Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 242143 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 443 PM EST Fri Feb 24 2017 .SYNOPSIS... A strong cold front will move through the Mid-Atlantic on Saturday. High pressure will build into the region on Sunday before moving into the Atlantic on Monday. A surface low will develop in the central U.S. on Tuesday and move northeast to the Great Lakes by Wednesday and southeast Canada by Thursday. An associated warm front is forecast to move through the Northeast on Wednesday before another strong cold front sweeps across the East Coast on Thursday. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... Big question tonight will be when and where low clouds and fog develop. Winds are expected to shift out of the southeast overnight. This slight shift with more of a onshore component, could result in low clouds advecting from the ocean on shore. Having said that, guidance was fairly poor with the extent of low clouds this morning, so not sure if it will be quite as widespread as what some guidance indicates. With the onshore flow and potential cloud cover, temperatures should stay quite mild, with low generally in the 50s across the region. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/... The first question will be how quickly early low clouds will dissipate. Even if clouds persist into late morning though, models have recently had a cold bias in warm air advection events, so have gone slightly above guidance for highs during the day. If clouds hang on longer or if the cold front moves in earlier, current forecast would be too high. The big story in the short term will be the cold front. Latest guidance suggests it should be moving across our area from 18Z (for far western Berks Co and the southern Poconos) to just approaching the shore by evening. Forecast still looks on track for a line of showers with embedded thunderstorms along and just behind the front. The biggest question mark with thunderstorms will be surface based or mixed layer instability. CAPE values will be marginal at best so if we do end up being cooler than forecast, instability (especially surface based instability) could be negligible. Given the model cold bias though, I think we will have enough instability to support thunderstorms. Primary hazard continues to be gusty winds given the high shear environment (0-6km bulk shear values could be above 50kt). Will continue mention in the HWO and add mention to the forecast for this potential. Although there could be brief periods of heavy rain with the line, the showers and storms should be moving through fast enough to limit any flooding risk. && .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/... The long term period continues to look active with a period of unsettled weather likely during the early to middle portions of next week. A pattern change is likely toward week`s end. By 00Z Sunday, a negatively-tilted shortwave trough in the Northeast and adjacent portions of Canada will continue to pivot northeastward to the maritime provinces of Canada by 00Z Monday. A narrow cold- frontal rain band will likely be moving offshore during the evening hours Saturday as deep ascent downstream of the associated vorticity maximum progresses to the northeast. Strong cold air advection will commence upstream of the front, and winds will sharply turn west or northwest Saturday evening, becoming gusty as the fast flow aloft mixes to the surface underneath the frontal inversion. Sprawling (but progressive) high pressure moves through the eastern U.S. on Sunday. In advance, gusty winds will continue during the day in the Mid-Atlantic as boundary-layer mixing diurnally maximizes underneath mostly clear skies. However, winds will quickly diminish late Sunday afternoon and evening as the surface high pressure center approaches the coast. Temperatures will be much colder than the anomalous warmth experienced in recent days. Consensus of the guidance has edged downward a bit for highs on Sunday, and given the strength of the cold air advection in play -- leaned a bit in this direction, despite the relatively clear skies expected. May see some stratocumulus development in the Poconos, as is typically observed in this setup. The upper-level pattern next week appears to start out as fast west- southwesterly flow with several embedded vorticity maxima ejecting from a longwave trough in the West. Operational models remain in quite a bit of disagreement timing these perturbations as they move through the central and eastern U.S. The aggressive appearance of the first perturbation seen in yesterday`s GFS is nowhere to be found with today`s 12Z simulation. In fact, the GFS overall looks like an outlier with the overall pattern next week. Given this as well as this model`s poor run-to-run continuity compared to the ECMWF/CMC/UKMET of late, pretty much disregarded it with today`s forecast. As such, it looks like there are three main systems that may affect our region next week. The first occurs some time on Tuesday, with the most likely timing in the morning hours, as a vort max quickly moves into the Northeast during the day. Strong isentropic ascent along a quasi-zonal baroclinic zone in advance of this system produces a swath of precipitation moving through the region. Temperatures may be cold enough for snow or a wintry mixture in the southern Poconos (at least in the morning), but confidence in the details remains low. As this first system moves through, transient cold air advection should follow before a stronger low moves into the Great Lakes on Wednesday. Winds should quickly switch to a more easterly component, which may favor a period of cold air damming Tuesday night. Isentropic ascent should commence again during this time, with another round of precipitation possible by Wednesday. The details of this second round of precipitation are even murkier, as much will depend on the eventual position of the warm front via cold air damming processes not well simulated this far in advance, the track of the surface low in the Great Lakes, and the track of a southern stream vorticity maximum to the south/west of the surface low. At the moment, it appears three main regions of precipitation will develop. The first will be tied to the surface low and the associated cold conveyor belt, which looks to be in the vicinity of the Great Lakes and New England. The second will be associated with the isentropic ascent along and poleward of the baroclinic zone positioned roughly in the Ohio Valley and southern Mid Atlantic. The third will be along the developing cold front that will sweep across the East Wednesday night and Thursday. The exact locations of these precipitation maxima are anyone`s guess, but there are some indications that a relatively decent precip event could occur in an east-west band from the Ohio/Tennessee Valleys to the vicinity of Delmarva Wednesday and Wednesday evening. Bumped up PoPs in the southern CWA during this time frame. The cold frontal passage Wednesday night or Thursday could also be tied with some heavier precipitation -- possibly with localized enhancement via convection. Also bumped PoPs across the CWA late Wednesday night and Thursday morning, though confidence in the timing is well below average. The temperature forecast is of ... below average confidence, to say the least. Temperatures look generally warmer than average Monday through Wednesday, but there is some question regarding the observed warmth Tuesday and Wednesday owing to the potential for precipitation and the establishment of cold air damming in portions of the area. For now, kept temps on the warmer side of guidance, but there is a possibility these numbers are overly optimistic on Tuesday/Wednesday. By week`s end, a longwave trough becomes established across the East. This means a return to much colder (i.e., more typical) conditions across the region. Establishing northwest flow could bring a few clipper-like systems to the area late next week through early the following week. && .AVIATION /22Z FRIDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR conditions are expected to continue through at least 03Z. After 03Z, as winds shift to more southeasterly (resulting in more of an onshore component), could see low stratus move over the TAF sites. As it does so, could also see some fog development underneath the stratus. Timing is uncertain, so have mentioned the period of greatest risk (08 to 14Z) in the the TAFs. After 14Z, any fog should dissipate and ceilings should begin to slowly lift. The next concern will be after 18Z as a cold front approaches from the west. An abrupt shift to westerly winds, widespread showers (and associated restrictions in visibility), and scattered thunderstorms are possible with and just behind the front. KRDG and KABE may see this between 18 and 21Z, while the rest of the TAF sites will be impacted 21Z or later. Outlook... Saturday night and Sunday...Conditions quickly improving to VFR behind the cold front, with showers/storms moving off the coast. West or northwest winds 10-20 knots with gusts to 30 knots likely. Above average confidence. Sunday night and Monday...VFR with west winds around 10 kts becoming southwest by Monday afternoon. Average confidence. Monday night through Wednesday...Periods of showers with associated sub-VFR conditions possible, especially during the day Tuesday and again Wednesday afternoon. Primarily light southerly or southwesterly winds, but a more easterly component may develop on Tuesday north of a developing warm front, and gustier southwest winds may occur Tuesday night or Wednesday as the warm front moves through the region. Very low confidence on timing and occurrence. && .MARINE... Seas will slowly build overnight and should be above 5 feet on the ocean waters near or shortly after sunrise. Winds will also increase after sunrise with gusts above 20kt possible. The strongest winds (and an abrupt shift to westerly winds) are expected to come with and behind a cold front which will move over the Delaware Bay late tomorrow afternoon. The front is not expected to arrive on the ocean waters before tomorrow evening. Outlook... Saturday night...Cold front will move through the coastal waters with showers/storms likely. Strong/gusty winds near storms. Small craft advisory (SCA) conditions likely as winds become west or northwest behind the front. Gusts may approach gale force. Average confidence. Sunday...Continued SCA conditions likely with winds slowly diminishing during the afternoon. Above average confidence. Sunday night and Monday...Sub-SCA conditions likely. Above average confidence. Monday night through Wednesday...Generally sub-SCA expected, though winds and seas may approach criteria after potential warm frontal passage Tuesday night or Wednesday. Periods of rain/showers likely, especially during the day Tuesday and again Wednesday afternoon. Very low confidence. && .CLIMATE...
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Record event reports have been issued for 6 of our 8 climate sites, including an all time monthly for the shorter POR at Allentown. Please see each for detail. ABE 77, RDG 77 GED 76 TTN 75 MPO 67 PHL equal at 74 We may add more information by 6PM.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 7 AM Saturday to 6 PM EST Sunday for ANZ450>455. Small Craft Advisory from 6 PM Saturday to 6 PM EST Sunday for ANZ430-431. && $$ Synopsis...CMS Near Term...Johnson Short Term...Johnson Long Term...CMS Aviation...CMS/Johnson Marine...CMS/Johnson Climate...Drag 442P

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