Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 200835 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 335 AM EST Mon Feb 20 2017 .SYNOPSIS...
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High pressure will build over the Mid Atlantic today and tonight before shifting offshore. A warm front will slide through the region Tuesday night. We may see a cold front then nudge into eastern Pennsylvania and northern New Jersey Thursday night, but it should quickly lift back north as a cold front. As a low pressure system lifts into eastern Canada this weekend, the associated cold front will sweep through our region on Saturday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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High amplitude mid-level ridge axis over the Great Lakes will continue to build Canadian high pressure south into our region today. Upper level jet axis will be positioned just northeast, with our area within the confluent region, promoting large scale subsidence through the column. Expect a sunny day with some cirrus, given the proximity of the aforementioned jet. We went above guidance, given the expectation of greater boundary layer mixing than depicted in model soundings, as well as a recent cold bias. High temperatures will average around 10 degrees above normal. With continued surface pressure rises and mixing within the diurnal boundary layer, northwest winds will gust 15 to perhaps 20 mph at times.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/...
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The mid-level ridge axis moves to the Appalachians tonight, with the surface ridge situated across our area. Winds will back from the northwest this evening to the east-northeast overnight, becoming light and variable. High cloudiness will be increasing, especially after midnight, and to the west of the NJ turnpike/I-95. The model trend is to bring this upper level moisture in faster, which will inhibit to some extent ideal long wave (radiational) cooling. At this point, given the dry air mass in place and initially clear sky until at least midnight (especially the Pine Barrens), 2-meter temperatures will fall quickly. Some of the MOS guidance that was initially bullish on low temperatures has backed off some. For now, we have increased temperatures slightly across the typically colder locations by a couple of degrees to account for this, but they will still average around 5 degrees above normal.
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&& .LONG TERM /TUESDAY THROUGH SUNDAY/...
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For a relatively quiet pattern, the long term is shaping up to have quite a bit to discuss. Tuesday and Tuesday night...main question through this period will be how quickly will the surface high and upper level ridge shift off shore. As once that happens, the warm front and rain showers will move into our region. For now, this looks to occur primarily Tuesday night. Thus, it still looks like Tuesday will be the coldest day of this week, with highs generally in the 40s and 50s (this is still 5 to 10 degrees above normal). Wednesday and Thursday...Big story through this period will be the warmth! Significant warm air advection pattern is expected to develop. Stayed close to the previous forecast with highs generally in the 60s on Wednesday and in the 60s and 70s on Thursday. This is several degrees higher than any guidance, but with our most recent warm air advection event (Saturday and yesterday), all guidance showed a cold bias anywhere from 5 to 10 degrees for the MOS guidance and up to 15 degrees for the operational runs of the GFS. It looks likely that some more records will be broken, especially on Thursday (climate section below has the record highs for both dates for our climate sites). As for precip through this period, GFS continues to show some QPF over the region on Thursday, but this seems unlikely as we will be at or near the saddle point between three surface lows (one in eastern Canada, one off the coast of FL, and one over the high plains). For now, just have a slight chance across Delmarva. Friday...There remains poor agreement between models and poor run to run consistency with how far south the cold front will get on Thursday night/early Friday. With the 00Z runs, the GFS shows the front briefly sinking into E Central PA and NW NJ Thursday night, before lifting north of the region by mid day Friday. ECMWF shows the front staying well north of the region. This looks to be primarily related to differences in the tracks of the previously mentioned surface low which will be lifting from the High Plains to the Great Lakes region through this time. ECMWF keeps the low on a more progressive and northerly track versus the GFS. Hesitate to make many changes to the forecast given the uncertainty, but if models trend closer to the ECMWF solution, this could mean that the current forecast is too cool (temps could be close to Thursday highs with the ECMWF solution) and too wet (with a further north track, our region would stay in the dry slot through the day on Friday and Friday evening). Will have to continue to watch this as we get closer. Saturday...Regardless of the previously mentioned differences between the models, it looks like the prime period for precip with this low will be Saturday morning and afternoon in the warm sector just ahead of the cold front. The cold front looks to sweep through our region later in the day (meaning we should have warm air advection through much of the day time hours). In the upper levels, it looks like we will have a jet over our region through this period. Looking at the GFS model soundings through this period, instability is lacking, but there are large bulk shear values. As mentioned previously, the operational runs of the GFS have had a very strong cold bias, so accounting for that, we could see modest instability during the day on Saturday. If all of this timing holds, and if we do warm more than what the GFS is depicting, then it could set the stage for strong thunderstorms. There are still too many what ifs to include in the hazardous weather outlook at this time, but we will continue to monitor as we get closer. Sunday...strong cold air advection behind the cold front could mean the first day in over a week of near normal temperatures. Could see lake effect snow showers propagate into the Poconos while we have strong northwesterly flow, but that doesn`t look to last too long as a surface high should build over the region late in the day resulting in light boundary layer winds.
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&& .AVIATION /08Z MONDAY THROUGH FRIDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR. Northwest wind gusts 15 to 20 knots late this morning into the afternoon. Tonight...VFR. Northwest winds will gradually veer to the east, as they become light and variable after midnight. Outlook... Tuesday...Ceilings could lower to MVFR with rain showers continuing into Tuesday night. Wednesday and Thursday...Mostly VFR conditions are expected. Friday...starting VFR could lower to MVFR if rain showers move in earlier than forecast.
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&& .MARINE...
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Today and tonight...Canadian high pressure continues to build south over the waters. Northwest winds today will veer to the north this evening, then east-northeast overnight. Sub-SCA conditions forecast. Outlook... Tuesday through Friday...Winds and seas should stay below SCA criteria. A gradual shift from southeasterly to southwesterly is expected on Tuesday. By Thursday night, winds could be light and variable shifting to southeasterly on Friday.
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&& .FIRE WEATHER...
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Relative Humidity values will fall in to the low to mid 30s this afternoon across portions of northern DE, NJ, northeast MD, and PA. In addition, fuel moistures are forecast to range from 7 to 10 pct outside of the Poconos. But at this point, we expect winds to largely remain below 20 mph, in terms of sustained winds and gusts.
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&& .CLIMATE...
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New records for this date have been established at PHL ILG ABE TTN and RDG. Here are/were the previous record highs for Sunday: PHL 68-1948 now 70-2017 ILG 67-1997 now 71-2017 ABE 63-1997 now 68-2017 TTN 68-1948 now 69-2017 GED 74-1976 ACY 73-1961 RDG 67-1997 now 68-2017 MPO 58-1981 Wednesday 2/22 Thursday 2/23 ACY 68-1991 72-1985 PHL 68-1997/1974 75-1874 ILG 69-1997/1974 72-1985 ABE 68-1974 71-1985 TTN 66-1997 74-1874 GED 72-1997 no data RDG 70-1974 72/1932/1922 MPO 56-1997 60-1977
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Johnson Near Term...Franck Short Term...Franck Long Term...Johnson Aviation...Franck/Johnson Marine...Franck/Johnson Fire Weather... Climate...

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