Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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501 FXUS61 KPHI 200755 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 355 AM EDT Tue Mar 20 2018 .SYNOPSIS... A pair of low pressure systems today will merge into one large coastal low by tomorrow. This low will lift northeast away from our region Wednesday night into Thursday. High pressure builds in to start the weekend. Another low pressure system may approach our region early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Well, a rather challenging forecast as moisture moves up from the south while rather dry low-level air (colder) is seeping southward. As we start aloft, a strong short wave shifts eastward today from the Ohio Valley and eventually starts to shear toward the east due to an area of confluence in the flow to the northeast. Meanwhile, another strong short wave sharpens an upper-level trough across the Tennessee Valley by late today. At the surface, this translates to low pressure developing off of the Mid-Atlantic coast while high pressure centered near Hudson Bay Canada wedges down into the Mid- Atlantic region (cold air damming). So how does this impact our sensible weather? Low-level warm air advection is forecast to occur into our region above a shallow northeast flow. This will be augmented by increasing easterly flow above the surface, combining to produce ample isentropic lift which will be enhanced by short wave energy and areas of 850-700 mb frontogenesis. A concern continues to be the rather dry low-level air seeping southward from the Canadian high pressure system. This will tend to keep the wet-bulb temperature lower especially across the central and northern areas, but deeper drier air farther north will tend to halt the advancement of the precipitation shield. The precipitation types are complicated due to a warm layer in place for awhile, and some model guidance suggests that a zone of sleet occurs. Some model soundings hint at freezing rain, however a closer examination points to sleet due to a shallower warm layer between about 800-850 mb. Freezing rain in late March during the daytime hours is tough given marginal surface temperatures. If more sleet were to fall, especially if heavy as some guidance suggests, then accumulations would be more efficient. There is quite a bit of uncertainty on this evolution, so definitely look for updates during the course of the day. We opted to remove the Winter Weather Advisory for today, and start a Winter Storm Warning early this evening to simplify the messaging and will discuss the mixed precipitation for today within the Winter Storm Warning product. Any snow and sleet accumulations should be on the light enough side today, and we held off on any ice accumulation from freezing rain until this evening as late March during the day should limit any icing. It appears the northern edge of the precipitation may struggle to get to and north of about I-78. As low pressure develops off the Mid-Atlantic coast, the pressure gradient tightens due to high pressure to the northwest. This along with strengthening flow above the surface will result in a gusty northeasterly wind with the highest gusts closer to the coast. This looks to be focused across coastal southeastern New Jersey to southern Delaware, therefore a Wind Advisory is in effect there for today. For the hourly temperatures and high temperatures, we blended in the 2-meter temperatures with continuity overall. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... A rather complex setup continues to take shape tonight. A significant upper-level trough that closes off will approach from the central Appalachians late tonight. This will maintain downstream ascent, however there are two separate forcing areas one which weakens and peels away to the northeast this evening. This should allow the precipitation shield to weaken and perhaps becomes more spotty for a time. Due to the precipitation intensity becoming lighter and even hints at some drying aloft occurring for a time, some light freezing rain/drizzle and sleet should continue for parts of the area although more light rain farther south and east. As height falls arrive toward morning with the incoming closed low, large scale ascent will start to increase with some uptick in the frontogenesis. This will maintain surface low pressure development off the Mid-Atlantic coast with a strong northeast wind continuing especially closer to the coast. We ended the Wind Advisory at 6 PM, as the gusty winds are included in the Winter Storm Warning which starts at 6 PM (most impacts though occur starting early Wednesday morning). Given the idea of the column cooling from northwest to southeast overnight, the precipitation type from any mixing should start to transition over to snow toward daybreak for much of the area. As mentioned, we started the Winter Storm Warning at 6 PM to cover any areas of light icing and mixed precipitation before things really start to ramp up. This is a challenging forecast especially with it being two phases with some weaker lift between them. Low temperatures are a blend of continuity and model guidance. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Pretty much all of the focus in the long term is on Wednesday and Wednesday night. Wednesday...Strong signal for TROWAL of the now merged coastal low to be over our region. There is a very distinct Theata E ridge at 700 and 600 mb on the northwest side of the low in nearly all of the operational models. Exactly where in our region is uncertain, it could be over the I95 corridor or further east over the Coastal Plains. This, along with the placement of a band of frontogenesis is critical to determining where the highest threat for mesoscale banding will be during the day. Hazards on Wednesday: Heavy wet snow especially under mesoscale banding. There is the potential for a 6 to 12 hour period of very heavy snow with snow rates of 2 to 3 inches at times. We expect to get a majority of the overall snow accumulation in this period. As mentioned above, there are still some questions as to where the highest threat is, and thus, still some uncertainty with the snow amounts. Given the temperature profiles, expect a very heavy wet snow to be falling through this time as the snow to liquid ratio across much of the region will likely be less than 10:1. Strong winds, with gusts around 45 mph, primarily along the coast, and primarily during the morning. This will could cause power problems, especially if there is already a snow load on trees and power lines. However, the strongest winds may occur before the heaviest snow, although that is still somewhat uncertain. I also don`t think these strong winds will contribute too much to blowing snow; as mentioned previously, this will be a very heavy wet snow which should limit the blowing snow threat. Wednesday night...snow could linger through the evening, but it should be tapering off as the low moves further away and it looks like we lose the ribbon of low to mid level frontogenesis through this time. Thursday and Friday...High builds in, leading to a return to tranquil weather. The snow pack could limit day time heating, so have gone with the lower side of guidance for highs each day. Saturday...Models continue to depict a very fast moving clipper system through this period. If there is enough moisture advection, may see a brief window of opportunity for light snow, but anything with this system should not amount to much. Saturday Night - Monday... The mid-level large scale features include additional energy digging into the western conus trough, which amplifies a downstream ridge over the central conus, while high latitude blocking invof the Davis Strait slowly breaks down. This all influences the placement and amplitude of the trough across the Northeast conus and Southeast Canada, and the shortwave disturbances moving through this feature. In particular, there is alot of uncertainty regrading the placement and amplitude of the central conus ridge, with even an apparent omega or rex block appearance, depending on the model solution. This all leads to a considerable amount of spread in the deterministic and ensemble solutions regarding low pressure tracking from the Great Planes on Saturday, with the potential for redevelopment off the Mid- Atlantic coast on Sunday and into Monday. Some solutions favor a slower and more suppressed track, which would have less of an impact on our region, while others suggest a track across our region, which would obviously have a significant impact. But there is just too much dispersion in the solutions at this point to lock onto any one in particular. What we can say is that there is the potential for another impactful storm system this weekend. Some of these impacts could include wintry precipitation and coastal flooding. Depending on the just how impactful the storm system Tuesday into Wednesday of this week is (e.g., heavy wet snow impacts to trees, powerlines, and the extent of power outages), it`s important to keep in mind that any potential storm this weekend could inhibit those recovery efforts. Looking ahead to Monday, the pattern is favoring a return to fair weather with temperatures at or above average. && .AVIATION /07Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR ceilings northward, lowering to MVFR possibly IFR from PHL/PHL south and eastward later this morning and this afternoon. A mix of snow, sleet and rain is expected at many of the terminals, with this occurring later today at ABE to TTN. The precipitation type forecast is low confidence as is the timing. More rain should occur at MIV and ACY. Northeast winds increasing to 12-16 knots and gusts to 25-35 knots with the highest winds closer to the coast. Tonight...Variable MVFR/IFR conditions with times of snow, sleet, rain and perhaps freezing rain, which should start to transition to all snow from west to east toward daybreak Wednesday. Most of the precipitation should trend lighter for awhile tonight, which is anticipated to improve the visibility for a time. Northeast or north- northeast winds 10-20 knots with gusts 25-35 knots with the highest gusts closer to the coast. Outlook... Wednesday and Wednesday night...IFR and lower conditions especially during the day Wednesday in snow, which could be heavy at times. We may see some gradual improvement Wednesday night. Northeasterly wind becoming northwesterly Wednesday night with gusts 20 to 30 KT possible. Wednesday morning, gusts up to 40 KT possible at KACY. Moderate confidence. Thursday and Friday...Mostly VFR conditions expected. Northwesterly winds up to 15 KT. High confidence. Saturday...Mostly VFR conditions expected with light and variable winds. Moderate Confidence. && .MARINE... Gale and Storm Warnings in effect. Conditions will rapidly deteriorate on the waters through tonight as low pressure develops off the Mid-Atlantic coast. Winds will reach gale force quickly from south to north today, then storm force starting early this evening resulting in dangerous conditions. Seas will build rapidly through tonight given a strengthening northeast flow, reaching 10-17 feet through tonight (3-7 feet on Delaware Bay). Outlook... Wednesday...Intermittent storm force conditions most of the Atlantic coastal waters and lower De Bay with a NE wind Wednesday. Intermittent gale conditions on upper DE Bay. Wednesday night...Northwest flow with winds and seas slowly subsiding to Gale and then possibly SCA by Thursday daybreak. Thursday...SCA northwest flow. Thursday Night - Saturday...Sub-SCA conditions expected. && .TIDES/COASTAL FLOODING... The beginning of the upcoming coastal flooding event is now within 24 hours. As a result, we have transitioned from the watch to warnings and advisories. The Coastal Flood Watch was upgraded to a Coastal Flood Warning for the New Jersey counties of Ocean, southeastern Burlington, Atlantic and Cape May, and for the Delaware counties of Sussex and Kent. We added Cumberland County to the warning, as well. The Coastal Flood Warning is in effect from 8:00 PM this evening until 5:00 AM Thursday to cover all three high tide cycles of concern. Based on the wind trajectory, the coastal flooding may not quite reach moderate in Monmouth County and Middlesex County. Also, it may be delayed a bit in those areas. We have placed those two counties under a Coastal Flood Advisory and it is in effect from 9:00 AM Wednesday until 5:00 AM Thursday. The surge is forecast to push up Delaware Bay and into the far lower Delaware River, impacting the high tides there on Wednesday and Wednesday night. A Coastal Flood Advisory has been issued for New Castle County and Kent County. It is in effect from noon on Wednesday until 5:00 AM Thursday. Another prolonged period of tidal flooding appears imminent along the coasts of New Jersey and Delaware and along Delaware Bay. The high tide cycles of concern are tonight`s, and the ones on Wednesday and Wednesday night. The astronomical tides with the daytime high tide are about a half foot lower than those with the nighttime high tides. We continue to favor the more robust ETSS based on the model trends over the past few days. The guidance has been moving toward increasing impacts associated with the storm. An onshore flow has already developed and it will strengthen today. The surge is expected to build around 1.0 to 1.5 feet from Long Beach Island northward, and around 1.5 to 2.0 feet from the Atlantic City area southward for tonight`s high tide. Minor flooding should begin from Long Beach Island southward at that time. The surge for the Wednesday high tide is forecast to be in the 2.5 to 3.0 foot range. It is expected to produce moderate coastal flooding along much of the New Jersey coast, the Delaware coast and Delaware Bay. The exception should be the northern part of the New Jersey coast where widespread minor flooding is anticipated. Also, minor flooding should reach the the far lower part of the Delaware River. The wind is forecast to transition from northeast to north on Wednesday night. However, water is expected to remain trapped along our coast and we are anticipating another round of surge levels in the 2.5 to 3.0 foot range. The degree of flooding is expected to be similar to that on Wednesday. At this point, no tidal flooding is anticipated for the upper eastern shore of Chesapeake Bay. && .CLIMATE... Daily Record Snowfall Site 3/20 3/21 3/22 ---- ---- ---- ---- PHL 9.6" (1958) 4.7" (1932) 3.0" (1914) ACY 5.0" (1914) 5.9" (1889) 2.4" (1964) ILG 10.3" (1958) 5.4" (1964) 3.0" (1943) ABE 16.5" (1958) 4.3" (1964) 2.6" (1992) Snowfall as of (3/18/18) Site March `18 Rank Since 7/1 Rank ---- --------- ---- --------- ---- PHL 7.6" 26 22.2" 56 ACY 3.5" 17 28.0" 13 ILG 6.1" 24 19.9" 48 ABE 7.5" 26 32.8" 39 Daily Record Rainfall Site 3/20 3/21 3/22 ---- ---- ---- ---- PHL 1.76" (1958) 2.24" (2000) 1.90" (1977) ACY 2.56" (1958) 1.98" (2000) 1.54" (1903) ILG 1.99" (1913) 3.21" (2000) 2.22" (1977) ABE 2.12" (1958) 1.42" (1983) 2.49" (1977) RDG 3.03" (1958) 1.57" (1890) 2.70" (2000) TTN 1.74" (1958) 2.02" (1980) 2.25" (1977) GED 2.12" (1975) 1.94" (2001) 1.20" (1964) MPO 2.13" (1975) 1.28" (1950) 2.74" (1980) && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Winter Storm Warning from 6 PM this evening to 2 AM EDT Thursday for PAZ070-071-101>106. Winter Storm Warning from 6 PM this evening to 8 PM EDT Wednesday for PAZ054-055-060>062. NJ...Winter Storm Warning from 6 PM this evening to 2 AM EDT Thursday for NJZ001-007>010-012>027. Coastal Flood Warning from 8 PM this evening to 5 AM EDT Thursday for NJZ020>027. Wind Advisory from 8 AM this morning to 6 PM EDT this evening for NJZ023>025. Coastal Flood Advisory from noon Wednesday to 5 AM EDT Thursday for NJZ016. Coastal Flood Advisory from 9 AM Wednesday to 5 AM EDT Thursday for NJZ012>014. DE...Winter Storm Warning from 6 PM this evening to 2 AM EDT Thursday for DEZ001>004. Coastal Flood Warning from 8 PM this evening to 5 AM EDT Thursday for DEZ002>004. Wind Advisory from 8 AM this morning to 6 PM EDT this evening for DEZ003-004. Coastal Flood Advisory from noon Wednesday to 5 AM EDT Thursday for DEZ001. MD...Winter Storm Warning from 6 PM this evening to 2 AM EDT Thursday for MDZ008-012-015-019-020. MARINE...Storm Warning from 6 PM this evening to 6 PM EDT Wednesday for ANZ431-451>455. Storm Warning from midnight tonight to 7 PM EDT Wednesday for ANZ450. Gale Warning from 11 AM this morning to 6 PM EDT Wednesday for ANZ430. && $$ Synopsis...Johnson Near Term...Gorse Short Term...Gorse Long Term...Franck/Johnson Aviation...Gorse/Franck/Johnson Marine...Gorse/Franck/Johnson Tides/Coastal Flooding...Iovino Climate...Franck is the U.S. government's official web portal to all federal, state and local government web resources and services.