Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 190843 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 443 AM EDT Mon Mar 19 2018 .SYNOPSIS... High pressure centered over south-central Canada will extend southeastward into the Mid-Atlantic region through tonight. Low pressure will move eastward and emerge off the Mid-Atlantic coast Tuesday before shifting northeastward. A secondary low pressure system develops just off the Mid-Atlantic coast Tuesday night and tracks northeastward Wednesday into Thursday. High pressure builds in later Friday into Saturday, then low pressure tracks nearby Saturday night and Sunday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... An upper air analysis shows a closed low near northern New England, a sharp trough in portions of the Plains and some ridging in place from the Midwest southeastward. A 250 mb jet was positioned across southern Canada and across New England, with another segment from the southern Plains to off the coast of the Carolinas. At the surface, high pressure is parked over south-central Canada with low pressure in the southern Plains. A weak front was draped from the Midwest to the eastern Great Lakes then into our area. Through early this morning, quite the variability with temperatures with some locales hanging onto a light wind which has slowed the temperature drop thus far. Meanwhile, some areas have radiated quite well. As a result, the temperatures needed to be reworked. As we go through today, a weak surface front settles southeastward and may simply dissipate. This occurs as high pressure centered just south of Hudson Bay Canada starts to build southeastward during the day. This will begin to build into our area by the end of the day. As this occurs, a strong short wave trough ejects eastward from the Plains and should be nearing the Ohio Valley by evening. There is some downstream short wave energy moving through a confluence zone over our region, and this combined with upper-level flow will lead to some mainly high level clouds (more of this across the southern zones with time). Given high pressure starting to build down from the northwest, the low-level winds turn from the northeast today. This should be fairly light overall and with enough heating temperatures should warm a decent amount mainly from Philadelphia on southward. Since the flow is turning from the northeast, did not go as mild as some of the guidance suggests and generally used a model blend of 2 meter temperatures. The developing onshore flow will also keep it cooler along the coast. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/... A rather complex setup begins to evolve tonight with a strong closed low sitting near the Canadian Maritimes, meanwhile plenty of energy shifts eastward and starts to interact. This will be setting the stage for significant trough amplification in the East. The main driver for tonight appears to be an incoming strong short wave trough from the Ohio Valley. This combines with high pressure still sitting just south of Hudson Bay Canada, which gets a cold air damming setup established. This will probably drive some low-level drier air southward and be setting up shop especially along and north of I-78. Meanwhile, increasing large scale lift augmented by warm air advection and isentropic lift begins to arrive across our southern zones toward daybreak Tuesday. This should allow for some rain to develop from south to north, however the northern extent is less certain although this may take some time to advance farther north given enough dry air advection in the lower levels. We therefore slowed down the northward increase in the PoPs. The thermal profiles across the southern areas suggest the precipitation starts as rain, however as the column cools enhanced by some evaporative cooling suggests some snow and/or sleet may occur. The bulk of the precipitation is expected to be after this time frame, therefore any snow/sleet accumulations down across portions of the southern areas are anticipated to be minimal. As for the temperatures, we continued with a model blend of the 2 meter temperatures overall. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Complex storm system consisting of multiple coastal lows are expected to impact the region on Tuesday and Wednesday. The extent of the impacts are yet to be determined... Forecast models continue to exhibit poor run-to-run continuity as they struggle to handle the complex interactions between the train of shortwave troughs ejecting from western North America (there are at least four distinct disturbances seen on water vapor satellite imagery that will come into play). There continues to be disagreement with the idea that the midweek event will consist of two distinct waves of precip (round 1 Tuesday-Tuesday night and round 2 Wednesday) with enough separation distance between the two coastal lows to allow for a break in the precip late Tuesday afternoon and night. Round (1)- Tuesday and Tuesday Night... The latest trends point toward a favorable synoptic pattern for a big winter snowstorm for our region. However, there are several factors working against that notion at least for the first part of the event: (1) A high sun angle in late March and marginally cold surface temperatures (especially I-95, S/E) will make it very difficult for snow to accumulate effectively during the day time unless the rates are moderate to heavy. Aside from the initial band of overrunning precip that targets the southern half of the forecast region Tuesday morning-early afternoon, the consensus from the 00Z guidance is for the precip to be light through Tuesday night. (2) A key ingredient in this event in the position of the high, which is forecast to be located over northern Ontario and a bit farther northwest than what is ideal for an all snow event, especially along and S/E of I-95. Consequentially, warm air aloft wrapping cyclonically around the system should advance into the S/E portion of the region. This would imply mainly rain in SE PA, C/S DE and E MD (along and south of the Delaware Bay) through at least Tuesday evening with a mix of rain/sleet/snow nosing close to Philadelphia (especially its S/E suburbs). (3) While areas farther N/W of the city favor snow Tuesday and Tuesday evening, precip amounts quickly taper off as there should be a sharp cutoff on the northern edge of the precip shield. Accordingly, snowfall accumulations are around an inch or less for Tuesday; same for Tuesday night. The exception is the higher elevations (above 400 ft) in SE PA and Cecil County, MD where 1-2" of snowfall is possible each period. A Winter Weather Advisory may be needed for these areas. Round (2)- Wednesday... This is the period of greatest concern for a potentially significant snowfall is Wednesday. This is also the period of greatest uncertainty. The 00Z ECMWF and UKMET have shifted considerably toward a high-impact event with warning-level snowfall accumulations and windy conditions as the second coastal low takes a track closer to the coast. The NAM and GGEM showed a similar snowy scenario but the latest 00Z runs have backed off from that idea. The GFS is somewhere in between but generally shows a minor event. There continues to be a large spread among the GFS, ECMWF and GEM ensemble prediction systems. All this equates to is a high degree of uncertainty. We`ve seen models trend northwest closer to the event with several significant snows in recent years, so it`s a bit premature to rule out this scenario. Given the uncertainty, the probabilistic snowfall graphics (available on our winter webpage) provide much more value than any deterministic forecast. IF the high- end snowfall scenario were to verify, extensive tree damage and power outages could occur once again. A Winter Storm Watch may eventually be needed. High pressure builds in behind the storm later Thursday into Friday. Cold (5-10 degrees below normal) and dry conditions would be expected in this pattern. Yet another storm system could impact the region next weekend. There has not yet been a signal for coastal redevelopment with this system, so the rain/snow line would be determined by the track of the primary low. && .AVIATION /09Z MONDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR with clouds mainly above 15,000 feet. Light/variable to light southwest winds, becoming northwest to north early this morning, then northeasterly 5-10 knots. Tonight...VFR as clouds increase and gradually lower, especially near and south/east of PHL. East to northeast winds less than 10 knots. OUTLOOK... Tuesday...Widespread MVFR/IFR conditions likely from KPHL/KPNE southward. There is high uncertainty as you go farther north toward ABE regarding if snow will make it that far north with a sharp cutoff in precip expected. For MIV and ACY, a considerable amount of mixing of rain, snow and perhaps sleet is likely while snow is favored farther northward toward PHL. NE winds 10-20 kt with gusts 25-35 kt (higher end of range near the coast). Moderate confidence. Tuesday night through Wednesday night...Additional precip is possible, mainly snow (except along the coast). Confidence in precip occurring is high along the coast, but decreases farther inland. MVFR or IFR conditions would be possible if snow reaches the terminals. Gusty N-NE winds expected, especially near the coast. Thursday...VFR. NW winds gradually relax. High confidence. Friday...VFR. NW winds. Moderate confidence. && .MARINE... The conditions are expected to be below Small Craft Advisory criteria today through the first half of tonight, then the southern waters will see an increase in winds and seas late tonight (nearing gale force gusts toward daybreak Tuesday). Otherwise, high pressure extending southeastward from south-central Canada through tonight will turn the winds from southerly to north and northeast today and continue tonight. OUTLOOK... Tuesday through Wednesday...The Gale Watch for the coastal waters (except ANZ450) was upgraded to a Gale Warning. There is a potential for gusts to peak near storm force off the S NJ and DE coastal waters briefly during the day on Tuesday and again on Wednesday. Capped gusts to 45 kt for now. The Gale Watch remains in effect for the northernmost NJ ocean waters with the onset of gales delayed until Tuesday night. A Gale Watch was also issued for the Delaware Bay from Tuesday afternoon through Wednesday morning though there is greater uncertainty in winds reaching gale force in the Delaware Bay than the coastal waters. Wave heights in our southern coastal waters are forecast to build to 10-13 ft and to 6 to 10 ft for northern coastal ocean waters late Tuesday into Wednesday. These wave heights were adjusted several feet above WaveWatch guidance given the model`s low bias in strong NE flow. Wednesday night and Thursday...SCA conditions likely. Friday...Sub-SCA conditions expected. && .TIDES/COASTAL FLOODING... An extended period of strong northeasterly winds are expected to occur from late tonight through Wednesday with two coastal storms tracking south and east of the region. Positive tidal anomalies will increase with each successive high tide as water piles up along the coast. The threat for coastal flooding looks to develop as early as the Tuesday evening high tide along the NJ- DE coast and continue through the Wednesday night high tide. The degree of coastal flooding will depend on the track and strength of both coastal lows, which is still uncertain especially with with the second one Tuesday night into Wednesday. ETSS, ESTOFS and other tidal prediction guidance from Stevens Flood Advisory System indicate minor to possibly low- end moderate coastal flooding is most likely. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Warning from 6 AM Tuesday to 6 PM EDT Wednesday for ANZ452>455. Gale Watch from Tuesday evening through Wednesday afternoon for ANZ450. Gale Warning from noon Tuesday to 6 PM EDT Wednesday for ANZ451. Gale Watch from Tuesday afternoon through Wednesday afternoon for ANZ430-431. && $$ Synopsis...Gorse Near Term...Gorse Short Term...Gorse Long Term...Klein Aviation...Gorse/Klein Marine...Gorse/Klein Tides/Coastal Flooding... is the U.S. government's official web portal to all federal, state and local government web resources and services.