Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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371 FXUS61 KPHI 231336 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 936 AM EDT Sun Oct 23 2016 .SYNOPSIS... An intense low pressure system will continue to move northward across eastern Canada today into tonight. A cold front will slide across our region late tonight and early Monday morning. High pressure from central Canada will then build toward our region through mid week. A warm front, followed quickly by another cold front, is expected in our region at the end of the week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... The water vapor imagery this morning shows a well developed midlevel cyclone just north of Maine. At the surface, deep low pressure is also centered just north of Maine. As we go through this afternoon, an upper-level trough weakens across the area as it shifts more into the Canadian Maritimes. A tight pressure gradient remains across our area for much of today, which will result in a gusty westerly wind. However, the airmass is warming aloft (not as steep low-level lapse rates overall) which should temper the mixing at least some. The 12z Sterling, VA RAOB shows the warming that has taken place since last evening below about 700 MB. Overall though, some increase in the winds are expected then the gradient relaxes toward late afternoon with the expectation of the winds diminishing. The combination of the cold air advection shifting to our north and east with incoming warm air advection has helped to strengthen a low- level inversion. This is evident on the 12z Sterling, VA RAOB just below 700 MB. Just to our west, moisture is trapped beneath this inversion in the form of stratocumulus and some of this should be pulled eastward. May need to add more cloudiness to at least the Poconos for awhile. Given the warming aloft, a milder afternoon is expected. The hourly temperature and dew point grids were adjusted based on the latest obs, then the LAV/LAMP guidance was blended in through early afternoon. The winds were adjusted down initially for many areas, then a continued increase was shown for awhile. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... A weak disturbance will move acrs nrn areas tonight. This feature will bring some showers, but any rain will be over with before daybreak. The vast majority of the guid keeps precip confined to nrn sections, while the ECMWF wants to bring it a bit further s. For now will go with the more nrn soln. && .LONG TERM /MONDAY THROUGH SATURDAY/... Monday and Tuesday...models have sped up the development of the northwesterly flow pattern, and thus the cold air advection. Therefore, expect a bit of a cooling trend with highs tomorrow, and then even more pronounced on Tuesday. Monday night begins what will likely be the first of several nights with frost/freeze potential for much of the area where the growing season continues. With another day of cold air advection, and even better radiational cooling conditions, Tuesday night looks to be even colder, with a freeze likely for much of southeastern PA (with the exception of the Philly metro area), and the remainder of NW NJ. the high moves closer, flow shifts more northerly, but still quite cool with highs about 10 degrees below normal. There will be another risk for freeze on Wedensday night. However, unlike Tuesday night, increasing clouds late may somewhat limit radiational cooling. If the clouds come in early enough, this could inhibit frost development. For now, kept a mention of patchy frost for areas with a forecast min of 36 degrees or less, but may have to adjust both the min temp forecast and mention of frost if the trend continues with low and mid level clouds moving in before dawn. Thursday and Friday...Models came into much better agreement with the track of the extra tropical low lifting across the Great Lakes region and into Quebec through this time. However, there remain some timing differences, with the GFS being the more progressive solution, while the ECMWF and CMC are slower. Still, with this track, it looks like a warm front will lift through our region sometime on Thursday. Within the warm sector, we should see increasing chances for precipitation across the region. Then, a cold front should sweep through the region on Friday. Those are the factors that the models agree on. However, the GFS is depicting a mid level short wave trough lifting through the region ahead of the warm front on Thursday morning, providing enough lift for some pre frontal precip. This is significant because GFS model soundings across the Poconos indicate that if this does happen, it could be a rain/snow mix. However, it is a very small chance especially considering that neither the CMC and ECMWF are depicting this and there would be limited opportunity for moisture advection ahead of the front. For now have only mentioned a chance of rain and snow across the Poconos through Thursday morning, before switching precip to all rain. Saturday and Sunday...models are depicting the cold front stalling near or just south of our Delmarva counties. However, this seems unlikely given the upper level northwesterly flow pattern. Thus...have kept the forecast dry beyond Friday. && .AVIATION /14Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of Today...VFR. West (locally west-southwest winds early) at 10- 15 knots, becoming West-northwest with gusts to around 25 knots by late morning, then diminishing late in the afternoon. Tonight...VFR with an increase in clouds. Lower VFR ceilings from about KABE to KTTN on northward with some showers arriving after 04z. West to southwest winds diminishing to 10 knots or less, although some increase may occur toward daybreak Monday with a wind shift to the northwest. Outlook... Monday through Wednesday...mostly VFR conditions expected. Wednesday night and Thursday...ceilings should lower through the overnight hours to MVFR (with possible localized IFR) by mid day Thursday. Visibility restrictions possible with rain showers later in the day. && .MARINE... We held onto the Gale Warning for now (all areas except Upper Delaware Bay) as a few gusts, based on the available obs, are to 35 knots. The trend is expected to be down through the day for much of the area given the airmass is now warming (thus reducing the mixing with time). Once the Gale Warning is removed it will be replaced with a Small Craft Advisory for a time. Seas were lowered a bit based on the available obs and a continuation of the strong offshore flow. Outlook... Monday and Tuesday...SCA conditions are expected to continue as winds should continue gusting above 25 kt (though seas may drop off below 5 ft. Wednesday and Thursday...winds and seas should stay below SCA criteria. Thursday night...winds could increase above SCA criteria on the coastal waters late. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Warning until 6 PM EDT this evening for ANZ450>452. Gale Warning until noon EDT today for ANZ431-453>455. Small Craft Advisory until 4 PM EDT this afternoon for ANZ430. && $$ Synopsis...Gorse/Johnson Near Term...Gorse Short Term...Nierenberg Long Term...Johnson Aviation...Gorse/Johnson Marine...Gorse/Johnson is the U.S. government's official web portal to all federal, state and local government web resources and services.