Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 200237 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 937 PM EST Fri Jan 19 2018 .SYNOPSIS... Surface high pressure will remain across the Southeast and adjacent western Atlantic Ocean this weekend before moving east next week. A surface low progressing across southeast Canada this weekend may bring a slow-moving cold front into southern New England and the northern Mid-Atlantic on Sunday and Sunday night before retreating as a warm front on Monday. A cyclone in the central plains will race northeastward through the Great Lakes Monday night to the Canadian Maritimes on Tuesday, bringing a strong cold front through the region late Monday night and Tuesday. High pressure will build into the eastern United States for the rest of the week. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... 935 PM Update...No major changes to the forecast. Just some adjustments to hourly temperature grids based on latest obs and trends. 3 PM Update...Overnight, our area will be in between the large high pressure to our south and the low progressing east across Ontario and Quebec. The low will be far enough north that we don`t expect any impacts save for a few high clouds. Temperature wise, models have been underestimating the valley radiation effects across the southern Poconos and NW NJ the last two mornings. Therefore, undercut guidance in these areas, with teens likely in the valleys. Otherwise, expect mostly 20s across the region. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/... The small warming trend is expected to continue into tomorrow as the low will move further away from the region, allowing 1000-500 mb thicknesses to increase. Highs are expected to be about 10 degrees above normal, ranging from the mid 40s to mid 50s. && .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/... A strong storm system will move through the region early next week, bringing a good chance for rain to much of the area. Otherwise, the pattern will be relatively benign during the long-term period. There are indications that a surface low moving across southeast Canada this weekend will bring a slowing cold front into New England and (perhaps) the northern Mid-Atlantic on Sunday. How far south this progresses is somewhat in question, but I suspect given the strength of the surface high encompassing the western Atlantic and southeastern U.S. that the front will struggle to make much progress into our area. Nevertheless, there is some potential for a high-temperature bust on Sunday should the front progress farther south than forecast. Currently, highs on Sunday are forecast to be about 10-15 degrees above seasonal averages, though a little bit lower in the northern CWA to at least account for the potential for the front to progress a little farther south. Saturday night and Sunday should be dry. Operational models have trended slower with the cyclone in the central plains Sunday and Monday, and this means the earlier- progged isentropic ascent along the developing baroclinic zone in the northern Mid-Atlantic during this period would be strongest farther west. Previous shift reduced PoPs Sunday night and Monday to adjust for this trend, and I have continued this trend with today`s update. I also fine-tuned the precipitation type forecasts based on somewhat increased confidence in hourly temperatures during this period (though this should not be assumed to be high confidence). The setup is favorable for liquid precipitation, but temperatures will be close to or even somewhat below freezing at times in the far northern CWA, so any precipitation that falls in the southern Poconos and adjacent northwest New Jersey may be freezing rain. Of course, given the trends in the cyclone track to the west, the odds of precipitation occurring are lower...too low to introduce ice accumulations into the grids at this time. We will continue to monitor this potential as the event approaches. The cyclone makes the move northeast into the Great Lakes and southeast Canada Monday afternoon through Tuesday, and a strong cold front will sweep across the eastern half of the U.S. during this period. Widespread frontal lift will be accompanied by larger-scale ascent via warm advection and considerable differential cyclonic vorticity advection downstream of a potent midlevel trough. Models continue to generate widespread rain near and ahead of the front, but the timing of the front still has about a 6-12 hour difference between the faster GFS and slower ECMWF. The CMC remains in the middle ground, but using it as consensus is troubling because its QPF field makes little sense in our area given the strong lift and moisture advection preceding it. Here, the GFS and ECMWF are in stronger agreement with widespread 0.50-1.00 inch totals in most of the CWA. As a result, weighted the forecast toward an ECMWF/GFS blend, which includes the timing of the frontal passage. This continues to favor the 06Z-18Z Tuesday time window for highest PoPs. Temperatures should be warm enough for all rain during this period. Most models keep the area dry Tuesday night through Friday with strong high pressure developing into the region. However, there continue to be indications of at least one perturbation moving through the zonal to northwest flow upstream of the trough, the timing of which would be Wednesday into Wednesday night. This system would be moisture-starved, but the lift may be strong enough to generate a few snow showers in the southern Poconos and vicinity (though the ECMWF/CMC/GFS are not indicative of this at the moment). No PoPs were included at the moment, and given the surface high the GFS/ECMWF depict late this week, have the forecast dry through the whole period. The origin of the surface high is western/central Canada, so the near-average temperatures MOS is giving for this period may be too conservative (i.e., too warm). Euro MOS trends are down, and MEX MOS is at least edging that way. Though I did not stray from consensus too far this period, I have a suspicion the forecast may end up being too warm Wednesday through Friday. && .AVIATION /03Z SATURDAY THROUGH WEDNESDAY/... 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 (less than 10 kt) southwesterly winds through the night. Tomorrow...VFR conditions with few, if any, clouds. Westerly winds near or below 10 kt. OUTLOOK... Saturday night and Sunday...Predominantly VFR, though clouds will be increasing Sunday, potentially to sub-VFR late (and especially north of Philly). Winds generally west or southwest and light. Average confidence. Sunday night and Monday...Periods of sub-VFR possible, with a chance of rain primarily north of the Mason-Dixon Line. Winds predominantly south or southeast below 10 kts. Below average confidence. Monday night and Tuesday...Periods of sub-VFR likely with a good chance of rain, especially from around midnight to noon. South winds 5 to 15 kts (potentially gusty at the coast) becoming west 10 to 20 kts (potentially gusty everywhere) subsequent to a frontal passage some time during this period. Slightly below average confidence, particularly regarding timing of CIGs/VSBYs/wind shift. Tuesday night and Wednesday...VFR with west to northwest winds 10 to 20 kts during the day and 5 to 15 kts at night. Some potential for gusts Wednesday afternoon. High confidence. && .MARINE... For the Delaware Bay, Delaware and far southern NJ coastal waters: winds and seas should stay below small craft advisory criteria tonight and tomorrow, though gusts near or just above 20 kt are possible Saturday morning. For the northern and central NJ coastal waters: winds will increase late tonight, with gusts to 25 kt expected before sunrise Saturday morning. OUTLOOK... Saturday night and Sunday...Sub-advisory conditions and fair weather expected. Sunday night and Monday...Sub-advisory conditions, though there is a slight chance of rain off the New Jersey coast during this period. Monday night and Tuesday...Strong south winds and building seas will likely generate at least advisory-level conditions, if not marginal gales before frontal passage on Tuesday. Thereafter, winds should become westerly and may again reach gale-force for a time thereafter. A good chance of rain during this period, with some visibility restrictions possible. Tuesday night...Residual gale-force gusts possible with strong northwest winds. Wednesday...At least advisory-level conditions likely, though the trend in winds and seas will be downward. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 4 AM to 6 PM EST Saturday for ANZ450>452. && $$ Synopsis...CMS Near Term...Fitzsimmons/Johnson Short Term...Johnson Long Term...CMS Aviation...CMS/Johnson Marine...CMS/Johnson

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