Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 271034 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 634 AM EDT Mon Mar 27 2017 .SYNOPSIS... Northeastward moving low pressure from the Upper Great Lakes to Missouri will pull a warm front northward through the Delmarva and southern New Jersey today. The warm front should stall across southeastern Pennsylvania and central New Jersey, awaiting the passage of the Missouri low, through New Jersey, Tuesday evening. Thereafter, Canadian high pressure will build into our area Wednesday and Thursday. Low pressure in the Ohio Valley on Friday will move east and off the mid Atlantic coast on Saturday. Weak high pressure should follow on Sunday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Few minor tweaks at 630 a.m. Added Coastal Atlantic county to Dense fog advy. It was mistakenly left out earlier. Also dropped Sussex DE from Advy since vsbys there have improved. Minor downward adjustment to temps in metro Philadelphia and n/w for today. A slow moving warm front will be tracking across the area today. Our forecast has trended slower with the frontal passage and includes a slower trend in improvement (sunshine) behind the front and slightly lower temperatures for today. We have issued a dense fog advisory for Delmarva and parts of SE/PA and srn NJ for the early part of the morning, since the VSBYS continue to trend downward and are 1/2 SM - 3/4 SM attm. The onshore flow will continue in these areas until the front passes later today. The low clouds and areas of dense fog will be over the area thru the first part of the morning before a S to N slow improvement trend is expected today. Areas across the Lehigh Valley and Southern Poconos may see improvement late this afternoon. Confid in the N/W areas is limited attm. The low which the warm front is attached to is weak, so it`s possible that the topography of these areas may play a factor slowing the motion. Highs today are expected to reach the upper 60s/low 70s over south NJ and Delmarva, low/mid 60s in central NJ , metro Philadelphia and Lehigh valley and 50s north of that.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/... Hopefully the warm front will have moved far enough away to have a lull in the precip for the evening. More showers will arrive overnight as the next short wave approaches from the W/SW. Pops increase back to the chc range overnight, favoring slightly the srn NJ and Delmarva areas. Mild with lows in the low 50s S/E and mid/upper 40s N/W. Light winds expected. && .LONG TERM /TUESDAY THROUGH SUNDAY/... 500 MB: A split flow regime will continue across the United States during this period. A short wave crosses Pennsylvania Tuesday and then phases with a northern stream system into a closed low over the Maritimes Wednesday night. The next southern stream short wave in the pipeline over the Mississippi Valley Thursday night, weakens eastward off the mid Atlantic coast Saturday. Yet another southern stream short wave will be moving eastward from the lower Mississippi Valley early next week. Temperatures: Calendar day averages should be about 5 degrees above normal Wednesday, near normal Thursday and normal to 5 degrees below on Friday, and then normal or several degrees above on Saturday and Sunday. Forecast basis: 50 50 blended 00z/27 GFS/NAM MOS Tuesday- Wednesday, then 00z/27 MEXMOS Wednesday night-Thursday and thereafter WPC guidance Thursday night-Sunday. The dailies... Tuesday...The position of the warm front Tuesday will probably result in a wide range of temperatures across our area with large error potential I-78 region northward where global models are warm but the ever improving 12km NAM and its downscaled 3KM temperatures, are much cooler. DREARY north of that warm front with patchy dense fog possible and a likelihood of heavy showers during the afternoon and evening as the Missouri low cuts east along the Mason Dixon line, heading for a NJ seaward exit Tuesday evening. There could be afternoon and evening thunderstorms with this short wave. Within the warm sector in advance of the cold front, we expect sunshine to break out south of the warm front with temperatures reaching the upper 70s. A low- level moist tongue will also be in place across this area, ML Cape (400-800J), along with negative Lifted and Showalter Indices, within an environment characterized by poor lapse rates and weak shear. Thunder is expected but despite the warm front,right now SVR is not. Do follow later PHI and SPC discussions. Precipitable water values around 1.25 inches, should yield some heavy showers near the path of the low. Movement should be east northeast and there could be some training and backbuilding along the warm front so that this may need to be monitored for narrow bands of poor drainage street flooding rain Tuesday afternoon/evening. Winds light northeast, north of the warm front (near I-78) and southerly in the warm sector Delmarva and s NJ. Confidence: above average except the location of the warm front and the associated temps near the warm front. Tuesday night...Showers still may be heavy in the evening (still thunder possible?), then end from west to east late and the wind turns north everywhere. Confidence: above average. Wednesday...a gusty northerly flow to 25 mph of cold air advection. Dry. Confidence: above average. Thursday...A sunny start, then increasing clouds and cool ahead of the next storm system. Light wind. Confidence: above average. Friday...Increasing model consensus that this actually looks like a decent little mid Atlantic coastal storm with one half to 1.5" of qpf, and currently, only a slight chance of a little wet snow near and north of I-80 since models have trended slightly northward. Friday evening tides may exceed the minor advisory threshold along the NJ Atlantic coast provided the sfc low passes south of ACY. overall Friday confidence: above average., Saturday...Precipitation(mainly rain) ends, but clouds may linger since there doesn`t seem to be much in the way of high pressure following the low. Confidence: average Sunday...Considerable cloudiness. Low confidence on what might transpire this day. && .AVIATION /12Z MONDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. The latest issuance of the tafs are somewhat more pessimistic with regards to how fast the transition back to mvfr then vfr occurs today. Warm fronts are usually known for their stubbornness for improving conditions and this seems to be the case for today. A decent SW flow aloft argues for the fact that when things begin to improve, it should happen quickly, but what time that happens is the problem for the day. The showers that the models show arriving this morning may mix up the low clouds and fog enough to get the process going. Winds will be ERLY prior to the warm front the SW behind it. VFR will probably be across the Del Valley and Delmarva/south NJ this afternoon, conditions may remain MVFR across KRDG/KABE longer. Probably a period of VFR this evening, the more low clouds/showers late tonight. OUTLOOK... Confidence: Above average all days. Exception being the details of the warm frontal position Tuesday and timing of poorest conditions where there is less than average confidence. Tuesday...IFR or MVFR conditions Tuesday morning at all TAF sites, with improvement to VFR by afternoon at all but ABE, RDG, and TTN where IFR may may linger all day. Showers become widespread in the afternoon/evening with light northeast flow at ABE, RDG, TTN and light south or southeast flow elsewhere. Isolated Tstms expected. Tuesday night...MVFR possible early in showers, otherwise becoming VFR late. Winds becoming north everywhere late. Wednesday...Mostly VFR. north wind gusts 20-25 kt. Thursday...VFR. Light north wind. Friday...IFR or MVFR conds in periods of rain. Mostly east wind ahead of the low. && .MARINE... We will continue with the SCA flag for seas around 5 ft seas today. The Erly flow will be weakening with time as the warm front slides northward today. I could see the flag being dropped earlier, but we will keep it for now. Dense fog on the waters this morning with the warmer and more humid air arriving across the cold water. Scattered showers later this morning may stir vsbys up. More Sct showers expected tonight. Patchy fog tonight. OUTLOOK... Tuesday...Fog may be marine hazard? otherwise no SCA expected at this time. Thunderstorms could occur, especially late in the day. Confidence: average. Tuesday night...SCA northerly flow may develop late. Confidence: average. Wednesday...Northerly flow SCA. Confidence: above average. Thursday...No headlines. Confidence: above average. Friday...SCA potential as low pressure heads for the mid Atlantic coast. Confidence: above average. && .CLIMATE... Georgetown 3/28 RER max is 80 set in 1960. PHL temperatures continue to project near 1 degree F below normal for the month as a whole. && .EQUIPMENT... KNEL appears to be reading 10F too warm the past couple of days. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Dense Fog Advisory until 10 AM EDT this morning for PAZ070-071- 101-102-104. NJ...Dense Fog Advisory until 10 AM EDT this morning for NJZ013-014- 016>018-020>027. DE...Dense Fog Advisory until 10 AM EDT this morning for DEZ001-002. MD...Dense Fog Advisory until 10 AM EDT this morning for MDZ008-012- 015-019-020. MARINE...Dense Fog Advisory until noon EDT today for ANZ430-431-450>455. Small Craft Advisory until 7 PM EDT this evening for ANZ450>453.
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&& $$ Synopsis...Drag Near Term...O`Hara Short Term...O`Hara Long Term...Drag Aviation...Drag/O`Hara Marine...Drag/O`Hara Climate... Equipment...

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