Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 282027 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 427 PM EDT THU JUL 28 2016 .SYNOPSIS... Scattered showers and thunderstorms with locally heavy rainfall will persist at times through the weekend. Drier and less humid weather is expected for much of the next work week as high pressure builds over the region. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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The start of what will be a very active 24 hour period of weather is underway. The primary threats over the area will be severe thunderstorms (locally damaging winds, frequent lightning, an isolated tornado or two) and flash flooding. These threats are covered with weather watches: 1) a Severe Thunderstorm Watch is in effect through midnight for approximately I-78 southward. 2) a Flash Flood Watch (FFA) is in effect through noon Friday. After collaboration with surrounding offices, the FFA was expanded northward to include the entire CWA. The majority of the forecast area should see rainfall amounts of 1-2" with isolated amounts of 3-4"+. In this setup, the greatest flash flood threat will exist along and south of the weak frontal boundary that is expected to not move much tonight from its current location near the I-95 corridor. Heavy rainfall rates of 2"+ per hour would be favored across the southern half of the CWA where inflow of moist unstable air resides. PWATs are currently above +2 SD (2-2.2) and are forecast to increase further to near record observed values based on PWAT climatology of nearby RAOBs. Note, that there is still considerable disagreement between models regarding where the axis of heaviest rainfall. Based on the above mesoanalysis and model reliability in these types of excessive rainfall events, we are favoring a southern solution for where the heaviest rainfall occurs, which is better supported by the hi-res CAM guidance. Updated hourly PoPs to try and time the multiple rounds of precip. After the diurnally driven convection through early evening, another round of heavy rain looks to move in late tonight as a wave of low pressure tracks northeastward along the stalled front. Heavy rainfall with this second round may be more widespread. Also, backed flow in the low levels ahead of the low may provide a source of low- level helicity. Given very low LCL heights, a weak tornado or two may occur toward early morning near the front as long as instability remains near surface based.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/... Precipitable water values of 2.2 to 2.4 inches will overspread our forecast area from the south for tonight. Conditions are expected to remain unstable, especially across our southern counties. Also, surface low pressure approaching from the west will result in the development of a favorable low level wind profile for organized convection tonight. The surface wind should begin to favor the east to south quadrant late tonight, especially on the coastal plain, while winds around 800 HPa will be from the southwest. The very high precipitable water values should result in heavy rainfall in any organized convection. As a result, the Flash Flood Watch remains in effect for southeastern Pennsylvania, central and southern New Jersey, Delaware and northeastern Maryland. The cloud cover and high humidity will keep temperatures from falling below the 70s for tonight except in the Poconos and far northern New Jersey. && .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... Highlights... * Scattered showers/t-storms w/locally heavy rainfall this weekend * Drier/less humid weather for much of the next work week Details... Friday night... Dry weather expected Friday night with subsidence behind departing shortwave. May see some patchy fog develop overnight with wet ground and lingering low level moisture. Saturday and Sunday... A couple of additional shortwaves will move across our region this weekend along with a frontal boundary in the vicinity. Timing is uncertain this far out, but more scattered showers and thunderstorms are expected at times this weekend. Pwats will still be between 1.5 and 2 inches, so the threat will exist for locally heavy rainfall. Clouds and the threat of precipitation should hold high temps mainly in the 80s both days. Monday through Thursday... High pressure will build over the region, bringing us drier and less humid weather. There still will at least be a low risk for a few showers early in the work week, but either way dry weather should dominate through Thursday. High temps will mainly be in the 80s. && .AVIATION /21Z THURSDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Flying conditions will continue to deteriorate through tonight as thunderstorms with gusty winds and heavy rain will periodically impact all taf sites, but especially KPHL, KPNE, KTTN, KMIV, KACY through early Friday morning. Added vrb15g30kt winds to these TAF sites to account for potential for gusty winds in and near thunderstorms. Confidence is low for the timing of convection at any particular TAF site, but high that storms will be in the region through early Friday morning. Showers and storms will expand in coverage tonight and early Friday morning. This will result in ceilings dropping to MVFR at most sites around or after 04Z. Southeast winds will back to the east late tonight generally 10 knots or less, except in and around thunderstorms, but shift to the northwest and increase in speeds, gusting at 10-20 kts after 11Z Friday. Conditions gradually improve by late morning/early afternoon Friday with precipitation ending, and ceilings lifting Outlook... Sat through Sun...Shra/tsra will lower cigs/vsbys at times Otherwise VFR expected. Moderate Confidence. Mon...Mainly VFR. Moderate Confidence. && .MARINE...
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A severe thunderstorm watch is in effect for most of the waters. There is a potential for storms through this evening to produce locally strong winds. An isolated waterspout is also possible through early Friday morning. S to SE winds 10-15 kt will become light tonight. Seas will generally be around 1 ft. S-SW in the morning will increase to 10-20 kt and become NW in the afternoon and then N late in the day across our northern coastal waters as a cold front passes thru. Expect seas to respond by building to 2 ft in the morning and possibly 3 to 4 ft during the afternoon in our coastal waters. Outlook...Friday night through Tuesday... There is a low risk for marginal small craft seas Friday night but for now kept them just below 5 feet. Otherwise, winds/seas should generally remain below small craft advisory thresholds through the period. Biggest concern for mariners will be the risk for thunderstorms this weekend. Drier weather should arrive early next week. RIP CURRENTS... There is a low risk for the development of dangerous rip currents for today due to the relatively light wind and tranquil seas. The probably risk for the development of dangerous rip currents on Friday should primarilly be low but it may approach moderate along the NJ coast late in the day as alongshore N-NE winds increase and surf builds to about 3 ft.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...Flash Flood Watch through Friday morning for PAZ054-055- 060>062-070-071-101>106. NJ...Flash Flood Watch through Friday morning for NJZ001-007>010- 012>027. DE...Flash Flood Watch through Friday morning for DEZ001>004. MD...Flash Flood Watch through Friday morning for MDZ008-012-015- 019-020. MARINE...None. && $$ Synopsis...Frank Near Term...Klein Short Term...Klein Long Term...Frank Aviation...Klein/Frank Marine...Klein/Frank

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