Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 211328 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 928 AM EDT Fri Jul 21 2017 .SYNOPSIS...
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A weak frontal boundary will move off the coast later today, with high pressure from the midwest building in behind it through early Saturday. Another front and associated low pressure will cross the area Saturday night through Monday. High pressure is expected to move in on Tuesday and remain into the latter part of next week.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Lee troughing will persist in vicinity of the I-95 corridor, but there is a noticeable lack of large-scale lifting mechanisms across the northern Mid-Atlantic today, particularly as the perturbation moving through last evening leaves subtle transient shortwave ridging in its wake. With the region remaining to the south of the stronger west-northwesterly midlevel flow, hot and humid air will remain entrenched. However, mixing should be somewhat more effective today in drying the boundary layer (given influence from the aforementioned shortwave ridging). Surface dew points will likely be a degree or two cooler yesterday, though my suspicion is that temperatures will be quite similar. Moreover, with effective mixing may come a somewhat more pronounced cloud deck this afternoon, which should prevent temperatures from increasing any more than those seen yesterday. Given these factors, I do not think a heat advisory is required surrounding the excessive heat warning, particularly since observed heat indices yesterday marginally reached criteria, at best. Of course, it will still be hot and humid, and heat stress/exhaustion is quite possible from prolonged outdoor exposure. Given the duration of the current hot spell, the excessive heat warning for the urban corridor is still warranted and did not change, though afternoon heat indices may not exceed 100 degrees in most locations. Convection cannot be entirely ruled out today, but think the best chances are in the Delmarva Peninsula, in closer proximity to a subtle perturbation approaching from the west late this afternoon. Think most of the region will remain dry, though. Lastly, with fairly light winds today, sea breeze penetration inland may be somewhat more pronounced, which poses a risk for some max temperature errors in coastal portions of New Jersey.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/...
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The main concern tonight is precipitation chances, primarily in Delmarva. A perturbation will move through the southern Mid- Atlantic during this period, and subtle large-scale ascent will occur in advance. With the proximity of surface troughing aiding in (albeit weak) near-surface convergence and potential for buoyant parcels within the residual mixed layer, showers and/or storms cannot be ruled out entirely in this region overnight. The 00Z NAM Nest, with some support from the WRF-ARW/NMM simulations, is indicative of scattered precipitation breaking out after midnight in this region. Kept the highest PoPs during this time frame, though think an isolated storm is possible during the evening as well. The rest of the region is expected to be dry, with no apparent lifting mechanisms aiding in the development of convection. Another sultry night is anticipated, with temperatures and dew points mainly in the 70s (60s north of I-78). Some spotty fog may occur in favored locations, but with the lack of rain expected today, not confident enough to include in the forecast at this point.
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&& .LONG TERM /SATURDAY THROUGH THURSDAY/...
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A mid-level zonal flow over the northern portion of the U.S. early Saturday will eventually yield to a more trough-like pattern Monday through Tuesday, then evolve into a west- northwest flow for the middle and latter portions of the week. This means the heat wave will continue into Monday, but with temperatures more in line with normal for this time of year beginning Tuesday. At the surface, medium range models indicate a warm front will lift north through the area late Saturday, bringing with it increasing humidity and the chance of showers and thunderstorms later Saturday into Sunday. SPC has placed a large portion of the CWA under a slight risk for severe weather during this time period. A strong cold front is then expected to move through the area Monday night into Tuesday, along with showers and thunderstorms. High pressure behind this front will bring with it more seasonal temperatures for this time of year and lower humidities for the latter half of next week.
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&& .AVIATION /14Z FRIDAY THROUGH TUESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Mostly VFR through the period. Expecting SCT-BKN cumulus around 5000-8000 feet today with winds transitioning from northwest to west or southwest as the day wears on. Speeds should remain around or below 10 kts. No storms are expected through the TAF period. Outlook... Saturday morning...VFR expected. Saturday afternoon thru Monday...Mostly VFR. Sct showers and tstms with lower conditions possible. Tue and Wed...Mostly VFR.
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&& .MARINE...
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Looks like a quiet day for the coastal waters, with light and variable winds this morning becoming south to southwest 5-15 kts this afternoon and this evening. There may be a chance of showers or storms tonight, primarily for Delaware Bay and the Delaware coastal waters, but confidence is rather low. Seas should generally be three feet or less today. OUTLOOK... Sub-SCA conditions overall. The main hazard will be sct tstms mainly between Saturday afternoon and Monday. Locally higher winds and seas with tstms. Bring your NOAA weather radio along to get alerts. RIP CURRENTS... The primary wave period for the past several hours has been around 7 seconds. However, there is some concern that an underlying 10-12 second period will become more dominant during the afternoon today. Should this occur, the rip current risk may become moderate. Observations will be monitored closely the next few hours to see if an upgrade is required, but for now, the forecast is for a low risk of rip currents today.
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&& .EQUIPMENT... The KDOV dew point readings are unrepresentative of the area and should not be used when diagnosing humidity/heat indices and meteorological analysis of meteorological items which use dewpoint in a calculation. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Excessive Heat Warning until 8 PM EDT this evening for PAZ070- 071-102-104-106. NJ...Excessive Heat Warning until 8 PM EDT this evening for NJZ015- 017>019. DE...Excessive Heat Warning until 8 PM EDT this evening for DEZ001. MD...None. MARINE...None. && $$ Synopsis...Miketta Near Term...CMS Short Term...CMS Long Term...Miketta Aviation...CMS/Miketta/Iovino Marine...CMS/Miketta Equipment...Staff

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