Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 171636 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1136 AM EST Fri Nov 17 2017 .SYNOPSIS... High pressure builds over our area during today before quickly moving offshore tonight. Low pressure strengthens as it moves into the eastern Great Lakes region Saturday, then to near northern New England Sunday. A strong cold front sweeps across our area later Saturday night into early Sunday morning, then high pressure builds to our south Monday into Tuesday. A cold front moves across the region Tuesday night, followed by high pressure arriving later Wednesday and Thursday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Forecast mainly on track. Will adjust hourly grids based on latest surface obs and visible satellite images. Generally mostly sunny skies for the rest of the afternoon with a brisk northwest wind gusting to 25 or 30 MPH at times and seasonably cool temps...highs generally in the upper 40s to lower 50s... low or mid 40s Poconos. Several degrees below normal. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/... Mostly clear and a bit chillier than last night with radiational cooling in the countryside. Bands of cirrus become evident near or after midnight. Light northwest wind to start becoming light south toward morning. Lows near normal in the urban centers and about 5 or so below normal in the countryside. Forecast basis was a 50 50 blend of the 00z/17 GFS/NAM MOS. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Summary...Strong cold front moves through Saturday night and early Sunday with potentially strong winds, followed by colder air and windy conditions Sunday into Monday. Turning milder Tuesday followed by another cool down Wednesday and especially Thursday. Synoptic Overview...An upper-level trough arrives Saturday night and Sunday which takes on a negative tilt across the Northeast. This will drive deepening low pressure to our northwest, with an associated strong cold front sweeping across our area later Saturday night. As a ridge moves across west-central Canada, the next downstream trough is forecast to begin amplifying across the Great Lakes and Northeast during Tuesday. The flow aloft looks progressive at least for a time, however later next week may feature more trough amplification into the East. This still is less certain given the parade of short wave energy which may or may not result in the flow becoming highly amplified. Through this forecast period, temperatures look to be a bit on a roller coaster. For Saturday...Warm air advection strengthens in advance of an incoming sharpening upper-level trough. This will occur as a warm front lifts to our north. An area of showers or rain will accompany the warm front, although the strongest and more focused lift is mostly to our northwest. However, some showers should occur just to its south into the warm sector for parts of eastern Pennsylvania and northern New Jersey mainly Saturday afternoon. Low pressure is forecast to strengthen quite a bit as it lifts across the Ohio Valley and eastern Great Lakes region, then eventually into Canada. The associated strong cold front sweeps through our area later Saturday night. Increasing southerly flow ahead of the cold front Saturday will drive much milder air into most of the region. The deeper warm sector may not get all the way to our northern zones, especially if showers develop sooner. The flow aloft is forecast to increase substantially Saturday afternoon and especially Saturday night including a low-level jet, as the upper-level trough eventually takes on a negative tilt. There are decent 3-hour pressure rises right behind the cold front along with a rapid increase in cold air advection at 850 mb. The forecast soundings indicate as quick cooling occurs aloft combined with some lingering warmth in the boundary layer, mixing deepens and this could result in strong wind gusts along and just behind the cold front. There looks to be little in the way of instability, however strong frontal forcing may result in an enhanced band of showers with the front which could mix down some of the stronger winds. We continue to bring in categorical PoPs later Saturday and Saturday night from west to east. Given the timing and lack of instability forecast, we continue to not include thunder. Given the strengthening wind field and warm air advection, temperatures should hold steady or even rise some ahead of the cold front. For Sunday and Monday...The strong cold front quickly shifts offshore early Sunday morning with showers ending. Some clearing is expected, however a stratocumulus deck may occur as strong cooling aloft works its way downward. Deep low pressure tracks just north of New England Sunday with a tight pressure gradient in place. A continuation of cold air advection and strong flow will result in a windy day as mixing will be efficient. There could be a burst of wind first thing with the cold frontal passage accompanied potentially by a band of enhanced showers. For now, kept gusts just below advisory criteria however it is possible a wind advisory may eventually need to be issued. The airmass does turn colder with high temperatures probably occurring in the morning Sunday for most, with some drop off through the afternoon. Lake effect snow should develop off the eastern Great Lakes, and it is possible that a streamer attempts to approach the Poconos later Sunday afternoon and evening. It is possible a few showers make it farther southeast, however the main focus looks to be closer to the Great Lakes. The winds should diminish at least some Sunday night as the pressure gradient weakens a bit. High pressure starts to build in during Monday, however its center is to our south-southwest and this will keep a west-northwest breeze going. Overall, a colder day is expected Monday. For Tuesday through Thursday...Renewed upper-level trough development is forecast to take place into the Northeast. This drives low pressure once again near and north of the Great Lakes with a trailing cold front approaching from the west. As the flow backs in response to this system, some warming will be drawn northward. The main forcing with this system may end up lifting to our north although this is less certain. For now, opted to maintain a dry forecast despite a cold frontal passage Tuesday night. High pressure is forecast to gradually build in from the west later Wednesday and Thursday with a colder airmass settling in. && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR with few clouds aoa 5000 ft. NW wind around 15 kt with 25-30 kt gusts, with gust speeds decreasing late in the afternoon. Confidence high. Tonight...VFR few clouds aoa 10000 ft with thickening cirrus arriving toward sunrise Saturday and a light northwest wind this evening becomes light south toward morning. Very high confidence. OUTLOOK... Saturday...VFR during the day for most with clouds lowering some, however ceilings lowering to MVFR in the afternoon mainly at RDG and ABE with some showers. A period of MVFR/IFR conditions with showers at night. South to southwest winds increasing to 10-15 knots with gusts up to 25 knots, then becoming west to northwest and gusty toward Sunday morning. Moderate confidence. Sunday...Improving to VFR early as showers move offshore. Some flurries or rain/snow showers possible later in the afternoon and evening mainly north and west of ABE. Northwesterly winds 15-25 knots with gusts to 30-40 knots, diminishing some during the night. Moderate to high confidence. Monday...VFR. West-northwest winds 10-15 knots with gusts up to 25 kt, diminishing at night. Moderate confidence. Tuesday...VFR. Southwest winds 5-15 knots with afternoon gusts up to 20 knots possible. Moderate confidence. && .MARINE... SCA all waters today and for NJ into this evening for winds gusting to around 30 kt. Diminishing northwest winds early tonight becoming southwest or south toward morning. OUTLOOK... Saturday and Sunday...Gale Watch issued for Saturday afternoon through the day Sunday. Southwesterly winds increase Saturday especially in the afternoon and evening ahead of a strong cold front. There is lower confidence though with the amount of mixing that may occur within the southwesterly milder flow, with higher confidence of mixing along and behind the cold front for Sunday. Given the potential and enough confidence overall, will go with the watch for all zones. The winds should settle down some later Sunday night. Monday...Small Craft Advisory conditions due to winds during the day are expected to diminish at night. Tuesday...Small Craft Advisory conditions should develop within southwesterly flow ahead of a cold front. && .FIRE WEATHER... Dryer air will mix down into our area today and minimum RH will drop to near 30 percent. This combined with very little rain the past few days, wind gusts of 25 mph and fine fuels indicating considerable dryness, suggests concern for enhanced rate of spread this afternoon for any uncontrolled fires. We will check with our partners during mid morning with a small chance of a special weather statement issuance for s and central NJ. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Watch from Saturday afternoon through Sunday afternoon for ANZ430-431-450>455. Small Craft Advisory until 10 PM EST this evening for ANZ450>453. Small Craft Advisory until 5 PM EST this afternoon for ANZ430- 431-454-455. && $$ Synopsis...Gorse Near Term...Drag/MPS Short Term...Drag Long Term...Gorse Aviation...Drag/Gorse/MPS Marine...Drag/Gorse/MPS Fire Weather... is the U.S. government's official web portal to all federal, state and local government web resources and services.