Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 191137 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 637 AM EST Sun Nov 19 2017 .SYNOPSIS... Low pressure over upstate New York will lift northeast into the Saint Lawrence Valley today. The attendant cold front will move through the Mid Atlantic early this morning. High pressure will build in from the south and west tonight into Monday before moving off the Southeast coast on Tuesday. A cold front is expected to pass through the region late Tuesday night or Wednesday morning. High pressure builds back into the area for Thanksgiving. The high will eventually weaken and move offshore late in the week as a cold front approaches from the Midwest. The front will likely move through the Mid Atlantic on Saturday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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630 am update: Front is blasting through the area at this time, with several reports of 40 to 50 mph wind gusts and downed trees/power lines coming in near/behind the front. Showers are rapidly moving along/ahead of it and should be offshore within an hour or two. Changes to forecast include tweaks to hourly temperatures and dew points to match trends/frontal timing and to fine-tune wind speeds/gusts based on upstream obs. Made considerable edits to sky cover, as I expect fairly rapid improvement later this morning. Previous discussion... Complex forecast through today as a strong cold front moves through the area in the next few hours. Strong/deepening surface low in central NY will progress east-northeast along/just south of the Saint Lawrence River during the next few hours, with an equatorward- extending cold front racing eastward through the Mid-Atlantic, aided by strong pressure rises upstream. Showers continue to develop in the pre-frontal warm sector in the northern Mid-Atlantic, with widespread slab-like lift along the cold front itself contributing to low-topped convection in central PA. Given the very strong winds off the surface (850-mb winds 50-75 kts downstream of the front), these showers will not have a difficult time transporting these winds to the surface, particularly with the added effects of water loading and evaporative cooling below the cloud bases. As a result, pushed up the wind advisory for the entire CWA, which started at 1 am. Several gusts of 40 to 50 mph have been reported so far, with the reports generally clustered in the Lehigh Valley, the Delmarva Peninsula, and the immediate coast of southern New Jersey so far. The HRRR is strongly indicative of two waves of stronger wind gusts through the morning hours. The first is associated with the frontal band of showers moving through the area between now and 12Z (and perhaps with showers in advance of this band), and the second is after cold frontal passage, tied strongly to the 2-3 hour duration of strongest pressure rises immediately upstream of the front. Gusts of 40 to 50 mph are likely after frontal passage, with scattered power outages and downed trees expected. The strongest winds will ease only slowly during the late morning and afternoon hours, as strong boundary-layer mixing will continue, aided by cold-air advection and increasing insolation with time. Meanwhile, the showers should quickly exit stage east early this morning, with QPF generally in the quarter to half inch range for the overnight and early morning hours today north of the Mason-Dixon Line and perhaps a tenth of an inch or so in far southern portions of the CWA. Temperatures have risen slowly in the pre-frontal warm sector, though there has been sharp cooling in the rural valley locations thanks to higher clouds earlier this past evening. Guidance is handling this poorly, to say the least, and have had to make several edits to hourly temperatures already (with more likely to come before frontal passage). Thereafter, temperatures will likely begin a slow fall through the day, though increasing insolation and strong mixing may curtail this to some degree. General forecast is strongly weighted to hi-res 2-m temperatures, given the unsurprisingly poor performance of statistical guidance overnight
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... Main question tonight is the potential for some snow showers in the Poconos. With a strongly mixed boundary layer and saturation around 900-850 mb, there are indications that snow showers will develop periodically to the northwest, occasionally progressing into the southern Poconos. The best time window for this looks to be late this evening and overnight as the deep-layer flow becomes more northwesterly (more favorable fetch for lake-effect "streamers"). Additionally, the main vort max associated with today`s system will be moving through Sunday night, which will supply additional lift and cold air aloft to aid in the development of a shallow (nearly) dry-adiabatic boundary layer in the post-frontal environment. Have kept and even prolonged slight-chances or chances of snow showers in this region, with potential for snow squalls given the strong low- level wind field and favorable low-level thermodynamic profile. Meanwhile, temperatures will turn colder across the area, though the bottom will not fall out given the continued elevated winds (though much lighter than during the day). Current forecast is a mix of MAV/MET/ECS MOS, which are in reasonable agreement, and features temperatures generally a few degrees below seasonal averages. && .LONG TERM /MONDAY THROUGH SATURDAY/... A rather tranquil and predominately cool weather pattern is in store for next week. Monday and Monday night... High pressure builds to our south across the Southeast states on Monday. Westerly winds will still be breezy (gusts 20-30 mph) owing a tight pressure gradient to the north of the surface high but not nearly as strong as today. CAA thermal pattern in the low levels transitions to neutral Monday morning and eventually even to a WAA pattern once the cold pool progress downstream of the area late afternoon and night. Dry and cool conditions can be expected with high temps 6-9F below normal and lows 3-6 below normal. Tuesday... High pressure moves off the eastern seaboard. Southerly return flow around the high will draw milder air into the region. Highs return to near normal. Winds will once again be breezy, especially during peak afternoon heating with gusts 20-30 mph. Tuesday night and Wednesday... A cold front will approach from the Tuesday evening before moving through the forecast area late Tuesday night into early Wednesday morning. Models continue to hint at a band of rain developing over the Mid-Atlantic region Tuesday night in response to frontogenetical lift that occurs in between a northern stream shortwave trough and a southern stream disturbance that is tracking off the Southeast coast. These systems do not appear to phase in time to bring more significant rains to our area, instead provide us with a glancing blow. PoPs and QPF increase from northwest to southeast across the forecast area with models in agreement that the better dynamics pass near the coast. Rain is forecast to end from west to east during the first half of the morning as drier air arrives in wake of fropa. Similar to what happens today, temperatures initially in the 40s and 50s will struggle to rise much during the day Wednesday as CAA offsets daytime heating. Wednesday night through Friday...Quiet weather is in store for Thanksgiving with high pressure building overhead and even into Black Friday when the high starts to retreat offshore. Below normal temperatures return with highs in the lower to mid 40s on Thursday and mid 40s (N/W of the Fall Line) to lower 50s (S/E of Philadelphia) Friday. Friday night and Saturday...A cold front will approach from the Midwest Friday night and then move through the region sometime on Saturday. The setup with this system is similar to the predecessor one Tuesday night/Wednesday morning system where both northern and southern stream systems fail to phase in time to produce a significant rainfall event across the forecast area. The airmass moderates to within a few degrees of climo in pre-frontal warm sector on Saturday before colder air moves back into the region later in the weekend. && .AVIATION /12Z SUNDAY THROUGH THURSDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Rapid improvement to VFR with showers quickly exiting the area early in the morning. Very strong west or west- northwest winds 20 to 30 kts with gusts to 40 kts or so are likely through the day, with the strongest winds generally in the morning. Confidence is well above average. Tonight...VFR with west winds slowly diminishing to around 10 kts after midnight. Gusts to 25 kts or so possible through the evening hours. Confidence above average. Outlook... Monday through Tuesday...VFR. W winds 10-15 kt with gusts around 25 kt on Monday become SW and weaken to under 10 kt Monday night. SW winds increase on Tuesday to around 10 kt with gusts to 20 kt during the afternoon. Forecast confidence: High. Tuesday night and Wednesday...Mainly VFR. However, the chance for rain increases late Tuesday night into early Wednesday morning, especially closer to the coast. Cannot rule out MVFR CIGs if steadier rain develops along and especially east of I-95 terminals between 06-12Z Wednesday. Wind shift from SW to NW will in wake of cold fropa early Wednesday. NW winds gusts to around kt on Wednesday. Forecast confidence: Medium overall; low for MVFR CIGs. Wednesday night and Thursday...VFR. NW winds gradually diminishing. A storm system is currently expected to remain offshore. Forecast confidence: Medium to High.
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&& .MARINE... Gale conditions are generally occurring across the waters at this time, with the strongest winds/gusts expected to occur just after frontal passage from about 6 am to noon. Winds 40-45 kts are likely during this time window. Very choppy/erratic seas are likely through the day. Showers will continue through daybreak, with even a rumble of thunder possible, but should move out quickly by mid to late morning. Outlook... Monday and Monday night...SCA conditions likely with NW winds gusting to 25-30 KT around daybreak, gradually decreasing throughout the day. Seas in the coastal waters will also subside, starting out at 4-5 ft in the morning, then becoming 2-4 ft by late afternoon. Monday night...No marine headlines anticipated. NW winds drop below 25 kt and seas decrease to 2-3 ft. Tuesday and Tuesday night...SCA likely beginning late morning and continuing through the evening. SW winds increase to 15-20 kt with gusts 25-30 kt gusts. Seas in the ocean waters are forecast to peak in the 5-6 ft range by early Tuesday evening. Winds and seas diminish overnight. && .TIDES/COASTAL FLOODING... A Low Water Advisory was issued for the Delaware Bay north of a line from East Point, NJ to Slaughter Beach, DE (ANZ430 marine zone). This was based on the latest Total Water Level forecast of around -2 ft MLLW near high tide late this afternoon-early evening. A more westerly wind direction (vs northwesterly) of the strong winds should limit the blow out potential in the tidal portion of the Delaware River (including Philadelphia) this evening. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Wind Advisory until 4 PM EST this afternoon for PAZ054-055- 060>062-070-071-101>106. NJ...Wind Advisory until 4 PM EST this afternoon for NJZ001-007>010- 012>027. DE...Wind Advisory until 4 PM EST this afternoon for DEZ001>004. MD...Wind Advisory until 4 PM EST this afternoon for MDZ008-012-015- 019-020. MARINE...Gale Warning until 5 AM EST Monday for ANZ430-431-450>455. Low Water Advisory from 2 PM this afternoon to 7 PM EST this evening for ANZ430. && $$ Synopsis...Klein Near Term...CMS Short Term...CMS Long Term...Klein Aviation...CMS/Klein Marine...CMS/Klein Tides/Coastal Flooding...Klein is the U.S. government's official web portal to all federal, state and local government web resources and services.