Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 151150 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 650 AM EST Wed Nov 15 2017 .SYNOPSIS... High pressure over our area weakens and retreats toward the Canadian Maritimes later today and tonight. A strong cold front moves across our area during Thursday, then high pressure builds in 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. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Today...Its model rh time section battle between the NAM and GFS. An inversion near 800 MB caps moisture below it and overall am thinking the cooler more moist NAM will prevail. So this forecast was a blend of the 00z/15 GFS/NAM MOS but LOW confidence on having made the correct decision on more clouds than sun by Noon EST. For now...am fcstg temps no higher than 2F above the blended GFS/GGEM/NAM 18z 2m temps. Light northeast wind this morning veering to southeast this afternoon. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/... Cloudy and relatively mild with nearly steady temps, possibly rising a bit during the night along and east of I-95. Showers ahead of a cold front are expected to develop, mainly after 06z/16 as per HRRRX NAM/GFS...mostly likely toward 09z/16. That means a band of showers-rain moving from W to E across PA/MD into our western zones around 06z. Meanwhile showers sprouting ahead of the main band in DE/NJ sometime between 06z-09z. Amounts .05 to 0.25". Less than average confidence on rainfall ILG/MIV southward but the 00z/EC has .10 or greater over virtually all of our area 6z-12z Thu. The drivers for the rainfall area are 100m 500mb height falls over our area ahead of a modeled 160M 12 hr HFC vt max sliding sewd through sw NYS at 12z Thu, also a nice 850 trough with considerable absolute vorticity in its axis as it crosses our area by 12z. That and some pretty good jet cores at 850mb-40 kt 700mb-55kt in PA-OH aiding the forcing. This forecast was a 50 50 blend of the 00z/15 GFS/NAM MOS. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Summary...Strong cold fronts move through Thursday and again Saturday night with milder air ahead of them, followed by colder air and gusty winds. Synoptic Overview...A strong upper-level trough moves through the Northeast Thursday before lifting 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...A strong upper-level trough moves across the Northeast during Thursday. A surface low tracks well to our north, however a secondary low should develop offshore of Long Island early then track northeastward. Showers are expected to end early, with perhaps some lingering across the far north through about early afternoon. This will depend on the extent of wrap around moisture. Cold air advection sets in during the course of the day, however this is initially stronger at 850 mb and above. The result will be a milder day for much of the area. The more notable cold air advection closer to the surface is forecast to arrive later in the day. The forecast soundings indicate deeper mixing occurs during the day especially in the afternoon. Based on the soundings, there is about 25-30 knots of wind available to mix down and therefore the wind gusts in the forecast were increased. The winds will diminish some at night, 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 /12Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Reliance was on a visually blended GFS/NAM Tsection. That will mean generally VFR conditions...probably cigs based near 4000 or 5000 ft developing with heating later this morning. MVFR CIGS may develop late n the day from se to nw. Light northeast wind to start becoming southeast this afternoon. Confidence in CIG forecast height is well below average, and for winds is above average. Tonight...VFR or MVFR CIGS to start, then degrading to probable IFR conds toward 09z/16 in showers stratus fog with stratus fog contg even after showers end vcnty Philly westward near 11z (no mixing). Confidence above average on sub MVFR conditions late at night. Light southeast wind becomes light southwest or west toward 11z/16. OUTLOOK... Thursday...Areas of MVFR conditions possible early with showers, otherwise VFR with clouds thinning with the passage of a cold front. Southwest winds 5-10 knots, become northwest and increasing to around 15 knots with gusts to 25-30 knots in the afternoon. The winds gradually diminish some at night. 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. && .MARINE... SCA was extended parts of the waters through tonight and may need a final extension northward to the Sandy Hook area (for tonight) in an afternoon update. The NAM is pretty strong up there where as the GFS is sub SCA. Still to be determined. OUTLOOK... Thursday...The winds shift to the west and northwest as a strong cold front moves through. This will result in deeper mixing with time and gusts 25-30 knots especially in the afternoon. Northwesterly gale force gusts are probable Thursday night, and therefore a Gale Watch has been issued for all zones. The extent of the winds will depend on how quick the colder air arrives, enhancing the vertical mixing. 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. && .CLIMATE... November will probably average below normal throughout our forecast area. It is expected that the last 16 days will average at best normal, more likely...below normal...though how much below normal we`re unsure. I cannot at this time see the departures listed at the end of this section, warming...and suspect they`ll run colder by a degree or 3. The pattern as signaled for many recent days by the NAEFS/GEFS/ECEFS is basically a trough constantly reloading in the northeast USA through the end of the month. So the positive departure excesses from September 14 through November 6 are history. What we`re experiencing now is what fall is supposed to be like...chillier. And its probably going to be a big difference when compared to last November which averaged 1 to 3 degrees above normal. The numbers below represent the departure from average for the first 14 days of the month and those departures will probably go more negative by time the 30th arrives. ACY -0.2 55N -1.2 (ACY Marina) ABE -1.0 ILG -0.9 MPO -3.4 PHL -1.5 RDG -2.7 TTN -1.5 GED +1.0 Basically that says the coldest departures were across the northwest part of our forecast area and the mildest departures from normal, along our southeast edge. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Watch from Thursday evening through late Thursday night for ANZ430-431-450>455. Small Craft Advisory until 6 PM EST this evening for ANZ451>453. Small Craft Advisory until 6 PM EST Thursday for ANZ454-455. && $$ Synopsis...Gorse Near Term...Drag 650 Short Term...Drag 650 Long Term...Gorse Aviation...Drag/Gorse 650 Marine...Drag/Gorse 650 Climate...

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