Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 200104 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 904 PM EDT Mon Mar 19 2018 .SYNOPSIS... A pair of back to back noreaster`s Tuesday and Wednesday mornings will merge into a single intense blockbuster storm off the mid Atlantic coast by Wednesday afternoon and then head for Nantucket Island early Thursday. High pressure builds in later Friday into Saturday, then low pressure tracks nearby and probably to our south Saturday night and Sunday. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
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Few changes needed. It was noted that the dew points across the northern areas verified 5-10 degrees colder than earlier fcst. The cold dry air will spread southward tonight and affect p-type during the initial arrival of the deeper moisture. The 18Z models were continuing to show varying solutions with the GFS showing much less QPF that the earlier run (especially for Wed) and the NAM showing more for Tue and less for Wed. It shows more impacts for sleet/snow for Tue across the Delaware Valley. Few changes to the previous fcst. A wind advisory has been issued for SE NJ and Sussex county DE for Tuesday. Earlier... An absolute nightmare of a forecast today. Models have trended north and wetter with the systems affecting our region beginning late tonight and early tomorrow. Run-to-run discontinuity appears to be the only forecast continuity with the operational models for this event, which means that the forecast remains unusually low confidence, even within 24 hours. Tonight is basically all setup, as a potent midlevel vort max in the central plains moves into the Ohio Valley tonight while an upstream kicker shortwave trough digs rapidly southeastward into the Mid-South by 12Z Tuesday. Attendant surface low with the predecessor vort max should reach the central Appalachians by 12Z Tuesday. This will begin to weaken as a low regenerates near the Carolina coast. Substantial isentropic ascent will begin tonight in the southern CWA in advance of these features, but it will have a tough time seeping northward, as high pressure in Ontario will prove to be a stubborn obstacle for poleward progression of warmer/moister air. Notably, models have uniformly underestimated the dry air at the surface today in the northern CWA. As such, once isentropic lift proves sufficient for precipitation production in the southern CWA late tonight, I slowed the progress of higher PoPs northward considerably. Basically, this means I kept anything above low- chance PoPs confined to MD/DE during the overnight period. Temperatures will mostly be warm enough for rain at onset, though temperatures will approach or drop below freezing near sunrise near the Mason-Dixon Line, so chances for sleet/snow mixing in become higher by this point. Speaking of temperatures, used a blend of 2-m operation model temperatures (strong weighting) and colder statistical guidance (weak weighting).
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&& .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/... The forecasting nightmare begins in earnest for Tuesday. The 12Z guidance has shifted northward and is much wetter, and this led to numerous changes to the forecast. The setup is very complex. The vort max in the Ohio Valley pivots northward and weakens while the kicker southern stream vort max pushes eastward rapidly on Tuesday. This leads to the development of a second low near the coast during the day as the first coastal low weakens/becomes absorbed the intensification of the second. Prolonged east/southeast near-surface low will lead to considerable isentropic ascent in the Mid-Atlantic, and precipitation will be maintained through the day in much of the area (making very slow progress northward). The 12Z NAM is probably a worst-case scenario, with a veritable sleet storm for locations along/near the I-76 corridor. This is suggested by other guidance as well (NAM Nest and 12Z HRRR-X, for starters), though the GFS and RGEM are slightly less aggressive. Then there is the CMC, which looks snowier but potentially just as impactful. With concerns regarding residual dry air to the north, could not justify issuing an advisory along/north of I-195. Meanwhile, surface temperatures may prove too warm close to the coast, so kept coastal and near-coastal NJ zones out of the winter weather advisory. However, did issue a winter weather advisory for southeast PA, northeast MD, New Castle County in DE, and central/southwest NJ, where a mixture of snow, sleet, rain, and maybe even some freezing rain is likely. Accumulations will be limited by diurnal heating/sun angle and by inevitable mixing with rain/sleet. Nevertheless, the possibility of travel impacts exists, especially if the more aggressive models verify. Expansion of the advisory is possible should temperatures prove to be colder and/or the dry air retreats northward more readily than expected. QPF in the advisory looks to be around a half inch Rain should persist to the south, with QPF around an inch during the day south of the wintry mix axis. It will also be windy as the surface low intensifies, especially near the coast. Hoisted a wind advisory for Sussex County, DE, Cape May County, NJ, and coastal Atlantic County, NJ. Temp forecast is weighted to the colder guidance, with low confidence in general given the substantial variability in model output. It is absolutely critical in messaging for this event to emphasize the uncertainty. It will not take much near-surface warming for the precipitation to be mostly rain in the advisory area tomorrow (i.e., a forecast "bust"). Then again, colder air with more intense precipitation rates spell potential big trouble for the afternoon/evening commute, with (more) accumulating snow/sleet possible. Stay tuned to the latest forecasts this evening and overnight with this very complex and rapidly evolving first phase of the storm. && .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/... Snow and ice maps you see on our Monday afternoon web site posts are an attempt at a ballpark figure for a set of model guidance that still has variability. Its for the entire period...start to finish of the storm. It will be adjusted on the following shifts. I could see it too high I95 east and too low to the northwest. Just dont know for sure. Ptype and amts will play a role. Dangerous winter storm promises to produce major winter impacts for our forecast area Tuesday through Wednesday evening. Wind driven sleet/freezing rain and wet snow becomes drier from north to south late Wednesday, but too late to forestall what may be the most power outages from any event in our forecast area this March. Ice loads (hopefully sleet, harder to remove from pavement but less of a tree/wire load) starts this event out south of I-78, with precipitation changing to wet snow in that area Wednesday morning. Six inches of 32-33F is my own trigger for empirically derived notable power outage increase. We`ll try to use some minimal snow loads as a predictor late this afternoon in a social media post toward 6 PM. The primary threat region for this is not determined as of yet and may not be finalized til we see what happens Tuesday. For now with think south of I-78 and especially Wednesday afternoon-evening. To add insult to injury an Alberta clipper may add more wintry precipitation on top of the Tue-Wed excessive precipitation. The long term was split today at 00z/Friday with ability to concentrate on the two storms. his was a collaborated model blend for a dangerous multifaceted winter storm that could leave some folks with out power for quite a few days, and potentially nearly impassable roads during the height of storm Wednesday afternoon- evening, especially elevations where nearly 3 inch per snow fall rates occur. Still tbd but confidence is high for big banding. Long Term Hazards: Complex. We needed to capture the front end on Tuesday with a WXA where a warm above freezing layer aloft precludes much snow but mdt to heavy sleet may occur. The watch for Tuesday night and Wednesday looks to cover more of ice to heavy wet snow scenario as the low develops vertically and intensifies rapidly off the NJ-DE coast. Coastal flood watch was issued and covers the primary tide flooding threat. There will be flooding but we dont know yet to what degree. HIGH WIND Watch late Tue night-Wed morning for S DE (Sussex County) through Cape May County forsaken because of the WSWatch taking care of the high wind threat which is marginal. Tue night...Windy with sleet possibly mixes in during lighter pcpn rates to at least I78 then every body goes to ocnl heavy snow and blowing snow on Wednesday, collapsing eastward to the coasts. Wednesday and Wednesday evening...The big snowfall part of the storm for everyone! Windy (north gusty 20-30 MPH except higher coast) and turning slightly colder and drier by late in the day with 1-3"/hr snowfall rates possible, especially midday-aftn. As per the mid shift a high late March sun angle and marginally cold surface temperatures (especially I-95, S/E) may make it very difficult for snow to accumulate effectively during the day time except when the rates are moderate to heavy, which at a minimum means slush I95 sewd. AT 5PM added sleet to the grids I78 Tue night and S+ almost everywhere Wed aftn. Late Wednesday night...Clearing and brisk. Northwest winds gusty 20- 25 MPH. Temps 5 to 10F below normal with wind. Thursday...Partly sunny and brisk. Northwest winds gusty 20-25 MPH. Temps 5 to 10F below normal with wind. Below prepared by Lance Franck Thursday Night through Saturday... Fair weather is expected with temperatures at or below normal. Northwest winds will be gusty at times. 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 /01Z TUESDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR with increasing cloudiness. Northeast winds around 10 kts. A wintry mix may move into areas south/west of ILG/MIV by 12Z. Moderate confidence. Tuesday...Prolonged sub-VFR likely PHL/PNE/ILG/MIV/ACY with a wintry mix of precipitation, but VFR may persist at RDG/ABE/TTN through the morning before CIG/VSBY restrictions increase (with precipitation taking much longer to commence at these terminals). Precipitation type forecast is very low confidence, with potential for prolonged snow or sleet, a mixture of snow, sleet, and even some freezing rain, or if temperatures warm enough, transition to rain (especially at MIV/ACY). However, some potential exists for substantial winter weather impacts. Overall confidence is low. OUTLOOK... Tuesday night...Variable IFR conds Tuesday night in sleet/freezing rain (snow/sleet KRDG/KABE-KTTN) with ne-n wind gusts 20-30 kt except possible gusts 40kt KACY. Confidence: Above average. Wednesday...Variable IFR/LIFR conds in sleet/snow changing to ocnl heavy snow with possible 1-4"/hr snowfall rates midday-aftn. N wind gust 20-30 kt except possible gusts near 40 KACY. Confidence: Above average. Wednesday night...Variable IFR/LIFR conds in snow with possible leftover 1-2"/hr snowfall rates through about 03z/22 then rapid improvement to MVFR or VFR CIGS late with nw wind gusts 20-25 kt. Confidence: Above average. Thursday...VFR. NW winds gradually relax with gusts 20-25 kt and sct-bkn clouds aoa 3500 ft. Well above average confidence. Thursday Night - Saturday...VFR. Northwest winds around 10 kts, with gusts up to around 20 kts Thu Night into Fri. Above average confidence.
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&& .MARINE... Conditions will rapidly deteriorate on the waters during the next 24 hours, with small-craft advisory conditions expected everywhere by daybreak. Winds will reach gale-force quickly from south to north on Tuesday, and storm-force by late in the day. Seas will build rapidly during the period as well, reaching 8-15 feet by late afternoon (3-7 feet on Delaware Bay). Rain is likely, with visibility restrictions to be expected on Tuesday. OUTLOOK... Tuesday night through Wednesday...Intermittent storm force conditions most of the Atlantic coastal waters and lower De Bay with a ne wind Tuesday night turning north Wednesday. Intermittent gale conditions on upper DE Bay, especially lower 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. && .HYDROLOGY... Poor drainage flood potential Tuesday with 1 to 1.5 inches of rainfall possible parts of the Delmarva. && .TIDES/COASTAL FLOODING... Another coastal storm will bring a new round of tidal flooding to coastal areas of New Jersey and Delaware. The high tide cycles of concern are those on Tuesday night, 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 will favor the more robust ETSS guidance based on the model trends toward increasing the impacts associated with this storm. An onshore flow will begin to develop tonight and it should strengthen on Tuesday. The surge is expected to build around 1.0 to 1.5 feet from Long Beach Island northward to Sandy Hook, and around 1.5 to 2.0 feet from the Atlantic City area southward to the Delaware Beaches for Tuesday night`s high tide. Minor flooding should begin from Atlantic City 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 widesperad minor flooding from Long Beach Island northward, and moderate flooding for coastal areas to the south. The wind should 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. Moderate flooding is expected along the entire coast of New Jersey and Delaware with one exception. Areas up around Sandy Hook may remain at the upper end of the minor range. Also, minor flooding should work its way up Delaware Bay into the tidal Delaware River. At this point, no tidal flooding is anticipated for the upper eastern shore of Chesapeake Bay. Based on our forecast, we will issue a Coastal Flood Watch at this this time for the coastal counties of New Jersey and Delaware. It will be in effect from 8:00 PM Tuesday until 6:00 AM Thursday. && .CLIMATE... Prepared by Lance Franck 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 Watch from Tuesday evening through Wednesday evening for PAZ054-055-061-062-070-071-101-102-104-105. Winter Weather Advisory from 8 AM to 6 PM EDT Tuesday for PAZ070-071-101-102-104. Winter Storm Watch from Tuesday afternoon through Wednesday evening for PAZ060-103-106. NJ...Winter Storm Watch from Tuesday evening through Wednesday evening for NJZ001-007>010-012-016>019. Winter Storm Watch from Wednesday morning through Wednesday evening for NJZ020>027. Coastal Flood Watch from Tuesday evening through late Wednesday night for NJZ012>014-020-022>027. Wind Advisory from 8 AM to 8 PM EDT Tuesday for NJZ023>025. Winter Weather Advisory from 8 AM to 6 PM EDT Tuesday for NJZ016>019. Winter Storm Watch from Tuesday afternoon through Wednesday evening for NJZ013-015. DE...Winter Storm Watch from Tuesday evening through Wednesday evening for DEZ001. Winter Storm Watch from Wednesday morning through Wednesday evening for DEZ002>004. Coastal Flood Watch from Tuesday evening through late Wednesday night for DEZ002>004. Wind Advisory from 8 AM to 8 PM EDT Tuesday for DEZ003-004. Winter Weather Advisory from 8 AM to 6 PM EDT Tuesday for DEZ001. MD...Winter Storm Watch from Tuesday evening through Wednesday evening for MDZ008-012. Winter Storm Watch from Wednesday morning through Wednesday evening for MDZ015-019-020. Winter Weather Advisory from 8 AM to 6 PM EDT Tuesday for MDZ008-012. MARINE...Storm Warning from 6 PM Tuesday to 6 PM EDT Wednesday for ANZ431-451>455. Storm Warning from midnight Tuesday night to 7 PM EDT Wednesday for ANZ450. Gale Warning from 11 AM Tuesday to 6 PM EDT Wednesday for ANZ430. && $$ Synopsis...Drag Near Term...CMS/PO Short Term...CMS Long Term...Drag Aviation...CMS/Drag/Franck Marine...CMS/Drag/Franck Hydrology... Tides/Coastal Flooding...Iovino Climate... is the U.S. government's official web portal to all federal, state and local government web resources and services.