752 FXUS61 KPHI 062318 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 718 PM EDT Mon Sep 6 2021 .SYNOPSIS... High pressure builds in through Tuesday, then a strong cold front moves through Wednesday night and Thursday morning. High pressure arrives Friday and Saturday before shifting to our south and east Sunday. A cold front may then arrive from the north during Monday. && .NEAR TERM /THROUGH TUESDAY NIGHT/... It has been a fairly pleasant Labor Day over our region as High Pressure, centered over the southern MS valley, gradually migrates northeastward and drier W-NW flow prevails in the wake of the early morning cold front. Per usual the mid-lvl wave is lagging a bit behind the surface low (which is well to our NE), and modest lift from this wave, combined with diurnal heating and weak cold advection at 850mb has resulted in a strato-cu field developing and advecting into our area. As surface heating wanes through the remainder of the evening, these clouds will continue to dissipate. Overnight we will remain dry with any cu clearing the area by this evening. Lows will range from the low 50s up north to the lower 60s in the urban corridor/Delmarva. Lows could drop a few more degrees if we can completely decouple but don't have enough confidence in this occuring to really "crater" temps in the favored spots. The High to our south will shift offshore during the day on Tuesday, with winds backing to more SWrly by the afternoon. The return flow won't really be able to become established during the day so expect rather similar conditions to today (i.e. Highs in the low-mid 80s, and dewpoints mostly in the 50s). Cloud cover will mostly be of the diurnal-cu variety although there is some chance of more substantial mid-lvl clouds advecting in from the SW in the afternoon. Given the warmer airmass and increasing moisture, Tuesday night will be 5-10 degrees warmer than tonight, but it should remain dry as the better forcing holds off until Wednesday. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... A period of unsettled weather Wednesday afternoon through Thursday morning. A potent upper-level trough that closes off is forecast to be shifting eastward from the Upper Midwest and Great Lakes. This feature may weaken some as it arrives into the Northeast as the main energy tracks into north-central Canada, however some strong shortwave energy is forecast to be maintained in the base of this trough. Surface low pressure tracks well to our north, however the strong cold front is scheduled to arrive into our area Wednesday night. An axis of higher precipitable water (nearing 2 inches) is forecast to surge northward ahead of the advancing cold front Wednesday. Since the surface low tracks well to our north, the overall forcing for ascent typically is weaker across our region however some stronger energy is forecast to be moving through the base of the trough as it enters our area. Therefore, a ribbon of showers and thunderstorms are expected to occur ahead of the front Wednesday afternoon and evening. A weak lee-side trough may provide some focus for convective development well ahead of the cold front. There could be an additional tropical moisture connection from the Gulf of Mexico. If this were to occur, some local enhancement to the rainfall is possible. We will monitor for any heavy rainfall threat especially given how wet the ground is. Some strong thunderstorms are possible Wednesday afternoon and early evening given current forecast shear profiles, however there is some uncertainty with the amount of instability and the timing details of the cold front. The cold front should be clearing our coasts Thursday morning, however there still is some uncertainty regarding how quickly this occurs. For this reason, we included some low PoPs from about the I- 95 corridor on south and east for a while Thursday. Otherwise, cooler and drier conditions build in by late day with an increasing west to northwest wind. && .LONG TERM /FRIDAY THROUGH MONDAY/... Summary...Dry conditions overall with temperatures close to average. Synoptic Overview...A strong upper-level will be lifting out of the Northeast and Mid-Atlantic Friday, then a weaker trough slides across the region Saturday. The flow should go more zonal for a time, however a ridge building across the southern states may result in a trough moving through the Northeast late in the weekend and early next week. Hurricane Larry well out in the open Atlantic waters is forecast to remain there, however larger swells are expected to impact our coastline. For Friday...A strong upper-level trough is forecast to be lifting out with surface high pressure building in. Despite tranquil weather conditions, there will be large swells into our coasts associated with Hurricane Larry as it churns well out in the open Atlantic. Due to the presence of a lingering trough axis overhead Friday, there may be a decent amount of cumulus development during the daytime before dissipating in the evening. For Saturday through Monday...The flow attempts to go more zonal for a time, however a ridge building across the southern states may result in a few fast moving shortwave troughs across the Northeast. Surface high pressure is forecast to be over our region to start Saturday, then it shifts to our east and south through Sunday. One of the shortwave troughs later Sunday into Monday should have a cold front with it, although this boundary may not get all that far south Monday. It will probably be a battle between Canadian high pressure to our north and the ridge across the southern states. Despite a couple of shortwave troughs moving through, forcing for ascent looks to be overall lacking and therefore just have some slight chance PoPs Sunday night across the northern areas which are closer to the incoming cold front. Temperatures look to be about average for this time of the year. && .AVIATION /23Z MONDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Through Tonight...VFR with WNW winds decreasing to 5kts or less tonight. High confidence. Tuesday... VFR, initially light W-NW winds turning more SW, 5-10kts by the afternoon. High confidence. Tuesday night... Generally VFR, although can't rule out some lower stratus starting to advect in towards daybreak Wednesday. Southerly winds 5-10kts. Moderate confidence. Outlook... Wednesday...VFR should give way to times of MVFR/IFR as some showers and thunderstorms move through especially in the afternoon and evening. South/southwest winds 10-15 knots with some gusts to 20 knots, diminishing at night and becoming west to west-northwest. Moderate confidence. Thursday...Sub-VFR conditions possible early as showers move out, otherwise VFR. Northwesterly winds around 10 knots, then becoming light and variable at night. Moderate confidence. Friday and Saturday...VFR. Northwesterly winds near 10 knots, diminishing Friday night, then becoming southwesterly near 5 knots Saturday. Moderate confidence. && .MARINE... Sub-SCA conditions are expected through Tuesday night with seas gradually increasing from 2-3 ft tonight & Tuesday to around 4 ft Tuesday night. Winds will shift from westerly today to southerly Tuesday afternoon. Outlook... Wednesday, southerly winds increase and seas build. A Small Craft Advisory will probably be needed for a time mainly for the ocean zones. Thursday through Saturday...The winds are expected to be below Small Craft Advisory criteria, however incoming large swells (wave period around 15 seconds) from Hurricane Larry churning way out in the open Atlantic look to be hazardous. The swells are anticipated to be subsiding though Saturday. Rip Currents... A LOW risk of rip currents is currently forecast for Tuesday given low wave heights in the surf zone. However, the rip current risk will likely ramp up into the latter half of the week as long period and increasingly high amplitude E-SE swells from Hurricane Larry (churning well out in the open Atlantic) reaches the Mid-Atlantic coastline. We're anticipating a HIGH risk of rip currents for the New Jersey coast and a MODERATE risk for the Delaware beaches for Wednesday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Gorse Near Term...Carr/Staarmann Short Term...Gorse Long Term...Gorse Aviation...Carr/Gorse/Staarmann Marine...Carr/Gorse/Staarmann