though winds are expected to be southwesterly at 10 to almost 20 kts this afternoon and evening. There is a slight chance of storms today and tonight, especially after sunset in the northern New Jersey coastal waters. OUTLOOK... Friday thru Monday...Sub-SCA conditions overall. The main hazard will be sct tstms Fri night thru Monday. Locally higher winds and seas with tstms. RIP CURRENTS... The underlying 12 to 16 second southeasterly swell is forecast to persist today. As a result, we will continue the moderate risk for the development of dangerous rip currents. && .EQUIPMENT... The KDOV dew point readings are unrepresentative of the area and should not be used when diagnosing humidity/heat indices. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Heat Advisory until 7 PM EDT this evening for PAZ060>062-101- 103-105. Excessive Heat Warning until 8 PM EDT Friday for PAZ070-071- 102-104-106. NJ...Heat Advisory until 7 PM EDT this evening for NJZ009-010-012- 013-016-020>023-027. Excessive Heat Warning until 8 PM EDT Friday for NJZ015- 017>019. DE...Heat Advisory until 7 PM EDT this evening for DEZ002-003. Excessive Heat Warning until 8 PM EDT Friday for DEZ001. MD...Heat Advisory until 7 PM EDT this evening for MDZ008-012-015- 019-020. MARINE...None. && $$ Synopsis...Miketta Near Term...CMS/Iovino Short Term...CMS Long Term...Miketta Aviation...CMS/Miketta Marine...CMS/Miketta Equipment...Staff

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