Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 131114 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 614 AM EST Tue Feb 13 2018 .SYNOPSIS... High pressure will build towards the region before propagating off shore Tuesday night. A warm front moves into the region Wednesday. Low pressure passes northwest of our area on Thursday; the cold front associated with this low is expected to arrive in our area Friday. High pressure builds over the region this weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Will adjust hourly grids based on latest surface obs. Otherwise, forecast on track, so no significant changes will be made. High pressure over the Great Lakes will pass north of the region today and will move over Cape Cod by late afternoon. Sunny skies on tap for most of the day, but late in the afternoon, high clouds will increase from the south. Light north winds this morning will become east this afternoon. Highs today will be a few degrees below normal, generally topping off in the low 30s in the Poconos, and otherwise in the upper 30s to low 40s. Temps may be a few degrees cooler along the shores as winds turn to the east and flow comes off the cooler ocean waters.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
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Weak warm front south of the area lifts to the north tonight. There will be an increase in low level moisture and clouds will lower and thicken through the overnight. Meanwhile, some mid- level shortwave energy will move in from the west. The latest hi-res short range models, including the HRRRX, RGEM, and the HiRes-ARW have some light precip across parts of the Lehigh Valley and into central NJ. 00Z GFS is a bit more robust in terms of precip, but pretty much follows along the same track. 00Z NAM has some light precip across northern NJ, and another area across southern DE. Best chances will be in the pre-dawn hours of Wednesday, and not expecting much, if any, measurable QPF. But do think there will be a stripe from around I-78 to around I-195 where some light freezing rain or freezing drizzle is possible, and will have a few hours of slight chance PoPs to account for it.
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&& .LONG TERM /WEDNESDAY THROUGH MONDAY/... The large-scale pattern will be dominated by longwave troughing in the western U.S. and an oscillating subtropical ridge in the Gulf of Mexico and western Atlantic through the next week. Such a pattern favors frequent bouts of inclement weather and resultant large swings in temperatures in the Mid-Atlantic, which will be situated in the fast zonal to southwesterly midlevel flow on the poleward periphery of the subtropical ridge. Overall, operational models are in better-than-average agreement with the synoptic-scale details through the long term. The general idea is that several perturbations will move through the quasi-zonal midlevel flow during the latter portion of the week before a northern-stream trough moves through the Northeast Friday and Friday night. This will allow the subtropical ridge in the Gulf of Mexico to progress gradually eastward into the western Atlantic during the weekend as another vort max plunges into the Desert Southwest. As this occurs, substantial ridge amplification will occur in the western Atlantic, developing southwesterly midlevel flow in the central and eastern U.S. Such a setup is quite favorable for several storm systems to organize and lift northeastward through the eastern half of the CONUS, with very little progression into the amplifying ridge. Undoubtedly, some locations in the central/eastern U.S. will see a considerable amount of precipitation this weekend through next week, but pinpointing the exact locations (and timing) is challenging this far in advance, particularly as models begin to diverge with the strength/positioning of the deep trough in the Southwest. At this point, the most likely area is to our west, in the Ohio and Tennessee Valleys. The forecast was generally a blend of the operational guidance through the period, given the relatively predictable nature of amplifying ridges. However, the CMC becomes noticeably out of phase with the remaining guidance this weekend through next week with shortwave troughs progressing in the southwesterly flow on the northwest fringes of the ridging. As such, the CMC was given substantially lower weighting compared to the rest of the model suite. Temperatures were generally a statistical blend, with considerable weighting given to the better-performing bias-corrected MEX and Euro MOS guidance. A couple rounds of light rain look to occur Wednesday night and Thursday night/Friday. Model QPF is generally light with both of these events, as the stronger lift with the passing vorticity maxima will be to our north and west. Temperatures will be going into torch mode in advance of the attendant cold front on Wednesday and Thursday, with forecast highs 5-15 degrees above average on Wednesday and 15-25 degrees above average on Thursday. Most record highs look out of reach on Thursday (except for Atlantic City), but it will certainly feel springlike. In a long-duration period of warm advection/broad-scale lift, skies will likely be considerably cloudy for much of the period. The 00Z GFS/ECMWF have a weak surface low moving near or just north of the area Thursday night and Friday morning, which is when PoPs are highest during this period. QPF is generally a model blend, which unfortunately tends to wash out the higher swaths. Suspect there will be some places getting a quarter to half inch of rain during this period, but with uncertainty regarding the low`s track (and the smaller-scale lift in proximity to it), generating these fine-scale QPF swaths is a fool`s errand. Temperatures will be warm again on Friday but will tank Friday night as cold advection sweeps through. A broad surface high moves into the eastern U.S. this weekend, but with the amplifying ridge to our south, the cold front that passes the area Friday will remain in close proximity. Meanwhile, the southwesterly flow aloft will provide a favorable track for upstream perturbations to periodically progress through or near our region. One such perturbation looks to move through Saturday night, and this may generate another period of precipitation, especially for the southern CWA. Should this occur, surface temperatures on the north side of the front will be cold, potentially for a wintry mix. I increased PoPs considerably Saturday afternoon through early Sunday morning as a result, with the best chances south of the Mason-Dixon Line. The GFS is much farther south, but I have a suspicion it is way too aggressive in progressing the front into the Southeast. Notably, the CMC actually moves this system through the heart of our area Saturday night. Be prepared for changes to the forecast for this time period in subsequent packages, and stay tuned for clarifying details on the winter weather potential here given the cold air on the north side of the slowing front. As ridging in the western Atlantic intensifies, southwest flow becomes established over much of the U.S. Sunday into next week. A storm track from the Southwest to the Great Lakes and New England is favored early next week, with warm advection downstream possibly giving the area another round of precipitation Sunday night and Monday (some timing discrepancies noted by this point among the models as phase errors increase). Nevertheless, think boosting PoPs in this time period is also warranted given the consistent signal with the overall synoptic- scale pattern. The surface air will be warming, but temps may be cold enough for more wintry precipitation in the northwest CWA Sunday night. The GFS keeps the wet pattern going much of next week, but the ECMWF keeps the main storm track farther to the northwest. I am betting on the latter given the GFS`s tendency to dig into amplifying ridges too readily. Nevertheless, kept slight chances of precipitation through the end of the long term for now. && .AVIATION /11Z TUESDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR. North winds less than 10 kt, becoming northeast by midday, then southeast late. High confidence. Tonight...VFR. Increasing mid and high clouds. Light and variable winds. Outlook... Wednesday...May have some early morning CIGs nearing MVFR, but conditions should improve to VFR as the day progresses. Southwest winds 5 to 15 kts. Moderate confidence. Wednesday night...Periods of sub-VFR possible with a chance of rain. Light southwest winds. Moderate confidence. Thursday...Lingering sub-VFR in the morning with perhaps some brief improvement to VFR in the afternoon. A chance of rain in the morning. Southwest winds 5 to 15 kts. Moderate confidence. Thursday night and Friday...Periods of sub-VFR likely with a good chance of rain. Light southwest winds becoming west/northwest Friday afternoon, with some gusts to 20+ kts possible after the wind switch. Moderate confidence. Friday night...VFR with northwest winds decreasing to around 10 kts or lower. High confidence. Saturday...Predominantly VFR, though conditions may deteriorate in the afternoon, especially south of the Mason-Dixon Line. A slight chance of rain or snow. Winds becoming west or southwest, remaining near/below 10 kts. Low confidence.
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&& .MARINE...
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Today...NE winds around 15 kt with gusts up to 20 kt for the ocean waters and around 10 kt for DE Bay today. Ocean seas 3-4 feet. Tonight...Sub-SCA criteria. Light and variable winds. Outlook... Wednesday through Thursday night...Sub-advisory winds expected, but rain is possible both Wednesday night and Thursday night with potential for visibility restrictions. Seas may build to advisory levels on Thursday night. Friday...A good chance of rain and visibility restrictions, with winds/seas potentially above advisory criteria. Wind direction predominantly south/southwest. Friday night...Winds become northwest and may remain gusty during the overnights. Seas expected to subside below criteria by daybreak. Saturday...Potential for more rain (or even a wintry mix), though confidence is low. Winds/seas below advisory criteria.
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&& .HYDROLOGY...
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For the latest river forecasts, visit the AHPS web site at: https:/water.weather.gov/ahps2/index.php?wfo=phi Flooding continues to subside, but there are still some rivers and streams across New Jersey that are elevated and above flood stage and some that have yet to crest. Flooding continues along the Rancocas Creek in Burlington County (both the north and south branch). At Pemberton, water levels have crested, but the fall below flood stage will be slow. As a result, the level may stay above flood stage until Wednesday. The flood warnings (both river flood warning for the forecast point at Pemberton and areal flood warnings for non-forecast points) continue. Along the Millstone River in Somerset County, the river has crested and is falling. Flooding may continue along portions of the river through the day, however. The Maurice River at Norma in Salem/Cumberland Counties is now in flood with levels continuing to rise. Lowland flooding is occurring near Vineland, and road closures may be required this morning. The Manalapan Brook at Spotswood in Middlesex County has crested below flood stage. No flooding is expected here. Finally, the Great Egg Harbor River is nearing flood stage at Folsom. Gauges upstream have only just crested, so there is a good likelihood minor flooding will occur here. Should trends continue, a flood warning will likely be issued this morning.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...CMS Near Term...MPS Short Term...MPS Long Term...CMS Aviation...CMS/MPS Marine...CMS/MPS Hydrology...

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