Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 211928 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 328 PM EDT Sat Apr 21 2018 .SYNOPSIS... Large area of high pressure over the northeastern United States will continue to slowly build east, building off the New England coast by Monday. Meanwhile, a low in the Arklatx region will lift into the Mid Atlantic states by mid to late week. Another area of low pressure may come in right behind it by next weekend. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... An upper air analysis this afternoon showed a lingering trough in the Northeast with a closed low ejecting into the Central Plains. There is some ridging from Hudson Bay Canada toward the Midwest. At the surface, high pressure is centered from the eastern Great Lakes to the Mid-Atlantic region. While surface high pressure is producing some low-level subsidence cross our region, a 250 mb jet with its core positioned just to our north will continue to provide enough lift within the fast northwest flow aloft to produce areas of higher level cloudiness. This is thicker to our west, and this should move over especially the central and southern parts of our region for a time tonight. The winds will go light and variable to even calm tonight, which combined with a dry airmass is conducive for good radiational cooling conditions. However, the presence of the higher level cloudiness may play a role in keeping temperatures from really dropping off. Based on the expectation of this cloudiness and collaborating with our neighboring offices, we opted to mention patchy frost for parts of southeast PA and interior central/southern New Jersey where temperatures are currently forecast to be cold enough and the growing season has started. As a result, no frost advisory issued at this time. We leaned more toward some potential frost as opposed to a freeze based on the above and dew points creeping up some during the course of tonight. The hourly temperature, dew point and wind grids were adjusted based on the latest observations/trends, then blended into the tonight period. Low temperatures are a multi-model blend with continuity. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... As an upper-level trough slides across the Northeast and northern Mid-Atlantic regions during Sunday, its axis along with short wave energy crosses our area during the afternoon. As this occurs, a 250 mb jet streak will shift to our east and the higher level cloudiness should settle south and east with it. Meanwhile, a closed low slides across the Gulf Coast states which is tracking under a ridge to its north. This ridge will keep surface high pressure anchored over the Mid-Atlantic and Northeast through the day Sunday. Another dry day is expected and with a good amount of sunshine resulting in ample boundary layer warming, temperatures are forecast to be at least a few degrees warmer than what was observed today (Saturday). The flow is on the lighter side especially in the boundary layer, therefore a light wind combined with warming on the land (increasing thermal gradient between land and water) is expected to result in the development of an afternoon sea/bay breeze. This will turn the winds for the coastal areas to south and southeast and provide some afternoon cooling. High temperatures are mainly a blend of MOS and continuity, although the gradient was tightened up some along the coast due to the expected cooling sea breeze. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... The long term period begins with tranquil period as the dominant feature will be the large area of high pressure that continues to slowly build east across the northeastern U.S.. This high should build off shore by Monday, allowing the low level flow to become onshore, resulting in increasing moisture advection ahead of the next low. The next low currently over TX, will very slowly progress east and then northeast, reaching the Mid Atlantic by mid week. What is remarkable about this low is that while I wouldn`t consider it a true cutoff low, it will be making very slow progress east through the week as compared to systems along the northern stream. As mentioned by the previous shift, the main period of rain with this system looks to be Tuesday night into Wednesday, coincident with stronger lift associated with the warm front. Due to the slow progression of the low and low level on shore flow, there is some concern of heavy rain, primarily late Tuesday night into Wednesday morning. However, there are a few factors working against the heavy rain threat. First, the cloud layer looks to be rather shallow as southwesterly flow in the mid an upper levels may keep drier air at these levels. Secondly, the warm cloud layer is relatively shallow (generally 8000 to 9000 ft), thanks to continued below normal temperatures. Finally, depending on the track of the surface low, some models are depicting a dry slot developing over our area by Wednesday morning. Not sure if this is the start of a trend, or a temporary change, but something we will be watching over the next few days. Once that low lifts northeast away from our region by Thursday, we will likely have brief reprieve of weak high pressure, before a mid and upper level open wave trough (this time from the northern stream) approaches our region next weekend. The associated cold front could slide through our region as early as Saturday, though the trough may be in the process of becoming negatively tilted which could also slow the progress of the surface low and associated front. Temperatures during this time will generally be near or below normal levels for most of the week. && .AVIATION /19Z SATURDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of this afternoon...VFR with high clouds. North-northwest to west-northwest winds mainly around 10 knots. Tonight...VFR. Clouds above 10,000 feet. Winds becoming light and variable. Sunday...VFR with some lingering high clouds. Light and variable winds, becoming west to southwest less than 10 knots. Winds are anticipated to become southeast at ACY during the afternoon. Outlook... Sunday night through Monday night...Mostly VFR conditions are expected. There is a small chance of visibility reduced in fog at KMIV and KACY on Monday morning. Winds will be light and variable for much of the period. High confidence on most aspects of the forecast except the development of fog, which is low confidence at this time. Tuesday through Thursday...MVFR and even localized IFR conditions will be possible especially on Tuesday night and Wednesday as periods of rain are likely across the region. Easterly wind gusts up to 20 kt are possible Tuesday night into Wednesday. Otherwise, generally light winds (less than 10 kt) are expected. Medium confidence on flight categories, but low confidence on the timing of any flight category changes. && .MARINE... The conditions are expected to remain below Small Craft Advisory criteria through Sunday. High pressure over the area will result in lighter winds, which will be southerly into tonight overall but tend to turn northerly during the night. The lighter flow on Sunday along with plenty of warming on land will result in the surface winds turning from the south or southeast mostly in the afternoon. Some increase in the winds is forecast in the afternoon as the wind shift occurs. Outlook... Sunday night through Monday night...Winds and seas are expected to remain below SCA criteria. Tuesday and Wednesday...Easterly winds will increase resulting also in an increase of wave heights late Tuesday through the day Wednesday. Currently forecasting SCA conditions only on the Atlantic coastal waters. However, we will continue to monitor model trends on the Delaware Bay as models tend to underestimate ESE winds especially on the lower bay. Wednesday night and Thursday...Winds shift to northwesterly and subsequently decrease. The main question will be how long seas remain above 5 feet on the coastal waters. Current forecast is for elevated seas into late Thursday, but there is still considerable uncertainty with this. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Johnson Near Term...Gorse Short Term...Gorse Long Term...Johnson Aviation...Gorse/Johnson Marine...Gorse/Johnson

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