Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 161004 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 604 AM EDT Mon Apr 16 2018 .SYNOPSIS... The low pressure system that brought storms and a cold front to the Mid Atlantic this morning, will slowly propagate northeast away from the region on Tuesday. Another area of low pressure organizing over the Midwest will propagate towards New England on Thursday. As it does so, our region will first see a warm front, followed quickly by a cold front. Canadian high pressure will move into the area for the weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 545 AM Update...Temps still slow to rise across the southern Poconos so we extended the Winter Weather Advisory until 9am. 345 AM...Low pressure centered over Virginia with a strong warm front extending east from this feature across southern Delaware into southern NJ. South of the front, temps in the 60s...north of it, temps in the upper 30s to low 40s. In fact, still freezing rain in Mount Pocono with the Advisory for Carbon, Monroe, Sussex Counties extended to 10z. Ahead of the aformentioned low in the system`s warm sector, a very strong 60-70 knot LLJ from the SSE is advecting very moist air northward with PWATSs of 1.5+ inches. This is supporting a line of heavy showers with embedded storms that is beginning to close in on the Delmarva at this time. As we head through the next few hours towards the Monday morning commute, low pressure will advance northeast into PA lifting the warm front through the forecast area. At the same time, line of heavy showers with embedded storms will progress east into forecast area...reaching SE PA including the Philadelphia area as well as S NJ by around the 6am time frame and areas farther east across central NJ toward Trenton within an hour or two thereafter. At this time it appears the biggest concern may be very heavy rainfall causing ponding on roads, the threat of urban and small stream flooding, and even the risk of localized flash flooding as this line has a history of producing rainfall rates of over 1 inch per hour in places and this could continue as it moves in...especially if line slows down a bit. Needless to say, this will likely make for a difficult morning commute. The threat for severe weather in the form of strong, damaging winds is still there...especially from Philly southward. However the limiting factor for this is little in the way of surface based instability so that will make it harder for the very strong winds just off the surface to be mix down. For this reason, don`t think severe storms will be widespread. Farther north, where freezing rain has been ongoing well north of the warm front across the southern Poconos into far NW NJ, this should come to an end this morning as warmer air finally makes it in. Following main line of showers/storms that moves through early to mid morning ahead of a leading front that will eventually wash out, additional rounds of showers are likely through the late morning into this afternoon as a secondary front moves through. These could also contain brief, heavy rainfall, and strong, gusty winds further exasperating any issues that may occur early this morning. Temps rise to the 55 to 65 degree range this morning for most areas (40s in the southern Poconos) before slowly falling off this afternoon behind this second feature on gusty west winds. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/... By early this evening, low will be departing to the NE into NY State into New England dragging a final surface trough through the area. In addition, systems` upper low will still be centered near western PA with the broad circulation continuing to spin shortwaves through the area. The upshot of this is that mainly cloudy skies with some continuing scattered showers are likely to affect parts of the area at least into this evening and likely through the overnight across the southern Poconos. In fact as temperatures drop off into the low to mid 30s tonight across the far north, any rain showers will change over to snow showers with continuing cool, gusty, west winds. Temps will be a bit warmer farther south with lows mainly in the upper 30s to low 40s over SE PA through central, S NJ, and the Delmarva. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Made few changes to the long term forecast in order to focus on the ongoing weather tonight. Fair weather is expected from Tuesday night into Wednesday as high pressure arrives across the area. It will be a bit warmer than Monday/Tuesday, but still slightly below normal for mid-April. Highs will generally be in the mid/upper 50s north/west and low/mid 60s south/east. These values will be 2 o 4 degrees below normal. Another low pressure system will approach the area later Wednesday and last into Thursday evening. The system will be progressive, so even though the low is depicted by the models as being rather strong, it will not have large window of opportunity when our area will be in the warm sector, and thus have more moisture for precipitation. We therefore have mostly chc pops in the fcst for Wed night/Thu and slgt chc pops into Thu night. Temperatures will remain mostly at or a little below normal this period. The warmest day will be Thursday (near normal) with mid/upper 50s for highs across the north and low/mid 60s over Delmarva and srn NJ. High pressure from Canada builds in for late this week and into the weekend. Mostly fair weather is expected with a continuation of below normal temperatures. && .AVIATION /10Z MONDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...A line of heavy showers and some thunderstorms moves through the area between about 09Z and 14Z Monday with LIFR conditions, torrential rain and erratic winds likely as the heaviest portion of the line moves through. Following this first batch of heavy showers, some additional showers that could also be briefly heavy may affect the terminals through around midday. Beyond this time expect an improving trend to MVFR and eventually VFR by late afternoon. E/SE winds this morning shift to SW/W this afternoon around 10 knots with gusts to around 20 knots. Potential for some erratic winds/gusts around showers. Moderate confidence. Finally, strong LLWS will continue to be an issue this morning. Tonight...mainly VFR. Moderate confidence. Outlook... Tuesday and Wednesday...Mostly VFR conditions are expected. A few showers are possible, primarily for KABE and KRDG, which could lead to brief reductions in the flight category. Moderate confidence. Wednesday night and Thursday...Starting VFR, we may see conditions lower as a warm front lifts through the area and precipitation moves in. However, confidence is low at this time. Thursday night and Friday...Mostly VFR expected behind a cold front that will bring a shift to northwesterly winds and drier conditions. High confidence. && .MARINE... Gale Warning continues...Widespread gales are expected in advance of a line of heavy showers and storms moving through the area west to east this morning. Erratic/severe wind gusts and torrential downpours are possible with these storms. After the line of storms passes, winds should gradually diminish from south to north, with small craft advisory conditions likely during the afternoon on Delaware Bay and the Atlantic Delaware coastal waters. Gale conditions may linger on the New Jersey coastal waters for most of the afternoon before conditions improve by evening. Tonight...A relative respite as winds should actually drop below SCA levels this evening before starting to ramp back up again overnight...likely to near or just below SCA levels by early Tuesday. Outlook... Tuesday and Tuesday night...lingering elevated seas are possible on the ocean waters through this time. Otherwise, winds should stay below SCA criteria. Wednesday and Thursday...Winds and seas are expected to be below SCA criteria. Thursday night and Friday...Winds and seas should increase once again and may approach SCA criteria. && .HYDROLOGY... Flash Flood Watch continues for SE PA zones into central and northern NJ. As line of shower/storms moves in, should generally continue moving eastward, however there could be some training or semi-training of the line at times...especially northwest of the 95 corridor this morning. Flash flood guidance is on the high side (about 1-2 inches in an hour) in most of the area (lowest in the urban corridor); however, with anomalous precipitable water, considerable moisture convergence via a very strong low-level jet impinging upon the approaching cold front, and contributions from upslope flow, should any training convection occur within the main line, 1-hr FFG could be met in spots. One of the primary concerns is the timing. With the main convective line moving through just before or during the morning commute, heavy rainfall may cause some traffic headaches. The nature of this event suggests flood issues would be with flashy small streams and in poor- drainage urban areas. Mainstem river flooding is not anticipated at this time. Total forecast rainfall accumulations in the watch area are generally 1.5-3 inches, with 1-2 inches expected elsewhere. && .TIDES/COASTAL FLOODING...
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A strong and persistent onshore flow during the overnight hours is producing tidal anomolies in the 2 to 3 foot range this morning, just before high tide starts to move in. The Coastal Flood Advisories have therefore been upgraded to Coastal Flood Warnings, which now cover the entire NJ and DE coastlines, and the Delaware Bay. A Coastal Flood Advisory continues for the tidal Delaware north of the Commodore Barry Bridge. Our thinking here is that an offshore flow may develop before the next high tide occurs later today, so the anomolies may get cut off before moderate flooding can develop. We`re also keeping an eye on the Cheseapeake Bay. Current thinking is that water levels along the shores of the upper bay should remain below the treshold needed for a Coastal Flood Advisory, but we`ll issue an Advisory if we see anomolies jump higher than expected. An offshore wind shift later today is then expected to end the threat of coastal flooding across the area.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...Flash Flood Watch through this afternoon for PAZ054-055- 060>062-070-071-101>106. Coastal Flood Advisory from 11 AM this morning to 3 PM EDT this afternoon for PAZ070-071-106. Winter Weather Advisory until 9 AM EDT this morning for PAZ054- 055. NJ...Flash Flood Watch through this afternoon for NJZ001-007>010- 012-015-017>019. Coastal Flood Warning until 1 PM EDT this afternoon for NJZ012>014-020>027. Coastal Flood Warning until 2 PM EDT this afternoon for NJZ016. Coastal Flood Advisory from 11 AM this morning to 3 PM EDT this afternoon for NJZ015-017>019. Winter Weather Advisory until 9 AM EDT this morning for NJZ001. DE...Coastal Flood Warning until 1 PM EDT this afternoon for DEZ002>004. Coastal Flood Warning until 2 PM EDT this afternoon for DEZ001. MD...None. MARINE...Gale Warning until 5 PM EDT this afternoon for ANZ450>453. Gale Warning until 11 AM EDT this morning for ANZ430-431-454- 455. && $$ Synopsis...Johnson Near Term...Fitzsimmons Short Term...Fitzsimmons Long Term...Johnson Aviation...Fitzsimmons/Johnson Marine...Fitzsimmons/Johnson Hydrology... Tides/Coastal Flooding...jjm is the U.S. government's official web portal to all federal, state and local government web resources and services.