Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 220734 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 334 AM EDT Sat Jul 22 2017 .SYNOPSIS... 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. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Concerns for today include temperatures/heat and then potential for severe weather. First, storms continue to develop/progress through central/southern Delaware, Delaware Bay, and far southeast New Jersey early this morning in an environment characterized by MUCAPEs > 2000 J/kg, PWs near or above 2 inches, and substantial isentropic lift in the low levels downstream of a weak perturbation in the southern Mid-Atlantic. Rain rates have been impressive with the strongest storms (1-3 inches per hour), with totals exceeding 0.5-1.0 inch(es) in portions of central Delaware. Storms are not moving very quickly, so accumulations greater than an inch are likely in southeastern Cumberland and Cape May Counties during the next hour or so. Storms should move offshore after 6 am or so. Wind gusts, so far, have reached 30-35 mph with the strongest cores, and this should likely be the case for the rest of the night. Meanwhile, the forecast today has been updated to reflect 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/...
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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.
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&& .LONG TERM /SUNDAY THROUGH FRIDAY/...
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Extended forecast delayed owing to convection that has developed south of the Mason-Dixon Line. Forecast/discussion hopefully updated by around 430 am. The extended period starting Saturday night features a transition in the flow aloft from roughly zonal flow across the northern US to a somewhat more amplified pattern with troughing over the east US by the latter part of next week. In terms of weather this translates to very warm/hot and humid conditions this weekend transitioning to a milder and somewhat drier period for at least part of next week. A quasi-stationary east-west frontal boundary is expected to persist across the area through the weekend, although it may lift north a bit on Sunday. Low pressure along the front will favor low-level convergence and areas of showers and thunderstorms. PWAT values near 2 inches will support some heavy downpours and possible flooding. Also SPC has a slight risk of severe for Saturday and Sunday. Timing is tricky but there is some indication of more likely precip for Saturday night and possibly again Sunday night. By Monday morning a fairly pronounced shortwave trof is forecast to be over the Great Lakes and move east to New England by Tuesday morning. The GFS is a bit slower than the ECMWF, but either way the trof should force a cold front across the mid- Atlantic and offshore by Tuesday morning. With the frontal passage on Monday there will be a continued good chance for aftn/eve t-storms. Max temps on Monday ahead of the front look to be still a few degrees above normal. Tuesday and Wednesday should be relatively pleasant with shrtwv ridging aloft and surface high pressure moving across the area. The high is associated with a somewhat cooler and drier air mass. Looks like some low-level warm advection develops Wednesday night and the forecast carries a chance of showers at that time. Chance of precip increases on Thursday as another shrtwv trof and associated cold front approach from the NW.
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&& .AVIATION /07Z SATURDAY THROUGH WEDNESDAY/... For the 06Z TAFs...VFR through the morning, except for potential/temporary visibility restrictions at KMIV/KACY through 12Z. 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, though confidence is too low at this point for TAF mention. Expect frequent updates to the TAF today as details become clearer. There is potential for strong to severe storms today with gusty to damaging downburst winds and torrential rainfall. OUTLOOK... Saturday Night through Monday...Generally VFR conditions expected, but temporarily lower cigs/vsbys in scattered showers and thunderstorms are likely at times. Tuesday and Wednesday...VFR conditions. && .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. 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, especially for the New Jersey coastal waters. The risk remains lower for the Delaware coastal waters.
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&& .EQUIPMENT...
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Dew point readings at KDOV continue to measure too high compared to surrounding locations and should be treated as unrepresentative of the area.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...AMC Near Term...CMS Short Term...CMS Long Term...AMC Aviation...AMC/CMS Marine...AMC/CMS Equipment...CMS

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