Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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899 FXUS61 KPHI 160701 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 201 AM EST Thu Nov 16 2017 .SYNOPSIS...
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A cold front moves across our area this morning, then high pressure builds during Friday. A warm front lifts to our north Saturday, followed by a cold front Saturday night. High pressure then builds to our south Monday into Tuesday.
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&& .NEAR TERM /THROUGH TODAY/...
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1230 AM ESTF. New 330 am forecast issued. Please see below. Prior to 7 AM...Bands of showers preceding the cold front will probably intensify as they exit NJ around 6 am. Patchy fog may develop around 6 or 7 am as the drying aloft arrives e PA and e MD with the light south wind trending west at that time in e PA and e MD. Mild. Today...any remaining showers and sprinkles exiting e Nj at 12z is followed by clearing and with the mild start...temps will rise into the upper 50s/lower 60s I95 corridor eastward...not quite as mild to the west of I95. A 150m 12 hr HFC will pass through ne PA or se NYS around 18-20z and both the HRRR and HRRRX have a band of showers developing ne PA around 17z exiting NNJ around 20z. This band is in the grids-fcst (WINDEX like with steep somewhat moist low lvl lapse rates - Tsection pf both NAM and GFS show this nicely--tops below 15000 ft) and thereafter, pretty strong cold advection ensues with gusty west northwest winds to 30-35 MPH mid and late afternoon just about everywhere. Forecast basis was the 00z/16 50 50 blended GFS/NAM MOS. I had to raise the wind gust tool manually by about 5 kts during mid and late afternoon per the NAM transfer signal. (I do realize the wrong decision ydy at 3 am on skycover - NAM was biased high in the morning resulting temps 1-2F warmer than fcst and the NAM 2m temps were poor ydy).
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&& .SHORT TERM /TONIGHT/...
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Mostly clear and breezy with rising heights at aloft (500mb) and not much radiational cooling. West-northwest wind gusts 20-30 mph most of the night... well up in the 30s over the Poconos and terrain above 1500 feet far nw NJ. There could be some sprinkles and flurries in the Poconos during the evening. The basis of this part of the forecast was a 50 50 blend of 00z/16 GFS/NAM MOS tending to just use the 3hourlies to fcst the low temp. This forecast may be a degree or two on the cold side. Tsection shows this nicely with any echo tops probably below 12000 ft.
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&& .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Summary...Strong cold front moves through Saturday night with milder air ahead of it, followed by colder air and gusty winds. Synoptic Overview...A strong upper-level trough lifts out on Friday. Some weak ridging then arrives, but another upper-level trough quickly arrives Saturday night into Sunday. As more pronounced ridging moves across west-central Canada, a downstream trough is forecast to begin amplifying across the Great Lakes and Northeast Tuesday. The flow aloft overall looks progressive for awhile, however there are signals that next week may feature more amplification with a trough settling into the East. For Thursday night, the winds will diminish some, however given the continuation of cold air advection in the lower levels and the pressure gradient remaining tight this process may be slowed. For Friday and Saturday...The strong upper-level trough moves away during Friday with the flow becoming more zonal. High pressure at the surface builds across our region Friday which will result in less wind through the day. It is noted that the stat guidance especially is warmer for Friday, however the modeled 2 meter temperatures are colder. The latter is rather close to our going forecast, and therefore we opted to make little in the way of changes. Clouds increase Friday night ahead of the next incoming upper-level trough. There is fairly strong warm air advection in advance of this, allowing a warm front to become better defined as it lifts northeastward. This should produce a zone of showers or rain along and to its north, however this appears to be slower in the guidance overall as we go through Saturday. As a result, we slowed down the PoP increase from west to east Saturday. Low pressure is forecast to strengthen as it lifts across the eastern Great Lakes region Saturday morning and then into Canada. The associated cold front sweeps through our area Saturday night. Increasing southerly flow ahead of the cold front Saturday will drive milder air into the region, then be replaced by west to northwest winds behind the frontal passage. We continue to bring in likely to categorical PoPs through Saturday night. Given the timing, held off in adding thunder. For Sunday and Monday...While some showers may linger early Sunday as the cold front shifts offshore, drying is expected. Low pressure tracks near northern New England Sunday, with a tight pressure gradient in place along with cold air advection. Sunday is looking like another breezy/windy day as mixing deepens during the day. The air turns colder and therefore lake effect snow should develop off the eastern Great Lakes. It is possible that a streamer attempts to approach the Poconos later Sunday afternoon and evening. The winds should diminish at least some Sunday night. High pressure starts to build in during Monday, however its center is to our south- southwest. Overall, a cold day is expected Monday. For Tuesday...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 draw northward. The main forcing with this system may end up lifting to our north, therefore continued with a dry forecast at this time. && .AVIATION /07Z THURSDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Early this morning prior to 12z...VFR cigs with a light south wind except brief MVFR conds in showers. Wind turns light southwest to west near 11z over e PA and e MD and there may be some patchy MVFR fog. Confidence: Average. After 12z today...VFR sct-bkn bkn aoa 4000 ft with chc of a brief MVFR vsby shower in the 118z-20z time frame KRDG/KABE/KTTN/KPNE as the trough axis aloft passes by, then a west northwest wind develops with gusts to 25-30 kt by 21z most TAF sites. Confidence: Above average. Tonight...VFR clear or sct clouds aoa 5000 ft. a gusty west northwest around 20-25 kt should subside near 11z/Friday morning. Confidence: Above average. OUTLOOK... Thursday night...VFR with winds diminishing. Moderate confidence. Friday...VFR overall, then clouds increase and lower some at night. High confidence. Saturday...A period of MVFR/IFR conditions with showers, especially in the afternoon and at night. Some improvement in the conditions should occur late at night as a cold front moves through. Moderate confidence. Sunday...VFR overall. Some flurries or snow showers possible in the afternoon mainly north and west of ABE. Northwesterly winds 15-20 knots with gusts up to 30 knots. Moderate confidence.
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&& .MARINE...
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SCA being forsaken for simplicity to a gale warning beginning around 21z all waters and not ending til 11z Friday. Looks like steepest lapse rates are in the 00z-03z/17 time frame (this Thursday evening) with a last gasp burst of gale gusts expected around 09z/17 (4 AM Friday). greatest confidence for verifying is lower De Bay and the coastal waters of NJ. Least confidence is vcnty buoy 44009 (ANZ455). We appreciate LWX having issued the warning ydy aftn, in the interest of customer lead time planning for a probable gale event of 34-40 kt gusts for at least two hours in a 12 hour period. OUTLOOK... Friday...Gale force gusts within a northwesterly flow may linger Friday morning, however the winds are expected to be diminishing. Saturday and Sunday...Gale force gusts from the southwest are possible Saturday afternoon and evening, then gale force gusts from the west and northwest are possible later Saturday night and Sunday.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 6 AM this morning to 6 PM EST this evening for ANZ430-431-450>453. Gale Warning from 6 PM this evening to 6 AM EST Friday for ANZ430-431-450>455. Small Craft Advisory until 6 PM EST this evening for ANZ454- 455.
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&& $$ Synopsis... Near Term...Drag 2a Short Term...Drag 2a Long Term...99 Aviation...Drag/99 2a Marine...Drag/99 2a

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