Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 300748 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 348 AM EDT Thu Mar 30 2017 .SYNOPSIS...
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High pressure will move offshore today. Low pressure moving through the Ohio Valley on Friday will redevelop near the New Jersey coast Friday night and move offshore on Saturday. Candian high pressure will move into the area for Sunday and Monday. Low pressure over the Ohio Valley on Tuesday will move through the middle Atlantic Tuesday night and offshore on Wednesday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/... An upper air analysis this evening shows a trough in northern New England, a ridge from the Tennessee Valley northward to the Great Lakes, then a robust trough in the central to southern Plains. The latter is helping to build the downstream ridge, placing strong surface high pressure near Hudson Bay. There is also considerable 850 mb warm air advection indicated into portions of the southern Midwest this evening. High pressure centered near Hudson Bay this evening will continue to build southward into the northern Mid Atlantic through the overnight. This along with a cooling boundary layer will allow for less wind and cooler temperatures. Meanwhile, the aforementioned warm air advection in combination with a 250 mb jet positioned to our north will help pull some high level clouds across our area especially late. This may thin eastward for awhile as the ridge axis remains to our west. We still indicate some cloud cover arriving through the overnight from west to east. For the 930 PM update, the hourly temperature, dew point and wind grids were adjusted based on the latest observations. Some locales are experiencing a quicker drop in temperature thus far, therefore some adjustments were needed. There is drier low-level air advecting in from the north, therefore the dew points for many areas were lowered a little faster. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/... The center of the high tracks into eastern Canada on Thursday, pulling an upper level ridge onto the Eastern Seaboard late in the day. Meanwhile, low pressure continues to organize over the Central Plains and Midwest. The upper ridge should keep precip at bay for the daytime hours, but high level clouds initially will then lower and thicken throughout the day. Temperatures should be just shy of normal, topping off in the mid 40s up north to the mid and upper 50s elsewhere. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/...
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The primary players during the long term continue to be two southern stream systems that are expected to impact our region in the Friday- Saturday and Monday-Tuesday time frames. The first system will produce a wide swath of rainfall across the region, with localized amounts in excess of 1.5 inches, especially across the I-95 corridor. While urban and poor drainage flooding are possible, and have been mentioned in the HWO, we do not expect major impacts on rivers and streams. Looking ahead to early next week, the next system is expected to bring more precip to our region, especially Monday night into Tuesday. This setup continues to look warmer compared to the previous system, given the absence of high latitude blocking, and would favor predominantly rain. Temperature-wise, daily means are expected to average near normal Friday thru Sunday, then around 5 to 10 plus degrees above normal Monday thru Thursday.
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&& .AVIATION /08Z THURSDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR. Northerly winds near 10 knots, becoming light and variable later this evening and overnight at most terminals. Thursday...VFR. A ceiling at or above 10000 feet will develop mainly in the afternoon. Variable winds near 5 knots becoming northeast in the morning, then turning southeasterly. OUTLOOK... MVFR possible in low clouds Friday and Friday night, then again Monday night into Tuesday. Otherwise, predominantly VFR. Southeast winds may gust 20 to 25 knots on Friday, and northwest winds may gust around 20 knots on Saturday.
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&& .MARINE...
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High pressure builds across the waters tonight through Thursday. N winds may gust to 20 KT on the ocean waters late tonight through Thursday morning, but Small Craft Advisories should not be needed. OUTLOOK... We have issued a Gale Watch from 18Z Friday to 09Z Saturday for the northern NJ coastal waters (ANZ450-451) for southeasterly wind gusts up to 35 knots during this time frame. A Small Craft Advisory may be needed on Saturday for northwest wind gust of 25 to 30 knots and seas of 5-8 FT, especially across the NJ coastal waters. Another SCA may be needed Monday night into Tuesday, as southeasterly winds and seas increase.
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&& .TIDES/COASTAL FLOODING...
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Though astronomical tides will be gradually diminishing through the week now that we are past the new moon, the threat of minor tidal flooding along the NJ and DE Atlantic coasts increases by Friday. We currently have high tides about 0.5 feet above the astronomical tide. We expect this to continue through Thursday. Beginning Thursday night, a low pressure system will bring a prolonged period of onshore flow, further increasing the surge. The tide of most concern is still the high tide Friday evening and Friday night, particularly along the northern and central New Jersey Shore. By this tide cycle, it will take a surge of 0.8 to 1.0 feet to reach minor flooding thresholds, which is likely. However, not sure yet if we will have another 0.3 feet surge to reach advisory thresholds. The exact magnitude of the surge will be dependent on how quickly the onshore flow develops and how strong it will be by Friday. ETSS shows water levels at Lewes and Sandy Hook touching minor flooding thresholds with the high tide cycles tonight and Thursday night. This is unlikely though as with the expected wind direction, we should not see a surge any higher than what we currently have at least through Thursday night. Given the fetch and duration of southeasterly flow, some of the surge guidance is also raising water levels along the eastern shore of Maryland, particularly for the Friday evening high tide. At this time, none of the guidance reaches the minor flooding threshold, but this will bear watching.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Watch from Friday afternoon through late Friday night for ANZ450-451.
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&& $$ Synopsis...Franck Near Term...Gorse Short Term...MPS Long Term...Franck Aviation...CMS/Franck/Gorse Marine...CMS/Franck/MPS Tides/Coastal Flooding...

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