Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 180834 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 434 AM EDT Sun Mar 18 2018 .SYNOPSIS... High pressure currently over the Midwest will gradually build to the south and east on Sunday, and will move off the Mid- Atlantic coast sometime Monday or Monday evening. Canadian high pressure then re- establishes itself north of the Great Lakes. Meanwhile, low pressure will move east and emerge over the Mid-Atlantic Monday night. It then intensifies as it moves out to sea on Tuesday. Another low may form over the Mid-Atlantic Wednesday or Wednesday night. High pressure returns from the north to close out the work week. Low pressure then approaches next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Quiet, dry weather persists through the near term. The tail end of a backdoor cold front will push south into the area overnight but really won`t have much influence on our temperatures for Sunday as high pressure moves in bringing sunshine with highs generally reaching the upper 40s to low 50s, except a bit cooler over NW NJ and the southern Poconos. NW winds will generally be around 10 MPH or so. && .SHORT TERM /TONIGHT THROUGH 6 PM MONDAY/... High pressure maintains control over the area through this period with mainly clear skies leading to another cool night as lows will mainly be in the upper 20s to low 30s, except colder across the far north. For Monday, low pressure begins to approach as it moves east from the southern plain states towards Tennessee. This will result in mainly clear skies to start the day giving way to increasing mid and high clouds during the afternoon, mainly across the southern half of the forecast area. However, any precip will hold off through the day. Highs will be mainly in the mid to upper 40s except upper 30s to low 40s across the southern Poconos and NW NJ. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... A very complex setup for the midweek event with multiple waves of low pressure impacting the Mid-Atlantic region. It may be best to break the event down into two parts: Part (1): Monday night-Tuesday A primary low west of the central Appalachians transfers its energy to a coastal low that develops near Norfolk, VA. With the upper air pattern relative flat, the low should quickly progress to the E-NE and out to sea. Cyclonic flow ahead of the 850 mb low will induce strong isentropic lift along and north of the frontal boundary that should aid in precip expanding northeastward into Delmarva, SE PA and S NJ late Monday night into Tuesday morning. There will likely be a very sharp cutoff in the precip on the northern side with drier air originating from the high centered over Ontario draining southward and with a relatively zonal steering flow pattern preventing the deeper lift from expanding much poleward. Significant differences in the models regarding how far north the steadier precip advances and where this sharp cutoff of the northern edge sets up. The typical model biases are evident in the latest runs with the NAM and SREF wetter/farther north with the precip (heaviest QPF of 1/2-1" falling along and south of the PA Turnpike/I-195 corridors, then amounts dropping off quickly along and north of I-78) while the GFS is suppressed to the south (QPF amounts of 1/2-1" confined to the S half of DE and adjacent E MD with amounts quickly tapering off farther north toward the Mason- Dixon line). Note, there is significant spread among the 00Z GEFS with several members supporting closer to the NAM. The 00Z ECMWF was a good middle ground solution between the NCEP operational models and was blended with QPF guidance from WPC as well as the NAM (which I think is catching on to the potential for strong F-gen forcing north of the 850-700 mb baroclinic zone Tuesday morning). Despite the above kinematic and moisture (QPF) differences among the models, guidance is in better agreement with thermal fields, which gives us higher confidence on ptype. The one caveat with the above statement is the placement/intensity of mesoscale lift will influence ptype as dynamical cooling will allow for rain/mix to changeover to wet snow where areas of strong upward motion and heavy precip occurs. Thermal profiles from the 00Z operational NAM, GFS, GEM, and ECWMF generally indicate predominately snow late Monday night and Tuesday from roughly the Mason-Dixon line northward, mainly rain across southern DE and a rain/snow/sleet mix in between. The official snowfall forecast calls for a swath of 2-3 inches of accumulation within 30 miles on either side of the Mason-Dixon line from late Monday night-Tuesday morning. Since just about all operational models but the GEM show a break in the precip late in the day Tuesday and Tuesday evening, we will treat (1) and (2) as separate events with headlines focused on the former. Part (2): Tuesday night-Wednesday The next wave of low pressure developing on the lee side of the southern Appalachians will deepen as it moves off the coast somewhere near the SC-NC coastal border. The 00Z models show varying degrees of phasing with southern and northern stream shortwave disturbances, resulting in amplification of the upstream trough over the Ohio Valley-Mid South regions. The extent of the phasing and how quickly it occurs will determine if this second coastal low heads out to sea, passing well to our S/E (only fringe impacts near the coast) or turns up the coast (more significant impacts, including strong winds and heavy wet snow). With support for both of these polar opposite solutions in the ensemble systems and with high uncertainty in the forecast, it`s still premature to rule out either scenario for the second part of this event (i.e., a complete miss or a high- impact nor`easter). We opted to take a conservative forecast approach that was somewhere in the middle (closer to the 00Z ECMWF and WPC guidance). Note, the official storm total snowfall and probabilistic snowfall graphics on our winter web page includes the first round (1) of the event and only a portion of round (2) since it goes out through 8 AM Wednesday morning. The probabilistic snowfall products probably have more value than a deterministic forecast at this point because of the uncertainty in the forecast. If the high-end snowfall scenario were to verify, there is a potential for extensive power outages once again given the damage left behind from the previous nor`easters this month. 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 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, which looks to move in from the Ohio Valley region. && .AVIATION /08Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Through 12z...VFR with light and variable winds. Sunday...VFR continuing. Northwest winds 5-10 knots, gusts may reach 15-20 knots in the afternoon. Sunday night...VFR with with diminishing winds. OUTLOOK... Monday...VFR. High confidence. Monday night and Tuesday...Onset of precip still looks to be late evening over the Delmarva as rain. By late Monday night and Tuesday morning, widespread MVFR/IFR conditions likely from Phila terminals 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 the range near the coast). Moderate confidence. Tuesday night through Wednesday night...Additional precip possible with continued MVFR/IFR conditions. Gusty N-NE winds expected, especially near the coast. Low confidence. Thursday...VFR. NW winds gradually weaken. High confidence. && .MARINE... Sub SCA conditions with fair weather through Sunday and Sunday night. West to northwest winds will generally be around 10 to 15 knots with seas 2 to 3 feet over the ocean waters and 1 to 2 feet on the Delaware Bay. OUTLOOK... Monday...Sub SCA conditions persisting. NW winds shifting to east at around 10 knots by late day. Monday night and Tuesday...A Gale Watch was issued from late Monday night through Tuesday afternoon for the coastal waters excluding the northern coastal waters of NJ (ANZ450). Confidence in winds reaching gale force increasing considerably as you head south toward the southern NJ and DE coastal waters. Gusts in excess of 40 kt are possible at its peak on Tuesday. Tuesday night and Wednesday...The potential for gales continue Tuesday night into Wednesday but it all depends on how close a second coastal low tracks to our area and how fast it deepens. Wednesday night and Thursday...SCA conditions likely. && .FIRE WEATHER... Relative humidity values on Sunday will drop into the 20s again, but winds are expected to be less than Saturday. Even though fuels have been drying, they are expected to remain above critical levels as well. No enhanced statements are expected at this time. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Watch from late Monday night through Tuesday afternoon for ANZ451>455. && $$ Synopsis...Fitzsimmons Near Term...Fitzsimmons Short Term...Fitzsimmons Long Term...Klein Aviation...Fitzsimmons/Klein Marine...Fitzsimmons/Klein Fire Weather... is the U.S. government's official web portal to all federal, state and local government web resources and services.