Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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303 FXUS61 KPHI 161403 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 903 AM EST Sat Dec 16 2017 .SYNOPSIS... A cold frontal boundary across the Great Lakes will move southeastward through the region today and tonight. This front will then comes back northward through the region as a warm front on Sunday. Another cold front will then move through the region during the middle of next week. To end next week, an area of low pressure will track northeast into the Great Lakes bringing a warm front through our region. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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900 AM ESTF: Temperatures were a little slower to rise this morning across northern NJ and portions of eastern PA where cloud cover is increasing, so adjustments were made. A bit more sunshine is also expected across southern NJ and DE/MD. In terms of PoPs, delayed the snow showers a couple hours across the northern portion of the area. Otherwise, forecast remains on track. Previous Discussion... Today, the attention turns to locations generally along and north of Interstate 80 for snow chances. No major changes to the previous forecast with today`s event. Another mid level shortwave trough will approach our region through the day. In response, the cold front currently over the Great Lakes will approach the region this afternoon. Further, the right entrance region of an upper level jet will be over the northern half of the region. As mentioned by the previous shift, the best synoptic scale lift should stay north of our region. However, models continue to indicate a ribbon of low level frontogenesis across southern NY. This, combined with the nearly saturated boundary layer and equivalent potential vorticity values near or below 0 just south of the frontogenesis, indicates there should be enough instability for some snow showers to progress as far south as the I80 corridor. Moisture is limited, as is the window of opportunity for any snow showers, so any accumulations are expected to be less than an inch. Further south, the combination of a building surface ridge over the Coastal Plains and be further removed from any forcing, should keep the area dry. Forecast temperatures are below most guidance. The reason for this is that models tend to have a warm bias the day after a fresh snow pack. Thus, expect highs to range from the upper 20s in the southern Poconos and NW NJ, to the lower 40s across Delmarva.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... Overnight, the latest mid level short wave trough moves further away from the region. The cold front should continue to slide south through the region. However, by this point, models don`t depict any frontogenesis, but there should be subsidence thanks to the building east coast ridge. Thus, expect the overnight period to be dry across the region. Undercut guidance considerably for overnight lows as models have had a strong warm bias the last few nights. Though the snow pack may not be much of an issue (much, if not all of it could melt during the day today), clearing skies and light winds will promote radiational cooling especially through the second half of the night. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Sunday through Monday night: A wintry mix event is currently modeled across northern portions of the region with moisture overruning a warm frontal boundary. Overall, this system will be weakening as it comes northeast into our region. Through this event, thermal profiles look to support mainly rain across the Philadelphia area and locations to the south, while locations further north have modeled soudings which would yield a mixture of snow and freezing rain. This is noted in a warm nose advecting northeast at about 925 mb throughout Sunday night with 925 mb temperatures rising to between 1 and 2C with surface temperatures near or below freezing. QPF does look light and under 1/10th of an inch on most models and ensembles at this time. Generally opted temperatures Sunday toward the modeled two meter temperatures then incorporating some of the MET/MAV/ECMWF temperature guidance Monday and Monday night. Temperatures should rise into the 40`s for most of us with lows Monday night in the 30`s. Tuesday and Tuesday night: Our region will be between a high pressure offshore and an approaching cold front from the west. This will result in southerly winds and warming temperatures. Enough moisture and lift could be present ahead of the front for some scattered rain showers. However, the signal for precipitation is noisy on the various ensembles and fairly limited so we will maintain slight to low chance pops. Right now the temperature forecast is on the warmer end of the ensemble envelope with highs well into the 50`s, nighttime temperatures should fall back into the 30`s. Wednesday through Thursday night: After the frontal passage, we return to northwest flow with cold air advection into the region. Another period of gusty winds is not out of the question. With high pressure building in both days should be mostly sunny. Right now the wind direction off the lakes looks unfavorable for any increase in clouds or snow showers up in the southern Poconos. Bufkit analysis shows the potential for some gusts from the west at around 20 mph Wednesday afternoon. High temperatures should fall back into the 40`s with lows in the 20`s, opted to go on the cooler end of the ensemble guidance given the cold air advection into the region. Friday and Friday night: A warm frontal boundary lifts northward across the region as a low pressure system tracks into the Great Lakes. Another surge of southerly flow will come into the region leading toward a moderating trend with temperatures. This warm front will also bring some showers with it. Longer term: A quick look into Christmas week does still provide the chance of a white Christmas with an active weather pattern that looks to turn colder thanks to a EPO which is falling several standard deviations below normal. The - EPO will discharge cold from AK southeast into the United States just before Christmas. However, the colder air can dive into the western United States first leading to warmer conditions and a typical ridge in La-Ninas over the Southeastern United States. Exactly how this plays out is still outside the forecast window and a lot of possibilities are in play. Be mindful of any weather information you see about Christmas can still change quite a bit over the next few days. && .AVIATION /14Z SATURDAY THROUGH WEDNESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR. There is a small chance that snow showers along and north of I80 this afternoon, could move as far south as KABE. If this happens, there could be brief MVFR conditions. However, this is very unlikely so have not included it in the TAF. Winds generally out of the west around 10 kt. High confidence. Tonight...VFR conditions are expected overnight. A cold front moving through by 06Z will result in northerly or northwesterly winds, but speeds should be 10kt or less. After 06Z, winds will likely become light and variable. High confidence. Outlook... Sunday: VFR, winds under 10 knots. High confidence. Sunday night: Sub-VFR possible with patchy light rain, mixed precipitation possible at KABE, KTTN and KRDG. Winds under 10 knots. Moderate confidence. Monday and Monday night: Mainly VFR, winds generally under 10 knots. Moderate confidence. Tuesday through Wednesday: VFR. An isolated shower can not be ruled out on Tuesday. Southwest wind gusts around 15 knots then shifting to northwest and increasing to 20 knots on Wednesday. High confidence.
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&& .MARINE... Winds gusting above 25 kt are expected on all the waters through at least the morning hours. On the Delaware Bay and Delaware and southern New Jersey coastal waters, winds may drop below small craft advisory earlier than previously expected, possibly as early as this afternoon. However, I did not have enough confidence to change the end time on the small craft advisory. On the northern and central New Jersey coastal waters, though there may be a brief lull in the winds late this afternoon, wind gusts above 25 kt are possible behind a cold front this evening. Outlook... The main period for concern is northwesterly winds Wednesday afternoon which may approach or briefly exceed 25 knots which may warrant an SCA. Data is indicating seas stay below five feet throughout the outlook period. High confidence. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 1 AM EST Sunday for ANZ450>455. Small Craft Advisory until 6 PM EST this evening for ANZ430- 431. && $$ Synopsis...Gaines Near Term...Franck/Johnson Short Term...Johnson Long Term...Gaines Aviation...Franck/Gaines/Johnson Marine...Gaines/Johnson

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