Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 110211 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 911 PM EST Tue Jan 10 2017 .SYNOPSIS... A warm front will continue to lift through our region overnight tonight. A secondary warm front is expected tomorrow night. A large high pressure system will build off the coast of the southeastern United States before a cold front moves through Thursday night. High pressure will then build across to the north of our area through the weekend. A low pressure system will develop across the Ohio River Valley and move through the Mid Atlantic late Saturday into Sunday. High pressure returns for early next week. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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Have made considerable changes to the forecast for this evening and overnight. Removed mention of snow and sleet as strong low level warm advection is underway and 950-850 mb temps look to be too warm by the time precip gets here late this evening. Only question is rain vs freezing rain which depends on sfc temps, and these have continued to rise this evening. Temps should continue to gradually warm overnight with fairly strong S to SW surface winds. The winter weather advisory was trimmed back by removing the SE tier of counties across parts of SE PA and NW NJ. Remainder of the WSW remains in effect for now, but the threat for freezing rain seems to be confined to higher elevations, lasting longest overnight in the Poconos. Previous discussion: A warm front will cross the area tonight and a shield of clouds and precipitation will accompany it. The clouds will continue to advance across the area this evening and then the deeper moisture and lift will arrive later. Precipitation will begin across the wrn areas first as snow, then transition to sleet/freezing rain before becoming all rain by morning. Further east, the precipitation will arrive later (more towards midnight), but will likely be mostly all rain as warmer air and a decent low level jet arrive across the area. For now, we have kept the WSW flags as they already exist, since it seems that these Nrn/NWrn areas have the greatest chc for a light glaze to develop. Temperatures will slowly rise through the night and the colder air will probably be trapped in the valley across the north to some extent. Pops are in the likely to categorical range overnight. Qpf will be generally be 1/4 to 1/3 of an inch in most areas, most of it being rain after the changeover. Snow/sleet accums should be rather limited, perhaps 1/4 to 1/2 inch at most.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/... The warm front will have passed N/E of the area Wed morning. The precipitation will end across the ern-most areas early then a slow improvement is expected through the day as drier air arrives from the S/W. The MOS guidance is quite a bit different with what happens Wed with the NAM showing low clouds and fog much of the day and the GFS offering mid-day fair weather across the Del Valley. With limited confidence in any solution, I decided to bring the improvement to the area, but a few hours later than the GFS is indicating. Skies should become partly sunny by afternoon. Tomorrows temperatures will depend on how fast things clear out, readings rising to the upper 40s/low 50s are possible, so we have cautiously sided with the GFS attm. Winds will be southwest to west at around 10 mph. && .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/... Wednesday night and Thursday...A secondary warm front lifts through the region. As a result, Thursday will be quite warm - Forecast highs are generally 20 to 25 degrees above normal and near records at most of our climate sites (see the climate section for details). Thursday night and Friday...models continued the trend with bringing the cold front through Thursday night (a bit more progressive than what we saw yesterday). As noted by the previous shift, frontolysis is expected to be occuring as the front moves into our region. That, combined with the fact that the associated low will stay well north of our region, means that we probably won`t see a widespread precip event for our region. None the less, there will still be some significant cold air advection with the front as by Friday highs will be about 10 degrees lower. Saturday through Sunday...The cold front is still expected to stall through this period, though exactly where it will stall is quite uncertain as there remain large differences between the models on the location of the stalled front. A pair of surface lows along the front will result in precip chances in the vicinity of the front, but precip type is highly dependent on the location of the front, so highly uncertain at this time. Monday...By late Monday or Monday night, a surface low is expected to lift out of the Midwest towards the Great Lakes Region, lifting another warm front through our region. && .AVIATION /02Z WEDNESDAY THROUGH SUNDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Winds through tonight will be mostly South 10 to 15 knots but with occasional higher gusts. Increasing and lowering CIGS with precipitation arriving 04-05Z at ABE/RDG and a couple hours later in the Delaware Valley. Most areas will just have rain but a mix with freeing rain is possible at KRDG/KABE early. We went along with the guidance which steadily lowers conditions to IFR overnight. Wednesday, the speed at which things change from IFR/LIFR is the biggest challenge for todays tafs. The NAM MOS has low conditions at PHL for the entire day, while GFS MOS has a return to VFR by 15z. Not a fcst with any great degree of confid after 12Z Wed, but generally brought better conditions after 15Z-17Z across the area. Outlook... Wednesday night...Conditions falling to MVFR and IFR in rain. Thursday...Conditions slowly improving to VFR. Small chance for MVFR at KABE and KRDG with rain. Thursday night...Low clouds, fog, and rain may bring conditions back down to MVFR and IFR primarily for KRDG, KABE, and KTTN. Friday...Mostly VFR conditions expected. Saturday and Saturday night...Conditions expected to deteriorate as precipitation moves back into the area. A wintry mix of precipitation possible. Sunday...small chance that precip may continue through the day. Otherwise, conditions should gradually improve to VFR.
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&& .MARINE... Approaching low pressure and a warm front will cross the waters overnight. A warming flow associated with the low level jet will become established across the cold waters tonight. The degree of mixing is problematic and even a few well mixed gusts are enough for gale flag criteria overnight. We have thus decided to upgrade the Gale Watch for the northern NJ coastal waters to a Gale Warning and also keep the SCA flag for the southern waters. The SCA flag was extended into lower Del Bay also. Overall confid in fcst is moderate at best. Winds will be mostly srly tonight and then shift to southwest late and remain Southwest Wednesday. Gales will likely be scaled back to SCA after the low level jet passes, and SCA conditions will probably remain on the waters Wed morning. Rain and fog expected overnight. Outlook... Wednesday night...Once winds and seas drop below SCA criteria by mid day, no more marine headlines are anticipated through this period. Thursday through Friday morning...South to southwest flow over the ocean waters will start to build seas to 5 to 7 feet. Winds may gust above 25 knots at times especially on Thursday, diminishing towards Friday morning. A SCA will likely be needed. Friday afternoon through Sunday...Winds and seas should stay below SCA criteria. && .CLIMATE... Thursday should be quite mild with current forecast near record highs for many of our climate sites. Here are the records for January 12: GED, MPO, TTN were updated at 515P. Location/ Record/Year Atlantic City/63/1913 Wilmington /63/1913 Allentown /60/1932 Reading /61/1913 Georgetown /70/1975 Mount Pocono /56/1975 Trenton /68/1890 At this time, we are forecasting that Philadelphia will stay well below its record of 72 set in 1890. Also for todays 441PM CLI: we assumed .01 for the MPO 2 hrs of snow with vsby variable down to around 2mi. The .29 at ABE was an obvious error. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Winter Weather Advisory until midnight EST tonight for PAZ060>062. Winter Weather Advisory until 5 AM EST Wednesday for PAZ054- 055. NJ...Winter Weather Advisory until midnight EST tonight for NJZ001- 007. DE...None. MD...None. MARINE...Small Craft Advisory until 1 PM EST Wednesday for ANZ431- 452>455. Gale Warning until 7 AM EST Wednesday for ANZ450-451.
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&& $$ Synopsis...Johnson Near Term...AMC/CMS/O`Hara Short Term...CMS/O`Hara Long Term...Johnson Aviation...AMC/CMS/Johnson/O`Hara Marine...CMS/Johnson/O`Hara Climate...

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