Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 180920 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 420 AM EST Sun Feb 18 2018 .SYNOPSIS... High pressure briefly builds across our area today, then offshore tonight. A strong warm front lifts northeast across our area early Tuesday, followed by a cold front late Wednesday. That cold front stalls to our south with waves of weak low pressure of moving northeast along the front Thursday through Saturday. Eventually a larger low pressure system should move across southeast Canada on Sunday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Watch for some slippery spots on untreated surfaces early this morning due to residual moisture and/or slush. A strong short wave trough with an associated robust short wave will shift east of the New England coast this morning (a few snow showers possible early this morning across the far northwest zones), with the flow aloft going more zonal in its wake. As this occurs, a strong ridge centered in the southwestern Atlantic Ocean extends westward into the Gulf of Mexico. A sprawling area of high pressure at the surface will quickly build over our region during today. Some cold air advection occurs this morning, however this quickly diminishes this afternoon. As drier air along with increasing subsidence overspreads our area, the cloud cover is expected to quickly erode early this morning. There could then be some stratocumulus leftover for a time within a northwesterly wind which diminishes this afternoon as high pressure arrives. Temperatures could be a bit of a challenge today thanks to the expansive area of 4+ inches of snow last evening. While temperatures are expected to rise into the 40s for much of the area this afternoon, these could be tempered some due to energy going into melting the snow. We therefore went a little cooler especially north and west of I-95 but not to much lower given lots of sunshine expected. To the south and east of here, there is much less snow and there will be a downslope component of the wind. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... As a significant upper-level trough settles into the West, a ridge begins to build across the East. Surface high pressure will therefore shift offshore allowing a return surface flow to start developing. In addition, the flow aloft backs to more southwesterly through tonight along with an increase in warm air advection. There are some weak short waves riding the northern side of the gradually building ridge, and these along with an increase in warm air advection could toss some clouds our way during the course of the night especially late. This may have an impact on temperatures, as winds will be light to calm at the surface with dew points low enough to assist in radiational cooling. For now, we opted to go somewhat colder across the northern areas assuming at least some snow cover will remain. && .LONG TERM /MONDAY THROUGH SATURDAY/...
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**Record warmth possible Tue and expected Wed** Hazard potential worthy of monitoring: Dense fog may become a widespread hazard event in e PA and NJ Monday night. Icing, possibly an intermittent but prolonged event may be setting up for the far northern part of our area between Thursday and Saturday morning. 500 MB: Very warm and strong ridging builds along the east coast early this week then gradually weakens late in the week as a NEG NAO develops and may be starting to change the pattern here by the weekend. A series of short waves will be ejecting enewd out of the western USA trough late this week into the weekend setting us up for periods of pcpn. Temperatures: Please the climate section on all the upcoming temperature records that will be approached or exceeded and a reevaluation of PHL Feb monthly projection. Calendar day averages should be about 3 to 5 degrees above normal Monday, then 20 to 24 degrees above normal Tuesday, 25 to 30 degrees above normal Wednesday, cooling to 10 above normal Thursday, 5 above normal Friday and then an uncertain weekend in which average temperatures should warm to anywhere between 8 and 18 degrees above normal. Forecast basis...Unless otherwise noted a 50 50 blend of the GFS/NAM MOS for Monday-Tuesday, the GFS MEXMOS for Tuesday night-Wednesday and then the 06z/18 WPC D4-8 12 hr elements of max/min temp/pop and 6 hrly sky/wind/td were used Wednesday night-Saturday. We did apply the colder ECMWF to our fcst temps beginning 06z Thursday through Saturday morning, which means we are under virtually all guidance except a bit warmer than the ec 2m temps. The dailies... Monday...after a potentially frosty start to the day, rapidly lowering cloudiness. An instability burst from the southwest should develop-advect a band or bands of showers into the western and northern part of our forecast area during the afternoon. Cloud cover and a light southerly wind limits afternoon warming. Confidence: Above average. Monday night...any leftover showers should end around midnight. Nearly steady or slowly rising temps with stratus/fog-some of the fog may be dense, especially over the remaining snowmelt I78 region northward. Confidence: above average. Tuesday...skies will brighten and it will warm during the afternoon but thinning stratus may limit the warm up in some areas. Confidence: average with the uncertainty how fast skies partially clear in the afternoon. Southwest wind should permit the the status to break up. Confidence: average. Tuesday night...Stratus may continue in parts of our area or reform for a few hours late at night but for now, expect the boundary layer wind to be too strong to permit dense fog forming. Confidence: average. Wednesday...Record daily warmth forecast Wednesday and potentially near the all time February record max. Record daily warm minimums appear possible but will depend on the strength of caa near 1159 PM Wednesday. Highs well up in the 70s over SE PA as well as much of the Delmarva and NJ. Stratus may continue in parts of our area for a couple of hours during the morning before thinning to reveal partly to mostly sunny skies. An approaching cold front from the northwest could bring some showers to the region northwest of I-95, toward sunset. Wednesday night into Thursday...A fairly strong cold front with scattered or isolated rain showers pushes southeast through our area before stalling somewhere between southern DE and NC. From this point forward, confidence on details drops below average. There are several scenarios, and they all are cold. It would not be impossible to see some sleet Thursday I78 north if a wave of low pressure spreads its pcpn shield north of I78. Friday - Saturday...an even bigger forecast problem may be looming. Presuming sub freezing boundary layer cold air seeps south into the I78 region, will low pressure in southeast Canada become dominant and yank the cold air out. I`m tending to think not, especially with blocking starting to press southwestward. I may be premature on this but if the Low forecast across se Canada is alot weaker, then we will be in a situation near the top of the ridge with positive tilt short waves ejecting out of the western USA and creating periods of light to mdt pcpn here, which could be ice I80 north. Right now its a long shot but the EC has a hint and the GFS is not too far off. We`ll watch the trends in the modeling consensus the next few days. For now uncertainty but i think, worthy of monitoring. The two winter events that occurred this week were briefly hinted at one time or another a week prior to their occurrence Tuesday and yesterday. The GGEM was onto this ydy and todays GGEM op member has lost it. Its February and winter and hopefully this extensive discussion of a colder late week scenario is not useless.
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&& .AVIATION /09Z SUNDAY THROUGH THURSDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...MVFR/IFR ceilings are expected to improve to VFR by 12z. Light west to northwest winds increasing to around 10 knots this morning, then diminishing by late afternoon and becoming west- southwest. Tonight...VFR. Light and variable or light southerly winds. OUTLOOK... Monday...VFR early, lowering to MVFR or IFR during the afternoon or evening in showers and/or stratus. South to southwest wind. Confidence: above average. Monday night...Widespread IFR conditions in st/fog with showers or drizzle possible early. Conditions may deteriorate to LIFR in areas of dense fog late. Confidence: Above average for IFR, average for LIFR. Tuesday...IFR conditions may linger through midday in low clouds/fog. Conditions should improve to VFR during the afternoon. Southwest winds may gust around 20 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. Wednesday night through Thursday...Chance of showers with associated restrictions. A wind shift from southwest Wednesday evening to northwest and then north on Thursday. Confidence: Below average.
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&& .MARINE...
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A short surge in cold air advection should result in increased mixing for a time this morning. As a result, we will maintain the Small Craft Advisory through late morning with a round of wind gusts to around 25 knots. Otherwise, the conditions are expected to be below Small Craft Advisory criteria as high pressure builds in through today then offshore tonight. OUTLOOK... Monday...Sub Small Craft Advisory south winds. Confidence: Well above average. Monday night...Sub Small Craft Advisory conditions expected, although southwest 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. Wednesday night - Thursday...Winds should stay below SCA levels with seas persisting in the 3 to 5 foot range. Confidence: Below average because of a wind shift Wednesday night from southwest to northwest and then north northeast on Thursday.
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&& .CLIMATE...
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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. 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 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 sharp cool down behind the cold front at 1159PM Wednesday, there is a 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 of 5.81 inches currently ranks 3rd wettest in February history with 6.50" in 2010 the wettest and #2 is 1958 with 5.98 inches. Wilmington`s rainfall now ranks #12 and Philly #15. All rankings will increase as the rest of the month looks a little on wet side. Yesterday we calculated the February average temp for PHL to 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.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 11 AM EST this morning for ANZ430- 431-450>455. && $$ Synopsis...Drag 420 Near Term...Gorse Short Term...Gorse Long Term...Drag 420 Aviation...Drag/Gorse 420 Marine...Drag/Gorse 420 Climate...420

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