Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 271647 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1247 PM 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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1230 pm update: Quite a few changes to the forecast this afternoon. Warm front is making very sluggish progress northward, with 16Z placement roughly along a line from KBWI to KMJX. Models continue to be way too aggressive moving this northward, and am beginning to wonder if there will be much progress north of the Mason-Dixon Line given close proximity of an upstream shortwave trough and associated pressure falls that may promote backing of flow to a more easterly component with time. For now, only edged temperatures downward a little bit in the northern CWA, but thinking the I-95 corridor northwestward to the Lehigh Valley may be in "bust territory" with forecast max temps today. Wind forecast needed considerable adjustment as well, given the switchover to southwest winds south of the front has not occurred as far north as forecast to this point. Will continue to monitor temps/winds closely this afternoon. Think most of the precipitation is done for the daytime hours, with the band of rain having just pushed offshore. However, hi- res guidance continues to hint at the developed of an isolated shower or two, and there have been recent indications that shower development may occur in the warm sector (i.e., Delmarva), where some instability may be present. If these trends continue, I may be inclined to add isolated storms to the grids this afternoon. 930 am update: Cancelled dense fog advisory as area webcams and surface obs indicated visibilities generally over a quarter mile, with only localized patches of lower visibilities remaining (especially near the New Jersey coast). This can be handled with nowcasts and special weather statements for the rest of the morning. Radar is showing a decent band of light to moderate rain from the western suburbs of New York City to Philadelphia to Dover moving east into New Jersey. This rain will quickly eradicate any remaining patchy dense fog as it moves through. Hi-res guidance generally has this precipitation moving out of the area by noon, so adjusted PoPs/Wx grids for this timing. Transient shortwave ridging will move into the region this afternoon, but residual cold air aloft and mixing to the south of the poleward-bound warm front may generate a few isolated showers this afternoon. Forecast soundings look a little less enthused about thunder chances this afternoon compared to how they looked yesterday, so no mention for this afternoon. Meanwhile, given weak subsidence aloft, not overly optimistic about precipitation chances in general for the CWA after midday. Lowered PoPs during this time frame. Speaking of the warm front, it is beginning to make progress northward through Delmarva this morning. Models, as usual, are poorly forecasting its position/movement, generally way too aggressive with its northward progress. Kept temps/dew points on the low side of guidance, particularly in the north CWA, today. Suspect further modifications will be required through the day given the poor verification of guidance in general.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/...
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1230 pm update: Hi-res guidance has been consistently indicating elevated convection moving through the CWA after 09Z, with potential for isolated/scattered storms developing ahead of the main band in portions of SE PA and northern/central NJ. Model soundings show marginal buoyancy for parcels rooted in the 900-800 mb layer. If trends continue, will need to add thunder chances to the grids tonight. Did raise PoPs north of the Mason-Dixon Line during the overnight hours, and may need to raise them more based on the latest simulations. Also made considerable changes to the temp/wind forecast based on latest guidance, particularly given the uncertainties in frontal placement this afternoon having domino effects through the evening hours. At least patchy fog looks possible across most of the area, particularly if winds back to a more easterly direction, as the latest guidance is keying in on. Previous discussion... 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.
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&& .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 /18Z MONDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Several modifications to the TAFs this morning to account for faster timing of the precipitation moving through the region. General IFR conditions continue this morning north of the Mason-Dixon Line with rapid improvement to MVFR/VFR in Delmarva and far southern New Jersey (including KMIV/KACY). Expect general improvement to VSBYs after rain passes the I-95 corridor by 15Z, but much more gradual improvement to CIGs. Expecting MVFR conditions to occur by mid-afternoon, but very low confidence in timing. VFR conditions likely most of the day at KMIV/KACY. Any improvements observed this afternoon will be short-lived, as sub-VFR conditions will return by late this evening across the region. 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...
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1230 pm update: Dense fog advisory has expired, as visibilities have improved on the waters. Rain is currently moving through, and the warm front has pushed north through most of the marine zones. South winds will increase to around 10 kts this afternoon, but should remain well below advisory thresholds. Seas are slowly dropping, and look to be below advisory threshold by mid-afternoon. Small craft advisory may be cancelled early. Previous discussion below... Seas remain elevated off the New Jersey coast, so the small craft advisory remains in effect until 7 pm. Beach webcams and satellite continue to indicate poor visibilities on the coastal waters, so the dense fog advisory continues through noon for all marine zones. There is a chance that fog may persist all day off the northern/central New Jersey coast, but improvement is expected by early this afternoon on Delaware Bay and off the Delaware coast. Another night of fog potential exists 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.
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&& .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...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 7 PM EDT this evening for ANZ450>453.
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&& $$ Synopsis...Drag Near Term...CMS Short Term...CMS/O`Hara Long Term...Drag Aviation...CMS/Drag Marine...CMS/Drag Climate...Drag Equipment...Drag

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