Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 161313 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 913 AM EDT Tue Jul 16 2019 .SYNOPSIS... High pressure over the area continues to move offshore this morning, and then a warm front will lift through the region this afternoon. The remnants of Barry will pass through the region late Wednesday through Thursday. Bermuda high pressure becomes entrenched off the Southeast coast as several weak cold fronts pass through the East Coast into this weekend. A stronger cold front approaches early in the new week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 910 AM Update...Based on latest obs and trends, made some slight adjustments to T,Td grids for this morning but otherwise no changes at this time. Previous Discussion...Surface high pressure currently just off the VA Coast will continue to move away from the area today with increasing southwesterly low- lvl flow behind it. At mid-lvls a low- amplitude shortwave ridge will initially be centered upstream of the area, but will be pushed over and then eventually east of the area today/tonight in response to the mid-lvl trough associated with Barry`s remnants. A weak warm front associated with a low passing well to the north of the area (over northern Quebec) will move across the area through the day. Deep moisture will increase considerably today with current PWAT values near 1 inch increasing to around 2 inches by day`s end (with afternoon dewpoints running about 10 degrees higher than yesterday). Temperatures will also warm a few degrees (with highs generally in the lower 90s outside of the Poconos and immediate coast (will a sea breeze likely developing). Unfortunately in terms of heat index today will actually likely be the most "pleasant" day of the upcoming week or so. Some diurnal showers/thunderstorms will likely develop towards the middle of this afternoon over the higher terrain west of the area and these will likely drift eastward towards the area. These storms will be fairly weakly-forced given that the best dynamic lift (right entrance region of upper jet, ejecting energy ahead of Barry`s remnants) will remain to our north and west. Consequently generally went with Slight Chc to low end Chc PoPs focused on our western areas. Can`t rule out a microburst or two given inverted-V profiles and but think storm coverage and still modest instability will be limiting factors for any more notable threat. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... The threat for showers and thunderstorms over primarily the western half of the area will continue into the evening. Instability remains sufficiently elevated to support thunder for a few hours after sunset but still expect limited coverage due to lack of forcing (am discounting the NAM-nest at this point which seems to be over- convecting). Overnight lows will be considerably warmer than Monday morning, thanks to increased dewpoints and mid-lvl cloud cover, with lows generally remaining in the 70s. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... An Excessive Heat Watch is now in effect from Wednesday through Sunday. A tropical airmass spreads into the Northeast and Mid-Atlantic ahead of the remnant low of Barry on Wednesday. Bermuda high pressure will be anchored offshore, and a hot and humid airmass begins to spread into the region. Surface dewpoints will be in the mid 70s or so, along with highs in the low to mid 90s. This results in Heat Index values generally 100 to 105 for all but the southern Poconos. With the airmass becoming quite unstable due to the heat and humidity, and PWATs increasing to over 2 inches, showers and thunderstorms will become likely as weak low pressure associated with the remnants of Barry passes through northern zones. Models are showing several bullseyes of high QPF, but are not consistent among the models as to where the bullseyes line up. A max of 2-3 inches of rain is possible Wednesday afternoon and Wednesday night, but right now confidence is low as to where. Flooding is possible. Thursday will remain humid with dewpoints well in the 70s, but temperatures may not be quite as high due to abundant cloud cover and rainfall associated with the remnants of Barry passing through. Highs will generally be in the 80s to around 90 in Philadelphia. Thursday will feature the lowest Heat Indices of the stretch, but will still be around 100 in Philadelphia, and from 100 to 105 in Delmarva. Another round of moderate to heavy rain is possible with the passage of the low on Thursday, with another round of 1-2 inches of rain and flooding are possible. From there, a prolonged period of excessively hot and humid air will be in place from at least Friday through Sunday. Highs will be in the low to mid 90s Friday, then mid to upper 90s on Saturday and Sunday. Surface dewpoints will be in the 70s, however, there are some indications that dewpoints may mix out in the afternoon hours. This results in dewpoints generally in the mid to upper 70s at night and in the morning, and in the low to mid 70s during the peak heating of the day. In terms of the heat index, the max heat index values look to be up to 105 on Friday and Sunday, and as high as 110 to 115 on Saturday. Due to high confidence in this occurring, and the dangerous impacts of a prolonged heat wave, will go ahead and issue a rare prolonged Excessive Heat Watch that starts on Wednesday and continues through Sunday for the Philadelphia area. For areas outside of the urban corridor, confidence is no less in this occurring, but there is still some time, as values do not look to reach criteria until Friday. Criteria for Excessive Heat Watches and Warnings are a bit more stringent for the urban corridor due to the cumulative effects of a heat wave on an urban center. It is important to note that overnight lows in the Watch area will be in the 70s to around 80. So there will not be much, if any, relief from the oppressive heat at night. Some weak mid-level shortwave energy may spark off some afternoon convection Friday through Sunday, but nothing widespread or organized. Will cap PoPs at slight chance. If convection does fire up, it may briefly result in some relief from the high temperatures, but the rainfall will also result in locally higher humidity values. A cold front approaches on Monday, and this should bring convection into the region along with some relief from the heat and humidity for the start of the work week. && .AVIATION /13Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Generally VFR with SW winds around 5-10 kts. Showers and Thunderstorms will be possible late in the afternoon around RDG and ABE. A sea breeze will be possible at the coastal sites Tonight...VFR with lingering showers and thunderstorms possible early. Winds generally 5 knots or less and favoring a southerly direction (if not variable). Outlook... Wednesday...Mostly VFR conditions expected. MVFR or lower in late afternoon/early evening SHRA/TSRA. South to southwest winds around 5 to 10 knots. Thursday...MVFR or lower in SHRA/TSRA. South to southwest winds around 5 to 10 knots. Friday through Saturday...Mainly VFR conditions expected. MVFR or lower possible in isolated SHRA/TSRA in the afternoon and early evening. West to southwest winds around 5 to 10 knots. Excessive heat index values expected. && .MARINE... Sub-SCA conditions are expected today/tonight with seas around 2-3 feet and SW winds 10-15 kts gusting to 20 kts or less. Outlook... Wednesday through Thursday...Generally sub-SCA conditions, however, showers and thunderstorms will result in dangerous conditions on the waters. Friday through Saturday...Sub-SCA conditions. Rip Currents... Due to seas around 3 feet, a somewhat long wave period of 9 seconds, and southerly winds 10-15 kts there will be a moderate risk of rip currents today over the northern NJ beaches. Over the southern NJ and DE beaches waves and winds will be a bit less so there is a low risk of rip currents forecast at this time. However the risk could be higher if winds and/or waves materialize stronger than what is currently forecast. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Excessive Heat Watch from Wednesday morning through Sunday evening for PAZ070-071-102-104-106. NJ...Excessive Heat Watch from Wednesday morning through Sunday evening for NJZ015-017>019. DE...Excessive Heat Watch from Wednesday morning through Sunday evening for DEZ001. MD...None. MARINE...None. && $$ Synopsis...MPS Near Term...Carr/Fitzsimmons Short Term...Carr Long Term...MPS Aviation...Carr/MPS Marine...Carr/MPS is the U.S. government's official web portal to all federal, state and local government web resources and services.