Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 120258 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 958 PM EST Wed Jan 11 2017 .SYNOPSIS... Low pressure will move into the Great Lakes area tonight and then down the Saint Lawrence valley Thursday night and Friday. An associated cold front will cross the Middle Atlantic region Thursday night. Large high pressure will affect the area Friday through early Saturday. Low pressure waves will move to the south of the region this weekend before more high pressure arrives for Monday. Another disturbance may affect our region for the middle of next week. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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high pressure was parked off the East Coast this evening. Meanwhile, low pressure was analyzed over the Midwest states with a cold front extending southwestward from the low across the Southern Plains. South-Southwest return flow around the offshore high and ahead of the front will continue to feed warm, moist air up the eastern seaboard tonight. A few tweaks were made to the PoPs as the grids from the 630 PM ESTF were matching up well with current observations (radar/satellite) and hi-res models (HRRR/RAP). At 930 PM, the rain was widespread/steady across eastern PA, NJ and northern Delmarva (PoPs were 100 percent). The precip will taper off from S to N overnight as the best isentropic lift and convergence at the nose of a SWly low-level jet streak shifts downstream of the area. There is still considerable uncertainty regarding our fog potential in wake of the rain late tonight. SWly flow will continue to advect a higher dewpoints into the region but clouds are likely to be persistent tonight across much of the region. If any breaks in the clouds were to develop, it would more likely occur across our the Delmarva and southeastern PA, where the surface winds are also forecast to be strongest. This does not appear to be a setup for a more widespread, dense fog event tonight. It would be a difference story if the snowpack hadn`t melted away today. Hourly temperatures were updated with the evening forecast update to reflect minimum temperatures occurring shortly after the onset of rain (due to evaporative cooling at the onset); i.e., this evening. Expect temperatures to gradually warm across the coastal plain as SW winds increase in the boundary layer. North and west of the Fall Line, where the CAD wedge is more pronounced, expect a more gradual rise overnight with light winds.
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...
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Temperatures were adjusted with the 930 PM update. The thinking is there will be a tighter S-SE to N-NW thermal gradient on Thursday- with highs ranging from the low/mid 50s N of I-78 and W of I-287 to the mid 60s from I-95, S and E. Think that what`s left of the cold air wedge will be slower to retreat than models are showing, especially if low clouds hang on into the afternoon across NE PA and NW NJ. Certainly a challenging temperature forecast. Another vorticity maximum will progress into the Northeast during the day Thursday. However, downstream ridge amplification in advance of this perturbation will prevent deep/sustained lift from reaching (most of) the area. Associated cold front will slow as it reaches the stronger ridging nosing into the area with precipitation generally dissipating as it approaches the CWA. Did leave some PoPs in the southern Poconos and adjacent Lehigh Valley and northwest New Jersey, in closer proximity to this next system. Meanwhile, for the rest of the region, strong warming will occur as SW low-level flow is expected to mix to the surface. Temperatures will warm 7-12 degrees above today`s values. Near-record highs are anticipated for several of our sites in this regime. While mid and high clouds should be present at least to some degree during the day, may see some partial sunshine in southern portions of the CWA at minimum, which would only add to the heating provided by mixing. May see some southwesterly wind gusts 25-35 mph tomorrow, especially near the coast and in the higher terrain of the southern Poconos and northwest New Jersey.
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&& .LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/... Thu night...Low pressure will continue to move down the Saint Lawrence valley while high pressure builds down across the northern U.S. A cold front, accompanied only by scattered showers, will move through the area overnight. We only have chc pops across the far N/W and slight chc pops adjacent to that for the frontal passage. Most models are similar with the lack of organized precip with this feature. The strong high across the upper Great Lakes builds east for Friday and Friday night. We expect fair weather for these periods along with mild temperatures Friday. Temperatures will fall Fri night back closer to normal values for early January. Highs Friday... and overnight lows... The weekend will be somewhat unsettled across our area. The front that crosses the area Thu night will not have moved too far south of our area Fri and early Saturday. The front will be close to VA/nrn NC early Saturday and a few disturbances will move along it Sat and Sun. These waves will bring increasing chc for precipitation over the weekend. We have pops are highest across Delmarva and srn NJ areas where pops are mostly in the chc range for Sat - Sun evening periods. Temperatures will be a little below normal Saturday and then close to normal Sunday. P-type will mostly be snow north/west and rain south/east in general. It doesn`t looks to be too much of an accumulating snow event with the systems moving thru rather quickly, but there is some window of opportunity Saturday for some accums. We expect better detail with regards this overnight or tomorrow. More upper ridging begins across the ern part of the country next week. Temperatures look to be above normal much of the time. A deep low across the Great Plains will usher in more moisture and cause increasing chc for rain (mostly) over the area Tue-Wed. && .AVIATION /03Z THURSDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Conditions will deteriorate late this evening with rain moving in. MVFR/IFR ceilings and MVFR visibilities are likely for a period between 03-08Z after a couple hours of steadier rain. Rain should end from S to N around 05-06Z in our southern terminals (ILG/MIV/ACY) and 07-08Z in our northern terminals (ABE/TTN). Most of the MOS guidance (LAMP/MAV/MET) show ceilings and visibilities lowering to LIFR late tonight. The NAM-based guidance keeps us socked in low clouds with IFR conditions for much of the day Thursday. Meanwhile, rapid refresh guidance from the RAP/HRRR/TL- NARRE are not nearly as aggressive with the low clouds and fog. 00Z TAFs were not quite as pessimistic as the MOS guidance but did contain a period of IFR between approx early in the morning before clouds/fog mix out and conditions improve from S to N between 14-18Z. Confidence for the 09Z-18Z Thursday portion of the TAF is low, especially considering it`s in the near term. Northern terminals (RDG-ABE-TTN) are at a greater risk of seeing low clouds and IFR CIGs linger later into the day than currently forecast. Light S-SW winds tonight. SW winds increase to 10-20 kts with gusts to 25 kts on Thursday (although we may not see the gusts develop at our northern terminals if the low clouds hang on into the afternoon).
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&& .MARINE...
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SCA continues for the coastal waters of NJ/DE through Thursday evening and has been issued for Delaware Bay during the day Thursday for strong south to southwest winds and high seas as the next system moves through the region. Wind gusts were lowered a bit for tonight and Thursday with poor mixing profiles owing to warm air above the colder waters. OUTLOOK... Thu night...Winds and seas continue to subside. SCA flag on the Ocean until around midnight Fri night. Fri thru Sunday night...Sub-SCA conditions. Good chances for rain Sat and Sun mostly over the srn coastal waters & Del Bay. Monday...Sub-SCA conditions expected.
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&& .TIDES/COASTAL FLOODING... Model guidance continues to show low tides tomorrow approaching blow out tide thresholds. With the full moon tomorrow, astronomical low tides will be between -0.8 and -1.0 ft MLLW. However, local regression equation suggests that the tidal departures should be between 0 and -0.5. Additionally, the south southwest or southwest winds expected tomorrow are not optimal for blow out tides. Therefore, we still expect the total water level to stay above blow out tide thresholds. && .CLIMATE... Thursday should be quite mild with current forecast within a couple of degrees of record highs for many of our climate sites. Here are the records for January 12: Location/ Record/Year Atlantic City 63/1913 Wilmington 63/1913 Allentown 60/1932 Reading 61/1913 Georgetown 70/1975 Mount Pocono 56/1975 Trenton 68/1890 At this time, we are forecasting that Philadelphia will stay well below its record of 72 set in 1890. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 1 AM EST Friday for ANZ450>455. Small Craft Advisory from 7 AM to 7 PM EST Thursday for ANZ430- 431. && $$ Synopsis...O`Hara Near Term...Klein Short Term...CMS/Klein Long Term...O`Hara Aviation...Klein/O`Hara Marine...CMS/Klein/O`Hara Tides/Coastal Flooding...Johnson Climate...Staff

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