Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 161324 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 924 AM EDT Sun Jun 16 2019 .SYNOPSIS... High pressure will continue moving farther offshore today as a cold front approaches from the north. This front will stall across the northern Mid-Atlantic and stick around through much of the upcoming work week. Several waves of low pressure are expected to develop along this boundary and pass across the region leading to several rounds of showers and thunderstorms through late this week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 930 am update: the earlier showers have mostly dissipated over our region, and areas from the western suburbs of Philly to Central and southern NJ to much of Delmarva have mostly clear skies with the exception of high cirrus clouds. I adjusted the sky and PoPs through the remainder of the morning to match these trends. That being said, the bigger concern is what implication this may have for the afternoon/evening convection. I am concerned that the earlier clearing will expand the risk of severe storms further north. However, extensive clouds are just west of the region and appear to be propagating east. Further, despite the clearing, temperatures are still very close to the forecast and most short term guidance. Thus, I didn`t make any significant changes to the forecast this afternoon and evening at this time. Previous discussion... Several weak perturbations are moving into/through the Northeast early this morning. One is moving through NY/PA at this time, with an area of light rain/showers generally affecting areas north of I- 78 early this morning. In general, models are handling the southern extent of the precipitation poorly, and made earlier adjustments to the forecast to increase PoPs southward to the urban corridor. Precipitation amounts with this first batch of precipitation should be a tenth of an inch or less (highest amounts in northern New Jersey). A second/stronger convectively-maintained perturbation is in Ohio, with heavier and more extensive precipitation near/downstream of it. This precipitation has moved into northern/central Pennsylvania, perhaps a little south of where most convection-allowing models had placed it, with the WRF simulations seemingly handling it best and the HRRR handling it worst. The track of this perturbation will be east-northeast, which should allow the heaviest precipitation to move just north of the area this morning. However, expect some precipitation northwest of the Fall Line at least and possibly a little farther southeast given radar trends. As a result, increased PoPs this morning for about the northwest half of the CWA and trended it a little faster given what has transpired so far overnight. A third perturbation arrives late today, and this is the one that should produce stronger convection. There are two main forecast questions associated with this perturbation. Where will it track, and how will the downstream environment destabilize? Regarding the first question, the 00z NAM tracks it along the Mason-Dixon Line. The 00z GFS tracks it across northern Virginia eastward to Delmarva. The 00z ECMWF is much farther north (generally through PA/NJ). These differences are critical because convection will likely develop in close proximity to this vort max. Hi-res models are similarly variable, with HRRR runs developing strong convection anywhere from eastern PA/central NJ to central/southern MD eastward through Delmarva. Additionally, extent of destabilization is questionable given the showers occurring in the northern half of the area this morning. This will undoubtedly mitigate diabatic heating to some degree today, but it is unclear how far to the south this will occur. My suspicion is that it is a little farther south than consensus given this morning`s trends, but this will ultimately depend on the precipitation moving into/through Pennsylvania this morning. Where the environment remains relatively unperturbed by the predecessor precipitation/clouds, BUFKIT soundings show MLCAPE approaching 1000- 1500 J/kg this afternoon in an adequately sheared (30-35 kts deep- layer bulk wind difference) environment. Low-level helicity will also improve through the day, with hodographs exhibiting modest curvature (0-3 km SRH approaching 150 J/kg by evening). Where convection develops, mixed convective modes may occur initially with all types of severe possible, though convection-allowing guidance is tending to develop a mesoscale convective system rather quickly. Timing of the stronger storms looks to be after 3 pm and may peak during the evening hours based on the latest HRRR/NAM Nest simulations. Currently, think the best chances for severe storms this afternoon are generally south of a Reading to Trenton to Asbury Park corridor, and have included enhanced wording in the public forecasts with this update. Heavy rainfall is certainly possible with stronger storms, but today`s convection looks fairly progressive as well, so not expecting a widespread flooding threat. Used a consensus blend for the temperature forecast today, given the uncertainty with cloud cover/precipitation. Generally, this lowered temperatures about 1-2 degrees from the previous forecast. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... 615 am update: Convection-allowing models have shown less and less of the convective regeneration during the early overnight hours discussed below, so this is becoming a lower-probability solution, as suspected. However, there are indications that if a mesoscale convective system moves through the area this evening, there may be some training of convection on both the north and south sides. This will require watching as the event unfolds to determine locations at greatest risk for heavy rainfall and any potential flooding. Previous discussion... Main forecast concern tonight is the ultimate evolution of strong convection developing this afternoon/evening... Generally, models (coarser and hi-res) are indicating storms congealing into a mesoscale convective system that moves through portions of the CWA during the evening before progressing offshore overnight. Where this occurs is a big question mark (see the near- term forecast discussion), but another concern is the tendency for some models to regenerate precipitation upstream of the primary MCS overnight. The NAM Nest and WRF simulations are both depicting a second round of convection developing near and somewhat north of where the first round of storms moves through. This appears to be tied to low-level warm advection downstream of a fourth (yes, fourth) perturbation approaching the area late. This prompted me to keep PoPs rather high through about 06z before trending them to slight or low-chance values thereafter. Should this second round of storms occur, heavy rainfall becomes a bigger concern. I am not biting on this potential yet, since models are not exactly agreeable on the first round of convection (much less the second). However, will need to monitor convection-allowing model trends closely today to determine if this is a plausible threat. Lows tonight should be in the 60s to around 70. && .LONG TERM /MONDAY THROUGH SATURDAY/... Overview: Generally and unsettled pattern this week and into next weekend with a decent westerly flow aloft between a persistent low/trough across eastern Canada and high pressure across the Gulf of mexico and the SE states. Disturbances from the Plains will continue east across the Ohio valley and then across the Middle Atlantic. This will bring rounds of showers and tstms with the possibility of both heavy rains and some severe weather also. The timing of the systems vary from model to model and from run to run, so we will update each day with the latest pop/qpf trends and not get too specific at the greater time ranges, since the models struggle to remain consistent at these time ranges. Precipitation: Overnight, we have reduced POPS for the first part of Monday and increased then for later Monday and Monday night. This is shaping up to be a period of excessive rains across some part of our CWA. The NAM/GFS seem to be favoring the Delmarva region while the latest EC and CMC show the greater totals across PA and into nrn NJ. The SPC also placed parts of the CWA in a slight risk for severe weather for Monday also. Another period for possible showers/tstms is Tue night into Wed morning, so we kept the high chc/low likely pops for this period. Beyond that, few changes were made, except to improve the look of t he maps between the offices. Mostly chc pops with the wavy front still nearby. Temperatures: Generally at or slightly above normal thru the period. Monday will probably be the warmest day of the week, but right now the differences between the days is only 1 or 2 degrees. Highs mostly in the low/mid 80s for most areas. A trend towards higher readings will begin towards next weekend. Overnight lows will be generally above normal with frequent clouds expected. && .AVIATION /13Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Generally VFR with at least a slight chance of showers this morning near/north of the Philly terminals (with negligible impacts expected). However, chances for stronger storms (and sub-VFR conditions as well as strong/erratic winds) increase this afternoon, though location/timing is a very low-confidence forecast at this point. Think the threat is high enough for thunder mention at the terminals late this afternoon, with fine-tuning of the TAFs to be expected once the convective evolution becomes clearer. Winds generally southwesterly 7 to 15 kts with stronger gusts in the afternoon. Tonight...Chances for storms continue during the evening hours but are expected to diminish after midnight. However, there is a chance storms could linger for a while longer, especially at ILG/MIV/ACY. Generally VFR outside of storms. Southwest winds near/below 10 kts. Low confidence. Outlook... Monday/Monday night...VFR during the morning, then showers and tstms with lower CIGS/VSBYS overnight. Tue...Early showers/tstms then improving conditions. Wed/Thu...Mostly VFR but sct tstms with lower conditions at times. && .MARINE... Seas will remain near/above 5 feet today on the Atlantic waters, with southwest winds likely gusting to 20 to 25 kts this afternoon. Models have trended a little longer with elevated seas, so have extended the advisory for the New Jersey waters through 10 pm. The advisory for the Delaware Atlantic waters continues until 6 pm. Sub- advisory conditions are expected on Delaware Bay through tonight. A chance of storms exists on the waters this afternoon and tonight. Some of the storms could be strong/severe, with erratic/strong wind gusts and locally higher waves possible in their proximity. Outlook... Mostly sub-SCA conditions. The main hazard will be sct tstms with higher winds and seas. Mon night and Tue night/Wed. are periods that have better chc for tstms. Rip Currents... A moderate risk for the formation of dangerous rip currents exists today, as south to southwest winds increase to around 20 kts with higher gusts and seas remain elevated (3 to 6 feet). && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 10 PM EDT this evening for ANZ450>453. Small Craft Advisory until 6 PM EDT this evening for ANZ454- 455. && $$ Synopsis...Staarmann Near Term...CMS/Johnson Short Term...CMS Long Term...O`Hara Aviation...CMS/O`Hara Marine...CMS/O`Hara is the U.S. government's official web portal to all federal, state and local government web resources and services.