Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 280211 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 911 PM EST Sun Nov 27 2016 .SYNOPSIS... High pressure centered over the Mid-Atlantic region through tonight will move offshore Monday. Low pressure strengthens over the Northern Plains on Monday and then slowly drifts eastward across the Great Lakes region midweek. Several waves of low pressure system will develop ahead of this low and will impact our area Tuesday and Wednesday. A cold front moves through on Thursday. High pressure then builds in from the west Friday through next weekend. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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No significant changes to the forecast tonight. Mainly just hourly temperature/dewpoint adjustments. Now that skies are clearing out, temperatures are dropping faster in some areas compared to others, so we`ve tried to adjust for this. Temperatures will drop down into the mid/upper 20s to mid 30s across the forecast area.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/... High pressure will slide off the Carolina coast early Monday and continue to push offshore through the day. As the high shifts to the east, the flow will become more southerly across the area. Skies will generally be cloudless at the start of the day but cloudcover will increase, especially by late afternoon and into the early evening in advance of an approaching low pressure system. Temperatures will gradually warm and we should highs reach into the upper 40s to mid 50s, with some low to mid 40s across portions of Carbon and Monroe counties. && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/... Low pressure that is organizing over the Front Range of the Rocky Mountains today is forecast to deepen into a 975 mb surface low over the Northern Plains on Monday. The low will gradually fill out and drift eastward into the upper Midwest midweek. Even though the center of the low will be distant from our area, our region will see impacts from this system in terms of wet weather. As discussed in previous AFDs, rainfall midweek looks to come in two distinctive waves and are tied to secondary low pressure development along the system`s frontal boundaries. Models have come into better agreement with the onset and ending timing of each round of rain, though there are still some details that need to be worked out. The first round of precipitation should arrive in conjunction with a warm front from the west late Monday night into Tuesday morning. Southerly winds associated 65 kt low-level jet streak will enhance isentropic lift and moisture flux convergence across the warm front. Models also indicate some additional lift in the mid-levels from a leading shortwave trough aloft that gets ejected ahead of the primary synoptic trough over the central CONUS. A well-defined atmospheric river (AR) with a moisture connection from the tropical Pacific and Gulf of Mexico looks to advance poleward up the eastern seaboard. PWATs within this AR are forecast to climb to 1.25-1.5 inches, which is +2-3 SD for this time of year. Accordingly, the rain could be moderate to locally heavy during the day Tuesday. The rain then tapers off from northwest to southeast during the late afternoon and evening on Tuesday. The second round of rainfall looks to move in from southwest to northeast during the day on Wednesday as a cold front approaches the Appalachians. Some of the guidance shows a secondary wave of low pressure forming along the system`s triple point that tracks northeastward through the mid-Atlantic states Wednesday night. This feature could enhance rainfall across the area Wednesday afternoon and night, especially if the low is able to deepen quickly prior to it`s arrival. However, models seem to be struggling with how much the secondary low strengthens as evident in the ensemble spread and run to run inconsistencies. PWATs are forecast to peak above 1.5 inches, which is +3-4 SD and approaching the max observed values on record for OKX RAOB. See the hydro section for details on expected rainfall amounts and potential hydro impacts. Most of the forecast area becomes situated in the warm sector of the system, resulting in unseasonably warm conditions Tuesday and Wednesday. The warm front may get hung up across the far north (near the I-80 corridor). High temperatures both days in the cool sector on the northern side of the boundary would only be in the 40s and 50s as opposed to the 60s farther south. There is a legitimate shot at 70 degrees on Wednesday, particularly for the Delmarva. The rain likely ends from southwest to northeast early on Thursday after a cold front moves through. Guidance has trended faster with the fropa and exit of precipitation compared to yesterday`s runs. It is looking more optimistic that the daytime hours on Thursday will be dry across the majority of the CWA. PoPs were adjusted accordingly. Gusty W-NW winds will develop behind the cold front Thursday afternoon. Breezy conditions will likely persist into the weekend as the pressure gradient remains tight between low pressure to our north and northeast and high pressure to our west and south. Temps will trend below normal by Friday through the weekend. Highs Saturday and Sunday may only be in the 40s area wide. This part of the long-term period should dry with the exception of lake effect snow showers/flurries in the higher terrain of NW PA and far NW NJ. && .AVIATION /02Z MONDAY THROUGH FRIDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR conditions will continue. Mid level clouds are expected to continue to clear out this evening, with only some scattered high level clouds overspreading the area overnight. Westerly winds of 5-10 knots are expected to become light and variable overnight. Monday...VFR conditions expected. High level clouds are expected to thicken and possibly lower some through the day. Light and variable winds during the morning will increase to 5-10 knots during the day and vary from south-southeast to south-southwest. OUTLOOK... Monday night through Tuesday...Rain expected to overspread the area from west to east late Monday night and early Tuesday morning. MVFR restrictions likely to develop early Tuesday once the steadier rain arrives. Periods of IFR possible during the daytime where the heavier rain sets up. LLWS is a concern early Tuesday prior to the arrival of a warm front. In the lowest 2 kft above the surface, forecast sounds show winds speeds increasing from 5-10 kt at the surface to 50-55 kt at 2 kft AGL while the wind direction veers with height about 50-60 degrees (from SEly at the surface to S-SW at the top of the shear layer). Tuesday night...A lull in precip is expected. However, MVFR or IFR restrictions may persist with low clouds hanging around. Any breaks in the cloud cover would allow for fog to form. Wednesday and Wednesday night...Precip moves back into the area on Wednesday. Rain looks to be heaviest and most widespread Wednesday afternoon and night. Flight restrictions likely. Thursday...Improving to VFR early in the day as rain tapers off. Westerly winds gusting to 25 kt are expected in wake of cold fropa. Thursday night through Friday...VFR and breezy W-NW winds continue.
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&& .MARINE...
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West to northwest winds around 10 to 15 knots will continue this evening. While the pressure gradient does tighten up just a bit this evening, wind gusts look like they will max out around 20-23 knots and therefore we will not be issuing any headlines for tonight. Seas on the ocean will be around 3 to 4 feet and around 1 to 3 feet in Delaware Bay. OUTLOOK... Monday night and Tuesday...Sly winds increase Monday night. Winds sustained 20-25 kt kt will only gust a few knots higher on Tuesday with weak lapse rates residing over the cooler waters. Seas will also respond, increasing to 5-7 ft in our coastal waters and 2-4 ft in the DE Bay by the afternoon. With high confidence of SCA criteria being met, a late 3rd period-4th period SCA was issued for all of our waters starting at 4 AM Tuesday. Tuesday night and Wednesday...Sly winds decrease and should drop below SCA levels during the evening. Wave heights in our coastal waters will take longer to subside to below 5 ft so the SCA will likely need to be extended into Tuesday night and possibly into Wednesday (for seas). Wednesday night through Friday...Sly winds and seas increase once again ahead of a cold front Wednesday night. Winds turn sharply out of the W-NW on Thursday in wake of fropa. A SCA will likely be needed for the period. A rapid pressure surge behind the front may produce gusts to near gale force on Thursday.
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&& .HYDROLOGY... QPF guidance from WPC and operational NWP models were in good agreement regarding the placement of the axis of heaviest rainfall from this midweek event setting up somewhere in the CWA (consensus has the axis along the Delaware Valley) although there are some differences in QPF amounts. Storm total rainfall will most likely average 1-2 inches but isolated higher amounts of 3+ inches are certainly possible if the heaviest rain from both waves fall over the same area. Conversely, some areas may receive less than an inch of rain; there is ensemble clustering in the QPF fields for a local min near the Delmarva coast. We are not expecting main stem flooding or even smaller basin flooding since these rainfall amounts will be spread out over a 48-hr period and the antecedent conditions are dry. Poor drainage flooding is always a possibility if the localized 3+ inch rainfall amounts materialize but impacts from this type of flooding are generally minor. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 4 AM to 6 PM EST Tuesday for ANZ430- 431-450>455. && $$ Synopsis...Klein Near Term...Robertson/Meola Short Term...Meola Long Term...Klein Aviation...Klein/Robertson/Meola Marine...Klein/Robertson/Meola Hydrology...

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