Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 181107 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 607 AM EST Mon Dec 18 2017 .SYNOPSIS... A warm frontal boundary will continue to lift north of the region today with high pressure building into the southeast United States. A cold front will then move through the region late Tuesday into Wednesday. High pressure will then build southeastward into the region for the middle of the week. On Friday, an area of low pressure will track northeast into the Great Lakes bringing a warm front through our region. This will be followed by a cold frontal passage at the end of the week or early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... The warm front has been far slower to lift north than previously expected. As of early this morning, it was still in Delmarva. We still may have a few more hours left for a window of opportunity for any precip, but thus far radar trends have been underwhelming. There is very limited, if any, lift associated with the front, and the 00Z IAD sounding indicates a very dry boundary layer south of the front. Once the front lifts north of the area, expect clouds to scatter out. Agree with the previous shift that the NAM model snow depth is likely causing a cold bias with temperatures. We should see clearing skies through the day and temperatures S of the front are already in the upper 30s and low 40s even as of 2 AM. Therefore, trended on the warmer side of guidance, with forecast highs from the upper 30s across the southern Poconos to lower 50s in S NJ and Delmarva. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/... I am expecting this period to be dry. A few sprinkles are possible ahead of a surface trough that digs southeast into the Great Lakes region, but through the overnight hours, the latest models keep this trough well to our west. There will be hardly any directional shear, as flow from the boundary layer through the upper levels will be westerly. Forecast lows are slightly below GFS MOS guidance given the light winds which should promote radiational cooling. However, if low clouds build in, as one model is depicting, lows may be several degrees higher. This appears unlikely given the off shore flow. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Tuesday through Tuesday night: Our region will be between a high pressure off the southeast coastline and an approaching cold front from the west. This will result in southerly winds and warming temperatures. The signal for precipitation on both the operational runs and ensembles continues to be fairly limited and confined to locations north and west of our CWA. Will maintain a precipitation free period. It looks more like the increase in moisture may just lead to more in the way of low stratus early Tuesday. Used a MAV/ECMWF blend for temperatures, the MET guidance looked too cool because of overdone cloud cover, maybe snow cover mentioned in short term? Right now the temperature forecast highs on Tuesday well into the 50`s. Nighttime temperatures Tuesday night should fall back into the 30`s. Wednesday through Thursday night: After the frontal passage, we return to northwest flow with cold air advection into the region. Also, with high pressure building in, both days should be mostly sunny. Another period of gusty winds is not out of the question. Bufkit analysis shows the potential for some gusts from the west at around 20 mph Wednesday afternoon. High temperatures both days should fall back into the 40`s with lows in the 20`s, opted to go on the cooler end of the ensemble guidance given the cold air advection into the region. Friday through Sunday night & longer term: A warm frontal boundary lifts northward across the region as a low pressure system tracks into the Great Lakes on Friday. Another surge of southerly flow and increase in southerly winds will come into the region leading toward a moderating trend with temperatures, likely reaching well into the 50`s and 60`s by Saturday. A cold frontal boundary will then approach the region from the west this weekend. The approach of the front has trended slower on the modeling over the past day or so. However, model and ensemble solutions have been rather unstable from one run to the next on the timing. The speed of the this front depends on where the discharge of very cold air from a -EPO goes, most likely into the plains or western United States at first. A response to this would to increase high pressure off the east coast which slows the approach of the cold front and continues to stream warm air northward into the region. This would result in a large temperature contrast along the frontal boundary this weekend. Even with a slower approach, rain chances would enter the picture either Friday or Friday night and last through much of the weekend. If the front moves through quicker than forecasted currently, temperatures would be a good deal cooler with more wintry concerns (00z GFS 12/18). Depending on the frontal timing, a non-diurnal temperature is possible but not included given we are quite a ways away. Modeling is keying in on the potential for quite a bit of rainfall potentially with this system, pops were raised to likely on Saturday where rainfall confidence is highest. Right now, temperatures look too warm enough to prevent mixing with snow/ice on the front end of the system Friday afternoon and evening. The large uncertainty with the weekend front would also carry into Christmas as well. && .AVIATION /11Z MONDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Once the low clouds over KABE, KACY, and KTTN dissipate, VFR conditions are expected for the remainder of the TAF period. The low clouds currently are expected to dissipate by 15Z. However, higher clouds have masked the trends on satellite, so confidence is below normal on this. Ceilings around or above 5000 ft AGL through the day. Light and variable winds are expected through the day. Below normal confidence on the dissipation of low clouds, otherwise average confidence. Tonight...VFR conditions are expected. One model (NAM) is depicting low clouds developing late tonight, but this appears very unlikely given the off shore flow. Winds will still be light (less than 10 kt), but should settle out of the southwest or west by early this evening. Moderate confidence. Outlook... Tuesday through Tuesday night: Mainly VFR, A brief interval of lower celings possible around sunrise. Wind gusts from the southwest at 15 knots Tuesday afternoon, decreasing at night. Moderate confidence. Wednesday through Friday: VFR. West to northwest wind gusts around 20 knots Wednesday afternoon decreasing gradually into Thursday. Winds shifting to southeast by Friday but staying near or under 10 knots. Ceilings may start lower later on Friday. High confidence. && .MARINE... Winds and seas are expected to stay below small craft advisory conditions today and tonight. Wind direction is expected to be variable, eventually settling out of the west this evening, when a few gusts near or above 20 kt will be possible. Outlook... Southwesterly wind gusts of 20-25 knots Tuesday afternoon and night from the southwest gradually becoming northwest on Wednesday. For the coastal waters off of northern New Jersey, gusts may be around 25 knots for several hours late Tuesday night. Gusts decreasing by Wednesday night on all ocean and bay waters. Data continues to indicate seas stay below five feet throughout the outlook period. High confidence. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Gaines Near Term...Gorse/Johnson Short Term...Johnson Long Term...Gaines Aviation...Gaines/Johnson Marine...Gaines/Johnson

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