Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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988 FXUS61 KPHI 171100 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 600 AM EST Sat Feb 17 2018 .SYNOPSIS... High pressure overhead this morning shifts offshore this afternoon. Low pressure rapidly intensifies off the mid Atlantic coast this evening then races out to sea by Sunday morning. High pressure briefly builds across the area again on Sunday, then offshore Sunday night. A strong warm front lifts northward across our area later Monday into early Tuesday, followed by a cold front late Wednesday. High pressure builds to our north next Thursday with a complex frontal system developing to our south and west at the end of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Winter Weather Advisory issued for much of the region. Cold and noticeably drier air continues to advect into the area this morning setting the stage for a quick hitting winter storm. As a closed low remains near Hudson Bay Canada, a strong short wave zips eastward from the Plains with some additional short wave energy to its south and east. This will enhance a short wave trough embedded within the broad trough as it moves into the Northeast tonight. As this occurs, a baroclinic zone to our south and east will serve focus for surface low development just of the Mid-Atlantic coast later today. This low will then track northeastward. The model guidance has generally sped up the arrival time of the precipitation shield as large scale lift looks to arrive quickly by late this afternoon. As a result, we sped up the PoP increase some from southeast to northeast. While the surface low is forecast to not be all that strong, forcing aloft is rather potent but it is quick moving. As we examine the forcing aloft, the Mid-Atlantic continues to get within the right entrance region of a potent 250 mb jet. In addition, lower to mid level frontogenetic forcing ramps up quickly toward evening from the southwest in response to a tightening thermal gradient northwest of the surface low. Given the expected large temperature to dew point spreads, the wet bulbing process will begin. Therefore while some rain may start across the south to southeast areas, snow could also occur for a time before enough warming takes over here. This will be different farther to northwest. More on the mesoscale component to this storm in the short term section below. Clouds are expected to increase and thicken quickly today which will help put a lid on the overall warming. While many places are expected to be several degrees above freezing this afternoon, the dryness of the airmass will assist in some cooling effect of the surface especially if thicker cloud cover rapidly moves in. Given this, we sided closer to the cooler guidance for high temperatures and also hourly temperatures. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... Mother Nature looks to put on a show tonight as a quick moving storm affects our region. As mentioned in the near term section, a potent short wave will move across the Northeast tonight with height falls to its southeast. This will help sustain a surface low off the Mid- Atlantic coast as it quickly tracks northeastward and away from our area overnight. The model guidance overall is in good agreement with a widespread precipitation event occurring mainly through the first half of tonight. The large scale ascent will be augmented within a right entrance region of a potent 250 mb jet. In addition, lower to mid level frontogenetic forcing will enhance the precipitation especially through about 06z. Some of the high resolution guidance hints at some banding potential, however the quick movement of the system may result in any heavier embedded bands to be transient. Where this occurs is a tough call at this time, however somewhere in the I-95 corridor may be the zone where this may try and become more focused. It is here where the better transition takes place with the thermal fields, resulting in stronger forcing associated with frontogenetic mechanisms. It is within this enhanced forcing where snowfall rates of up to an inch per hour is very possible. The entire system and lift however are on the move with the associated lift moving out of the area after midnight. As a result, a 3-6 hour window of enhanced precipitation rates along with embedded heavier elements look to occur. The snowfall forecast is still challenging as the heavier rates will help overcome any lingering warmth close to the ground, although this is occurring at night. It is during the heavier snowfall rates where road conditions will deteriorate quickly especially where air temperatures hover right near freezing. The liquid to snow ratios will also be lower as one goes southeastward across our region. This will result in a zone of larger wet snowflakes especially during the time frame of enhanced lift where accumulations may end up being the most efficient. Given the extent of the dry air in place initially, wet bulbing should be a factor in allowing temperatures to drop a bit more. We therefore blended closer to the cooler guidance such as the NAM. The snow accumulations are mostly from the I-295/NJ Turnpike corridor westward, with amounts dropping off quite a bit to the south and east. The far southeastern areas and southern Delaware into portions of northeast Maryland should see little to no snow. The forecast soundings still look to not have a pronounced warm layer aloft, at least enough to produce much sleet therefore we did not factor this into the forecast all that much. Looks like rain, snow, rain/snow mix. Some mixing may get northwestward into Philadelphia before the precipitation ends. Given the forecast snow amounts and less confidence on where some banding may enhance local accumulations, we went with a Winter Weather Advisory for the Watch area and added a tier to the south and east. It is near and west of I-95 where the greatest snow accumulations are forecast, however the I-95 corridor needs to be monitored as a period of enhance lift, higher QPF and increased snowfall rates may result in a zone of higher amounts of wet snow. If 2-4 inches of wet snow falls and sticks efficiently in a short period of time, there could be some power outages. This potential impact is less certain. The storm quickly moves away overnight with precipitation ending quickly, and clouds should start to thin some from the west toward daybreak. && .LONG TERM /SUNDAY THROUGH FRIDAY/... **Record warmth possible Tue and expected Wed** 500 MB: a very strong ridge develops along the east coast during mid week, then slowly weakens Friday and next weekend as a positive tilt trough develops across the United States. Temperatures: February through the first 16 days is averaging 3 to 6 degrees above normal and this weeks record warmth will likely make this February a top 10 warmest in our Mount Holly area history. In the mean time...the warming trend begins on Monday with calendar day averages 3 to 7 degrees above normal, then jumping 20 to 25 degrees above normal Tuesday and maybe 25 to 28 degrees above normal Wednesday, before cooling to between 15 and 20 above normal Thursday and still about 10 degrees above normal next Friday and Saturday. Forecast basis: Except as noted generally a 50 50 blend of the 00z/17 GFS/NAM MOS was applied for Sunday-Monday, the GFS MEXMOS for Monday night-Tuesday and then the 06z/17 WPC D4-8 12 hr elements of max/min temp/pop and 6 hrly sky/wind/td were used Tuesday night- Friday. The dailies... Sunday...A nice day. Seasonable. Gusty northwest winds in the morning to 20 mph quick diminish by midday as the high moves overhead. Confidence: Well above average. Sunday night. Frosty with light wind and generally clear sky atop the melting snow and remaining moisture from the Saturday night event. Confidence: Well above average. Monday...A p/sunny start quickly becomes overcast with a period of rain during the afternoon or evening in eastern PA but lesser chances elsewhere. South wind. Still sort of cool due to skycover and little in the way of mixing. Confidence: Well above average. Monday night...after rain ends, areas of dense fog possible late at night. Confidence: average. Tuesday...Stratus probably limits potential warmth. It becomes partly sunny during the afternoon. Record max temps possible, especially Georgetown DE. Confidence: average. Wednesday...Partly sunny and very warm! Widespread records likely. Mid 70s. Potential for equaling record warm Feb max temp. See climate section at 6 am for the records. Chance of showers late day or evening with the cold frontal passage. Confidence: Above average. Thursday and Friday...Considerable cloudiness. chance of showers. Confidence: Below average on both rain and max temps because of model differences. The primary scenario for Thu/Fri as outlined ydy at 330 PM by our previous long term forecaster: Thursday through Friday the cold front pushes to our south while high pressure builds to our north. Thursday should be mostly dry except across the far south where there could be some lingering lift/moisture behind the cold front that may combine to bring some light rain to the area. Friday has the better chance of rain as return flow develops as the high to our north builds offshore, and a warm front may lift across the area at the same time a couple of short waves/vorticity impulses do as well. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR through early/mid afternoon as clouds lower, then MVFR/IFR conditions develop after about 20z from southwest to northeast as snow and rain arrive. Snow is expected at the terminals, except MIV and ACY where it will mix with rain before changing to all rain. Northerly winds less than 10 knots, becoming east and then southeasterly. Moderate confidence on the timing. Tonight...Widespread precipitation including some moderate to heavy snow at times. IFR or lower conditions, especially with heavier snowfall rates. Some rain may mix in up to the PHL metro. Snowfall rates of up to 1-inch per hour possible and this is when accumulations would be the most efficient. Snow/rain ends quickly after 05z from southwest to northeast with conditions improving to VFR by daybreak. High confidence regarding precipitation occurring, but low confidence on a wintry mix into the I-95 terminals. Light and variable winds overall, becoming west-northwest overnight less than 10 knots. OUTLOOK... Sunday-Sunday night...VFR. Northwest winds may gust around 15-20 knots Sunday morning-midday. Confidence: Well above average. Monday...VFR early, lowering to MVFR or IFR during the afternoon or evening with rain. South to southwest wind. Confidence: above average. Monday night...MVFR/IFR conditions continue with rain possible in the evening, and low clouds/dense fog possible overnight. Confidence: Above average. Tuesday...MVFR/IFR conditions possible through midday in low clouds/fog. Conditions improving to VFR during the afternoon. Southwest winds may gust around 20-25 knots in the afternoon. Confidence: average. Tuesday night...VFR early, then possibly lowering to MVFR/IFR as low clouds/fog may develop overnight. Confidence: average. Wednesday...MVFR/IFR conditions possible early with low clouds/fog. Conditions improving to VFR during the day. chance of showers late in the day. Southwest winds may gust around 20-25 knots. Confidence:Above average. && .MARINE... The conditions through tonight are expected to be below Small Craft Advisory criteria. High pressure over the area this morning will shift offshore, allowing a northerly wind to turn northeast, east and then eventually southeast. Low pressure develops to our south by later today, then quickly moves out to sea tonight. This will allow the winds to eventually shift back from the west and northwest later tonight with some increase. OUTLOOK... Sunday..may need a brief SCA for nw winds gusty 20-25 kt early in the day then sub advisory nw winds in the aftn. Confidence: average. Monday...Sub Small Craft Advisory south winds. Confidence: Well above average. Monday night...Sub Small Craft Advisory conditions expected, although winds may gust around 20 knots. Confidence: Above average. Tuesday-Wednesday...Winds should stay below advisory levels, but seas may increase to near 5 feet. Confidence: Average. && .CLIMATE... Record high temperatures could be challenged on Tuesday, February 20th and probably will be equaled or exceeded Wednesday February 21st. These records are listed below. As of our 330 AM Saturday forecast...we are forecasting records at GED/MPO,/ABE record equaling at PHL/ILG and 1 below record at RDG/ACY (3 below at TTN). February 20: Atlantic City...70 in 1930 Philadelphia....70 in 1939 Wilmington......71 in 1930 Allentown.......68 in 1930 Trenton.........70 in 1930 Georgetown......68 in 2002 Mount Pocono....59 in 1930 Reading.........72 in 1930 As of our 330 am Saturday forecast....we are forecasting records at all locations except possibly ACY (matter of wind direction there and this fcst may be a bit too cool at ACY). February 21: Atlantic City...74 in 1930 Philadelphia....72 in 1930 Wilmington......70 in 1953 Allentown.......67 in 1953 Trenton.........70 in 1930 Georgetown......71 in 2014 Mount Pocono....60 in 1930 Reading.........71 in 1930 We are not posting the monthly records at this time since our forecast is 3 to 7 degrees below the monthly record. We`ll check again Sunday and Monday to see if our forecast edges warmer. The following are the record warm mins for Wednesday Feb 21. Barring a sudden immediate cool down behind the cool front at 1159PM Wednesday, there is a good chance of record warm daily minimum temperatures as well the previously referenced record highs. Record high mins for 2/21 ABE 46-1981 ACY 49-1954 PHL 49-2002 ILG 47-2002 RDG 48-1930 TTN 48-2002 Atlantic City rainfall already ranks 11th wettest in February history with 6.50" in 2010 the wettest. By sunrise Sunday, Atlantic City should rank about 8th wettest in its historical database. The February average temp for PHL projects 41.0 or 5.3F above normal or 8th warmest on record. This placement can change anywhere from 3rd to 12th depending on a 1 degree change in the eventual average. Bottom line...we`re likely on our way to a top 10 warmest February on record. The warmest was last year with a 44.2F average. Years 2002 and 2012 flank our currently projected 8th warmest. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Winter Weather Advisory from 4 PM this afternoon to 4 AM EST Sunday for PAZ054-055-060>062-070-071-101>106. NJ...Winter Weather Advisory from 4 PM this afternoon to 4 AM EST Sunday for NJZ001-007>010-012-013-015>019. DE...Winter Weather Advisory from 4 PM this afternoon to 4 AM EST Sunday for DEZ001. MD...Winter Weather Advisory from 4 PM this afternoon to 4 AM EST Sunday for MDZ008. MARINE...None. && $$ Synopsis...Drag Near Term...Gorse Short Term...Gorse Long Term...Drag Aviation...Drag/Gorse Marine...Drag/Gorse Climate...6a

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