Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 231039 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 639 AM EDT THU JUN 23 2016 .SYNOPSIS... Low pressure in western Pennsylvania will move southeastward and off the Atlantic coast later today. High pressure will then build in from the north for Friday into early next week. A slow moving cold front is forecast to approach from the northwest on Tuesday and move through the region on Wednesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Made some minor edits to the PoP and weather grids with the 630 AM ESTF update. Took into account a slightly faster exit of the widespread showers and embedded thunderstorms late this morning. Also, ramped down the threat of strong and heavy rainfall/flooding with this morning`s round as the greater instability and deeper convection in on track to stay suppressed to our south. Previous Discussion... Overnight surface analysis shows an area of low pressure located along over the Midwest (near the IA-IL border). This low was located along a stalled frontal boundary that extends eastward along the OH Valley and then arcs southeastward across the southern mid-Atlantic (near the VA-NC border). An MCS developed last night just downstream of the surface low. Based on the orientation of 1000-500 mb thickness contours and the flow in the mean cloud layer, this MCS should move to the east- southeast and through the Chesapeake Bay/Delmarva region this morning. The timing of the precip from this MCS closely follows the HRRR, WRF ARW/NMM as well as the latest radar trends upstream. Expect showers to arrive across our far western zones in southeast PA as well as eastern MD shortly after sunrise (6 or 7 AM) and then progress eastward during the mid morning. The back edge of the widespread showers is forecast to advance quickly eastward, moving off the coast around midday. There will be a sharp northern cutoff of precip somewhere near the PA Turnpike/I-195 corridor. Note, there were considerable changes to the PoP and weather grids compared to the previous shift. The stronger convection associated with severe storms (tornadoes, damaging winds and flash flooding) were present on the southern flank of the MCS and along or just south of the frontal boundary. Therefore, we expect these stronger storms to track to our south and west this morning. However, numerous lightning strikes were noted farther north on the cool side of the boundary. Kept a chance for thunderstorms from about RDG-PHL-ACY southward later this morning, where model soundings indicate a 2-3 hour period of 250-750 J/kg (highest south) of elevated instability. SPC Day 1 outlook still has the southern third of the CWA in a slight risk of severe storms. There will be a strong wind field aloft ahead of the MCS but the threat of damaging winds looks to be limited assuming these storms remain elevated in nature by the time they reach our area. Greater uncertainty in the forecast revolves around how much will the atmosphere be able to rebound/destabilize this afternoon in wake of this morning`s MCS. Based on the latest guidance, there will likely be scattered convection developing during peak heating across eastern PA and central/northwest NJ along the residual outflow boundary and/or differential heating boundary to the north of where MCS tracked earlier. Majority of the model forecast soundings show the atmosphere capped, which would hinder convective updrafts and our thunderstorm potential. Still kept a slight chance of thunder in the weather grids for the mid to late afternoon. High temperatures should generally in the in the upper 70s and lower 80s. The warmest conditions are expected across the northern half of the CWA as well as the lower Delmarva, where the greatest potential to see extended breaks in the clouds reside. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/... Some of the hires convective-allowing model guidance advertises a second MCS developing along or just south of the Ohio River. If it develops, it should track mainly to our southwest tonight given the distant position of the frontal boundary to our south and with weak high pressure building in from the north. Kept low chance PoPs in the forecast for our Delmarva/Delaware Bay zones tonight to account for the possibility of the northern periphery that these showers and storms clip us. There is still a limited risk that a storm or two becomes strong to severe this evening mainly across the Delmarva. The threat would transition from winds to hail late in the evening as convection becomes increasingly elevated. Patchy fog may also develop late this evening and especially overnight. There greatest potential will reside across the southern portion of the area, where the ground remains wet after today`s precip. However, convective debris clouds may linger across the lower Delmarva, which should inhibit the formation of radiational fog. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... High pressure will be the dominant wx feature for the extendd period. The high will build down from Canada on Fri and influence our wx into early next week. THe current fcst is for dry and pleasant wx durg this time. There is one potential fly in the ointment though. A mid level trof is fcst to knock back the ridge a bit on Fri. The majority of the guid keeps any precip assocd with this trof to our w and s. However, the ecmwf is more robust and brings precip acrs our area and then develops a sfc low which moves out to sea on Sat. For now, will maintain a dry fcst, since the ecmwf is the outlier and go with the majority of the mdls and wpc. By Mon, an area of low pres will be movg acrs ern Canada. Its assocd cdfnt will approach from the w late Mon and cross the area on Tue, with a secondary fropa Tue night into erly Wed. Both cfp will bring the chc of shwrs/tstms. High pressure will then build in for the remainder of Wed. Temps look to be near seasonal values with comfortable dew points. && .AVIATION /12Z THURSDAY THROUGH MONDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR thru 11Z. Then a complex of showers and embedded thunderstorms will move across SE PA, S NJ, E MD and DE between 11Z and 15Z. MVFR were included in the 06Z TAFs from RDG to PNE, southward. A 2 hr tempo group from TSRA was added across these southern terminals. MVFR cigs possible in wake of this morning`s precip before improvement to VFR occurs between 17-20Z. More scattered showers could develop after 18Z before ending around 02Z. Added a 2-3 hour prob30 group for most of the terminals for late in the day. While an isolated thunderstorm is not out of the question this afternoon, confidence too low to include in TAFs. Patchy fog could develop late this evening and overnight. Winds will be light (under 10 kt) and variable thruout the period. OUTLOOK... Friday through Monday...VFR under high pres. Mdt confidence Friday. High COnfidence for the rest of the period. && .MARINE... Winds and seas below SCA today and tonight. A sly swell around 2-4 ft will continue in our coastal Atlantic waters thru the period. One potential impact on our southern waters could be locally gusty winds in thunderstorms this morning as well as this evening. Coverage of storms will be low. OUTLOOK... Friday through Monday...No marine headlines are anticipated. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Nierenberg Near Term...Klein Short Term...Klein Long Term...Nierenberg Aviation...Klein/Nierenberg Marine...Klein/Nierenberg is the U.S. government's official web portal to all federal, state and local government web resources and services.