Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 202109 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 509 PM EDT Tue Mar 20 2018 .SYNOPSIS... A pair of low pressure systems to our south today will consolidate into an intense nor`easter just east of the mid-Atlantic coast on Wednesday, then pass southeast of Nantucket Island Wednesday night. A blustery cold northwest flow follows in its wake Thursday and Friday. Canadian high pressure builds in to start the weekend. Another low pressure system may pass to our south early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... A mixed bag of precipitation is encompassing much of the area this afternoon, with combination of snow/sleet/freezing rain beginning to accumulate on paved and non-paved surfaces west of the Delaware River and at least on elevated surfaces in central/southern New Jersey. Plain rain dominates south of Kent County MD/DE and in Cape May County, NJ. Travel impacts are beginning to occur. As such, feel that ramping up the start time of the winter storm warning to the present is a good idea, generally between I-80 and New Castle/Queen Anne`s Counties. However, I delayed the start time of the warning until 6 am for far northern New Jersey and for the rest of Delmarva, where precipitation will either be sparse (north) or mostly liquid (with temps above freezing; south). The evening commute may be difficult, especially near/west of Philly, where quick bursts of snow/sleet/freezing rain have already caused some problems. Several reports of 0.5-2 inches of snow/sleet accumulation have been received, and several ASOS sites are reporting freezing rain mixing in with the snow/sleet (including ILG and PHL). Worried that some roads will become quite hazardous during rush hour. Updated grids expanded freezing rain mention along the I-76/I-95/I-295 corridors. Also lowered temperatures a little bit, since wet bulbing was still keeping temps lower than what models are insisting upon. Meanwhile, wind gusts 40-50 mph are ongoing in Sussex County in Delaware and Cape May/Atlantic Counties in NJ. Wind advisory looks good. Based on delaying the winter storm watch here, may end up extending the wind advisory through the night to account for the continued wind threat. So far, though, have kept the advisory through 6 pm. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... Potent vort max moving through the Ohio/Tennessee Valleys this afternoon will pivot into more of a negative tilt overnight, with considerable large-scale ascent downstream in much of the Mid-Atlantic. Downstream surface low off the coast will be absorbed/overtaken by a strengthening/kicker vort max late tonight. Long-duration onshore flow and low-level isentropic lift will occur north of the low in the northern Mid-Atlantic. This should permit continued precipitation in much of the area tonight, though some gradual lessening of rates will occur as the strongest/deepest lift with the predecessor vort max moves out of the area. However, another ramp-up will likely begin by the end of the night in the region as considerable differential cyclonic vorticity advection begins, aided by left-exit region upper-level divergence as a cyclonic jet streak reaches the Southeast coast. As mentioned above, precipitation during the late evening and early overnight hours will become somewhat lighter and potentially even drizzly. This is concerning, because this may permit liquid precipitation to expand northward/westward as the precipitation intensity lightens. With temperatures near or even below freezing, freezing drizzle/rain may become more widespread between the I-195 and I-76 corridors this evening and in northern/central Delmarva through much of the overnight. I expanded icing potential considerably during the evening/early overnight hours, with potential for up to tenth-of-an-inch accumulations, especially in far southern New Jersey and northeast MD/northern DE. Just how far south this threat expands is a tough question, as temperatures should stay marginal in the Dover/Stevensville areas southward. This may require frequent grid edits overnight, based on nowcasting. The precipitation will ramp up again late, and dynamic cooling aided by considerable large-scale lift should get the snow machine cranking from west to east late in the night. The main show will begin tomorrow. For temps/dew points, used a blend of 2-m NAM/ECMWF with only some weighting to the previous forecast, given these models` generally superior performance during the past 12 hours. Winds at the coast may stay elevated overnight, which may require extension of the wind advisory. Winds will likely stay elevated tonight -- I kept gusts 25 to 30 mph through much of the area, and they may actually remain higher than this in the higher terrain, especially late. && .LONG TERM /WEDNESDAY THROUGH TUESDAY/... The following is stated with a request to please check back on our 4 AM products tomorrow morning. But as of this writing...we continue to need to sound the siren for major or extreme impact for the I95 corridor to I80 Wednesday afternoon and night. Major and dangerous probable record breaking March winter snow storm (pasting!) for both the 2 day total and the month of March for parts of our area (NNJ for the month, PHL the storm) with impassable roads expected and possibly the greatest number of power outages from any storm so far in March. This continues to look HUGE and potential extreme impact I95 corridor up to I80. Take this storm seriously and prepare for changing your daily plans, especially Wednesday. Unnecessary travel is not recommended Wednesday in eastern PA and the northwest two thirds of NJ. It could be life threatening if disabled in rural areas. If you have to travel, plan for much much longer commutes. Potential exists widespread cancellations in eastern PA and much of nw NJ. This forecast today was split between myself Wednesday through Friday, and Lance, Friday night through the weekend. Growing 12z/20 model consensus (exception the seemingly lagging op 12z/20 ECMWF?) with heavy reliance on the NAM 12 and 3KM as well as the bountiful model qpf from the RGEM/GGEM, UKMET. We expect widespread 1 to 3 inches of qpf from this, heaviest se PA thru northern and central NJ, Banding will be a huge contributor. The northwest fringe of our area from KMPO to High Point may escape with only 6 to 10 inches of snow but it is now expected that widespread storm totals 10-20" will occur I95 corridor nwwd with isolated 25 inch amounts possible some where between I95 and and I80...probably best chance hilly areas of northern NJ where some 30 inches has already occurred this month. The forecast snow total map includes todays sleet/snow mix and the anticipated much greater snowfall on Wednesday. Please realize that pavement amounts will be considerably less but we expect at least 7 inches on all pavements in NJ/e PA from this storm as a minimum and at least 3" pavements elsewhere to the se of I95. We forecast for the traditional accumulating surfaces including homes/trees/wires and that is what will cause the power 6" or more of wet snow (32-33F) should trigger a sharp increase in power outages Wednesday afternoon. Also wind driven wet snow or freezing rain clinging to branches and wires will be part of the power outage problem. Our forecast amounts are presented on our winter wx page and social media briefing packages serves as a good start! Have been uncomfortable forecasting these amounts the past two days in our area, late season and what not, but the recent advances in modeling and snow forecasting science tells us these amounts are probable including running qpf from snow ratios. Wednesday and Wednesday night through 2 am Thursday...By 00z/22 (Wednesday evening), our forecast area will have been in constant 3 to 4 standard deviation 850MB easterly flow with pretty high values of moisture flux for 24hours!!, beginning late this afternoon and continuing through Wednesday afternoon. This less extreme easterly flow inflow might normally mean less qpf, but the unusually long duration has to make up for the normally faster newd movement of the 850MB ely inflow. I think this justifies the NAM and GGEM QPF and resultant excessive late season snowfall. As the 500 mb associated heights lower, whatever remains of the remaining above freezing 880-800MB layer is cooled to freezing around 12z/Wednesday along and nw of I95, and any mixed precipitation will change to snow. (We think the NAM warm nose is a bit overdone but respect it becoming more pronounced this evening before cooling tomorrow morning). Bands of cirrus will increase the seeding potential (potential thunder snow) and there is considerable agreement that FGEN banding will develop on the northwest periphery of the deepening mid Atlc coast cyclone. That will mean 1-3"/hr accumulations at times Wednesday afternoon and evening, especially I95 westward. This with snow water ratios of 11 or 12 to 1 e PA and nNJ (near 9 or 10 to 1 I95 should result in record breaking (see climate section). If thunderstorms occur (we saw lightning in clouds in nw VA and around 11z and recently se of DC around 1320z), then we may have a repeat of 3-4" hr snowfall rates. There is still uncertainty on whether thunder will occur. 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. 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 prior to the evening commute. This will be a very heavy wet snow which should limit the blowing snow threat until sundown Wednesday. For now consider this heart attack - power outage snow, pasted street signs. Late Wednesday night after 2 am Thursday...snow tapering off late with gusty northwest winds to 25 MPH. Thursday and Friday...Blustery and cold with northwest winds gusting to 35 MPH during the day and a little less at night. Blowing snow. The snow pack could limit day time heating, so have gone with the lower side of guidance for highs each day. One note: a sewd moving cold core across our area seems likely to trigger sct snow showers and there is a snow squall signal so the hilly areas of ne PA and nw NJ may pick up a a little bit of a dusting. Friday Night through Tuesday... 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 continues to trend down. This continues to influence the axis and amplitude of the trough across the Northeast conus and Southeast Canada, and the shortwave disturbances moving through this feature, as it gradually closes off well south of Nova Scotia early next week. There remains disagreement in the deterministic and ensemble solutions regarding low pressure tracking from the Great Planes on Saturday, with redevelopment off the Mid-Atlantic coast Sunday and into Monday. The trend has been to suppress the low further to the south, yet the extent of dry air associated with a high pressure ridge axis located east of the Appalachian Mountains remains in question, and therefore the northward extent of overunning precipitation associated with this system. WPC has also noted the lack of deterministic and ensemble run to run consistency, and is favoring a blended approach of today`s 06Z GEFS and 00Z ECENS. While this would focus precipitation across portions of eastern PA and Delmarva from late Saturday into at least Sunday, it could certainly be more widespread across the remainder of the area, given the level of uncertainty. In addition, an early look at some of the mass fields indicates an environment favorable for wintry precipitation outside of southern Delmarva, especially Saturday Night. For Sunday, there is a lot of disagreement in temperatures, e.g., evaporative cooling effects and dynamical cooling, particularly during the final week of March with the increasing sun angle. Bottom line...there is still the potential for wintry precipitation to accumulate across much of the area late Saturday into Sunday. Renewed coastal flooding is also possible depending on the strength and duration of onshore flow. Furthermore, any impacts realized this weekend could inhibit recovery efforts from the ongoing storm system (e.g., power restoration). Looking ahead to Tuesday, the pattern is favoring a return to fair weather with temperatures at or above average. && .AVIATION /21Z TUESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Low pressure developing off the mid-Atlantic coast will bring gusty NE winds and a mixed bag of wintry precip to the area through tonight and Wednesday. So far this afternoon it has been mostly rain at MIV/ACY, mostly snow at RDG/ABE and a mix in between for TTN/PHL/ILG. For the rest of this afternoon and tonight...fairly light mixed precip should continue over the area with MVFR to occasional IFR conditions. Confidence in precip type and timing is below average. There should be a lull in precip during the evening into overnight resulting in better vsbys. Winds will remain from the NE and rather gusty, especially towards the shore (i.e., ACY). Outlook... Wednesday and Wednesday night...IFR and LIFR conditions especially during Wednesday afternoon and evening in snow, which should be heavy at times. The change to all snow occurs at KMIV and KACY during late Wednesday morning. We may see some gradual improvement LATE Wednesday night (after 06z/22). Northeasterly wind Wednesday becoming northwesterly Wednesday night with gusts 20 to 30 KT. Wednesday morning, gusts up to 40 KT possible at KACY. HIGH confidence. Thursday and Friday...VFR with northwest wind gusting to 30 kt, especially day time. Blowing snow where accumulations are still more than 2 inches which should be most TAF sites. Friday Night through Saturday...VFR conditions expected. Northwest winds around 10 kts. Moderate confidence. Saturday Night and Sunday...MVFR possible in low clouds and possible wintry precipitation at all terminals. Low confidence. North winds shifting to Northeast 10 to 20 kts. && .MARINE... Storm warning through tonight except for Upper Delaware Bay, where a gale warning is in effect. Northeast winds nearing storm force now and will continue on and off through the night. Seas will be building through the period, likely reaching 15+ feet in the Delaware/southern NJ Atlantic waters by dawn. Waves of 3-7 feet are possible in Delaware Bay. Rain (possibly mixing with sleet and snow near the coast) will continue for much of the night, with visibility restrictions likely. 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 through Friday...SCA northwest flow. Friday Night - Sunday... SCA conditions are expected in a northwest flow on Saturday, then continuing into Sunday, as winds veer to the northeast. && .TIDES/COASTAL FLOODING... A strong northeast wind continued along the coasts of New Jersey and Delaware at mid afternoon. The surge had already built to around 1.5 feet along the coasts of Sussex County, Delaware and Cape May County, New Jersey. The surge was lagging a bit along the northern part of the New Jersey coast. The across-water fetch there is limited by Long Island and southern New England. The surge was running around a half foot in the Sandy Hook vicinity. We will not be making much change to the total water level forecasts for our tidal areas. We will continue to favor the more robust ETSS due to the strength of the coastal storm. The ETSS has remained rather consistent from run to run over the past couple days. The Coastal Flood Warning for the New Jersey counties of Ocean, southeastern Burlington, Atlantic, Cape May and Cumberland, and for the Delaware counties of Sussex and Kent remains in effect. The warning runs from 8:00 PM this evening until 5:00 AM Thursday to cover all three high tide cycles of concern. The Coastal Flood Advisory for Monmouth County and Middlesex County remains in effect from 9:00 AM Wednesday until 5:00 AM Thursday. Due to the lag in the surge there, those areas are forecast to fall short of the minor flooding threshold tonight and they should experience widesperad minor flooding but little or no moderate flooding on Wednesday and Wednesday night. 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 is in effect for New Castle County and Salem County from noon on Wednesday until 5:00 AM Thursday. The surge is expected to build around 1.0 to 1.5 feet from Long Beach Island northward, and close 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 2.0 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 north to northwest on Wednesday night. However, water is expected to remain trapped along our coast and we are anticipating another round of surge levels in the 2.0 to 3.0 foot range. The degree of flooding is expected to be similar to that on Wednesday. At this point, only spotty minor flooding is anticipated for the tidal Delaware River between the Commodore Barry Bridge and Trenton. No tidal flooding is expected for the upper eastern shore of Chesapeake Bay. && .CLIMATE...
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Record breaking March snowfall accumulation forecast at PHL. The record is 12" as checked by Ray Kruzdlo of our office. March 13-14, 1993. #9 is 8.8 inches March 3-4, March 4-5 in 1981. Allentown record March two day storm total snow 19.9 inches March 19-20 1958 (close to a 60 year anniversary). # 10 is the 10.2" event of 3/29-30/1970. We have a very good chance of top 10 at ABE, barring any last minute model surprises. Wilmington record March two day storm total snow is 17.9 inches 3/19-20/1958. # 10 is 7.9" 3/2-3/1960. We have a good chance of cracking top 10 list. Atlantic City record March two day storm total is 11.6 inches 3/1-2/69 and #7 is 7 inches 3/4-5/15 and 3/6-7/53. There is a chance of cracking top 10. We`ll update again at 5 PM Wednesday once we know whats occurred and whats coming. Also from Rutgers-Dave Robinson and Mat Gerbish at our request below. Since we think northern NJ per the Rutgers areal definition will exceed 5 inches of snow (areal average), probably end up closer to 15 inches or more... this would put northern NJ by Thursday morning 8 AM as the highest ever in our recorded history. Right now the average is 18.9 there and we expect it to rise to ~25 or 30" by 8AM Thursday (this has yet to happen), which would propel us to the snowiest March on record in NNJ. The monthly MARCH record for a single station in NJ to our knowledge via Rutgers is 43.0 in March 1958 at Canistear Reservoir. Oak Ridge had 42.0 and Greenwood Lake 36.8 also in 1958. This storm may drive the current Jefferson Township and Rockaway values which are near 33 inches, above those records...we just dont know yet but by this time! We`ll let the reality determine the outcome. This is all stated to place some perspective on what we think this upcoming event can produce for us. 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)
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...Winter Storm Warning until 2 AM EDT Thursday for PAZ070-071- 101>106. Winter Storm Warning until 11 PM EDT Wednesday for PAZ054-055- 060>062. NJ...Winter Storm Warning from 6 AM Wednesday to 2 AM EDT Thursday for NJZ001-007-008-020>027. Winter Storm Warning until 2 AM EDT Thursday for NJZ009-010- 012>015. Coastal Flood Warning from 8 PM this evening to 5 AM EDT Thursday for NJZ020>027. Wind Advisory until 6 PM EDT this evening for NJZ023>025. Winter Storm Warning until 2 AM EDT Thursday for NJZ016>019. 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 AM Wednesday to 2 AM EDT Thursday for DEZ002>004. Coastal Flood Warning from 8 PM this evening to 5 AM EDT Thursday for DEZ002>004. Wind Advisory until 6 PM EDT this evening for DEZ003-004. Winter Storm Warning until 2 AM EDT Thursday for DEZ001. Coastal Flood Advisory from noon Wednesday to 5 AM EDT Thursday for DEZ001. MD...Winter Storm Warning from 6 AM Wednesday to 2 AM EDT Thursday for MDZ012-015-019-020. Winter Storm Warning until 2 AM EDT Thursday for MDZ008. MARINE...Storm Warning until 6 PM EDT Wednesday for ANZ431-451>455. Storm Warning from midnight tonight to 7 PM EDT Wednesday for ANZ450. Gale Warning until midnight EDT Wednesday night for ANZ430. && $$ Synopsis...Drag Near Term...CMS Short Term...CMS Long Term...Drag/Franck Aviation...AMC/Drag/Franck Marine...CMS/Drag/Franck Tides/Coastal Flooding... Climate...509 PM is the U.S. government's official web portal to all federal, state and local government web resources and services.