Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 161042 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 542 AM EST Thu Nov 16 2017 .SYNOPSIS... A strong cold front moves across our area today, then high pressure builds in during Friday. A warm front lifts to our north Saturday, followed by a strong cold front Saturday night and early Sunday. High pressure then builds to our south Monday into Tuesday, followed by the next cold front moving through Tuesday night. High pressure should then be building toward our region Wednesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 630 AM ESTF: Not anticipating much change from what was posted prior to 330 AM. Everything seems to be well modeled. Prior to 7 AM...Bands of showers preceding the cold front will be exiting NJ around 12z having brought .01 to .10 to virtually all of E pa and NJ with the cutoff of widespread measurable from roughly ILG to MIV, with scattered measurable along and south of that line. So the NAM was too robust as also some of the ECMWF cycles leading up to this overnight period now ending. Today...Clearing 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. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/... Mostly clear and breezy with rising heights aloft (500mb) and not much radiational cooling. West-northwest wind gusts 20-30 mph most of the night... gusts 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. Tsection shows this nicely with any echo tops probably below 12000 ft. 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. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... 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. Synoptic Overview...A strong upper-level trough lifts out with surface high pressure moving across our area during Friday. Some weak ridging then arrives, but another upper-level trough quickly arrives Saturday night and Sunday. As more pronounced ridging moves across west-central Canada, a downstream trough is forecast to begin amplifying across the Great Lakes and Northeast by late Tuesday. The flow aloft overall looks progressive for awhile, however next week (especially the second half) may feature a more persistent trough settling into the East. This has uncertainty though with it, based on less agreement among the guidance on whether the flow becomes highly amplified or not. For Friday and Saturday...A strong upper-level trough moves away during Friday with the flow becoming more zonal. High pressure at the surface builds across our region by later Friday which will result in less wind through the day. It will be cooler Friday along with a much drier airmass and a breeze, however the breeze will diminish during the afternoon and at night as high pressure arrives. As we move into Saturday, there is strong warm air advection in advance of an incoming sharpening upper-level trough, 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 even slower in the guidance overall as we go through Saturday. As a result, we slowed down the PoP increase quite a bit from west to east Saturday as much of the area may remain dry during the day. Low pressure is forecast to strengthen quite a bit 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 much milder air into the region, then be replaced by west to northwest winds behind the frontal passage. The flow aloft is forecast to increase substantially Saturday night especially along and behind the cold front. There are decent 3-hour pressure rises right behind the cold front along with a rapid increase in cold air advection. The forecast soundings indicate as cooling occurs aloft combined with some lingering warmth in the boundary layer, mixing could promote 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 likely and categorical PoPs mainly Saturday night. Given the timing and lack of instability forecast, no mention of thunder was included. Temperatures may hold steady for awhile Saturday night 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. Strong cold air advection combined with 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 below advisory criteria Sunday however it may be close. 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. It is possible some showers may 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 and Wednesday...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 should be approaching from the west during Wednesday with a cooler airmass. && .AVIATION /12Z THURSDAY THROUGH MONDAY/... 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 or cigs. Confidence: Average. After 12z today...VFR sct-bkn bkn aoa 4000 ft with chc of a brief MVFR vsby shower in the 18z-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: Well above average. Operational HRRR has both the shower and wind gust elements nicely and I think accurately modeled. Tonight...VFR clear or sct clouds aoa 5000 ft. a gusty west northwest wind around 20-28 kt should subside near 11z/Friday morning. Confidence: Well above average. OUTLOOK... Friday...VFR with mainly high clouds increasing at night. Northwest winds 10-15 knots with gusts to 20 knots, becoming southwest and diminishing to around 5 knots late in the afternoon and at night. High confidence. Saturday...VFR during the day with clouds lowering some, then a period of MVFR/IFR conditions with showers at night. South to southwest winds increasing to 10-15 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 snow showers possible late in the afternoon and evening mainly north and west of ABE. Northwesterly winds 15-25 knots with gusts to around 35 knots, diminishing some during the night. Moderate confidence. Monday...VFR. west to northwest winds 10-15 knots. Moderate confidence. && .MARINE... 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). OUTLOOK... Friday...Northwesterly gale force gusts may linger for a time Friday morning for parts of the area, however the winds are expected to be diminishing. Given some uncertainty, held off on extending the Gale Warning into this time period. Saturday and Sunday...Gale force gusts from the southwest are possible Saturday afternoon and evening especially on the ocean zones, then gale force gusts from the west and northwest are probable later Saturday night and especially Sunday. Monday...Small Craft Advisory conditions expected for awhile. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Warning from 4 PM this afternoon to 6 AM EST Friday for ANZ430-431-450>455. && $$ Synopsis...Gorse Near Term...Drag 541 Short Term...Drag 541 Long Term...Gorse Aviation...Drag/Gorse 541 Marine...Drag/Gorse 541 is the U.S. government's official web portal to all federal, state and local government web resources and services.