Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 221021 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 621 AM EDT Tue Aug 22 2017 .SYNOPSIS... The Mid-Atlantic region will be located today between high pressure that is parked offshore and a cold front that will be moving through the Great Lakes and Midwest states. The cold front will move into the region late tonight and slowly exit off the Delmarva coast on Wednesday. Canadian high pressure builds into the region on Thursday and should continue to be the primary influence on our weather into early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Quick update to expand fog to the entire CWA early this morning. Fog remains mainly patchy and transient, but numerous surface obs indicating its occurrence in the past hour. Visibilities remain generally above a mile, but some particularly susceptible valley/rural locations may observe brief sub-mile visibilities through about 9 am. Rest of discussion below remains relevant... 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 and adjacent portions of New Jersey, 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 through daybreak, particularly north of the Mason-Dixon Line. Kept slight chance PoPs across the region (given at least a couple of simulations that generate a shower or two in Delmarva around daybreak), with subtly higher values in eastern PA and NJ. Patchy fog is also occurring this morning and should continue through around daybreak. There appears to be a window of low clouds developing around 12Z this morning as strong isentropic ascent occurs downstream of the much stronger vort max moving through the Great Lakes. As such, boosted cloud cover considerably in about a 4- 5 hour period, mostly north of the Mason-Dixon Line, as this ascent peaks around daybreak. Thereafter, boundary layer mixing should increase ceilings and scatter the clouds out fairly rapidly mid-to-late morning as low pressure intensifies well to the north. Southerly surface flow will increase as the surface pressure gradient increases, and this may allow for some gusts to around 20 mph or so, especially near the coast. With strong mixing and (presumably) strong insolation late this morning and this afternoon, temperatures will soar to well above average values. Latest MOS guidance has highs in the mid 90s in Philadelphia, but strong mixing suggests that dew points will be considerably lower than those observed last Friday. Nevertheless, heat indices should flirt with or exceed heat advisory criteria in the urban corridor this afternoon. Did not change the current advisory with this morning`s forecast package. Aforementioned Great Lakes vort max marches east through the day. A prefrontal trough will develop/progress into Upstate New York and Pennsylvania this afternoon, with convection rapidly developing in a region of focused ascent. However, this convection should remain west of the area through the near-term period. Downstream of the prefrontal trough, mechanisms for storm-scale ascent are limited to orographic effects and differential heating. With stronger large- scale ascent to the north and west of the CWA, suspect that convective coverage in our region will be isolated/widely scattered at best and primarily confined to areas northwest of the urban corridor. Continued the trend of reducing PoPs this afternoon, with chances primarily confined to north/west of I-95. Regarding severe potential, CAPE-shear parameter space is more than adequate for severe storms. MLCAPE could approach/exceed 2000 J/kg this afternoon in the western CWA, with effective bulk shear of 35 to 45 kts. With a deep layer of positive buoyancy and unusually strong veering wind profiles for this region, the potential for hail in addition to strong/damaging winds is present. Main limiting factor (aside from nebulous/weak ascent and a lack of near-surface convergence) is weak midlevel lapse rates, which may prevent the development of stronger negative buoyancy/cold pools in convective downdrafts. Nevertheless, the ambient environment is quite supportive of severe storms. The risk for hail and even an isolated tornado is nonzero, especially with any storms that remain discrete.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... As the aforementioned pre-frontal trough approaches the region tonight, a band or bands of loosely organized storms should approach/enter the region. However, with nocturnal stabilization commencing, the chances for severe storms are diminished in our CWA versus points to the north/west earlier in the day. Additionally, the vort max approaching the area will be weakening as it lifts northeastward into New England. One more limiting factor is the potential for fairly weak cold pools, given weak midlevel lapse rates and a deep layer of substantial atmospheric moisture. All of these factors suggest convection may struggle to survive as it progresses eastward through the CWA overnight. This looks to be especially true south of I-78, where large-scale ascent rapidly weakens. One potential modulating factor, however, is the nocturnal increase of a low-level jet, which may enhance downward momentum transport on the upstream side of linear convection. With residual instability and strong shear profiles, high-resolution guidance may be too quick to diminish convection after dark. One other factor to consider: Winds will be slow to diminish after dark, which will keep boundary layer mixing elevated. This would slow the process of increasing convective inhibition, and models do tend to forecast this process too quickly, in general. With the above in mind, kept PoPs primarily slight chance south of I- 76, with much higher chances north of I-78 through the night. The synoptic cold front should be on the doorstep late in the night, but temperatures/dew points will be quite high in advance of it. Kept forecast numbers close to MAV guidance, though confidence is low given potential impacts from convection. One other note: The progression of the convection and upstream cold front appears to be slowing with the latest set of model guidance. The updated forecast slows the progression of higher PoPs in accordance with this trend. As a result, PoPs are generally unmentionable in the far southern CWA until late in the night. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... The cold front will most likely reside along or just south/east of I- 95 by daybreak Wednesday morning. The movement of the front will then likely slow down as it progress southeastward through the Delmarva during the day as it approaches downstream ridge blocking over the western Atlantic Ocean. Accordingly, conditions should dry out rather nicely on Wednesday along and especially north/west of the I-95 corridor while locations in southeastern NJ and Delmarva will continue to be under a chance for showers. If the front slows down even more and some breaks in the clouds develop, there may be enough instability to fuel thunderstorms toward the middle Delmarva peninsula in the afternoon. Consensus from the latest suite of model guidance keeps the more favorable environment for severe storms just south of Delaware. An isolated strong to severe storm and heavy rain producers is possible if the front gets hung up north. High pressure centered over central Canada midweek shifts southeastward into the Great Lakes Thursday through Saturday, then eastward into southeastern Canada/Northeast U.S. Sunday and Monday. This expansive high will control our regional weather pattern across the Mid Atlantic during this time. Barring any major changes to the synoptic pattern, the upcoming Thursday-Monday period could potentially be only our second (virtually) rain-free stretch that spans at least five days in what has otherwise been a very wet summer. I say "virtually rain free" because 1) cannot completely rule out an isolated shower Sunday-Monday with the trough axis of the upper low expected to move through at a time when onshore flow could act to moisten the low-levels and 2) 8-13 June, which was the other 5+ day dry stretch this summer, was technically not rain free as MPO reported 0.01 inches of rain on both 10 June and 13 June. The last time all eight of our climate sites have not recorded any measurable rainfall for at least five consecutive days was 2-6 March 2017 (honorable mentions: 15-19 May except for 0.02 inches at MPO on 15 May and the previously mentioned 8-13 June period). Below normal temperatures can also be expected Thursday-Monday with highs in the mid 70s-lower 80s and lows in the 50s- lower 60s. This will provide very comfortable temperatures for outdoor activities. && .AVIATION /10Z 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 CIGs/VSBYs have set in at all sites except ACY. Expect this to continue through about 12Z-13Z before improvement. Though the VSBY restrictions should be variable/transient, the CIGs may hover in MVFR/IFR with little improvement until about 12Z or so. Thereafter, VFR is likely through the day, with chances for storms gradually increasing north/west of KPHL during the late afternoon/evening. Timing of storms is noticeably slower with model runs the past few hours. 12Z TAF update will accordingly delay onset at all terminals by at least a couple hours. Not at all clear if storms will affect the urban corridor tonight, but the chances are high enough for at least a PROB30 mention after 00Z-03Z. Outlook... Wednesday...Predominately VFR. Any leftover showers should be isolated and end by midday for I-95 terminals, N and W. Farther S/E toward MIV and ACY, chances for showers and even a thunderstorm could linger through the afternoon as a cold front slowly progresses offshore. Localized/brief restrictions possible in thunderstorms. Winds veer from W to NW during the morning with speeds 5-10 kt. Wednesday night through Saturday...VFR. Generally light winds from the N or NW, possibly becoming N-NE on Saturday.
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&& .MARINE... No changes to the marine headlines this morning, as south to southwest winds should continue to increase today and tonight on the waters. Speeds should reach advisory criteria by mid-afternoon and continue through tonight. Gusts may approach/exceed 30 kts during the evening hours. Seas should range from 4-6 feet this afternoon and tonight. There is a chance of showers and thunderstorms tonight, with the best chances in the New Jersey coastal waters. Winds and seas will be locally higher near any storms that occur. Outlook... Wednesday...Did not extend the SCA into Wednesday as available wave guidance shows seas in our coastal waters falling below 5 ft by daybreak. However, will still leave the door open for an extension into the morning if seas take a bit longer to subside. Winds change direction out of the NW and decrease steadily during the day behind cold fropa. Wednesday night through Saturday...Winds and seas below SCA criteria. RIP CURRENTS... Reevaluated the rip current risk and have retained the moderate for today and this evening. However, conditions will be near the high category late this evening as southerly to southwesterly winds peak and seas increase to around 5 feet. Though short- period swell should be the primary mode through the period, an underlying longer-period swell may be present. Should this be more dominant than currently progged or should conditions worsen earlier than forecast, an upgrade to high risk may be required later this morning. Without question, if swimming in the waters today, use extra caution and common sense. Swim only in the presence of lifeguards, and do not swim alone! Heed any and all restrictions by area beach patrols. && .PHI WATCHES/WARNINGS/ADVISORIES... 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. && $$ Synopsis...Klein Near Term...CMS Short Term...CMS Long Term...Klein Aviation...CMS/Klein Marine...CMS/Klein is the U.S. government's official web portal to all federal, state and local government web resources and services.