Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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055 FXUS61 KPHI 011654 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1154 AM EST Wed Mar 1 2017 .SYNOPSIS... A strong cold front will move through the region tonight. A weak low will progress through the Mid-Atlantic on Friday. A surface high will build into the eastern U.S. this weekend. A warm front will develop and track northeastward into New England on Monday, and a cold front will move through the region on Tuesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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The initial batch of showers and embedded thunderstorms was moving away to our northeast late this morning. There will be a brief break in the precipitation in our region. An area of convection was located in western Pennsylvania and West Virginia late this morning and it will continue to move eastward. It should reach our region during the afternoon hours. This line will bring the threat of damaging wind gusts along with locally heavy rain, frequent lightning and possibly some hail. The initial batch of precipitation and the low clouds was keeping our region somewhat stable this morning. However, some clearing from the southwest will continue ahead of this afternoon`s convection. The extent of the severe weather threat will be dependent upon how much clearing takes place and how much the instability increases. It continues to appear as though the better chance for severe weather will be across the southern half of our forecast area. Temperatures were in the upper 50s and 60s in much of our region late this morning, with some readings in the middle and upper 60s in central and southern Delaware and in the adjacent counties of eastern Maryland. Highs should be in the 70s in much of southeastern Pennsylvania, central and southern New Jersey, Delaware and northeastern Maryland. Maximum temperatures should be in the 60s in the Poconos and in far northern New Jersey. A southwest wind is forecast for today at speeds around 10 to 15 MPH with gusts into the 20s.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/... Unstable airmass in place in the evening ahead of the approaching cold front. With loss of differential heating, as line of storms pass through after sunset, may lose some surface instability, but elevated instability parameters remain fairly high. MUCAPE will be near 1000 J/kg initially, but will diminish a bit going through the evening. 0-6 KM Bulk Shear magnitude will be around 60-70 KT, and a 50-60 KT LLJ that will pass through the region along with the pre-frontal trough. Will not take much to mix those winds down to the surface, thus, primary hazard will be for damaging winds. PWATs increase to just over 1.25", and locally heavy rain is possible. Although storms should move quickly enough to prevent widespread flash flooding, cannot rule out localized urban and poor drainage flooding. Storms taper off from west to east by midnight or so tonight. A few lingering showers are possible ahead of and with the passage of the cold front, which should happen after midnight tonight. Winds shift to the W-NW behind the front and will increase to 15-20 MPH with gusts up to 30 MPH. CAA will be underway, and temps should fall off into the 30s and 40s prior to daybreak Thursday. && .LONG TERM /THURSDAY THROUGH TUESDAY/... By 12Z Thursday, the cold front is expected to have passed the CWA, with the associated vort max pivoting northeastward through southern Quebec. Strong cold air advection will be present across the CWA, allowing for much colder temperatures to work their way into the area. The surface pressure gradient will be relatively strong during the morning hours, allowing for breezy northwest winds to continue. BUFKIT soundings from the GFS suggest a small window near daybreak where wind gusts could reach 45+ mph with deep mixing to 850-800 mb. However, other guidance is a little less aggressive, and the GFS has been on the hot side with winds of late. Not confident enough to issue a wind advisory at this time, and with the expected weather during the short term period, not sure issuing an advisory at this point is prudent anyway. Temperatures are expected to warm little from the morning lows. A transient surface ridge should be moving through during the evening hours, allowing winds to relax and temperatures to plummet via favorable nocturnal radiational cooling effects, at least during the evening hours. Forecast confidence decreases markedly Thursday night and Friday owing to unusually large spread with a northwest flow system moving through the Great Lakes and the Northeast during this period. The 00Z GFS looks like the main outlier here, with a much deeper evolution of a shortwave trough progressing from the Dakotas during the day Thursday to the vicinity of the Virginia coast by Friday afternoon. Operational GFS develops a relatively (compared to other guidance, that is) strong surface low in the vicinity of Chesapeake Bay by 12Z Friday, which places areas generally north of the Mason- Dixon Line on the colder northwest side of the low and associated precipitation shield. With favorable differential cyclonic vorticity advection and at least weak isentropic ascent east of the Appalachians, a swath of precipitation develops along and north of the low`s track. Temperatures would be cold enough for snow (again, generally north of the Mason-Dixon Line), and with QPF on the order of a tenth to two tenths of an inch during the morning hours Friday, this spells a couple of inches of snow for locations with sufficiently cold surface temperatures. Notably, the operational GFS was on the south side of the ensemble envelope, with most perturbations showing a farther north depiction of the vorticity maximum and associated surface low. Meanwhile, the 00Z NAM shows a much more zonal progression of the shortwave trough across the Great Lakes/Northeast, with the associated surface low tracking across northern PA. This would leave most of the CWA (at least outside the southern Poconos) on the south side of the low with considerably warmer surface temperatures and noticeably lower QPF on Friday. The 00Z CMC sheds no light on the matter, as it produces virtually no precipitation across the CWA -- keeping light rain predominantly to the south. Given that its 500- mb height field clusters with the north side of the guidance, the surface reflection so far south compared to the rest of the guidance makes little sense, and the CMC was discounted for this forecast. Finally, the 00Z ECMWF did trend south and somewhat stronger with the system, at least giving some leverage to the GFS. However, its timing is about 6 hours slower than the GFS, which allows for more surface warming to occur before the heaviest precipitation occurs. Probabilistic guidance (via SREF/GEFS, e.g.) expresses the uncertainty quite well, both with the chances of precipitation in general (generally in the 20-40% chance range region wide) and with precipitation type (nearly equal chances for rain and snow Friday morning south of the Poconos). As such, felt a consensus approach to the forecast was appropriate, hinging largely on ensemble guidance and with somewhat higher weight to the more clustered NAM/ECMWF versus the anomalous GFS and the physically inconsistent CMC. So the forecast has at least some chance of a rain/snow mix Friday morning generally north of the Mason-Dixon Line, but with lesser accumulations than the GFS is suggesting owing to the timing/strength/low track uncertainties. Importantly, the main message here is that the possible weather outcomes on Friday are widely varying -- and the potential impacts could be high given the timing of the system (rush hour Friday morning) or negligible. Hopefully, model agreement will improve during this period in later model cycles. From Friday night through Sunday, relatively cold northwest flow becomes established. Guidance has trended noticeably colder with temperatures during this period, which makes sense given the fairly strong cold air advection behind the Friday system and the origins of the incoming surface high. Followed suit with the temperature forecast during this period. Saturday looks downright cold, with temperatures several degrees below average for this time of year. Sunday should see some warming as the area comes under return flow on the upstream side of the surface high. Another potent vorticity maximum will straddle the U.S./Canada border early next week, generating a strong surface low in the northern plains of the U.S. tracking northeastward to Manitoba/Ontario by Tuesday. This will sweep a cold front through the area on Tuesday, but the strongest lift looks to be well north of the region. Did increase PoPs on Tuesday given the decent model consensus during this period, but right now, QPF looks generally light for our area. Given the far north track of the system, our CWA would be well within the warm sector on Tuesday, which suggests convection is a decent bet with any precipitation that occurs. For now, kept thunder out of the forecast, but if the trends of the associated system continue, suspect such inclusion will be required in later forecasts. Temperatures look to be above to well above average early next week. && .AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Low clouds persisted at most of our TAF sites this morning as an area of rain with embedded thunderstorms moved across eastern Pennsylvania and northern New Jersey. Some clearing will continue to build into our region from the southwest during the early afternoon hours with some locations improving to VFR. A line of showers and thunderstorms from the west is forecast to sweep across our TAF sites between about 1800Z and 2200Z. Some of these thunderstorms may be accompanied by strong wind gusts, frequent lightning and heavy rain. Another batch of showers with isolated thunder is expected to pass across our region this evening, followed by clearing for late tonight. A southwest wind around 10 to 15 knots today will gust into the 20s. The southwest wind is forecast to shift to the west around 0400Z to 0600Z as a cold front pushes across our TAF sites OUTLOOK... Thursday...VFR with strong northwest winds 15-25 kts with gusts to 35 kts. Confidence above average. Thursday night and Friday...Sub-VFR conditions possible with light precipitation, possibly snow, especially late Thursday night and Friday morning. Generally west or northwest winds becoming gusty again by Friday afternoon as the system departs. Conditions should improve to VFR by late in the day. Confidence well below average. Friday night through Sunday...VFR with decreased winds compared to Thursday and Friday.
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&& .MARINE... A line of strong to severe thunderstorms will impact the waters late afternoon/early this evening. Wind gusts in excess of 40 KT are possible, along with heavy rain and lightning. Small Craft Advisory conditions otherwise in place today and tonight with SW winds 15-20 KT with 25-30 KT gusts, shifting NW behind the front later tonight. OUTLOOK... Thursday...Gales likely during the day. Gale watch upgraded to a gale warning. Confidence above average. Thursday night through Friday night...Small craft advisory criteria likely on Thursday evening. Sub-advisory criteria likely Thursday night and Friday morning, but northwest winds should quickly increase, approaching/exceeding gale force late Friday afternoon through Friday evening. A chance of rain showers during the day. Confidence average. Saturday...Small craft advisory criteria will likely be met with breezy northwest winds continuing. Confidence average. Saturday night and Sunday...Sub-advisory criteria is expected. Confidence average. && .CLIMATE... Both Philadelphia and Wilmington tied their daily record high temperatures for February 28th. See RERPHL and RERILG for more details. February 2017 will be the warmest February on record for our climate sites. See the table below for more details. WARMEST FEBRUARY ON RECORD [Based on monthly avg temp (F)] Site 2017 Previous Record (Year) Normal POR PHL 44.2 42.2 (1925) 35.7 1874 ABE 39.2 38.6 (1998) 30.7 1922 ACY 43.0 40.6 (1954) 35.3 1945** ILG 43.1 41.2 (1976) 35.1 1894 It will also be a top 6 warmest meteorological winters (defined as December-January-February) on record for our climate sites. TOP WARMEST WINTERS ON RECORD [Based on DJF avg temp (F)] Site 2016-2017* 2016-17 Ranking*** Record (Year) POR PHL 40.5 6th 43.3 (1931-32) 1874 ABE 36.0 6th 37.3 (1931-32) 1922 ACY 39.9 Tied 3rd w/ 2001-02 40.9 (2011-12) 1945** ILG 39.5 Tied 3rd w/ 2011-12 41.5 (1931-32) 1894 **Period of record (POR) that includes complete and reliable monthly temperature data for ACY differs from the daily climate data listed in the Daily Climate Report (CLI) for each station. Monthly climate records go back to 1958 at Atlantic City Airport and then back to 1945 at the Atlantic City NAS. ***Shows how high this winter ranks on the list of warmest winters on record (e.g., #3 means 2016-17 is the third warmest meteorological winter on record) at each site. Record high temperatures that are in jeopardy for Wednesday are listed below. Location Daily High Temp Record for 3/1 (Year) ABE 67 (1972) ACY 72 (1972) GED 73 (1976) ILG 75 (1972) MPO 67 (1972) PHL 76 (1972) RDG 74 (1972) TTN 74 (1972) && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EST Thursday for ANZ430-431- 450>455. Gale Warning from 6 AM to 4 PM EST Thursday for ANZ430-431- 450>455.
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&& $$ Synopsis...CMS Near Term...Iovino Short Term...MPS Long Term...CMS Aviation...CMS/Iovino Marine...CMS/MPS Climate...Staff

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