750 FXUS61 KPHI 161636 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1236 PM EDT Fri Jul 16 2021 .SYNOPSIS... High pressure located over the western North Atlantic will lose its influence over our weather gradually through Saturday. A cold front approaching from the northwest is forecast to arrive on Sunday. The boundary is expected to stall and slowly dissipate in or near our region early new week. Another cold front is anticipated to reach eastern Pennsylvania, New Jersey, eastern Maryland and Delaware around Wednesday night. && .NEAR TERM /THROUGH TONIGHT/... West-southwesterly flow will prevail this afternoon with a stalled or very slowly moving frontal boundary positioned across the southern Great Lakes region to our northwest. With heights rising a bit aloft and southwesterly warm air advection continuing, expecting widespread high temperatures in the low to mid 90s along with dewpoints in the low 70s. Resulting Heat Indicies are forecast to rise into the low 100s mainly along and south/east of the urban corridor. The Heat Advisory thus continues for portions of the area. Winds sill be from the southwest around 5-10 mph. While there won't be a whole lot of mid-level support for convection, instability will rise to near 2000 J/kg near and north/west of I-95. Shear will be weak, only around 15-20 kts at best where the instability and forcing will be the greatest. Thus, expect some isolated showers and thunderstorms to develop, mainly north/west of I-95. Wouldn't be surprised to see a few showers or storms fire up along the sea breeze either. Severe potential appears low given weak forcing, weak shear, and poor mid-level lapse rates (~5 C/km). Nevertheless, SPC has highlighted areas along and north of I-78 in a Marginal Risk for isolated damaging wind gusts with any stronger cores that are able to develop. Expect a muggy night with lows in the low to mid 70s and light to calm winds. A weak mid-level wave may allow for some isolated convection to continue into the overnight period, mainly near and north of the I-78 corridor. && .SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/... On Saturday, a mid-level shortwave trough will approach from the Ohio Valley. The trend with this feature appears to be slower, which makes sense given that some guidance attempts to close off a weak mid- level low somewhere across the southern Great Lakes region. I suspect this will result in convection not developing until late in the afternoon or evening and potentially lingering past the midnight hour. This will also be dependent on the timing and placement of a slowly approaching frontal boundary from the north, which has also trended a bit slower. Nevertheless, the forcing is forecast to arrive at some point late in the day with instability building once again to near or over 2000 J/kg. The approaching shortwave will also result in stronger mid-level flow and thus greater forcing than Friday with effective shear increasing to near 30 kts. This should result in at least an isolated severe threat, but the mid-level lapse rates remain poor which should preclude a more widespread damaging wind or large hail threat. SPC has highlighted the majority of the forecast area in a Marginal Risk for damaging winds with any stronger or more organized convection that develops. The bigger story for Saturday will be the potential for locally heavy rainfall with the storms developing into the afternoon and evening. As with any convectively driven heavy rainfall and flash flooding potential, confidence is low on exactly where the heaviest rainfall will occur at this time. In addition to the modest instability developing into the afternoon, Precipitable Water values will rise to near or over 2". Unlike the flash flood environment we had this past Monday, the convection will be moving, but could begin to train over the same area with any boundary interactions. WPC continues to highlight areas along and north/west of I-95 in a Slight Risk for excessive rainfall, and this may need to be upgraded to a Moderate Risk in a future update as confidence increases on the placement of heaviest rainfall. A Flash Flood Watch may thus also be needed for this threat as well if confidence increases enough that portions of the area may receive flash flooding on Saturday. Long story short, widespread convection is expected in the forecast area, especially along and north/west of I-95. Temperatures will once again rise into the low 90s with low 70s dewpoints. The Heat Advisory remains in effect for Saturday with maximum Heat Indicies rising to near 100 degrees near and south of Philly, and values possibly nearing 105 degrees across portions of Delmarva. The showers and thunderstorms should linger for some time into the overnight period, but should generally wane in intensity and coverage after midnight. Expect another muggy night with lows in the low 70s. && .LONG TERM /SUNDAY THROUGH THURSDAY/... Sunday morning a cold front will remain near the CWA with another impulse approaching from the west. There are some discrepancies on where exactly the front will be Sunday, with the GFS being the most progressive and having the cold front clear the coast Sunday morning. The ECMWF, CMC, and NAM all have the front hung up somewhere near the Delaware River Sunday and this overall seems to make sense. The upper level forcing responsible for pushing the cold front through NJ remains mainly back to our west at the time. Ahead of the frontal zone, PWATs remain around 2" which is at or slightly above the 90th percentile for this time of year. Convective initiation will also be aided by an approaching jet streak across central PA. Given the above, another round of locally heavy rainfall will be possible Sunday. Monday, guidance continues to struggle with the evolution of a shortwave diving southeast out of the Great Lakes region. The ECMWF closes the wave off into an area of low pressure, while the GFS keeps the wave open. Either way, this also favor a slower progression east of the cold front. The CMC actually has another round of convection Monday in association with the stalled boundary near the PA/ NJ border. The ECMWF stalls the boundary more towards the NJ coastline. This solution makes sense, as convective outflow will likely help to push the front towards the coast, but not be strong enough for the front to completely clear the DE and NJ coastlines. Tuesday into Thursday, a longwave trough axis will center across the east coast of the United States with a mid-level ridge building over the Mountain West. The overall consensus is for several more rounds of embedded shortwaves to traverse the region, keeping the chance of precipitation in for the remainder of the work week. There is some disagreement on the amplitude of the wave, but as of now, seasonable temperatures and humidity are forecast. && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. This afternoon...VFR. Some isolated showers or thunderstorms are possible at RDG/ABE. Winds southwest around 5-10 kts. High confidence. Tonight...Prevailing VFR. Winds becoming light and variable after 00Z. Some patchy fog is possible after 06Z, but confidence is not high enough to include in the TAFs. Moderate confidence. Outlook... Saturday...Prevailing VFR, but thunderstorms with heavy rain and gusty winds are expected to develop after 21Z, especially near and northwest of the I-95 corridor. Restrictions likely in these areas associated with convection. Patchy fog possible late. Winds southwest around 5-10 kts becoming light and variable after 00Z. Moderate confidence overall. Sunday...Some low clouds and fog are possible in the morning, with another round of showers and thunderstorms in the afternoon. IFR cigs and vsbys likely at times. Southwest to west wind 5 to 10 knots. Medium confidence. Monday...Mainly VFR. A chance of showers and thunderstorms in the afternoon and evening. Westerly wind 5 to 10 knots. Low confidence. Tuesday...Mainly VFR. Westerly wind 5 to 10 knots. Medium confidence. && .MARINE... Sub-advisory conditions expected through tonight with southwesterly winds around 15 kts. Seas 2-3 feet. Outlook... Saturday...Sub-advisory conditions expected with southwesterly winds around 15 kts increasing to around 20 kts late. Showers and thunderstorms expected Saturday evening with heavy rain and locally high winds and seas possible. Seas 3-4 feet. Sunday through Tuesday...Seas 2 to 3 ft are expected on the ocean and 1 to 2 ft for the Delaware Bay as winds remain below 25 kts. Chances for showers and thunderstorms each day, especially Sunday. Rip Currents... A relatively light southwest to south flow is expected through Saturday with wind speeds around 15 MPH or less. Breaking waves are forecast to be 1 to 2 feet with a medium period southerly swell. As a result, the rip current risk is LOW through Saturday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Heat Advisory until 8 PM EDT Saturday for PAZ070-071-102-104- 106. NJ...Heat Advisory until 8 PM EDT this evening for NJZ008-013-014- 020-027. Heat Advisory until 8 PM EDT Saturday for NJZ010-012-015>019. DE...Heat Advisory until 8 PM EDT Saturday for DEZ001. MD...None. MARINE...None. && $$ Synopsis...Haines Near Term...Iovino/Staarmann Short Term...Staarmann Long Term...Haines Aviation...Haines/Staarmann Marine...Haines/Staarmann