Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 182230 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 530 PM EST Thu Jan 18 2018 .SYNOPSIS... A broad surface high will build into much of the eastern U.S. late this week before moving offshore this weekend. A strong surface low will develop in the central plains this weekend, lifting northeast into southeast Canada early next week. This will bring a strong cold front through the area Monday night and Tuesday. High pressure will build back into the region during the middle of next week. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... The only thing of note to talk about in the near term period is an upper level short wave trough which is expected to pass well north of our region, but could bring in some high clouds to the area. Otherwise, rather tranquil pattern will continue. Light northwesterly winds should back to westerly as the high continues to build closer to the area. With few clouds, light winds, and dew points in the single digits and teens across the area, expect prime radiational cooling conditions. Lows are expected to range from the teens to low 20s across the area. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/... A modest warming trend is expected tomorrow as the 1000 to 500 mb thickness increase. Pressure gradient will be rather weak tomorrow with the high to our south. Consequently winds should be light (10 mph or less) through the day. && .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... A warm and dry weekend followed by an inclement start to the new week... Model solutions remain rather stable through Tuesday of next week, with the midlevels featuring generally zonal flow through the weekend followed by some ridging downstream of a negatively tilted trough moving through the Midwest/Great Lakes late this weekend into early next week. The trough passes the region Tuesday and Tuesday night, with reinforcing perturbations in transient northwest flow Wednesday. Model discrepancies become large by this time, as phase differences among the ECMWF, GFS, and CMC become pronounced. However, in terms of sensible weather, think the Tuesday night through Wednesday night period should be dry before uncertainty becomes high enough at the end of the long term for at least mentionable (slight chance) PoPs. Not much to discuss sensible-weather wise this weekend, as a strong surface high in the Southeast moves offshore. Statistical guidance remains consistent in bringing temperatures near or slightly above 50 for the Philadelphia metropolitan area this weekend but remain muted in warming it up much more than this. Given the stubborn zonal flow (i.e., lack of increased amplification downstream of the developing trough in the central U.S.) depicted, the plateau of temperatures around 10 to 15 degrees above average Saturday through Monday makes sense, though I do wonder if models are underdoing the amplification somewhat. As such, I kept things close to MOS guidance, but did bump maximum temperatures a degree or two during this period. Isentropic ascent downstream of the negatively-tilted trough on Monday may lead to warm-advection precipitation in the northern Mid-Atlantic and Northeast, which required keeping/increasing PoPs Sunday night and Monday, especially north of the Mason- Dixon Line. Such regimes tend to be favorable for freezing rain or sleet (i.e., a low-level warm nose), and did retain mention of this in the far northern CWA Sunday night and Monday morning. As the strong cyclone in the plains moves northeast into the Great Lakes and southeast Canada Monday night and Tuesday, a strong cold front will sweep through the Mid-Atlantic during this period. Models continue to show decent QPF in vicinity of the front, with some disagreement on timing (GFS fast; ECMWF slow; CMC in between). Timing continues to favor Monday night, though it has slowed in the past couple of days to include Tuesday morning (and perhaps most of the day in the northern/eastern CWA). Given the strong/consistent agreement among models Monday night, introduced categorical PoPs during this period and increased PoPs a little bit through the day Tuesday. Precipitation may be showery, especially close to the front, but large-scale lift will be pronounced, suggesting a more widespread rain may occur. Should showers/convection develop, QPF may be locally enhanced, and this will need to be watched as the event approaches. A rumble of thunder cannot be ruled out either, given cold midlevels with the approaching trough, though this threat looks too low for mention at this time. There are some indications of upstream perturbations moving through the transient northwest flow Tuesday night and Wednesday, which may lead to flurries or snow showers in the Poconos, but the signal is not agreed upon amongst the operational models. Kept Tuesday night through Wednesday night dry as a result, but temperatures will fall back to seasonal averages as colder high pressure builds into the area. The 12Z ECMWF keeps the high entrenched in the Mid-Atlantic through Thursday, but the 12Z CMC is quick to bring a developing low through the area late in the long term (with some indication of this in the GFS as well, though pushed off to our southeast). Will add slight chance PoPs for Thursday at this time, though this may be on the fast/aggressive side. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR conditions are expected with few high clouds. Light northwesterly winds this evening are expected to back to westerly. Wind speeds through the night are expected to be less than 10 kt. Tomorrow...VFR conditions with few, if any, clouds. Westerly winds near or below 10 kt. OUTLOOK... Thursday night through Sunday...Primarily VFR, though increasing clouds should occur Sunday. Winds generally west around or below 10 kts. High confidence. Sunday night and Monday...Sub-VFR conditions possible with increasing chances of rain or showers during the period. Generally light and variable winds becoming south or southeast around or below 10 kts. Low confidence. Monday night and Tuesday...Periods of sub-VFR likely with a good chance of rain/showers, especially through Tuesday morning. Southeast winds becoming west or southwest after frontal passage on Tuesday (probably by the afternoon). Winds may become gusty after frontal passage and in vicinity of showers. Medium confidence. && .MARINE... Winds and seas on the coastal waters are expected to slowly diminish through the evening, and should fall below small craft advisory criteria for several hours overnight. Gusts above 20 kt will again be possible, primarily for the NJ coastal waters during the day tomorrow. On the Delaware Bay, winds and seas should stay below SCA criteria tonight and tomorrow. OUTLOOK... Friday night through Saturday night: Marginal advisory conditions may occur off the northern/central NJ coast; otherwise, sub-advisory conditions and fair weather expected. Sunday through Monday: Sub-advisory conditions, with an increasing chance of rain or showers by Monday. Monday night and Tuesday: Advisory-level winds possible (south or southeast before frontal passage; west after frontal passage), with a good chance of rain or showers. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 1 AM EST Friday for ANZ450>455. && $$ Synopsis...CMS Near Term...Johnson Short Term...Johnson Long Term...CMS Aviation...CMS/Johnson Marine...CMS/Johnson

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