Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 230731 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 331 AM EDT Mon Apr 23 2018 .SYNOPSIS...
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Large high pressure over the Northeast United States will continue to move east today and then move off the New England coast tonight. Low pressure over the Gulf Coast states will lift to the north and east and affect the Mid-Atlantic region during the mid-week period. Another low may affect the region late in the week, followed by a cold front next weekend.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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A midlevel vort max will be moving offshore this morning while an upper low over the Mid-South drifts eastward into the Tennessee Valley today. In between, pronounced midlevel ridging will seep eastward into the Mid-Atlantic. This will permit mostly sunny skies and warmer conditions today (as midlevel heights and 1000-500 mb thicknesses increase). GFS/NAM BUFKIT soundings show about 2-3 degrees of warming today versus yesterday with similar mixing heights. Surface temperatures were about 3-6 degrees warmer than the simulated values as depicted in these soundings on Sunday. A blend of MAV/MET guidance represented these displaced values well for today (and are in excellent agreement, in general). Combined this with a blend of the previous forecast and 2-m HRRR-simulated temperatures for today. Dew points tanked yesterday during the afternoon, which was poorly simulated by virtually all models (a typical bias this time of year). Model soundings indicate mixing that reduces dew points about 2-4 degrees below a statistical/consensus blend. I dropped dew points by this amount in the grids for today, basically everywhere. This may not be enough in some spots and will likely require some monitoring/editing during the day. In addition, the sea (bay) breeze will again develop and move onshore today. It appears high-resolution simulations were somewhat too aggressive in moving the sea/bay breeze onshore on Sunday, but there are a couple of differences in today`s environment. First, surface temperatures will be warmer, allowing for a stronger temperature gradient from the ocean to land. Second, synoptic-scale flow will permit a return of southerly/southeasterly flow, which should allow the sea/bay breeze to move inland more readily. So, the forecast used a blend of the more aggressive WRF-ARW/NMM to the somewhat slower- to-materialize HRRR. This will again have impacts on temps/dew points during the afternoon, particularly in southern/eastern NJ and coastal Delaware, which was also considered in the forecast today.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/...
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The upper low in the Tennessee Valley will continue to trudge toward the Appalachians tonight, with downstream ridging progressing through the area tonight. A predecessor vort max will progress into the central/southern Mid-Atlantic late tonight, and attendant lift will allow mid and upper-level clouds to increase late tonight in southern/western portions of the CWA. In advance of the increasing clouds, relatively clear skies and light winds will allow temperatures to drop readily during the evening hours. Therefore, even with the increasing cloudiness allowing for the temperature drop to slow/stop after midnight, I went a little below the consensus statistical blend for lows tonight given its predilection for being too conservative with the nocturnal cooling (especially of late). Think the potential for patchy frost exists again in the usual trouble spots (Pine Barrens; valley locations to the west/northwest of the Philly metro), but this will certainly be more limited than the past few nights. Forecast lows are about 3-5 degrees warmer than those seen early this morning. Models keep precipitation in association with the upper low and the predecessor vort max to the south/west of the CWA through the night. Slowed the onset of mentionable PoPs considerably late tonight as a result. I also slowed the onset of increased cloudiness to some degree, with the thinking that models are probably on the aggressive side with the movement of the low (typical bias with these setups).
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&& .LONG TERM /TUESDAY THROUGH SUNDAY/...
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Low pressure will be over central Kentucky and Tennessee Tuesday morning, and a warm front will extend out ahead of it, and that warm front will be just southwest of the Delmarva. That warm front lifts to the north and east during the day Tuesday, and developing rain ahead of that front will lift into the Delmarva, southeast Pennsylvania, and southern New Jersey in the afternoon. Upper trough with closed low digs into the Tennessee Valley in the evening, and a secondary low forms off the Southeast U.S. coast Tuesday night. This system lifts along the coast Tuesday night, and then it appears as if the 2 systems merge over the Delmarva Peninsula Wednesday morning. With strong onshore flow, PWATs will range from 1.25" to 1.50" after midnight Tuesday night, and as a result, there may be a period of heavy rain. Low pressure lifts to the north and east during the day. Dry air will build along the coast in the afternoon, and rain tapers off from south to north by Wednesday evening. From there, unsettled weather looks to continue as an upper low passes through the region. Will keep chance PoPs in the forecast Thursday night and Friday. For the weekend, the CMC-GDPS has low pressure affecting the region while the ECMWF has a low forming as a cold front passes through the region, but the low stays well offshore. The GFS is similar to the ECMWF. Will carry chance PoPs in the forecast through Saturday. High pressure builds back into the region for the start of the new week.
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&& .AVIATION /08Z MONDAY THROUGH FRIDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR with winds becoming southerly or southeasterly around 5 to 10 kts. Sea/bay breezes will develop and move inland this afternoon, with somewhat stronger south to southeast winds once the boundary passes. These boundaries will pass MIV/ACY/ILG during the afternoon and may reach the Philly terminals late in the day. High confidence in VFR; moderate confidence with winds. Tonight...VFR with increasing mid/upper cloudiness late. Winds becoming light and variable. High confidence. Outlook... Tuesday through Wednesday...MVFR/IFR conditions, especially Tuesday night/Wednesday morning as periods of rain will develop across the region. East wind gusts up to 20 kt are possible Tuesday night into Wednesday. Otherwise, generally light winds (less than 10 kt) are expected. Moderate confidence on flight categories, but low confidence on the timing of any flight category changes. Thursday and Friday...Generally VFR conditions, but scattered showers could result in occasional MVFR conditions. West winds on Thursday eventually shift to the south on Friday. Low confidence on flight categories and wind.
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&& .MARINE...
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Winds/seas will remain below advisory levels through tonight, but southeast winds will be increasing through the period, becoming 10 to 20 kts this afternoon and tonight with gusts approaching advisory levels late tonight. Seas will slowly build to 2-4 feet through the period. There is some chance of patchy fog early this morning and again late tonight, with some visibility restrictions possible but expected to be fairly localized. Outlook... Tuesday and Wednesday...East to southeast winds will increase to 15- 20 kt with 25-30 kt gusts, and seas build up to 10 feet on the Atlantic coastal waters. Wind gusts to 25 kt are also expected on the Delaware Bay. Winds shift to the west and diminish to 10-15 kt late Wednesday. Seas remain elevated. Wednesday night and Thursday...Winds shift to the northwest. The main question will be how long seas remain above 5 feet on the coastal waters. Current forecast is for elevated seas into late Thursday, but there is still considerable uncertainty with this. Friday...depending on if a coastal low develops, we may see increasing southeasterly winds. If this happens, we could once again reach SCA criteria.
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&& .FIRE WEATHER...
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Dew points were quite low on Sunday, allowing relative humidity values to drop well below 30 percent across the area for a lengthy period of time during the afternoon. Though winds were generally light, the combination of low relative humidity and fairly dry fuels allowed for the development of some wildfires. Similar conditions will exist today. Winds will be below red flag warning thresholds, but much like Sunday, sea and bay breezes will develop and move inland during the day. As these boundaries pass, winds may shift and increase rather suddenly. For any preexisting fires, the passage of the sea/bay breezes may cause erratic conditions affecting fire growth/spread. Should winds be stronger than projected, a special weather statement will likely be required indicating the increased potential for wildfires.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from noon Tuesday to 6 AM EDT Wednesday for ANZ450>455. Small Craft Advisory from noon Tuesday to 2 AM EDT Wednesday for ANZ430-431.
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&& $$ Synopsis...MPS Near Term...CMS Short Term...CMS Long Term...MPS Aviation...CMS/MPS Marine...CMS/MPS Fire Weather...CMS

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