Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 210538 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 138 AM EDT Mon Aug 21 2017 .SYNOPSIS... High pressure over the area moves offshore tonight, then slowly drifts out to sea through Tuesday. A cold front approaches from the west on Tuesday, then works its way across the region on Wednesday. Canadian high pressure builds into the region late in the week and into next weekend. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... Made a few last-minute changes to the winds/sky cover/Wx grids to increase the fog potential a little bit late tonight and to bring a light but more persistent west or southwest flow to locations with climatologically stronger winds at night. Current obs suggest transient lower visibilities in susceptible valley locations (K22N and KSMQ), with other problem spots starting to show reduced visibilities as temps meet dew points (e.g., KRDG, KABE, KPTW, and KMIV). Relatively tranquil weather conditions will persist through tonight as high pressure remains over the area. Scant cloud cover and light and variable winds will lead to favorable radiational cooling tonight, with temperatures falling to near or within a few degrees of climatological normals. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM MONDAY/... Quite a few adjustments to the forecast for Monday, with more likely on the way. Biggest change was to slow the onset of cloud cover a little bit during the day, especially from the Delaware River eastward. Latest model guidance shows a somewhat slower onset with increased clouds, both with the developing cumulus field during the day and with high-level cirrus as upper-level moisture progresses eastward. That would be good news for solar eclipse fans, but confidence is not particularly high with the sky cover forecast given the uncertainties associated with the extent of the cumulus field that develops (a planetary boundary layer process, which in forecast land is typically low-skill, even in the short range). However, confidence is increasing that storms will break out to our west in the afternoon and move into western portions of the area by late afternoon or early evening (likely after eclipse time). With aid from a low-amplitude vort max rapidly progressing through Pennsylvania during the afternoon, smaller- scale lift provided by differential heating (perhaps via increasing cloud cover to the west versus little to the east), terrain effects, etc. will likely be sufficient to develop scattered convection. Storm-scale environment is not too shabby either, with MUCAPE (SBCAPE) approaching/exceeding 2000 (1500) J/kg in the western CWA by late afternoon and effective shear increasing to 30-40 kts. With a deep layer of strongly positive buoyancy (i.e., relatively "thick" CAPE profiles) and wet bulb zero heights potentially as low as 10-11 kft during peak heating, such magnitudes of shear may generate updrafts sufficiently strong for hail in addition to strong winds. Will evaluate the environment further, but may add some enhanced wording to the Wx grids during the afternoon hours west of Philadelphia. && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/... Some mid-level shortwave energy will pass through the region Monday night. Onshore flow will usher a warm and humid airmass into the region, as surface dewpoints will climb into the upper 60s to low 70s, and into the mid 70s across the Delmarva and extreme southern NJ. This will result in isolated to scattered showers and thunderstorms in the evening, gradually tapering off after midnight. Patchy fog may form in the pre-dawn hours going into Tuesday morning. On Tuesday, low pressure over the Great Lakes will lift to the northeast towards eastern Canada. This will drag a cold front through the Great Lakes and into the Ohio Valley late Tuesday. Pre-frontal trough sets up ahead of the front, and that looks to trigger afternoon and evening showers and thunderstorms. Abundant low level moisture will be in place with dewpoints in the low to mid 70s, and with several strong shortwaves out ahead of the upper trough/surface cold front, can expect some fairly strong thunderstorms during the overnight hours Tuesday night into Wednesday morning. Cold front works its way to the south and east Wednesday through Wednesday evening, then becomes nearly stationary over the Mid-Atlantic and Delmarva for the end of the week. Quite warm and humid for the first half of the long term period, with highs in the upper 80s to low 90s on Tuesday, then in the mid to upper 80s on Wednesday. Max heat index values on Tuesday look to be in the upper 90s for areas along and south of the I-95 corridor, and around 100 for the Delmarva. Several waves of low pressure may develop on this boundary, but latest model guidance keeps those systems well offshore. Canadian high pressure then builds in from the north and west to close out the work week, with the center of the high gradually passing north of the region on Saturday before moving offshore on Sunday. Dry conditions expected for the end of the week and into the weekend with temperatures running several degrees below normal. Highs will top off in the upper 70s to around 80 from Thursday through Sunday, and lows will drop into the upper 50s to low 60s, except cooler up north. && .AVIATION /05Z MONDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. For the 06Z TAFs...Added some TEMPO fog to KRDG, KABE, and KMIV based on latest trends, with brief sub-VFR conditions possible, especially at KRDG/KMIV, through 12Z. Cumulus field will develop during the day, with bases generally in the 4-6 kft range. Added PROB30 -TSRA to KRDG/KABE given somewhat increased confidence that convection will move into eastern PA late this afternoon into the evening hours. Generally increasing cloud cover from this afternoon through tonight, and latest guidance shows at least some potential for sub-VFR CIGs/VSBYs Monday night. Winds generally light/variable through 15Z, then becoming mostly southerly 5-10 kts. Outlook... Monday night...Sub-VFR conditions possible in scattered SHRA/TSRA. Fog/stratus also possible with sub-VFR conditions. Tuesday...Morning fog/stratus burns off, then predominantly VFR. Scattered SHRA/TSRA develop late in the day and at night with sub-VFR conditions. Wednesday...Sub-VFR conditions in showers/thunderstorms, which taper off late in the day. A wind shift from S to NW likely late in the day with the passage of a cold front. Thursday through Friday...VFR and light winds. && .MARINE... Winds and seas remain tranquil across the waters through Monday as high pressure gradually shifts offshore. Seas will remain two feet or less through Monday afternoon, with a few 3 foot seas possible off the northern NJ coast by evening as southerly winds begin to strengthen. Outlook... Monday night...Winds and seas below SCA thresholds. Tuesday...SCA possible late afternoon and night with S winds increasing to around 25 kt ahead of cold front. Building seas to 4-6 ft are forecast for the Atlantic coastal waters. Wednesday...Winds should decrease to below 25 kt but SCA conditions may continue into the morning as it may take longer for seas in the coastal waters to drop below 5 ft. Thursday through Friday...No marine hazards expected. Rip currents... At this point we expect the risk for the formation of dangerous rip currents on Monday to begin low. However, with winds increasing out of the south and the dominant period trending longer, the risk could increase as we go through the day depending on how quickly the wind shift occurs. For now we will mention a low risk in the surf forecast but continue to monitor closely especially as we go through the day on Monday. && .TIDES/COASTAL FLOODING... Based on observations Sunday evening and current projections for the high tide Monday evening, will hold off on issuing a coastal flood advisory this morning. Current model projections continue to indicate levels reaching near minor flood thresholds but consistently below advisory thresholds. Patchy minor flooding is probable Monday evening, but confidence is too low at this point to issue an advisory. There is at least some concern that a more onshore component of the wind (south to possibly south-southeast) may contribute to somewhat higher levels than guidance suggests Monday evening. Should this be observed during the day, the threat for minor flooding would increase. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...MPS Near Term...CMS/Johnson/MMD Short Term...CMS/MMD Long Term...MPS Aviation...CMS/Johnson/MPS/MMD Marine...Johnson/MPS/MMD Tides/Coastal Flooding...CMS is the U.S. government's official web portal to all federal, state and local government web resources and services.