Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 230800 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 400 AM EDT Sun Jul 23 2017 .SYNOPSIS... A frontal boundary extended from the southern Great Lakes to the mid Atlantic states this morning with one low pressure system south of Long Island, moving seaward. Another low will follow tonight, passing south of Long Island on Monday. The cold front will trail down into yet another low over the Delmarva Peninsula Monday night. Canadian high pressure builds down into the northeast United States Tuesday and Wednesday. A cold front approaching from the northwest late Thursday may be very slow to pass through the mid Atlantic states next Friday and Saturday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Quite a bit to talk about in the near term. Currently, MCS is slowly propagating southward/southeastward off the coast of Delmarva, but cells continue to develop via upwind propagation processes on the southwest flank. A large shield of lighter precipitation has developed to the north of the main convective band, still providing a nice steady rain and anvil lightning to far southern portions of the CWA. Meanwhile, showers continue developing on the west side of Chesapeake Bay, aided by convectively-induced perturbations moving through the southern Mid-Atlantic region at this time. Have prolonged PoPs in this region through the overnight hours to account for these trends. Farther north, long-lived MCV is moving offshore south of Long Island, which provided a fairly sizable swath of 0.5-2 inch(es) of rain tonight from Lehigh County eastward through Hunterdon/Somerset/Middlesex/Monmouth Counties in New Jersey. This will be an important consideration with regards to tonight`s forecast (see next section). The aforementioned perturbations will be moving offshore this morning, and with convective overturning affecting much of the area, see very little in the way of precipitation chances later this morning. Cloud cover is going to hang tough, I suspect, owing to some northeasterly flow that will seep into the region. KDIX is indicating an outflow boundary from the aforementioned MCV that is sagging slowly southward through central New Jersey (as of 230 am located generally from Trenton to Manasquan. Winds become northeasterly to the north of this boundary, and dew points are a few degrees lower. Higher-resolution model output suggests this may continue southward to approximately the Mason- Dixon Line through a considerable portion of the day (at least a chunk of the morning hours) - in addition, some showers may develop along this boundary (as seen in the past hour). Models are notoriously too eager with dissipating such boundaries or returning them northward. As such, I bumped down dew points considerably (around 3-5 degrees) north of the Mason-Dixon Line today and re-oriented the winds to a more northeasterly direction for a longer duration for these areas as well. Guidance is noticeably cooler with highs today, and this makes sense given the current observational trends. Will be interesting to see how much clearing, if any, occurs today. Regarding convective potential, the next in a series of midlevel perturbations moves toward the region this afternoon and tonight, and with somewhat more focused large-scale ascent downstream in a weakly to negligibly capped environment, scattered to widespread convection should develop this afternoon to our west. The timing of its approach to our region looks rather late in the day, and held off on higher PoPs to mostly the tonight period. However, some storms may move into the far western CWA by late afternoon. With the approach of stronger westerlies as a larger-scale trough begins to amplify today, vertical shear profiles will continue to improve. Think the main question today will be degree of instability, with remnant cloud cover and outflow boundaries likely making model depictions of the thermodynamic environment mere pipe dreams. However, southwesterly flow to the south/west of the area should advect warm/moist air northeastward, allowing boundary- layer based parcels to attain CAPE values 1000-2000 J/kg by afternoon near/south of the convectively-reinforced surface boundary in the southern Mid-Atlantic. Where this boundary ends up is anyone`s guess (mine is south of consensus, based on the past 24 hours of analysis). Should such instability materialize, CAPE-shear parameter space is favorable for organized severe, with downburst winds associated with bowing segments and/or mesovortices embedded within convective segments/clusters the main threat. Highly heterogeneous convective environment would suggest that more organized (lengthier) lines of convection may be hard to materialize, especially given the very moist boundary layer hindering the development of somewhat stronger cold pools. CAM guidance hints at such a scenario playing out, and given pattern recognition and the past 24 hours as "analog guidance", this increases confidence in at least the overall mode of strong to severe convection this afternoon/evening - again, should the stronger instability pan out. With PWs 1.75+ inches (somewhat lower than the past 24 hours, aided by somewhat drier midlevels and a southward push of the convectively-induced surface boundary) and the potential for strong updrafts should near-surface based instability materialize, heavy rainfall is a threat with any of the stronger storms today. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... Tonight`s forecast could be metaphorically described as throwing darts in the dark. As convection from the west moves into the region overnight, there is a clear signal from CAM guidance of developing a mesoscale convective vortex on the north side of the convection, with larger-scale influence via a developing/eastward-moving surface low translating across the Mid-Atlantic during this period (as the trough in the eastern Ohio Valley continues to amplify). Some CAM depictions (and even some of the coarser guidance) produce very alarming QPF during this period (with 3 to 6+ inches indicated in a corridor generally between I-78 and I-80) as the MCV trudges eastward. This seems way overdone, and the CAMs have had a history of that in the past 24 hours. The concern is that the thermodynamic support just will not be there (or, alternatively, will set up farther south) since the convection occurring the past few hours has displaced the surface boundary to far southern Virginia at this time. My suspicion is that the higher side of the QPF distribution is a low probability, but I suspect there will be a swath of heavy rainfall totals (maybe 1-3 inches rather than 3-6) where convective organization combines with large-scale support. Regarding the latter, as the trough to the west amplifies, a downstream 250-mb jet streak in New England provides substantial upper-level divergence. Considerable warm/moist air will be lofted above the zonally- oriented convectively-reinforced boundary ... somewhere ... in the Mid-Atlantic region and advected northward. Meanwhile, with antecedent heavy rainfall in portions of the region (the Lehigh to Monmouth County corridor, for instance), ingredients are in play for a flood event. Too much uncertainty for a watch at this time, but flooding potential is definitely a concern tonight. Stay tuned. Temp forecast is a blend of MET/MAV MOS with some addition of CAM 2-meter temperatures to account for effects from precipitation. && .LONG TERM /MONDAY THROUGH SATURDAY/... 500 MB: A trough crossing New York State on Monday reaches eastern New England Tuesday with ridging height rises to follow Wednesday. The next trough...complex...moves into the northeast USA for the end of the week Temperatures: Calendar day averages about 5 degrees above normal Monday near normal Tuesday through Thursday...and then may run several degrees above normal Friday and possibly into the weekend. Forecast basis: a 50 50 blend of the 00z/23 GFS/NAM MOS was applied Monday-Tuesday (except mainly GFS MAV TT/TD), then 00z/23 GFS MEX MOS Tuesday night before relying heavily on the 05z/23 WPC D4-8 elements for Wednesday - Saturday. The dailies... Monday... Unsettled conditions into Monday evening. The frontal boundary is expected to drift south through our area with lingering shower and thunderstorm potential along and ahead of the front. Precipitable water values should be decreasing slowly from north to south... to on average 1.5 inches. Partial clearing develops from north to south later at night, with patchy dense fog possible. PWAT is modeled to decrease to 1.2 inches by Tuesday morning. Tuesday...The axis of a mid level short wave trough has passed by in the morning with height rises expected. It should be drying out. A fly in the ointment may be onshore flow and a sprinkle on the coasts? Wednesday...Partly sunny, possibly after morning stratus or fog? Again there is some modeling hint of sprinkles on the coast. Thursday...Warm air advection develops late in the day. Chance of showers and isolated tstms late. Friday...a chance of showers and thunderstorms with the next cold front, especially morning. Saturday...Hopefully dry weather returns but models are slowing the front and possibly stalling it as a tricky complex mid level pattern develops. One short wave passes to our northeast while another digs to our west, building the Atlantic ridge westward and making southwest flow aloft tend to stall the front. If that happens...showers would be in our area. && .AVIATION /08Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Sub-VFR CIGs (with locally patchy fog as well) this morning generally along and north of the Mason-Dixon Line should slowly improve to VFR in the 12Z-15Z time frame with northeast winds 5-10 kts. Thereafter, winds may become easterly or southeasterly (or even transition to southwesterly this afternoon for a time), remaining around 5-10 kts. Stray showers may occur this morning, but best chances for precipitation are after 00Z, where scattered to widespread strong thunderstorms are expected to move through the area. Erratic winds/gusts are likely with any shower/storm. OUTLOOK... Monday...VFR. A chance of showers and thunderstorms with sub VFR conditions along a cold front. Monday night...Conditions lowering to MVFR and possibly IFR in fog patches with clearing after a chance of evening showers and thunderstorms. Tuesday...MVFR and IFR conditions in morning stratus/fog improving to VFR. Tuesday night through Thursday...Mainly VFR though some MVFR fog or stratus possible Tuesday night and Wednesday morning. && .MARINE... Sub-advisory conditions are expected to continue through tonight. Storms are in the forecast again tonight, and some of these may be strong and will likely produce heavy rainfall. Locally rough seas should be expected near precipitation. Outside of any precipitation, seas will generally be 2-4 feet with a northeast to east flow developing this morning veering to the southeast tonight, generally 5-15 kts. OUTLOOK... Monday through Thursday...No marine headlines are anticipated. RIP CURRENTS... A moderate risk of rip currents is expected to continue through today as the new moon combined with an underlying swell of 10-12 seconds is occasionally showing up in buoy observations (though less so overnight so far). && .TIDES/COASTAL FLOODING... We have issued another coastal flood advisory for this evening`s high tide for the Atlantic coastal waters and for adjacent portions of the coast of Delaware Bay, as the new moon combined with increased onshore flow today should provide conditions at least as favorable, if not somewhat more so, for minor coastal flooding. With last evening`s high tide, some minor flooding occurred in most of this area (though generally below advisory levels). Thus, with the somewhat higher forecasts observed with this evening`s high tide, an advisory seems warranted. && .CLIMATE... ABE month of July rainfall as of 1 am...6.46",20th wettest in the POR dating back to 1922. July record 10.42" 1969. && .EQUIPMENT... Dew point readings at KDOV continue to measure too high compared to surrounding locations and should be treated as unrepresentative of the area. Hibernia PA and Hamburg NJ transmitters are off the air (lost Hibernia this evening around 0015z/23). Both have trouble tickets. No known rts. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...Coastal Flood Advisory from 7 PM this evening to 1 AM EDT Monday for NJZ012>014-020>027. DE...Coastal Flood Advisory from 7 PM this evening to 1 AM EDT Monday for DEZ002>004. MD...None. MARINE...None. && $$ Synopsis...Drag 4a Near Term...CMS Short Term...CMS Long Term...Drag 4a Aviation...CMS/Drag 4a Marine...CMS/Drag 4a Tides/Coastal Flooding... 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