Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 190820 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 420 AM EDT Wed Sep 19 2018 .SYNOPSIS... High pressure will build in from the west today. The high will slide off the northeastern coast on Thursday. A cold front will cross the region late Friday into Saturday and then stall to the south of our area through early next week. Meanwhile, high pressure will build across the northeast for the start of the week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... For such a benign weather day, a rather difficult forecast exists. The main forecast concern is cloud cover and resultant effects on max temperatures. Model soundings are all over the place with diurnal evolution of boundary-layer profiles today, and this will have rather large implications on resultant sky cover/temperatures today. The NAM and GFS are in their usual camps (NAM develops a saturated capping inversion rather quickly this morning; GFS is drier), with consensus of the statistical guidance a little more on the pessimistic side. Additionally, a weak vort max moving into the area this morning will produce sufficient large-scale ascent for midlevel saturation/clouds (as depicted by satellite imagery upstream of the region). Given these factors, think there will be more cloud cover than the inherited forecast was suggesting, at least for portions of the day. Have attempted to time the increase in sky cover based on extrapolation of upstream midlevel clouds and on the expected diurnal heating as progged by some of the statistical/hi-res guidance. One of the big questions is just how much this will affect temperatures. The statistical guidance provides some hints, with the MAV considerably warmer than the MET overall (which is typical), and the ECS siding with the warmer MAV given its much more optimistic projected sky cover. My suspicion is that the cooler guidance will win out, particularly given my overall confidence on the midlevel cloud deck moving through during the day given upstream observations (less confidence with the low-level instability cloud deck). As such, strongly favored MET guidance for highs today. Complicating this is the southwestward progress of a low-cloud deck in New England (readily depicted in infrared/derived satellite imagery this morning). This has potential to move into north/east portions of the area late today and will likely inhibit or completely stall the warming process. The result of this is somewhat improved sky cover late this afternoon in south/west portions of the area as the vort max moves offshore and heating wanes (reducing the instability low-cloud deck), whereas sky cover generally remains much higher in northern/eastern New Jersey. Not overly confident on any of this, and given the sensitivity temperatures will undoubtedly exhibit as a result of the cloud-cover evolution today, there is some fairly significant bust potential to max temperatures today. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/... The main concern tonight is the extent of low stratus across the area. Models suggest low-level flow becomes more northeasterly tonight on the upstream side of the vort max moving through the northern Mid- Atlantic today and a developing surface low offshore. This will very likely allow the low-stratus deck to our northeast to set in over much of the area overnight, and increased sky cover considerably to account for this trend. Nevertheless, the more northerly flow in northern/western portions of the area will act to prevent westward progress of the stratus deck to some degree, which would permit more substantial cooling in these areas overnight. Lowered minimum temperatures here by about 5 degrees from the previous forecast, but tempered this cooling trend south/east of the Interstate 95 corridor given the expected increased cloud cover. Patchy fog is possible across the area, but suspect this will be curbed to some degree by the low stratus moving in. Did make mention of this in the forecast, but think any impacts would be rather localized. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Thursday...High pressure will build across the northeastern United States on Thursday. Ridging aloft along with dry air in the mid levels in the north to northwest flow, should be enough to keep the region dry through the day. A layer of moisture is present in model soundings but without much support, do not see precipitation falling. More of an indicator of a cloudy day across the region. Winds at the surface may become more east to southeast as the high noses down into our region from the north. Overall, expect Thursday to be a much cooler day with highs remaining in the 70s across the region. Friday through Saturday...The high shifts to the northeast of the region, returning us to more south to southwest flow. This flow will allow for increasing low level moisture (higher dew points) and as a result a return to higher humidity. Temperatures on Friday will rise into the upper 70s to lower 80s across the region. The main low pressure system will cross through eastern Canada Friday night. This low will drag a cold front across the region late Friday through early Saturday. The front sags southward and then stalls to the south of our forecast area. Showers and thunderstorms are expected to accompany the frontal passage, with conditions starting to clear on Saturday. Uncertainty with respect to how far south the front stalls will impact the remainder of the weekend as waves traverse the boundary. Scattered showers and thunderstorms will remain possible through at least Sunday, especially across our southern areas which will be closer to the stalled boundary. With the increased moisture, PWAT`s will start to rebound and rise to 1.5-2.0 inches by Friday night, which means some storms may produce higher rainfall rates as the front moves through. Monday through Tuesday...High pressure builds across New England late in the weekend and persists through Monday. The front to the south will try to push northward as a warm front but that will be tempered by the strength of the high to the north which may suppress any northward progression by blocking the boundary. The front may start to push northward on Tuesday as the high starts to shift towards Newfoundland. Scattered showers and thunderstorms will remain possible through this period as considerable uncertainty remains. Best chances for precipitation will remain across our southern areas with a gradual expansion northward through Tuesday. && .AVIATION /08Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of tonight...Predominantly VFR, but patchy fog has developed in some of the more susceptible rural and valley locations in northern New Jersey and eastern Pennsylvania. This may continue to develop southward and affect most of the terminals the rest of tonight, though restrictions may be on the brief side. Additionally, patchy fog has developed near the coasts of Chesapeake Bay and Delaware Bay, and this occasionally is affecting ILG. Winds generally light/variable, though they may stay stronger from the north at PHL/ACY. Fog should dissipate after 12Z. Moderate confidence. Wednesday...Generally VFR, but low VFR or high MVFR CIGs may develop at most of the terminals by late morning, especially if winds are more northeasterly than forecast. For now, kept CIGs as VFR, but this may not end up being the case, especially in the morning and especially at ACY/MIV (closer to the coast). Winds north to northeast around 10 kts. Low confidence. Wednesday night...Conditions may deteriorate to MVFR as winds become more northeasterly, with speeds generally below 10 kts. Patchy fog may also develop, but this looks less likely than development of low stratus. Low confidence. OUTLOOK... Thursday...MVFR in status to start the day. Ceilings should lift to VFR by early afternoon. Northeast winds around 5 to 10 knots, becoming southeast later in the day. Friday...Mainly VFR conditions expected. Light southeast winds early will become south around 10 knots. Gusts around 20 knots possible. Friday night through Saturday...Mainly VFR conditions with MVFR or lower possible in scattered showers and thunderstorms. South to southwest winds Friday night becoming northwest to north by Saturday morning behind a cold frontal passage. Sunday...Mainly VFR conditions expected. Northeast winds around 5 knots. Scattered showers possible. && .MARINE... Sub-advisory winds/seas are expected today. Winds will be northerly this morning but should become more northeasterly this afternoon, with speeds 10 to 15 kts and gusts to 20 kts. Seas of 3 to 4 feet are expected on the Atlantic waters (less on Delaware Bay). Winds will become more easterly with time tonight, with speeds remaining 10 to 15 kts with gusts to 20 kts. Seas may begin to increase on the Atlantic waters, approaching 5 feet after midnight. For now, held off on a small craft advisory, since forecast seas merely touch 5 feet rather than exceed them. Nevertheless, may need to issue an advisory if wave heights trend upward overnight. Outlook... Thursday...Seas may linger near 5 feet on the ocean waters through Thursday. Conditions look marginal and will hold off on issuing any Small Craft Advisories at this time. Friday through early Saturday...South to southwest winds will increase through the day Friday, around 25 knots ahead of an approaching cold front. Winds will turn more to the southwest and then west to northwest by Saturday. Seas will also start to build to around 5 feet. A Small Craft Advisory will likely be needed. Saturday morning through Sunday...West/northwest winds on Saturday will turn to the north and then northeast for late Saturday night through Sunday. Seas will be around 2 to 4 feet. Rip Currents... A moderate risk of rip currents exists today, as medium-period easterly or southeasterly swell is forecast with winds becoming more northeasterly with time. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Meola Near Term...CMS Short Term...CMS Long Term...Meola Aviation...CMS/Meola Marine...CMS/Meola

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