Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 221504 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1104 AM EDT Sat Jul 22 2017 .SYNOPSIS...
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A very warm and humid air mass will remain over the area through this weekend and possibly into Monday. Meanwhile, low pressure along a roughly west to east frontal boundary will result in periods of showers and thunderstorms. By Tuesday, high pressure will build over the area bringing somewhat milder and drier conditions through through the middle of next week. A seasonally strong cold front may cross the area later in the week.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Concerns for today include temperatures/heat and then potential for severe weather. The forecast today has been updated to reflect temperature trends and the latest thinking on timing of convection moving through during the late afternoon. Hi-res guidance is beginning to converge on the most favorable timing generally after 3 pm from west to east across the area. The 00Z NAM Nest is hinting at the potential for multiple rounds of convection, with perhaps another convective complex this evening. This increases hydro concerns, particularly given the troubling high QPF output from the NAM Nest (and NAM), RAP, UKMET, and CMC. Don`t think this is flood watch material (yet), but with more convection expected later this weekend ... think the concerns will increase with time. Would not be surprised to see a few flooding issues this afternoon/tonight as well, though this may be more of the urban /poor-drainage variety, for the most part. Chance of severe storms today is certainly present. Forecast soundings show 2000+ MLCAPE and deep-layer shear of 30+ kts this afternoon, with effective shear approaching/exceeding 40 kts by early evening. With moist midlevels and a well-mixed boundary layer this afternoon, wet microburst potential is present, with cold-pool organization likely given degree of instability/shear. Shear looks to increase this evening, so the potential will be present for embedded mesovortices within any convective lines/clusters that develop. Think the risk of severe is high enough for inclusion in the grids at this point. Regarding temperatures, statistical guidance is generally in the 90-94 range for KPHL today, with dew points around 70 during max heating. With uncertainty regarding maximum heating from approaching cloud cover, there is simply not enough confidence to justify issuing another heat advisory or excessive heat warning. The best potential appears to be for the Delmarva Peninsula, but even here, forecast heat indices look to be around or just below yesterday`s values...which were shy of advisory criteria. Nevertheless, it will be uncomfortable again outside, and heat stress from prolonged outdoor exposure or strenuous activity is possible.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... Low-confidence forecast overnight with potential for a round or two of storms in a moderately unstable environment. With adequate shear for severe storms, mention of this potential remains through the early morning hours. However, with the first round of convection likely to move through during the early evening, there is some question as to what occurs thereafter. If another convective complex enters the region, threat of heavy rainfall (with some suggestion of this in most model output currently) will be elevated, especially north of the Mason-Dixon Line (as a weak surface boundary slowly lifts northward). However, there is a decent chance there will be too much stabilization with the first round of storms to support additional convection. Decreased PoPs after midnight but did not eliminate them anywhere given the uncertainty. With a slow-moving boundary, midlevel flow oriented parallel to it, very high PWs (2+ inches, generally), and potential for training/upwind propagation effects, locally excessive rainfall is possible. Too much uncertainty exists for a flood watch at this time, especially given the relatively dry antecedent conditions. Nevertheless, some local instances of poor- drainage/urban flooding are probable and isolated flash flooding is possible. Temperature forecast very uncertain given likely impacts from precipitation. Large errors in the hourly forecast should be anticipated if/when convection passes through, and low temperatures may be augmented by rain-cooled air. Currently have lows just below values observed this morning. && .LONG TERM /SUNDAY THROUGH FRIDAY/...
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This long term section has not had the opportunity to develop confidence on the forecast scenario. Was working the thunderstorms the that produced at least 1.5" of rain in parts of Cape May County and reports of gusts near 30 kt early this morning. 500MB: A -1SD short wave in the Great Lakes region Sunday morning crosses NYS and New England Monday with a short wave ridge to follow Tuesday-Wednesday before short waves carve out a -1sd trough over Quebec and New England next Thursday - Friday. Temperatures: The first 21 days of the month were averaging about a degree above normal near Mount Pocono to about 2.5 degrees above normal elsewhere except almost 5 degrees above normal vicinity Georgetown DE. Overall, expect an above normal month of July temperature wise, despite a cool down to near normal next week. Calendar day averages: Should be 3 to 7 degrees above normal Sunday and Monday then near normal Tuesday and Wednesday, several degrees above normal next Thursday and possibly Friday. Forecast basis: a 50 50 blend of the 00z/21 GFS/NAM MOS was applied Sunday-Monday, then 00z/21 GFS MEX MOS Monday night and Tuesday before relying heavily on the 05z/21 WPC D4-8 elements for Tuesday night- Friday. Exception: POPS were basically continuity from yesterdays 330PM Mount Holly forecast through day 5, then WPC POPS days 6-7. The dailies... Sunday...Considerable cloudiness. A quasi-stationary east-west frontal boundary is expected across our area. Low pressure along the front will favor low-level convergence and areas of showers and thunderstorms. PWAT values near 1.75 inches north and over 2 inches Delmarva with 2400J MLC just to our west will support some heavy downpours and possible flooding. Also SPC has a slight risk of severe for Sunday. Timing is tricky. Monday...Partly sunny, very warm and still a bit humid (dewpoints upper 60s to mid 70s) with 1100J of MLC. SVR potential. A cfp should generate scattered afternoon and evening thunderstorms? Tuesday and Wednesday...should be relatively pleasant with ridging aloft and surface high pressure moving across the area. The high is associated with a somewhat cooler and drier air mass. Partly to mostly sunny Tuesday with a light northeast wind and Partly to mostly sunny Wednesday with a southeast wind. Wednesday night...Looks like some low-level warm advection develops and the forecast carries a chance of showers at that time. South wind. Thursday...Partly sunny. Chance of rain increases on Thursday as another shortwave and associated cold front approach from the northwest. MLC increases to 2300J vicinity I95 and dewpoints are near 70. Small chc of svr...timing dependent. Southwest wind shifts to northwest at night. Friday...Uncertainty and followed WPC.
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&& .AVIATION /15Z SATURDAY THROUGH WEDNESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR conditions will continue through the rest of this morning and into the early afternoon. Clouds increase through the day, but CIGs should remain VFR outside convection. Storms expected to move east through the area late this afternoon into early evening, with timing generally between 21Z and 03Z. More storms are possible thereafter, but confidence is low. Additionally, sub-VFR CIGs/VSBYs may develop after storms move through and during the overnight, mainly from 08Z onward. There is potential for strong to severe storms today with gusty to damaging downburst winds and torrential rainfall. OUTLOOK... Sunday through Monday...Generally VFR conditions expected, but temporarily lower cigs/vsbys in scattered showers and thunderstorms are expected at times. Tuesday and Wednesday...VFR conditions with a light northeast wind Tuesday becoming and then southeast Wednesday.
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&& .MARINE...
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Storms will impact the southern New Jersey and Delaware coastal waters, as well as Delaware Bay, early this morning. Wind gusts around 30 kts or so can be expected with the strongest storms, as well as frequent lightning and torrential downpours. More storms are expected this afternoon and tonight, with a higher chance of severe weather. Strong winds are likely with the strongest storms. Keep a close eye on the weather today if venturing to sea. Outside of convection, winds should generally be under 15 kts but may be quite variable. By this afternoon, some favoring of a south direction is likely. After storms pass tonight, some preference for a west or northwest wind may be observed. Seas should be 2-4 feet through the period, with locally higher winds/seas near any thunderstorm. OUTLOOK... Generally sub-SCA conditions are expected Sunday through Wednesday. However winds and seas may be locally higher in scattered thunderstorms over the waters from Sunday through Monday. Small chance of 5 foot hazardous seas advisory Sunday evening. RIP CURRENTS... The 10-12 second dominant period has returned early this morning, and there are indications that this will continue through the day. As such, the rip current risk is expected to be in the moderate category today for the New Jersey and Delaware coasts.
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&& .TIDES/COASTAL FLOODING...
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We have decided to issue a Coastal Flood Advisory for this evening`s high tide along the New Jersey and Delaware coasts and along Delaware Bay. The surge at this morning`s high tide was about +0.4 to +0.6 feet. There will be an onshore component to the wind today and it should bring the surge up around +0.6 to +0.7 feet by evening. The astronomical tides with the upcoming new moon are quite high, as was the case with the new moon in both May and June. As a result, the somewhat unimpressive surge values will likely result in some minor flooding. If heavy rain occurs coincident with the high tide, the potential for flooding will increase. Conditions may be similar for Sunday evening`s high tide. An onshore flow is anticipated for Sunday, especially in areas from Atlantic City up to Sandy Hook.
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&& .EQUIPMENT... Dew point readings at KDOV continue to measure too high compared to surrounding locations and should be treated as unrepresentative of the area. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...Coastal Flood Advisory from 6 PM this evening to midnight EDT tonight for NJZ012>014-020>027. DE...Coastal Flood Advisory from 6 PM this evening to midnight EDT tonight for DEZ002>004. MD...None. MARINE...None.
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&& $$ Synopsis...Drag Near Term...CMS/Meola Short Term...CMS Long Term...Drag Aviation...CMS/Drag Marine...CMS/Drag/Meola Tides/Coastal Flooding...Iovino Equipment...Staff

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