Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 210941 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 541 AM EDT Mon Aug 21 2017 .SYNOPSIS...
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High pressure centered over the coastal Mid Atlantic early this morning will begin to move offshore today. The high will shift farther out to sea on Tuesday while a cold front approaches from the Midwest. The cold front is expected to move through the region early Wednesday and then stall nearby to our south Wednesday afternoon and evening. Canadian high pressure building into the region on Thursday will continue to influence our weather through the upcoming weekend.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/... All eyes on the sky today (but hopefully not directly at the sun). A colleague here said that this was the most important sky cover forecast of my career. So of course it would not be an easy one. There are three issues that complicate the forecast of clouds today. The first, and probably most important, is the developing cumulus field with diabatic heating. Model soundings and cross sections indicate increasing low-level moisture through the day and are strongly suggestive of at least scattered to possibly broken cumulus by afternoon, especially from the Delaware River westward. Forecasting the extent of this deck of clouds is a challenge (and unfortunately, rather low-skill, even in the short range). The latest high-resolution guidance points to a somewhat slower onset of these clouds, which is obviously good, but has also trended somewhat denser with the clouds (given somewhat higher moisture content in the 900-700 mb layer). In the larger-scale, a low-amplitude vort max will be progressing into Pennsylvania today (issue #2), and there will be attendant ascent just downstream. With smaller-scale lift associated with orographic effects and differential heating (via sky cover gradients, e.g.), convection is expected to develop to our west this afternoon. The timing of convective initiation looks to be early afternoon, but the placement should be comfortably west of the CWA (at least at onset). However, with large-scale ascent present, this may further aid the development of a thicker low-cloud deck, and the anvils from developing convection may advect rapidly eastward into eastern Pennsylvania. General thinking is that the greatest area of concern is in the Poconos and near Reading. There are also some convection-allowing models (CAMs) that develop a couple showers this morning in Delmarva, and residual clouds from any showers that develop here may be a hindrance to eclipse viewing. Upper-level moisture will also be available, and high cirrus may be result (issue #3). However, latest model progs suggest a rather thin layer would be present, and this may be dwindling with time (at least during the day) given a departing jet streak. With all of the above in mind, the current sky cover forecast is fairly optimistic east of the Delaware River (generally mostly sunny), somewhat more pessimistic from the Lehigh Valley west and northwest (increasing cloudiness this afternoon) and in between in Delmarva (partly to mostly sunny). Enjoy the show. Other big concern today is storm chances. The environment is decently favorable for organized storms capable of isolated strong to severe wind gusts and hail. MUCAPE will approach/exceed 2000 J/kg this afternoon in the far western CWA, and effective shear will be increasing through the day as the aforementioned vort max approaches. CAPE-shear parameter space will become favorable for severe storms this afternoon and evening, and CAMs are suggestive of scattered, semi-discrete storms developing in the higher terrain of central Pennsylvania moving eastward into the area late this afternoon. Included mention of gusty winds, hail, and heavy rain with storms generally west of Philadelphia during this period. With scattered coverage and questions regarding timing, have no more than chance PoPs in the grids at the moment. Forecast temps are a blend of GFS/ECMWF/NAM MOS with a bit of continuity and CAM-based 2-m temperatures added to the blend. It should feel noticeably more uncomfortable today with increasing surface moisture. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/... Main forecast concerns tonight are precipitation chances and fog/stratus development. CAMs are not overly optimistic in maintaining convection after sunset, and this makes sense given little in the way of surface support for smaller-scale lift. Nevertheless, the presence of a passing vort max leaves me concerned, especially since these have been poorly simulated in general this summer. Notably, another vort max should be moving through northern Pennsylvania late tonight, and may provide glancing ascent to the northern portions of the CWA during this time. Global/coarser models are keeping precipitation chances around for much of the night in the area, with CAMs generally showing little to nothing after 03Z. With such disparity, I find it difficult to remove PoPs entirely or to elevate them beyond slight to low-chance during the period. Notably, MET MOS PoPs went way up with the 00Z package, and I have a hard time trending the opposite direction of statistical guidance. Meanwhile, some guidance (notably, the NAM, as per usual) develops low clouds and/or fog in portions of the area tonight. I am somewhat skeptical, given that a shortwave trough will be moving through during the evening/early overnight hours. With attendant lift associated with this and the aforementioned upstream vort max, cloud cover may be too prevalent to get more than patchy fog in the usual valley/rural spots. The chance for low stratus seems more pronounced as warm/moist advection will be in full swing during the overnight hours (downstream of a much more potent vort max moving into the Great Lakes region). Combined with uncertainty regarding precipitation forecast, only mentioned fog in the Lehigh Valley/Poconos/NW NJ region for now, though patchy fog certainly cannot be ruled out elsewhere. Low temps were a combination of MOS guidance and should be a few degrees above seasonal averages across the region. It will be muggy thanks to increased dew points. && .LONG TERM /TUESDAY THROUGH SUNDAY/... On Tuesday, a longwave trough associated with an upper low near James Bay is forecast to amplify over the Great Lakes region as shortwave energy digs around the backside of the trough. The surface low will deepen in response to increasing height falls as it tracks northeastward across Ontario and Quebec. The attendant cold front will progress eastward through the Midwest. Looking farther downstream and closer to our area, models show development of a pre- frontal/lee trough over central PA and Potomac Highlands during the afternoon hours. Tuesday will clearly be the hottest of the next 7 days with forecast highs in lower 90s (except cooler 80s near the coast and at higher elevations in NE PA/NW NJ). The humidity will also be a factor with dewpoints reaching the mid 70s across Delmarva and southern NJ and lower 70s just about everywhere else. Peak heat indices in the upper 90s to near 100F are likely for the urban I-95 corridor and points south/east. With the expectation that focused lift will remain to our west near the pre-frontal trough on Tuesday, convection over the forecast area will likely be disorganized and generally isolated in coverage (perhaps widely scattered across NE PA where the terrain could locally enhance lift) during the afternoon. Slightly better chances for showers and storms arrive Tuesday evening as convection along this trough propagates eastward into our area. PoPs are highest in our far western zones (west of the Delaware Valley and near the eastern shore of MD). Showers and storm are then expected to move eastward into the Delaware Valley and coastal plain at night as the cold front catches up with the pre-frontal trough and moves into the area. Even though the environment should be supportive of showers and storms continuing into the night owing to the arrival of deep synoptic lift, PoPs decrease slightly with eastward extent due to some uncertainty concerning the intensity and coverage of convection later with the nocturnal boundary layer tending to stabilize further into the night. The approach of a low- and mid-level jet ahead of the cold front will yield increasing shear profiles (0-6 km bulk shear 35-45 kt) by evening. Additionally, weak to moderate amount of instability (MLCAPE 1500 to 2000+ J/KG) is forecast to be available to fuel convective updrafts. Some storms could be strong to severe, primarily during the afternoon and evening hours, with this setup. The latest D2 Convective Outlook from SPC includes the southern Poconos in a slight risk for severe storms with a marginal risk extending southeastward into the I-95 corridor. The latest 00Z models have trended faster with the cold front as it moves through eastern PA and NJ early Wednesday morning. The front will then likely slow down when it reaches Delmarva and perhaps eventually stall just south of the area across southern VA Wednesday afternoon. Accordingly, it is looking more probable that the majority of the forecast area dries out on Wednesday, especially north of the Mason-Dixon line. While PoPs were also lowered farther south, the close proximity to the cold front means that there is still a chance that showers and storms linger into the day. Additionally, some of the models (most prominently, the 00Z NAM) develop a wave of low pressure along the stationary front in southeastern VA. More organized convection could shift back northward toward Delmarva and possible far southern NJ by evening. High pressure over Canada and the Great Lakes region then starts to build southeastward toward the area on Thursday. This expansive high will remain in control through the weekend. The end result will be an extended stretch of pleasant weather Thursday through Sunday. Highs in the upper 70s/lower 80s and and lows ranging from the mid 50s to lower 60s are forecast each day and night, respectively. While the forecast remains dry for these four days, there looks to be increasing cloud cover heading into the weekend and potentially a risk for an isolated shower with 1) the approach of the upper trough and 2) light onshore flow helping to moisten low-levels over time. && .AVIATION /10Z MONDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Kept TEMPO fog for KRDG, KABE, and KMIV based on latest trends, with brief sub-VFR conditions possible, especially at KRDG/KMIV, through 12Z. Cumulus field will develop during the day, with bases generally in the 4-6 kft range. Retained PROB30 -TSRA to KRDG/KABE given somewhat increased confidence that convection will move into eastern PA late this afternoon into the evening hours. Generally increasing cloud cover from this afternoon through tonight, and latest guidance shows at least some potential for sub-VFR CIGs/VSBYs Monday night. Winds generally light/variable through 15Z, then becoming mostly southerly 5-10 kts. Outlook... Tuesday...Early morning fog should dissipate by mid morning, leading to VFR for most of the day. Showers and storms develop during the afternoon but current thinking is coverage should initially be isolated and mainly confined to western terminals (ABE/RDG). Tuesday night...Potential for MVFR or IFR restrictions with slightly higher chances for showers and storms. This activity should progress to the east-southeast through the night. Wednesday...Scattered showers and isolated thunderstorms mainly for terminals S/E of PHL and earlier in the day. May start off MVFR with lower CIGs but improvement to VFR is likely from NW to SE during the day. A wind shift from SW to NW can be expected early in the day w/ fropa. Wednesday night through Friday...VFR and light winds. && .MARINE...
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Generally tranquil conditions on the waters today, though southerly winds should pick up this afternoon/evening. Speeds should attain 5- 15 kts with gusts to 20 kts or so, especially off the New Jersey coast. Seas will generally range from 2 to 3 feet. There may be some patchy fog early this morning, but this should dissipate rapidly after sunrise. Outlook... Tuesday...SCA was issued for the Atlantic coastal waters of NJ and DE for late Tuesday afternoon and night. S-SW winds are expected to strengthen late in the day to 15-25 kt. Isolated gusts to 30 kt are possible mainly during the evening when the winds will be strongest. Seas will quickly build in response to the wind field to 4-6 ft. Wednesday...A wind shift from SW to W-NW should occur on Wednesday behind a cold front. Winds will also decrease as well. There is still a possibility that the SCA may need to be extended into Wednesday morning if seas take a bit longer to subside below 5 ft. Thursday and Friday...Winds and seas below SCA criteria. RIP CURRENTS... The rip current risk has been upgraded to moderate for today, mainly from late this afternoon through this evening, when a longer-period swell develops (around 9-10 seconds), seas begin to increase, and southerly winds increase to around 15 kts with higher gusts. The combination of the above factors and the presence of a new moon suggests the rip current risk will be increasing through the day. As such, felt best to raise the risk to moderate.
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&& .TIDES/COASTAL FLOODING... Based on observations yesterday evening and current projections for the high tide this evening, will hold off on issuing a coastal flood advisory this morning. Current model projections continue to indicate levels reaching near minor flood thresholds but consistently below advisory thresholds. Spotty minor flooding is probable this evening, but confidence is too low at this point to issue an advisory. Nonetheless, there is some concern that a more onshore component of the wind (south to possibly south-southeast) may contribute to somewhat higher levels than guidance suggests this evening. Should this be observed during the day, the threat for minor flooding would increase. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 3 PM Tuesday to 6 AM EDT Wednesday for ANZ450>455. && $$ Synopsis...Klein Near Term...CMS Short Term...CMS Long Term...Klein Aviation...CMS/Klein Marine...CMS/Klein Tides/Coastal Flooding...CMS/Klein is the U.S. government's official web portal to all federal, state and local government web resources and services.