Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 192011 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 411 PM EDT Wed Apr 19 2017 .SYNOPSIS...
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A front extending from the Northeast to the Great Lakes will remain nearly stationary through Thursday night. Low pressure will move from the central plains tonight to the Northeast on Friday, which will bring a cold front through the Mid-Atlantic by Friday night. Another low will progress from the southern plains to the southeastern U.S. through the weekend. High pressure then builds into the region by the middle of next week.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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High pressure centered over Atlantic Canada today will continue to pull away to the northeast tonight. The surface flow in our region is forecast to veer from the southeast and south to the south and southwest as a frontal boundary approaches from the northwest. The front was located in the eastern Great Lakes this afternoon. It should reach northeastern and central Pennsylvania toward daybreak. Warm advection aloft, ahead of the frontal boundary, should maintain clouds over our region into tonight. There is an increasing chance of showers for tonight across eastern Pennsylvania and northern and central New Jersey as the front draws closer to those areas. The cloud cover is expected to keep temperatures from dropping much for tonight. We anticipate lows to be mainly in the upper 40s in the elevated terrain of the Poconos and northern New Jersey, and in the 50s in the remainder of our forecast area.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/...
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The progress of the frontal boundary is expected to slow across Pennsylvania and northern New Jersey on Thursday morning. A mid level short wave trough from the west is forecast to pass overhead around midday. Once the feature moves off the coast, it will help to pull the weakening front southward on Thursday afternoon. We continue to anticipate showers for Thursday morning in eastern Pennsylvania and New Jersey, as well as in northeastern Maryland and Delaware. The cloud cover should decrease somewhat in the wake of the mid level short wave. However, marginal instability is expected to develop from the Susquehanna River Valley of Pennsylvania down into the Chesapeake Bay region. As a result, there is a chance that showers and thunderstorms will develop in that area on Thursday afternoon possibly impacting the western and southwestern edge of our forecast area. Warm weather is expected to return for Thursday. Highs should favor the 70s with readings not forecast to get above the 60s at the shore and in the Poconos and far northern New Jersey. The wind should remain less than 12 MPH on Thursday. The direction is forecast to be variable in the morning with an onshore component developing in the afternoon.
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&& .LONG TERM /THURSDAY THROUGH WEDNESDAY/...
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Main forecast concerns revolve around precipitation chances with one system moving through Friday and another moving through late this weekend. At 00Z Friday, a surface low should be located near the Great Lakes with a baroclinic zone extending east-southeastward through the northern Mid-Atlantic and southern New England. As the low progresses eastward, the associated quasi-stationary front will struggle to lift northeastward away from the region, at least Thursday night. Though the strongest low-level ascent will be located along and north of the boundary, operational models indicate there may be sufficient lift downstream of a perturbation revolving around the potent midlevel low to generate scattered convection in the area Thursday night. Though marginal, elevated instability will exist with parcels rooted around 900 mb exhibiting CAPE 500-1000 J/kg (according to the latest NAM), allowing for a slight chance of storms. Though some small hail cannot be ruled out should convection develop, shear is rather weak and the near-surface layer looks too stable for organized/severe storms. Best chances for precip are in the northern CWA (closer to the baroclinic zone and associated maximum in isentropic ascent), with only slight chances in far southern NJ and Delmarva. Lows Thursday night will be about 5 degrees below average highs for this time of year (low 50s north to low-to-mid 60s south). As the low approaches on Friday, the area will become firmly entrenched in the warm sector. High temperatures will soar in this environment, with forecast values 15-20 degrees above average. Midlevel cooling downstream of the midlevel low will continue to aid in the presence of instability, but cloud cover may prevent the development of boundary-layer based instability. If diabatic heating and boundary-layer mixing are stronger than forecast, the environment would be supportive of an isolated severe storm or two with gusty winds the primary threat. Models are not overly aggressive in generating convection (keeping coverage isolated to scattered), so kept PoPs fairly low (slight to low-end chance). The cold front looks to sweep through the region late in the day through the evening hours, with winds switching to northwest and skies rapidly clearing thereafter. Low temperatures will be about 10 degrees colder than Thursday night. The upper-level pattern will be in quick transition on Saturday, with an upstream vort max moving through New England and a potent midlevel low moving through the lower Missouri Valley. Jet streak coupling will commence on Saturday night with a quasi-zonal 250-mb jet streak in New England and a developing jet streak downstream of the central plains midlevel low. Broadly divergent flow in the eastern U.S. will allow for the widespread development of rain in the Ohio Valley spreading eastward into the Mid-Atlantic by Saturday night. In advance, Saturday should feature increasing cloudiness and near-average maximum temperatures in our area. The rain should begin Saturday night, but models are not in stellar agreement regarding location of maximum QPF. The trend appeared to be southward (i.e., from the Delmarva Peninsula westward) with sharply decreasing amounts northward through eastern PA/NJ (12Z GFS/CMC), but the 12Z ECMWF has a northward- displaced swath of higher QPF (near and just north of the Mason- Dixon Line). Rainfall totals exceeding one inch are likely where this maximum ends up being, but there is still quite a bit of spread at this point despite fairly decent agreement on the synoptic scale amongst the model suite. One other consideration: Each model develops a pretty sharp gradient in precipitation totals. Thus, even though most if not all of the area will see rain from this system, there are some locations that may see very light amounts (most likely in the southern Poconos and adjacent far northern NJ). Hopefully, we will get a better handle on these details as the weekend approaches. After a rainy Sunday, precipitation will taper off or become more showery by Sunday night, with the best chances of lingering precipitation in Delmarva and coastal NJ. The upper low associated with the rain event this weekend may become detached from the faster northern-stream flow (12Z GFS) or may eventually lift northward through the western Atlantic (12Z CMC/ECMWF) with poor run-to-run continuity and model agreement during the early week period. There is general agreement, however, that a ridge will build in the eastern U.S., which suggests the early-to-midweek period will be dry with a gradual warming trend. For now, kept the forecast close to what was inherited given the degrading model agreement during this time frame.
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&& .AVIATION /20Z WEDNESDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Mainly VFR conditions are forecast into this evening with ceilings favoring the 4500 to 6000 foot range. Some MVFR ceilings should linger near the New Jersey coast. Spotty light showers are possible. We anticipate lowering ceilings and an increasing chance of showers for late tonight. Ceilings may drop into the 1000 foot neighborhood, either low-end MVFR or high-end IFR. The low clouds and showers should linger for much of the morning with improvement to VFR expected in the afternoon. There is a chance of showers and thunderstorms late in the day mainly in the western and southwestern parts of our area of responsibility, close to KRDG and KILG. The wind should decrease to less than 8 knots for tonight with the direction veering from the southeast to the southwest. The wind is expected to become light and variable on Thursday morning and it should settle into the east to south quadrant on Thursday afternoon mainly at 10 knots or less. OUTLOOK... Friday and Friday night...Lingering MVFR/IFR CIGs/VSBYs in the morning before a SCT/BKN deck around 4000-6000 feet becomes established ahead of a cold front moving through the area. Winds southwest 5-10 kts ahead of the front becoming northwest 10-15 kts with occasional gusts to 20 kts behind it. A chance of showers and isolated storms ahead of the front with brief MVFR/IFR conditions and gusty/erratic winds in their vicinity. Rapid clearing expected after frontal passage. Saturday...VFR with increasing mid and high clouds. Winds generally north or northwest around 10 kts. Saturday night through Sunday night...Conditions gradually deteriorating to MVFR/IFR with periods of rain and potentially fog. Winds generally east or northeast around or below 10 kts, except somewhat stronger/gustier near the coast. Monday...Improvement to VFR expected, though BKN-OVC CIGs around 5000 feet remain likely, especially at KMIV/KACY (where potential for residual showers remains). Generally northeast winds 5-15 kts, with occasional gusts near the coast.
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&& .MARINE...
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The lingering effects of the stronger east to southeast flow this morning were keeping wave heights near 5 feet on our southern waters this afternoon. We will keep the Small Craft Advisory going until 6:00 PM. Wind speeds are forecast to remain below 20 knots for tonight and Thursday and wave heights will remain less than 5 feet. The wind direction over our waters will favor the southeast to southwest quadrant. OUTLOOK... Friday and Friday night...Southwest winds 10-15 kts becoming northwest 10-20 kts after cold frontal passage (probably during the late afternoon and evening hours). Some gusts may approach advisory criteria shortly after the frontal passage; otherwise, sub-advisory criteria expected. A chance of showers and thunderstorms prior to frontal passage. Saturday...Sub-advisory conditions expected. Saturday night through Monday...A good chance of rain with advisory- threshold east-northeast winds and seas above 5 feet possible Sunday afternoon onward. Rain/winds will begin to diminish Monday, but winds/seas could remain above advisory thresholds through the afternoon.
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&& .CLIMATE... April will probably be a top 10 warmest in the period of record for multiple long term climate stations and depending on the warmth of this Thursday-Friday and how cold it is on Sunday when there is a potential chilly washout for at least part of the day...we could be heading for a top 2 warmest april on record. The usual analysis process first 18 days done, the 19th-25th per our 330 am fcst, the 26-29th from FTPRHA and the last day, normal max/min for the date. PHL projects #2, 4.9 above normal and 0.5degs from record. POR 1874 ABE projects #2, 5.9 above normal and 0.6degs from record. POR 1922 ACY projects rer 5.0 above normal,0.4F above prev 2010 56.3 POR 1874 So, there could be some slippage but its unlikely that any of these locations can slip below #10. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EDT this evening for ANZ452>455. && $$ Synopsis...CMS Near Term...Iovino Short Term...Iovino Long Term...CMS Aviation...CMS/Iovino Marine...CMS/Iovino Climate...Drag

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