Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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749 FXUS61 KPHI 261340 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 940 AM EDT Mon Sep 26 2016 .SYNOPSIS...
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High pressure centered over the New England this morning will shift offshore this afternoon. A cold front crossing the Appalachians later today will progress through the forecast area tonight. The front will stall offshore on Tuesday. An upper low is expected to meander over the eastern United States during the second half of the week before eventually lifting northeastward sometime during the weekend. Meanwhile, several waves of low pressure may develop along the stalled boundary and move up the eastern seaboard.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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High pressure continues to push to the east, resulting in a southerly flow developing across the region through this afternoon. Skies have started to cloud up a bit along the western edge of our forecast area this morning and we have increased the sky cover in the forecast to reflect cloudier conditions through the day. A cold frontal boundary with an area of low pressure across the Great Lakes will continue to move eastward toward our region. Rain should off till after sunset but clouds will be on the increase from west to east. With the southerly flow, areas that stay sunny east of the I-95 corridor should see highs in the mid 70`s, likely a few degrees warmer than MET/MAV. On the other hand, further west highs will likely be kept back in the 60`s due to afternoon cloudcover.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/... The previously mentioned cold frontal boundary moves through the region tonight. Enough lift and moisture will be present for a period of showers ahead of the front. Model agreement is fairly good in terms of QPF and timing of the rain. We are looking at a mid to late evening start in eastern Pennsylvania and Maryland with a midnight-ish start across New Jersey and Delaware. QPF totals have lowered slightly compared to the last few model cycles. This is something that typically happens when models are trying to account for drier than normal conditions. Still a 1/4-1/3 inch of rain is expected to fall regionwide. An isolated thunderstorm is possibile with LI`s approaching 0 across Delmarva and areas slightly further north. Lows should be around 60 or so in most spots. && .LONG TERM /TUESDAY THROUGH SUNDAY/... The cold front will move off the coast early Tuesday morning. In response to strong downstream ridge blocking over the western Atlantic Ocean, the southern tail of the front is expected to slow down or stall nearby to our east off the NJ-Delmarva coast and to our south near the VA-NC border on Tuesday. The vertically-stacked closed low still be located well upstream of us over the northern Great Lakes. A potent 125 kt upper-level jet streak positioned on the eastern flank of the trough will continue to provide synoptic lift near the front. Accordingly, we anticipate the back edge of the showers to not make much progress eastward past the I-95 corridor on Tuesday. Precipitation should be mainly light with the deeper moisture and instability shifting offshore, but it looks to persist through most of the day in the coastal plain and perhaps linger into the evening near Cape May, NJ and in southern DE. Lower than normal forecast confidence continues to plague the forecast for the second half of the week and upcoming weekend with a potential for a cutoff low to meander over the eastern CONUS and several waves of low pressure moving slowly up the eastern seaboard (somewhere between the lee side of the Appalachians and the coast). Models are having a difficult time resolving the details in this complex setup as evident by the large spread between the guidance and considerable run-to-run shifts seen with almost all of the global models. The high degree of uncertainty seems to be related to how much energy from the primary surface low (associated with the cutoff low over the Midwest) transfers to secondary low pressure and how quickly the transfer occurs. This will all determine if there is one organized secondary low or several disorganized waves as well as the strength and track of each disturbance. The good news is that the spread seems to be gradually diminishing with each run, allowing us to take some useful clues from these trends. We may be able to briefly squeeze out a dry stint Tuesday night into part of Wednesday with the closed low still over the Great Lakes/Midwest and before secondary cyclogenesis to the lee of the Appalachians. After that, it is looking more likely that we`ll see a prolonged period of unsettled conditions later Wednesday through the end of the work week and possibly extending into the weekend. This is when the upper low becomes cutoff to our west. Meanwhile, high pressure anchored over southeastern Canada will create a CAD setup with cool air draining down the eastern side of the Appalachians. This high anchored to our north would provide a blocking setup. Therefore, any secondary low that develops to our south would likely stall somewhere in the mid-Atlantic region. The end result would produce cloudy, cool and potentially rainy conditions for several days. Still too early to say how much precipitation we will get from this event but it can range from mainly light rain and drizzle to periods of moderate or even locally heavy rain. && .AVIATION /14Z MONDAY THROUGH FRIDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Mainly VFR conditions expected through late afternoon. MVFR ceilings have started to edge towards the region from the west this morning. Expect bases to remain around 3500 ft or higher at the terminals but they will start to lower as we head into this afternoon and evening. MVFR conditions are anticipated for the overnight hours. A cold front will cross the terminals tonight with rain showers lowering vsbys to MVFR levels. Timing looks to be around 03z for KRDG and more toward 07z at KACY (plus or minus a couple of hours). Southerly winds increasing throughout the day to around 10 knots with isolated gusts up to 20 knots in the afternoon. Winds decreasing and becoming more westerly overnight. OUTLOOK... Tuesday...Quickly improving to VFR in the western terminals (ABE/RDG) early morning and then I-95 terminals either late morning or early afternoon. MVFR/IFR conditions and showers may linger near the coast (MIV/ACY) through the afternoon or evening. Forecast confidence: moderate. Tuesday night and Wednesday morning...Mainly VFR. Forecast confidence: moderate. Wednesday afternoon through Friday...Sub-VFR conditions in showers likely. Best chances for more widespread rain appear to be on Thursday and Friday. Forecast confidence: low.
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&& .MARINE... Waveheights will build this afternoon, potentially reaching five feet for a period this evening ahead of a cold front. Southerly winds will also ramp up as well tonight with gusts from 25-30 knots looking likely based on sounding and model data. With an increased confidence for SCA gusts, we have issued an SCA for tonight into Tuesday morning. OUTLOOK... Tuesday morning...SCA continues thru 15Z for both the ocean and the bay. SW winds ahead of a cold front will gust to around 25 kt early while seas in the coastal waters near 5 ft may persist later in the morning. Tuesday afternoon through Wednesday night...Winds and seas below SCA threshold. Thursday through Friday...NE winds 10-20 kt. Seas may build to 4-6 ft and thus a SCA may be needed. Low confidence in forecast for late in the week. RIP CURRENTS... Even though waveheights will likely be a bit lower today compared to yesterday, increased southerly winds by this afternoon will still likely contribute to a moderate risk for the formation of dangerous rip currents today. This moderate risk may continue into the first part of Tuesday before diminishing toward afternoon as winds and seas subside behind a cold front. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 6 PM this evening to 11 AM EDT Tuesday for ANZ430-431-450>455. && $$ Synopsis...Klein Near Term...Gaines/Meola Short Term...Gaines Long Term...Klein Aviation...Gaines/Klein/Meola Marine...Gaines/Klein

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