Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 020748 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 348 AM EDT Tue Jun 2 2020 .SYNOPSIS... A warm front is expected to lift through the region later today. It will be followed by a cold front approaching our region tomorrow as low pressure moves out of the Great Lakes. That front is expected to stall as a stationary front and remain over the Mid Atlantic region through the end of the week. Another cold front will then approach Friday or Saturday as another area of low pressure moves out of the Great Lakes. High pressure should finally start to build back in from the west by later in the weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... One last comfortable day with seasonable temperatures and reasonable humidity levels across the area. An approaching warm front will be close enough to bring occasional high and mid level clouds to the area today. A couple showers are possible for the wrn areas this afternoon (mostly) but only Berks County will have any pops higher than slight chc. High temperatures will be mostly in the low to mid 70s for most areas with some 60s up on the higher elevations across the far NW areas. Winds will turn SW and be 5 to 10 mph this morning and 10 to 15 mph this afternoon. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... Clouds will continue to increase and lower across the area tonight as a warm front approaches from the west. Scattered showers are expected later tonight, especially across the northern areas where chc pops are in the fcst. We have included a slight chc for thunder across the N areas towards dawn as upper CAPE increases. It will be a mild night as the humidity levels become more noticeable. Lows will only drop into the mid/upper 60s for Delmarva and srn NJ while up north lows will be in the upper 50s to low 60s. Winds will remain mostly from the S at 5 to 10 mph. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Overview: The focus in the long term period remains on the front which is expected to approach our region tomorrow as a cold front before stalling over the Mid Atlantic through the remainder of the week. As a result, expect several rounds of showers and storms from Wednesday through Saturday. Tranquil weather conditions should return at the beginning of next week as high pressure builds south. Details: Wednesday...On Wednesday, the primary focus is on the potential for severe storms, specifically with damaging winds as the primary hazard. There could be two main rounds of showers and storms on Wednesday. The first could come Wednesday morning/early afternoon as a short wave trough digs southeast and off shore. This will be well ahead of the main front. As mentioned by the previous shift, not all models depict this round developing. However, if storms do develop this early, it would primarily affect E Central PA and northern NJ, and should serve to decrease the instability those areas ahead of the arrival of the front, thus limiting the severe storm risk in these areas. The second round ahead of and with the arrival of the cold front is expected late Wednesday afternoon and Wednesday evening. For areas that don`t see earlier convection, staying in the warm sector most, if not all, day will allow temperatures to be in the upper 80s or even lower 90s by the time we see storms propagating into the region. Consequently, we should see surface based instability. It is interesting to note though that many of the operational models depicted a much more robust inversion between 800 and 700 mb (as compared to model soundings from yesterday`s runs). Consequently, instability profiles are skinnier, and most models are now depicting CAPE values in the 1000 - 1500 J/Kg range - still more than sufficient for strong updrafts but a far cry from the 3000 J/kg that the NAM was depicting at one point. 0-6 km bulk shear values near 40 kt should promote sustained updrafts. As for hazard types, as mentioned above the primary concern is damaging winds as 50kt + winds from 700 or even 850 mb may be able to mix down with a strong enough updraft. With a rather skinny instability profile withing the hail growth region and the melting layer above 10K ft AGL, the hail threat should be limited, though not completely gone. As for the tornado threat, model hodographs depict nearly unidirectional flow through the tropopause until well after 00Z, which would significantly limit the threat. Having said that, will have to watch how quickly the front sinks down as some directional shear will be possible right within the vicinity of the front. Thursday...By Thursday, attention shifts from severe winds to heavy rain potential. While the best synoptic scale lift will likely be west of our region, mesoscale lift along the stalled front could be enough for convective initiation. The big question then becomes where that front will be. At this point, it looks like it will be somewhere south of the PA turnpike/195 corridor. While the best lift will be west of us, and model QPF values aren`t especially impressive, there are a few big factors which could increase the heavy rain threat. First, mean flow looks like it will be nearly parallel to the stalled front, increasing the risk for training storms. Secondly, expect a deep warm cloud layer (as deep as 12K ft) by late Thursday. Precipitable water values also look to be well above normal. At this point, the highest risk for heavy rain in our area should be over northern Delmarva, where flash flood guidance is higher. However, if we get training storms just 30 to 50 miles further north, it could affect portions of the urban corridor where it will take much less heavy rain to cause flash flooding. Friday and Saturday...A few more rounds of showers and storms are possible Friday and Saturday, though there is considerably more uncertainty through this period. The stalled front weakens as frontolysis takes over, and the highest potential for additional showers look to come with a mid level short wave trough on Friday, followed by another cold front approaching the region on Saturday. Sunday and Monday...High pressure should build southward over our region leading to a return to tranquil, albeit cooler, conditions. && .AVIATION /07Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR conditions are expected through the day with slowly increasing clouds. A couple showers NW are possible this afternoon, but not enough of a chc to include in the TAFS attm. Winds will be light from the W this morning and then become SW at 8 to 12 knots this afternoon. Moderate confid. Tonight...VFR conditions are expected to continue as clouds bases continue to lower. Some OP models are showing some showers arriving across the N areas after midnight, so some lower conditions are possible there. The rains seem to stay north/east of KABE/KTTN so we will not include any lower CIGS or VSBYS in the present tafs. Winds will be S to SW around 10 knots. Moderate confid. Outlook... Wednesday...Prevailing VFR, though a period of MVFR or even IFR conditions will be possible primarily late Wednesday into Wednesday night as showers and storms cross the region. Variable and gusty winds will be possible with the strongest storms. Outside of the storms, winds should be westerly around 10 to 15 kt becoming light northwesterly overnight. Moderate confidence in the overall pattern, but low confidence in the timing and extent of storms. Thursday...Prevailing VFR, with a period of MVFR/IFR Thursday afternoon and evening with showers and storms. Visibility restrictions in heavy rain will be possible. Light northwesterly winds becoming light southwesterly in the afternoon then light and variable overnight. Low confidence. Friday and Saturday... Mostly VFR, but additional rounds of showers and thunderstorms are possible especially Friday, which could result in brief MVFR conditions. Light southwesterly winds Friday becoming light northwesterly on Saturday. Moderate confidence. && .MARINE... High pressure will continue to move away from the waters while a warm front approaches from the west. Sub-SCA conditions are expected today and tonight. Winds will slowly increase tonight though with some gusts around 20 knots expected by dawn. Overall, fair weather is expected today and much of tonight with only a small chc for a shower tonight. Outlook... Wednesday through Saturday...winds and seas should stay mostly below SCA criteria through the period with the following exceptions: First, on the coastal waters, wave heights could approach 5 feet especially Wednesday into Thursday. Secondly, several rounds of showers and storms will be possible through this period, locally higher winds and waves will be possible in the vicinity of stronger storms, especially on Wednesday. Rip currents... A LOW risk for rip currents is forecast for today and Wednesday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Johnson Near Term...O`Hara Short Term...O`Hara Long Term...Johnson Aviation...Johnson/O`Hara Marine...Johnson/O`Hara is the U.S. government's official web portal to all federal, state and local government web resources and services.