Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 191632 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1232 PM EDT Wed Jul 19 2017 .SYNOPSIS... A cold front will approach the region slowly today and Thursday before stalling in the Mid-Atlantic Friday and Saturday. A wave of low pressure along the front may lift it northward on Sunday before dragging a cold front through the area early next week. High pressure will build in behind the cold front by the middle of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Abundant sunshine as high pressure becomes entrenched over the Mid-Atlantic. Hot and humid conditions in place as highs top off in the low 90s for most areas, and in the mid to upper 80s along the coasts and up in the Pocono Mountains. Surface dewpoints start out in the upper 60s across the Poconos and NW NJ, in the lower 70s for most of the forecast area, and into the mid 70s in the Delmarva. Going through the afternoon, SW winds increase to 5-10 MPH, and this may be enough to allow for some afternoon mixing which would bring dewpoints down in the upper 60s to around 70. As a result, max heat index should fall just short of 100 (topping off at 97-99) across the urban corridor from around Somerville to Trenton to Philadelphia, and from 100-102 from Wilmington to Georgetown, DE. This should be just short of Heat Advisory criteria. Closed H5 low centered over the Carolinas opens up this afternoon and begins to drift offshore. This allows several strong H5 shortwaves associated with it to lift to the north and west from the ocean towards NJ. An axis of isolated to scattered showers and thunderstorms should develop across the region this afternoon, but highest chances will be mainly south and east of a line from Somerville, NJ to Doylestown, PA to Wilmington, DE. Afternoon SB CAPE values will be 1000-1500 J/kg, a Lifted Index as low as -2C, and 0-6 km Bulk Shear of 10-20 KT. PWATs will be from 1.5-1.75", which are not that high for this time of the year, but any storms that develop will be slow moving, and may result in localized poor drainage flooding. Given the scattered nature of these storms, will cap PoPs at low chance. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/... Any lingering showers and thunderstorms taper off this evening with loss of diurnal heating. Low level moisture increases once again as surface dewpoints rise back up into the lower 70s across much of the forecast area. Another warm and muggy night on tap with lows in the upper 60s to low 70s, and in the mid and upper 70s in Philadelphia. Patchy fog may develop once again late tonight. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Main forecast challenges in the long term will be the heat through this weekend and frequent chances for storms Thursday night through at least early next week. The general synoptic pattern includes amplification of a longwave trough in eastern North America during the period, aided by the progression of a fairly potent shortwave trough along the U.S./Canada border late this week, and the retrogression of subtropical ridging from the southeastern U.S. to the southwestern U.S. Main forecast uncertainties this period include the speed and rate of amplification of the shortwave trough as it approaches the Great Lakes this weekend and the subsequent impacts on the larger-scale trough early next week. The 00Z operational models are in fairly poor agreement regarding the speed of the shortwave trough, with about a 24-hour spread in timing between the fast ECMWF and slow CMC by Sunday in the Great Lakes region. These discrepancies only worsen as the upscale impacts occur with the longwave trough in eastern North America early next week, with the CMC anomalously deep/slow with the large-scale troughing along/east of the Appalachians and the ECMWF on the fast fringes of consensus in moving the trough offshore by next Tuesday. Though the GFS is in the middle of the pack with timing, the depth of the longwave trough is on the flat side of the model consensus, which does not gel with the amplification of upstream ridging particularly well. This is especially suspicious given its noticeably deeper upstream trough in the Pacific Northwest. Forecast rationale was to use mostly a blend of the ECMWF and CMC late this week and this weekend, given the suspiciously flat look of the GFS with large-scale troughing during this period. However, increased the weighting of the GFS and decreased the weighting of the CMC with time as the deep/slow progression of the longwave trough becomes more and more anomalous with the CMC solution. Specific forecast details: There appear to be three periods of elevated chances of convection through Monday. The first is Thursday night, as a convectively-enhanced perturbation rapidly progresses east-southeastward through the Great Lakes into the Northeast. The behavior of the perturbation will be strongly tied to the development of a mesoscale convective system in the Upper Midwest and its progression in the northern tier of the country thereafter. Forecasts of such convective evolution are low-skill, in general, and the fairly jarring shifts in track/speed with subsequent model runs and poor agreement among the models for a particular model start time are bearing this out. Nevertheless, the simulated convection in the GFS looks particularly suspicious, as it appears to occur too far upstream of the midlevel perturbation (unlikely). The northward shift of the ECMWF with QPF is plausible, however, given models` tendencies this year to underestimate the strength of subtropical ridging. As such, kept highest PoPs Thursday night generally north of I-78, with the highest chances likely to be to the north of the CWA. Transient ridging upstream of the perturbation will likely keep much of Friday and Friday night dry, so the lowest PoPs of the extended occur during this period. The second period of more elevated chances of convection occurs Saturday and Saturday night, as another perturbation moves rapidly eastward from the Midwest to the northern Mid- Atlantic region. With a stalling front across the area by this point, smaller-scale lift will likely aid in the development/maintenance of a progressive convective cluster moving along the quasi-zonally oriented boundary during this time. Though the CMC looks overdone with upscale growth, including the development of a compact surface low, suspect the prolonged southerly fetch of warm/moist air will promote considerable instability and moist convergence for potentially strong convection associated with the convective cluster (potential MCS). The ECMWF has a similar look to the overall evolution, which lends some confidence that this scenario will pan out. Main question is timing, as the perturbation looks to move through the region generally somewhat after peak heating. This suggests best chances for convection will be in the northwest CWA with gradually diminished chances southeastward (with convection generally waning as it moves through the area). For now, kept chance PoPs for most of the area during this time frame. The parent shortwave trough approaches the region late this weekend into early next week, with timing becoming more and more uncertain by this point. Nevertheless, an associated cold front will likely approach the area, and synoptic-scale lift/dynamical support improve for the development of fairly widespread convection in the Sunday-Monday period. Given uncertainty of timing, kept PoPs elevated (though capped at chance) through this period. Importantly, however, convection is not expected to occur throughout this time is just not possible to pinpoint the timing more so than this at this point. Tuesday and Wednesday, PoPs slowly decrease, but did not remove them entirely given the potential for the large-scale trough to stick around. Regarding temperatures...Thursday and Friday look hot. Forecast highs may be a little conservative, particularly Friday when shortwave ridging may provide extra subsidence/compressional warming. However, some mixing should also occur, so dew points may lower into the upper 60s to around 70 during max heating. With forecast highs in the mid 90s in the urban corridor, this would keep heat indices just shy of advisory criteria. Notably, the Delmarva region will likely have somewhat higher dew points, with heat indices easily exceeding 100 and approaching 105 during peak heating. An advisory certainly cannot be ruled out here. Convection or its debris suggest Saturday and Sunday may be somewhat cooler. After cold frontal passage early next week, temperatures may actually be somewhat less than average by the end of the period. && .AVIATION /18Z WEDNESDAY THROUGH MONDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Isolated to scattered SHRA/TSRA possible this afternoon. Best chances will be from KTTN-KPNE-KPHL-KILG SE, but confidence of a storm developing over any given terminal is low, so will not mention in the TAFs at this time. Generally VFR conditions continue late tonight into Thursday morning. Patchy fog possible once again. W-SW winds 5-10 KT this afternoon. Winds become nearly calm after 00Z Thursday. OUTLOOK... Thursday through Friday night...Generally VFR. Chance of storms generally north of KPHL Thursday night. Patchy fog possible near dawn in favored rural/valley locations. Winds generally west at or below 10 kts. Confidence slightly above average. Saturday through Sunday...Increased chances of thunderstorms, with sub-VFR conditions likely in their proximity. Winds generally light and somewhat variable during the period, except stronger/erratic near any storms. Confidence somewhat below average. && .MARINE... SW flow becomes S this afternoon and increases to 10-15 KT on the ocean waters, and remains 5-10 KT on DE Bay. Ocean seas will average 2-3 feet, and waves on DE Bay will be 1-2 feet. Winds diminish tonight. OUTLOOK... Sub-advisory conditions are forecast through the period. Chances for storms exist through the period, especially Thursday night in the New Jersey coastal waters and Saturday-Sunday everywhere. RIP CURRENTS... The underlying 12 to 16 second southeasterly swell is becoming more pronounced along our coast. As a result, we have upgraded Delaware to a moderate risk to match New Jersey. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...CMS Near Term...Iovino/MPS Short Term...MPS Long Term...CMS Aviation...CMS/MPS Marine...CMS/Iovino/MPS is the U.S. government's official web portal to all federal, state and local government web resources and services.