Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 180224 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 924 PM EST Sun Dec 17 2017 .SYNOPSIS... A warm front will lift northeast of our area early Monday, then a cold front moves through the Mid-Atlantic states Tuesday night. High pressure will follow by Thursday morning while an area of low pressure scoots out to sea to the east of the Carolinas. Thereafter, strengthening low pressure will track northeast from the Great Lakes region Friday, sending a warm front through our region, followed by a cold front on Saturday. That cold front may stall just south of our area on Sunday. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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For the 930 PM update, had to throttle back (delay) the PoPs some more as weak and unorganized ascent along with lingering dry air is keeping precipitation either at bay or from reaching the ground. Radar imagery is unimpressive, which matches guidance overall up to this time frame. The bulk of the radar returns are moving off the Delmarva coast. The 00z Sterling, VA raob showed lots of dry air below 700 mb. A little more ascent may develop across the northern areas over the next 3-6 hours, with perhaps some light snow. Still cannot rule out some light freezing rain/drizzle in parts of eastern Pennsylvania to parts of northern New Jersey, however this continues to look mostly spotty. As a result, no advisory will be issued at this time. Adjusted some temperatures and dew points based on the latest observations, then blended in the LAMP guidance for a few hours to help assist with trends. No other changes at this time. Otherwise, the next short wave disturbance moves from the Ohio Valley and through our region overnight. Meanwhile, a stationary front across far southern NJ and Delmarva will begin to lift north as a warm front overnight. Expect weak ascent to increase across the region overnight. The models are highlighting two areas in particular: Delmarva into southern NJ and the Lehigh Valley into the southern Poconos. While precipitation southeast of the NJ Turnpike and I-95 remains predominantly liquid with temperatures expected to remain at or above freezing, a light and spotty wintry mix is possible to the northwest. In particular, there is the potential for a corridor of spotty light freezing rain, especially between Interstate 78 and the Pennsylvania and New Jersey Turnpikes. North of this area, the potential for freezing rain is less, with a greater potential of light snow. There is still model uncertainty with regard to the extent of snow versus freezing rain. In general, the guidance appears too bullish on freezing rain north of I-78, where sufficient ice aloft should favor snow. Model soundings also indicate the absence of a warm nose, so any sleet potential should be very limited. There is the potential for snowfall up to a half of an inch. If the potential for freezing rain to produce ice accumulation at or above a trace of an inch increases, then a Winter Weather Advisory may be needed. Given the light and spotty nature of the precipitation, and uncertainty with regard to freezing rain versus snow, we opted for a Special Weather Statement to cover the potential impacts at this point. Overall, a low confidence forecast continues.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/... Any precipitation is expected to end around sunrise on Monday, with a return to generally fair weather. The warm front will move well to the north of our region, with a continued westerly flow aloft. This will maintain considerable cloudiness across portions of eastern PA and northern NJ, with skies gradually becoming partly sunny to the south. High temperatures will be above normal by several degrees, with light westerly winds. && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/... Hazards: None likely. A super week for the shopping economy in our forecast area as basically this week doesn`t show much sign of any significant winter weather...other than what might occur tonight to the northwest and north of Philadelphia. And yes, we have looked at the records for Tuesday and Saturday and we are not yet forecasting within 2F of record. 500MB: A notable warm ridge of high pressure over the southeast USA Monday night will weaken Wednesday in response to the passage of a strong short wave through southeast Canada. Ridging begins building back across the mid Atlantic states and southeast USA by next Friday. Thereafter, it further strengthens along the east coast and western Atlantic while a positive tilt trough develops from the Rockies into south central Canada. Temperatures: Well it was a nice stretch of winter from the 8th through 16th that drove the monthly average temps down. The first 16 days of the month averaged 3.6F below normal at KMPO and 2F below normal at KGED/KRDG and 1 to 1.8F below normal at our other 5 long term climate sites. Well, thats over and am unwilling to speculate whether the month will average above or below normal after the culmination of the next 8 days of generally above normal temperatures. Calendar day averages Tuesday should be 10 to 15 degrees above normal, near 5F above normal Wednesday (minimums occurring at 1159PM), near normal Thursday, then warming above normal again on Friday, to possibly 15 or even 20 degrees above normal Saturday and perhaps Sunday as well. The best chance for below normal temps between now and the 26th of December appears to be this coming Thursday. Forecast basis: Monday night-Tuesday night is a 50 50 blend of the 12z/17 GFS-NAM MOS with a tendency to favor the warmer GFS (see our note on snowcover below), Wednesday the 12z/17 GFS MEXMOS and next Thursday- Sunday (D4-8) is the 15z/17 WPC 12 hr elements of the max-min temps/pops and 6 hrly td/wind/sky. These D4-8 WPC max/min temps were modified higher via a compromise with the 12z GFS MEXMOS temps or the 12z EC 2m temps. SNOW depth and MOS temperature guidance. We have viewed NAM model snow depth and its biased far too high here in SNJ. Be alert for possible impact on BL temps even into Monday night. The 06z and 12x GFS snow depth also is high but not as much as the NAM. Nevertheless it too may impact the GFS MOS temp guidance cooler than what one would think for this pattern. The dailies... Monday night...Milder. Light south wind. slight chance of a sprinkle in ne PA but not really accounted for the in the forecast. Confidence: above average. Tuesday...Warming nicely ahead of the next cold front. Gusty southwest winds possibly to 20 mph in the afternoon. Confidence: above average. Tuesday night...maybe a sprinkle or light shower up north. Otherwise for now, odds favor a dry cfp. Confidence: above average. Wednesday...P/Sunny and cooler with a gusty northwest wind to 20 or 25 MPH. Minimum temperature for the day occurs around 1159 PM. The cirrus forecast (Partly Sunny) is in association with a southern USA short wave and sfc low heading east, of which the northern fringe of the high cloud shield is modeled up to about I78. Confidence: above average. Thursday...Probably the coldest temps of the week. Light northwest wind. Confidence: above average. Friday...WAA begins and it may yield rain late in the day or at night. Confidence: average on scenario but below average on whether rain will begin Friday. Saturday...Probably breezy and warm. It looks like at least a period of widespread showers ahead of a CFP. Forecast POPS are probably too conservative by 20 to 30 pct. Confidence: average. Sunday the 24th...Confidence rather low (well below average for a Day7), since model solutions vary regarding the timing of the next pcpn event. This day is primarily a blend of WPC, EC and GFS. A look into Christmas week indicates an eventual trend to below normal temperatures, but it probably is delayed til after Christmas. At this time...some uncertainty still exists regarding whether there will be snow cover or even snow on the 25th in our area. Recent model trends are less favorable for snow but that does not mean it cant happen. GEFS guidance as of 12z/17 suggests considerable warming aloft developing next weekend into early next week while a 1050MB high in the central Rockies is associated with a significant cold out break from the Rockies and Plains into the Great Lakes region. GEFS 500 MB height fields and associated anomalies, as well as ECEFS 2m temp fields suggest above normal temps continuing in our area through Christmas. After Christmas, trends-odds favor colder airmasses with sub freezing temps and potential for wintry weather events. A concern regarding teleconnections (tc)... my interpretation of the tc patterns shows very little evidence of blocking in the north Atlantic through at least the 27th (-PNA and +NAO). The EPO is modeled to drop to very very low values around the 23rd through the 27th, yet all our MOS and MOS ensemble guidance is showing aoa normal temps here in our area except for Thursday, even when the the EPO is negative. Thursday is currently modeled to be our coldest day of next 8 days (inclusive of Christmas). && .AVIATION /02Z MONDAY THROUGH FRIDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR ceilings lowering to MVFR after midnight mainly at RDG, ABE to near TTN. While some light precipitation is possible, the probability is low and the visibility may not be affected therefore a mention is not included in the TAFs. Northeast winds 5 knots or less to light and variable or calm. Lower confidence regarding the extent of the MVFR ceilings overnight. Monday...MVFR ceilings mainly at RDG, ABE to near TTN improves to VFR by mid morning. Elsewhere, VFR is anticipated. Light and variable winds early in the morning becoming westerly generally around 5 knots. Outlook... Monday night...VFR. West wind. Confidence: Average. Tuesday...VFR. Southwest wind may gust 20-25 kts early afternoon. Confidence: Average. Tuesday night...VFR. Northwest wind. Confidence: Average. Wednesday...VFR. Northwest wind should gust 20-25 kt. Confidence: Average. Thursday...VFR. Light northwest wind. Confidence: Average. Friday.... VFR. MVFR or IFR conds possible late. South to southeast wind. Confidence: Average.
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&& .MARINE... Winds shift from east tonight to southeast Monday morning and eventually to the west Monday afternoon, as a warm front lifts north across the waters. Both winds and seas well below SCA criteria tonight and Monday. Outlook... Monday night through Friday Southwesterly wind gusts may approach 20 knots Tuesday afternoon. However, the main period for concern is Wednesday when northwesterly winds should approach or briefly exceed 25 knots. Model guidance indicates seas stay below five feet throughout this outlook period. Confidence: Above average. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Drag Near Term...Franck/Gorse Short Term...Franck Long Term...Drag Aviation...Drag/Franck/Gorse Marine...Drag/Franck

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