Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 171507 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1007 AM EST Sun Dec 17 2017 .SYNOPSIS...
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A warm front extending from southern Pennsylvania through southern New Jersey will lift northeast of our area early Monday. A cold front will then move through the mid Atlantic states Tuesday night. High pressure will follow by Thursday morning. 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.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/... The 12Z surface analysis indicated the cold front has become stationary, now extending from far southern NJ into northern Delmarva. The surface flow is weak, with light northeast winds advecting in slightly drier air from the north, which is the sharpest indication of an otherwise diffuse boundary. Given the deep layer flow parallel to the front, expect it to remain stationary much of the day. Considerable cloudiness remains across the northern portion of the area, with partly cloudy skies across Delmarva. As the next short wave disturbance approaches from the Ohio Valley this afternoon, expect cloudiness to increase across Delmarva as well. Composite radar imagery indicates echoes developing aloft (virga) across the central Appalachians. Expect weak ascent to increase in the vicinity of the front, which could lead to some light showers late this afternoon, mainly confined to Delmarva. In terms of high temperatures, given the front and cloud cover, cooler north and warmer south. Overall though temperatures within a few degrees of average for this time of year. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... With the latest model run, almost all operational models backed off on the amount and extent of QPF, synoptic, and mesoscale forcing across our area as the warm front continues to progress north this evening. It appears this is in response to models depicting the surface high to weaken and move away from the region slower than previously expected. There is little, if any, frontogenesis depicted with the warm front, and it appears that the most lift comes from light onshore flow developing below the inversion, which is usually not enough for measurable precip. As a result, decreased PoPs across the northern half of the area to slight chance. Precipitation type: If we do have precip this evening, it looks like the mid level cloud deck should be persistent enough to allow ice crystals to already be present. Thus, if there is precip, it should be mostly snow. However, if the mid level cloud deck dissipates early, freezing rain would be possible for some of the area. Given how unlikely precipitation is at this point, there is not nearly enough confidence to issue anything, but we will keep the mention in the hazardous weather outlook. Temperatures: with the warm front moving through generally the first half of the night, expect lows to be 5 to 10 degrees above normal. && .LONG TERM /MONDAY THROUGH SATURDAY/... DRAFT for the 330PM issuance. Expect some modification based on receipt of the 12z/17 model suite and some of the 12z ensemble guidance. We`ll note where any significant changes have been made between this 955 AM issuance and the mid afternoon issuance. 930 AM DRAFT 12/17/17 .SYNOPSIS... A warm front will lift from southern Pennsylvania through southern New Jersey will life northeast of our area early Monday. A cold front will then move through the mid Atlantic states Tuesday night. High pressure will follow by Thursday morning. 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. && .LONG TERM /MONDAY night THROUGH Sunday/...
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Hazards: None likely. A super week for the shopping economy in our area as basically this week doesn`t show much sign of any significant winter weather. 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 builds back across the southeast USA next weekend as large positive tilt trough develops in the Rockies. Temperatures: Well it was a nice stretch of winter from the 8th through 16th that drove the monthly average temps down so that the first 16 days of the month were 3.6 F below normal at KMPO and 2F below normal at KGED/KRDG and 1 to 1.8F below normal at the 5 other 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 (mins occurring at 1159PM), near normal Thursday, then warming above normal again on Friday, and then 7 to 15 degrees above normal next weekend. The best chance for below normal temps between now and the 26th of December appears to be this coming Thursday. Forecast basis: Unless otherwise noted Monday night-Tuesday night is a 50 50 blend of the 12z/17 GFS-NAM MOS, 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. SNOW depth and MOS temperature guidance. We have just 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 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. The dailies... Monday night...Milder. Light south wind. Confidence: above average. Tuesday...Warming nicely ahead of the next cold front. 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. Min temperature for the day occurs around 1159 PM. 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. Saturday...Looks like at least a widespread period of showers or rain ahead of a CFP. POPS will probably be 60 to 80 pct for a period of rain. Confidence: above 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. A look into Christmas week indicates an eventual trend to below normal temperatures, but it may be delayed til after Christmas. At this time...considerable uncertainty exists regarding whether there will be snow cover or even snow on the 25th in our area except maybe the Poconos. GEFS guidance as of 06z/17 suggests considerable warming aloft developing next weekend into early next week while it turns rather cold from the Rockies and Plains into the Great Lakes region. If we are fortunate, we may still be cold enough for snow cover or a bit of snow between the 24th-25th, but at this time, I dont think ensemble model guidance is favorable for any remaining snow cover here or an all snow event. GEFS 500 MB height fields and associated anomalies, as well as ECEFS 2m temp fields suggest above normal temps continuing 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 strongly negative EPO that is modeled to begin the 23rd, might not be working east of the Appalachians, unless model MOS changes dramatically the next day or so. 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, even when the the EPO is negative. Thursday is currently modeled to be our coldest day of next 8 days (inclusive of Christmas). I probably spent too much time detailing the multiple solutions and contradicting signals for Christmas but wanted any readers of this long term discussion to be aware of what is modeled.
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&& .AVIATION /15Z SUNDAY THROUGH THURSDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR conditions are expected, though a mid level cloud deck around 5000 ft AGL may advect south later today. Winds are expected to be light and variable. High confidence. Tonight...Mostly VFR conditions are expected overnight. There is a small chance for precipitation at the TAF sites; if precipitation develops then brief MVFR conditions are possible. Winds should settle out of the east or southeast by this evening, but wind speeds should remain 5 kt or less. Moderate confidence. 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...
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Winds and seas are expected to stay below small craft advisory conditions today and tonight. Wind direction is expected to be variable, eventually settling out of the east or southeast by tonight. 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.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Drag 1006 Near Term...Franck/Johnson Short Term...Johnson Long Term...Drag 1006 Aviation...Drag/Franck/Johnson 1006 Marine...Drag/Franck/Johnson 1006

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