347 FXUS61 KPHI 211750 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 150 PM EDT Sat Aug 21 2021 .SYNOPSIS... A broad upper level trough will remain near our region through this weekend. Meanwhile, Tropical Cyclone Henri is expected to track northward towards the east coast this weekend, however it is anticipated to stay offshore of our coast as it approaches Long Island and the southern New England coast. Henri will then become absorbed by the upper trough early next week and move away from the area by late Monday. Weak High Pressure will then briefly build near the region through midweek before another cold front may approach our region late next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 845 AM Update...A plume of moisture that is associated with mid level moisture from Henri has resulted in an area of showers/storms off the coast that is currently moving N/NW. Think some of these showers are likely to get into at least coastal NJ by later this morning so POPs and QPF was bumped up some for today. By afternoon, rain chances will start to generally increase for all areas as diurnally driven pop-up showers and isolated storms will be more favored over the western half of the region in association with the upper trough to our west. So nearly all areas have at least chc PoPs today. With a good deal of clouds and the showers around, high temperatures will run near to a bit below average, favoring the low to locally mid 80s. However, heat index values will approach 90 thanks to high humidity. Light easterly flow is expected as we get into the far western periphery of Henri. && .SHORT TERM /6 PM THIS EVENING THROUGH MONDAY NIGHT/... Changes with this update: Flood watch has been issued for much of the region continuing through Sunday night. The focus remains on Henri and its interaction with the mid and upper level low centered to our southwest. Much of the 00Z guidance depicted a slight eastward trend with the track, in contrast to the westward track we saw yesterday. However, this is probably not going to make a big difference with our local impacts. It still appears that tropical storm force wind impacts will be mostly limited to Middlesex and Monmouth counties. The main hazard for our region (aside from the marine and beach hazards) is heavy rain. Heavy rain/Flooding: Aside from a maximum over E Central northeastern New Jersey, the distribution of precipitation across our region is uncertain thanks to the uncertainty with when and how Henri with interact with the low. For now have stayed close to a blend of guidance and especially WPC guidance. The HRRR, starting with the 00Z run depicted very high QPF values, not so much with the main rain shield from Henri, but a secondary area developing in response to the interaction. This is an outlier for now, and given its recent poor performance with convection, have not put much weight into this guidance. As for the timing of heavy rain, this is a relatively long duration of precipitation chances, but it appears the period for the heaviest rain will be late tonight through the day on Sunday. Further information on the flash flooding and flooding threat are in the hydrology section below. Wind: Have not changed much from the previous advisory issuance. It still looks like there will be a very sharp drop off away from Henri, so at this point it looks like Monmouth and Middlesex will be clipped by tropical storm force winds. Can't rule out tropical storm force winds edging into Ocean County if there is a trend westward again, but this is becoming less likely. Other hazards: As mentioned below in the marine section, rough surf and high rip current risk are the primary other impacts for this event. The tornado threat for this event should be low as a majority of tornadoes associated with tropical systems are to the right of the track and we will be staying to the left of the track of this system (opposite of the conditions on Thursday with the remnants of Fred). The remnants of Henri could remain in southern New England through the day on Monday. However, our rain chances should start to decrease as the mid/upper level low shifts further east from our region, and Henri nudges slightly further east. && .LONG TERM /TUESDAY THROUGH FRIDAY/... Overview: In the wake of the mid-lvl trough (merged with Henri's remnants) ridging centered over the Southern US will build eastward towards the region while the subtropical ridge also builds westward (these two ridges may bridge just south of the area). A mid-lvl low/inverted trough feature will begin to nudge northward towards the southern Mid-Atlantic Thursday into Friday, while guidance is in disagreement on how close a northern stream shortwave gets to our area later in the week. The surface pattern is fairly inconspicuous with weak high pressure prevailing Tuesday into Wednesday before guidance quickly diverges on whether or not the cold front associated with low pressure over southern Canada will impact the area Thursday-Friday. Details: Tuesday looks like a (rare) dry day as mid-lvl heights rise and weak high pressure progresses towards the area, Wednesday should remain mostly dry although some upstream convection may make it intact to far western portions of our area, so retained some PoPs. WNW flow on Tuesday will transition to W-SW on Wednesday with warm (temperatures getting into the low 90s by Wed) and fairly humid conditions prevailing. As mentioned in the Overview section, guidance begins to diverge considerably on how the pattern evolves heading into the late week period. Given the increasing heat/humidity (Heat indices perhaps approaching 100 degrees on Thursday) and resultant instability, Chc Thunder seems like a decent bet, although it remains to be seen if a more substantial forcing mechanism (e.g. the cold front with the Canadian Low) can move into the area, or if it would be more of the weakly-forced diurnal variety. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of Today...Mostly VFR conditions persist this afternoon, however, there could temporarily be some MVFR ceilings at times through the day. There will continue to be a chance of showers and even thunderstorms this afternoon. We do not have thunderstorms in the TAFS however as confidence in the timing and placement is low. Winds will remain out of the east to northeast around 5-10 knots. Moderate confidence overall. Tonight...Showers and thunderstorm chances will increase through this evening and overnight due to moisture associated with Hurricane Henri. The center of the storm will remain to our east overnight. However, rainfall associated with the storm will move across the area. Ceilings will lower to MVFR this evening and overnight, with IFR ceilings developing for many areas. Visibility restrictions are also likely due to periods of moderate to heavy rainfall. Winds will shift to the northeast this evening, then become northwest overnight with speeds around 5-10 knots, although some areas will become light and variable at times. Moderate confidence overall. Sunday...Periods of MVFR or IFR conditions will continue during the morning hours associated with rain from Hurricane Henri. Some improvement for southern areas is possible during the afternoon. Winds shift to west to northwest 5-10 knots, with stronger winds closer to the New Jersey coast 10-15 knots. Moderate confidence overall. Outlook... Sunday night...Conditions may begin to improve to VFR during the evening into the overnight hours for some areas. However, there remains a chance of showers overnight which could lead to lower conditions for some areas. West to southwest winds 5 to 10 kt. Moderate confidence. Monday-Monday night... Mainly VFR but still a chance for occasional restrictions due to rain showers especially during the daytime Monday. West-southwest winds 5 to 10 kt. Moderate confidence. Tuesday-Wednesday... Mainly VFR. Winds mostly west or southwest at 5 to 10 kt but light and variable at times. High confidence. && .MARINE... Through tonight... A Tropical Storm Warning is in effect for the coastal waters off Monmouth County. For the remaining Atlantic coastal waters of central and southern New Jersey and Delaware, a Small Craft Advisory is in effect. On Delaware Bay, conditions are expected to remain below SCA levels. Seas on the Atlantic coastal waters will build to 4 to 6 ft today, then 6 to 9 ft tonight, as Tropical Storm Henri moves closer. East-northeast winds around 10 kt today. Overnight, winds will gradually shift from northeast to northwest with speeds of 10 to 15 kt and gusts to 20 kt. Some higher winds are possible over the central and northern New Jersey waters by later in the night, especially over the waters off Monmouth County where the Tropical Storm Warning is in effect and tropical storm force winds are possible. Outlook... Sunday-Sunday night... SCA conditions will likely continue through the day Sunday. Seas will start to gradually decrease from south to north but remain above advisory levels. For the northern New Jersey Atlantic coastal waters, tropical storm force northwesterly winds are possible through the morning. Elsewhere, northwest winds gusting mainly 20 kt or less. By later Sunday and Sunday night, winds will shift to west-southwest with speeds falling to 10 to 15 kt. By the overnight hours, winds and seas should be below SCA levels on all the waters. Monday-Wednesday... No marine headlines are anticipated. Winds mainly southwest at 10 to 15 kt. Seas 2 to 4 ft. Rip Currents... Swells continue to increase on the coastal waters of New Jersey and Delaware as Henri continues its approach to southern New England. There is a high risk of dangerous, life-threatening rip currents through Sunday evening. Entering the surf zone is strongly discouraged this weekend. Conditions should improve early next week as the storm weakens and moves into the Gulf of Maine. && .HYDROLOGY... Heavy rain will be a concern through at least Sunday night due to the proximity of Henri and a mid level low. Many risk factors for heavy rain will exist in our region including precipitable water values well above normal, some slow storm motions (depending on the exact timing within this event), and a deep warm cloud layer. Additionally, portions of the region have wet antecedent conditions from recent heavy rains. As for the mode of flooding, flash flooding will be possible, especially in the prime period for heavy rain late tonight through the day on Sunday. However, there is also a risk, especially as we go later into the day on Sunday into Sunday night for slower progressing flooding as by that point any additional rain, even light rain could quickly become runoff if the ground is already saturated leading to additional flooding. && .TIDES/COASTAL FLOODING... 1145 am update: No significant changes to the forecast this morning, though recent model trends of a slightly more eastern track of Henri would suggest the "ceiling" of coastal flooding is lowering in our area. This increases confidence somewhat that mainly minor flooding is expected for the Atlantic coast of New Jersey and Delaware (more widespread for Ocean County northward; spottier farther south). For now, think that the current coastal flood advisories are well-placed and have made no changes. Previous discussion follows... The evening high tides associated with the full moon will be particularly high through the weekend. At this point the period of intersection of onshore flow (and thus positive surge values) associated with Henri and the high evening astronomical tides looks to occur this evening. Guidance remains in disagreement over the magnitude of the surge with realistic solutions ranging from a non-event to some locations along the northern NJ coast approaching moderate flooding. Consequently went with a coastal flood advisory this package for all the NJ coastal counties, apart from Atlantic (where advisory-lvl flooding looks unlikely) for this evening's high tide. Although this definitely isn't a slam dunk by any means it would only take some small departures to result in coastal flooding. Astro tides will remain elevated through the weekend but the flow direction will likely have switched to an offshore direction by Sunday, resulting in lowering water levels. For coastal Delaware, the Delaware Bay, tidal Delaware River, mainly sub-advisory flooding is expected this weekend, though cannot rule out more widespread minor flooding during the Saturday night high tide. For the eastern shore of Chesapeake Bay, no significant flooding is expected this weekend. Finally, surf will be approaching or exceeding 5 feet on the coast Saturday and Saturday night, especially in Monmouth and Ocean Counties. Some beach erosion is expected should swells reach these levels. A high surf advisory may be required if the forecast of Henri continues to trend westward. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Flood Watch through Monday morning for PAZ060>062-070-071- 101>106. NJ...Flood Watch through Monday morning for NJZ001-007>010-012>027. High Rip Current Risk through Sunday evening for NJZ014- 024>026. Coastal Flood Advisory from 7 PM this evening to 1 AM EDT Sunday for NJZ012>014-020-023-024-026. Tropical Storm Warning for NJZ012>014. DE...Flood Watch through Monday morning for DEZ001. High Rip Current Risk through Sunday evening for DEZ004. MD...None. MARINE...Tropical Storm Warning for ANZ450. Small Craft Advisory until 6 PM EDT Sunday for ANZ451>455. && $$ Synopsis...Carr Near Term...Fitzsimmons/O'Brien Short Term...Johnson Long Term...Carr Aviation...Carr/O'Brien/Robertson Marine...Carr/O'Brien Hydrology...WFO PHI Tides/Coastal Flooding...WFO PHI