Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 271354 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 954 AM EDT Tue Jun 27 2017 .SYNOPSIS... A weak cold front passes across the mid Atlantic states today. High pressure will follow tonight and Wednesday. This high moves offshore during the end of the week. Eventually, another weak cold front will cross the mid Atlantic states Saturday night or Sunday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 930 am update: Weak-amplitude vorticity maxima continue to wreak havoc with the forecast this morning. The vort max that moved through late last night (generating fairly impressive convection in much of the area) has moved rapidly northeastward, but another weak lobe of vorticity has progressed into the region this morning. Aided by a subtle midlevel jet streak, more showers developed in the southern Mid-Atlantic and in central Pennsylvania overnight and have moved into the CWA this morning. These showers were diminishing as large-scale lift has diminished owing to the weakening/shearing of the vort max as it moves northeastward. All of these perturbations lie just downstream of the main vort max pivoting northeastward through New York today. The large- scale lift from this vort max should remain north of the area today (for the most part), but yet another elongated lobe of higher vorticity will swing through the area this afternoon. As it does so, the vorticity will subtly strengthen...right around the time of peak heating. With large-scale support via differential cyclonic vorticity advection and smaller-scale lift provided by subtle convergence along an attendant surface trough, differential heating from variable cloudiness, and potential lift from residual outflow boundaries from overnight convection, think there remains a chance of storms this afternoon, especially north of the Mason-Dixon Line in closer proximity to the stronger large-scale ascent in New York/New England. As such, adjusted PoPs to align with current radar trends and with forecast thinking this afternoon. Confined most PoPs after 18Z to the north of Philadelphia, though a stray shower/storm cannot be ruled out farther to the south. Convection would be isolated/widely scattered, but questions remain regarding convective initiation at all given uncertainty with how the environment will recover after overnight precipitation. For example, temperatures are running lower than forecast across the area this morning thanks to increased cloud cover upstream of the precipitation, but at least partial clearing is occurring to the west and will likely move in late this morning and early this afternoon. Observations will be monitored closely the next few hours to get a better feel for the overall convective threat this afternoon. Expect further refinements to the forecast with time. Previous discussion... An upper-level trough is forecast to sharpen eastward today as it moves into the Northeast and Mid Atlantic. This will drive weak low pressure well to our north, however a few weak cold fronts or surface troughs will continue to push eastward. The first one early this morning along with a weak short wave is responsible for a band of downpours. These has now just about exited our area. We then turn our attention to the west. A more pronounced area of short wave energy is forecast to arrive from west to east later this morning and through the afternoon. This looks to be accompanied by either a weak cold front or surface trough. While the trough aloft is forecast to be sharpening some as it shifts eastward along with gradual cooling aloft, the bulk of the lift is more focused to our north and west. The majority of the model guidance is therefore showing much of the convection just to our north through west. There should however be enough moisture and lift along with at least some instability through the day to pop a few showers or thunderstorms. This appears to be focused initially early this morning just to our west, while a few other showers continue to initiate to our southwest which moves across parts of our southern zones through mid-morning. Since the overall coverage is less certain this afternoon, we kept PoPs in the slight chance to low chance range. High temperatures are a blend of continuity, MOS and some high-res guidance. Given lower heights and thickness values, the afternoon high temperatures are forecast to be a bit cooler than yesterday. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... The axis of an upper-level trough is forecast to move across our area during the overnight. Any shower or thunder is expected to end very early as the main short wave energy is offshore. There should then be some subsidence overspreading the area as surface high pressure starts to arrive toward daybreak. The sky is anticipated to become mostly clear as a result with any clouds due to daytime heating flattening and then dissipating in the evening. We are expecting enough drying along with wind above the surface to limit any fog development overnight. Low temperatures are mostly a blend of continuity and MOS. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Highlights: 1. Heat index values near 100 possible for the urban corridor Saturday. 2. Highest chance of thunderstorms centered on later Saturday into Sunday when isolated svr may occur. 500 MB: A cold trough over the northeast USA tonight weakens to the Maritimes by Thursday night as heights rise-warm over the mid-Atlantic states. The warm ridge aloft, almost 2SD above normal, on Saturday over the northeast gives way to a broad trough over our area early next week. Temperatures: Calendar day averages will be about 5 degrees below normal Wednesday average near normal Thursday then 5 to 8 degrees above normal Friday through Sunday, cooling slightly on Monday. For June, the result should be an average temperatures generally 1 to 2 degrees above normal at our NWS primary climate sites except around 1 to 1.5 degrees below normal at Mount Pocono. Forecast basis: a 50 50 blend of 00z/27 GFS/NAM MOS Wednesday- Thursday, 00z/27 GFSMEX MOS Thursday night-Friday and then only dewpoints/max/min temps from the 0601z/27 WPC grids for D4-8. Other guidance including POPS/Sky/Wind was continuity from the 330 PM Monday forecast. The dailies... Wednesday...Mostly sunny. Westerly wind. Wednesday night...Clear, then high clouds toward dawn. Light southwest wind. Thursday...Partly to mostly sunny (high clouds early, cu and ac fields afternoon-night in waa. Southwest wind gusty to 25 mph during the afternoon diminishes somewhat at night but still a stirring mild wind. Small chance of a shower/showers late or night except a thunderstorm north of I-78. SPC D3 consideration, especially with bulk shear. MLC is rather paltry Thursday so from my view, this a leftover gusty thunderstorm Thu night. Friday...Partly to mostly sunny. Southwest with gusts probably 20 mph or less. Heat Index should rise to 95 to 97 I-95 corridor. Saturday...After predawn patchy fog or stratus, partly to mostly sunny. Southwest wind. Thunderstorm potential ahead of cold front later in the day or at night. MLC 1800J. Bulk shear is currently modeled low. Widespread heat index of 95 to 100 I95 with a WPC around 30 percent probability of HI in PHL. Sunday...Partly to mostly sunny. Depending on psn of the cold front there could be leftover showers and tstms. West southwest wind. Monday...Chance of a shower in the fcst but for now odds favor a decent Monday. Low confidence on this day: && .AVIATION /14Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR overall with a ceiling at times at or above 6000 feet. A few additional showers possible this morning, then isolated to perhaps scattered showers/storms should occur this afternoon. Our confidence regarding timing especially this afternoon is low and therefore a mention is not included in the TAFs. Mostly light and variable winds, becoming west or west-southwest and increasing to around 10 knots later this morning and afternoon. Some local gusts up to 20 knots are possible this afternoon. Tonight...Any spotty shower/storm ends early, otherwise VFR with any lingering clouds tending to dissipate. West to northwest winds mainly 5 knots or less. OUTLOOK... Wednesday...VFR. Mainly a west wind. Wednesday night...VFR. light southwest wind. Thursday...VFR with occasional variable cigs aoa 5000 ft. Southwest wind gusts to around 25 kt in the afternoon. Chance of a shower. Friday...VFR. Southwest wind may gust to near 20 kt in the afternoon. Saturday...Patchy IFR conditions possible predawn in fog or stratus. Then VFR most of the day. Southwest wind. Thunderstorms ahead of a cold front are expected but timing uncertain. && .MARINE... The conditions are expected to remain below Small Craft Advisory through tonight. A southwesterly wind will become westerly late in the day and tonight as a weak cold front moves offshore. This offshore flow will increase at times tonight with some gusts to near 20 knots. A few isolated thunderstorms will be possible mainly this afternoon. OUTLOOK... marine headline. Rather quiet sea state. Thursday...SCA possible for the near shore waters in the afternoon then a possible Atlantic waters SCA for hazardous seas during Thursday night. Friday and Saturday...Just below SCA threshold. RIP CURRENTS... The forecast conditions for today should result once again in a low risk for the development of dangerous rip currents. && .CLIMATE... June average temperature: temperatures are expected to generally average 1 to 2 degrees above normal at our NWS primary climate sites except around 1 to 1.5 degrees below normal at Mount Pocono. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Drag Near Term...CMS/Gorse Short Term...Gorse Long Term...Drag Aviation...Drag/Gorse Marine...Drag/Gorse Climate... is the U.S. government's official web portal to all federal, state and local government web resources and services.