Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 162044 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 344 PM EST Sat Dec 16 2017 .SYNOPSIS... A cold frontal boundary along the northern Pennsylvania border with New York State will move southeast into our region tonight. This front will then return northeast of the region as a warm front late Sunday night. Another cold front will then move through the mid Atlantic states Tuesday night. High pressure follows Wednesday night. Thereafter, to end the week, an area of low pressure will track northeast from the Great Lakes and advance a warm front into our region. && .NEAR TERM /THROUGH SUNDAY/... Cold front continues to push south into NY/PA as of 20Z. The 12Z model suite is advertising some light snow showers across portions of the area north of I-80. As the airmass continues to moisten in advance of the front, expect this activity to gradually move into the aforementioned area late this afternoon and early this evening, ending by midnight. Only expecting a few hundredths of an inch of liquid equivalent precip, and with snow ratios higher in the cold airmass, this is expected to translate to amounts generally from a dusting to a half an inch. Cannot rule out an isolated one inch amount, especially across the highest elevations of the southern Poconos. The cold front will slowly cross our region overnight. Expect considerable cloudiness to continue invof and north of the PA Turnpike to I-195 corridor. South of this area, partial clearing is anticipated. With winds becoming light overnight, dew points higher compared to last night, and the moist low-level airmass, patchy fog may develop across portions of southern NJ, southeast PA, and Delmarva. Our min temperature forecast remains along the colder envelope of the guidance, given the warm bias exhibited recently, including the fresh snow cover around and east of I-95. But with cloud cover across portions of the region overnight (PA Turnpike to I-195 and points north), the forecast may need fine tuning. && .SHORT TERM /SUNDAY NIGHT THROUGH 6 PM SUNDAY/... Weak high pressure will be across the region on Sunday, with the cold front just to our south. This front will begin to lift north as a warm front, and cloudiness will increase during the day. For now, precip is expected to hold off until later in the evening. (see the long term section for more details on this system). Max temps will be a few degrees below normal with light winds. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... 500 MB: A warm ridge centered over the southern Appalachians will weaken Wednesday with the passage of a substantial short wave through se Canada. Thereafter...warmer that normal heights prevail over the se USA with a resurgence of the ridge over the se coast next weekend. Cooler than normal heights are near Hudson Bay, resulting in a fairly strong wsw jet from the southern Plains to the mid and north Atlantic coast. Temperatures: The first 15 days have averaged below normal, ranging from 3.5F below normal at KMPO to near 1 degree below normal elsewhere. (Monthly melted w.e. pcpn has been below normal but snowfall, at least in our area of the I95 corridor is already well above the monthly normal at both PHL and NYC!) In any case calendar day average temps are modeled near normal around 5F above Monday, around 10 above Tuesday, maybe as much as 5 above Wednesday, near normal Thursday then probably warming again next Thursday and Friday to at least several degrees above normal (the EC suggests 15F above normal average temps next Saturday-we used the cooler WPC/MEX blend). Forecast basis: Unless otherwise noted Sunday night-Monday night is a 50 50 blend of the 12z/16 GFS-NAM MOS, Tuesday the 12z/16 GFS MEXMOS and next Wednesday-Saturday (D4-8) is the 15z/16 WPC 12 hr elements of the max-min temps/pops and 6 hrly td/wind/sky. Sunday night...our grids have a 25 to 40 pct chance for a period(s) of light snow or mixed freezing-frozen pcpn in E PA and the n2/3rds of NJ in weak waa. There is considerable uncertainty regarding the extent and timing of any light pcpn so snow and ice amounts are not officially posted in storm total grids (less than 55% chance). Also, the forecast wording is also therefore fraught with uncertainty. Near 1/2" snow is possible near and north of I80 and spotty trace to .03 radial freezing rain icing is possible in the I78-I276 region. Subfreezing ground temps will need to be considered for any possible advisory. Otherwise mostly cloudy and milder. Light south to southwest wind. PWAT 0.55" Confidence: average. Monday and Monday night...Variable cloudiness or P/S and milder. PWAT dries out slightly from nw to se to 0.45". West wind. Confidence: average Tuesday and Tuesday night...a very very nice mid December day- warmup...southwest wind may gust 20-25 MPH during the afternoon, then shifts northwest by midnight. Above normal temps by 10 to 14 degrees. A CFP during the evening may generate a few rain showers-sprinkles far north (end as snow showers or flurries Poconos?) but PWAT is further drying out from nw to se so Confidence: average or below average on whether it precipitates. Wednesday...High pressure building in from the west. P/S cooler with a nw wind gust 20-25 mph during the morning diminishing mid afternoon. Thursday...Variable, mostly high clouds. Seasonable colder. Light wind. Saturday...conservative and much below average confidence on what may happen. Pretty much WPC grids. Longer term: A quick look into Christmas week does still provide the chance of a white Christmas with an active weather pattern that looks to turn colder thanks to a EPO which is falling several standard deviations below normal. The - EPO will discharge cold from AK southeast into the United States just before Christmas. However, the colder air can dive into the western United States first leading to warmer conditions and a typical ridge in La-Ninas over the Southeastern United States. Exactly how this plays out is still outside the forecast window and a lot of possibilities are in play. Be mindful of any weather information you see about Christmas can still change quite a bit over the next few days. The only additional note to Mitchell`s discussion from this morning is a concern about warming aloft. Should the se USA ridge build more than currently modeled... snow risks would tend to be limited to more of a transition into mixed phase (Ice is already in some of the long range member guidance) && .AVIATION /20Z SATURDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. .Tonight...VFR conditions. A cold front will cross the terminals overnight, with winds shifting from southwest to north-northeast. Winds early this evening will gust up to around 20 knots at times, then diminish to AOB 5 kt late. High confidence. .Sunday...VFR conditions. Easterly winds 5 to 10 kts. High confidence. Outlook... Sunday night: Brief Sub-VFR possible with patchy light rain, mixed precipitation possible at KABE, KTTN and KRDG. South to southwest winds under 10 knots. Confidence: Average or below. Monday and Monday night: Mainly VFR. Westerly sustained winds generally under 10 knots. Confidence: Above average. Tuesday through Wednesday: VFR. An isolated shower cannot be ruled out Tuesday evening north. Southwest wind gusts around 15-20 knots then shifting to northwest and increasing to around 20-27 knots Wednesday morning. Confidence: Above average. Thursday...VFR. Light wind. Confidence: Above average. && .MARINE... While seas across the Atlantic waters have subsided below 5ft, winds remain elevated. We continue the SCA for the DE Bay thru 23Z this evening and the NJ waters through 6Z Sunday. The SCA has been cancelled for the DE Atlantic waters, where winds have subsided. As the cold front crosses the region overnight, winds will veer from southwest to north-northwest, then to the north- east on Sunday (expect southeast on DE Bay), but winds are not expected to return to SCA levels late tonight into Sunday. Outlook... Sunday night-Thursday...No headlines anticipated except for Wednesday morning when a NW flow SCA of 25-30 kt wind gusts is anticipated. Confidence: Above average. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 1 AM EST Sunday for ANZ450>454. Small Craft Advisory until 6 PM EST this evening for ANZ430- 431. && $$ Synopsis...Drag 344 Near Term...Franck 344 Short Term...Franck 344 Long Term...Drag 344 Aviation...Drag/Franck 344 Marine...Drag/Franck 344

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