Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 101034 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 634 AM EDT Fri Apr 10 2020 .SYNOPSIS... An area of low pressure will drift through eastern Canada through the weekend, while high pressure builds across the deep south today and Saturday, before moving offshore Saturday night. Our area will remain under a tight pressure gradient between these two features today through the weekend. A strengthening low pressure system will move across the Mid West states Sunday night, then into the Great Lakes region Monday and into southern Canada Monday night. This will lift an occluding frontal system across the Mid Atlantic region Sunday night into Monday. Another system could affect portions of the Mid Atlantic region around Wednesday, with a clipper type system around Thursday. && .NEAR TERM /THROUGH TONIGHT/... 630 am update: No updates needed to the forecast at this time. Previous discussion... Way behind with the forecast overnight owing to a high-impact storm that produced a swath of significant wind damage from the Harrisburg, PA, area east-southeast to Wilmington, DE, and adjacent portions of southern New Jersey. A wind gust of 69 mph was reported at KILG and of 62 mph was reported at KACY. In addition, areas of snow developed in northern portions of the forecast area, with several spotters reporting brief wind gusts of 40 to 50 mph with some of these showers. Several reports of graupel/small hail were received from many of these showers, as well. Most of the shower activity for the CWA is coming to an end early this morning. The exception will be the southern Poconos and adjacent areas, as northwest-flow showers will occasionally reach these areas today. Temperatures will likely remain cold enough for snow or a rain/snow mix this morning, with potential for light accumulations in the highest elevations. However, chances will slowly dwindle tonight as a reinforcing shortwave trough digs southeast into the Atlantic during the evening, and upstream descent takes over. PoPs are generally negligible after 00z Saturday. Clouds will thicken fairly quickly during the day, as mixing and diurnal heating generate saturated parcels readily after sunrise. This should keep temperatures from warming (especially given the ambient cold advection). Nudged highs slightly lower today given these factors. The main story today will be the winds. Model forecast soundings indicate another day of mixing to 850-800 mb, with winds of 35-45 kt at these levels likely mixing to the surface rather efficiently in the cold-advection regime in place. Wind gusts of 40 to 50 mph will be likely at times today, especially near/south of Philadelphia in closer proximity to stronger winds aloft. However, the threat of advisory-level gusts persists across the area, and the wind advisory was unchanged with this forecast update. With the elevated winds and strong mixing, fire-weather concerns are elevated today. Please see the Fire Weather section for more details. Attention tonight turns to temperatures, as the strong cold advection occurring today will lead to a temperature plunge tonight. Model guidance is generally in agreement that areas northwest of the Philadelphia metropolitan area will reach or drop below freezing overnight. However, confidence is not particularly high, since winds will likely remain elevated through the night. In such regimes, models tend to be too aggressive with the temperature drop, especially in more urbanized areas. Nevertheless, with the growing season in full swing southeast of the Poconos and with consistent guidance showing temperatures falling below freezing in Berks County, the Lehigh Valley, the far west suburbs of Philadelphia, and northern New Jersey, hoisted a freeze watch for the overnight hours in these areas. Will need to watch the Pine Barrens as well, as temperatures often drop precipitously, even in otherwise elevated wind conditions. && .SHORT TERM /SATURDAY THROUGH SUNDAY/... Saturday will remain rather breezy, though considerably less so than today. Winds will likely diminish by late afternoon as high pressure approaches the area. With the upper-level trough axis moving offshore, should see thicknesses increase, allowing for a slightly warmer day compared to Friday. Used a statistical blend for highs, resulting in readings in the mid to upper 40s in the Poconos to the mid 50s southeast of the I-95 corridor. Unfortunately, the breezy conditions and resultant mixing will allow for another day of elevated fire weather concerns. As high pressure shifts offshore Saturday night, winds will become light and back to a more southerly or southeasterly direction. Subtle warm advection should prevent a collapse in temperatures, but I suspect the rural/valley locations will see another cold night, with potential for more frost/freeze products. By Sunday, the Mid-Atlantic will begin to see downstream midlevel ridging as a very potent shortwave trough ejects into the southern Plains. Warm advection will increase in earnest during the day, with an associated increase in cloud cover as well. Despite the dampened insolation, the height rises and warm advection will result in a considerable warmup. Highs may approach 70 in much of the area. The region will be well downstream of large-scale lift with the approaching system, so have unmentionable PoPs through the day. The sensible weather of interest will once again be the winds, as they increase from the south during the day. Could see some gusts of 25 to 35 mph by the afternoon, but fortunately, the warming temperatures will be compensatory to most. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... The main focus for the long term forecast will be Sunday night into Monday. An area of low pressure will move across the Mid- West states and into the Great Lakes area Sunday night, before lifting into southern Canada on Monday. This will lift an occluding frontal system across the Mid-Atlantic and northeastern states. Rainfall is expected to begin overnight Sunday, then continue into Monday, before ending later Monday afternoon into Monday evening as the cold front pushes offshore. Several concerns are beginning to take shape with this system. The first will be the potential for moderate to heavy rainfall overnight Sunday into Monday morning. 1-2 inches of rain will be possible, with the heaviest rainfall expected for portions of eastern Pennsylvania and northern/central New Jersey. Latest model guidance indicates that there will be some instability developing during day ahead of the approaching cold front, so there will be a chance of thunderstorms. Thunderstorms would mostly be confined to daytime hours Monday. This leads to the next concern, which may be the greatest threat. Low level winds with this system are forecast to increase significantly with 925-850 mb winds potentially increasing to 50-70+ knots. Not all of these winds would mix down to the surface, however, we will likely have some decent mixing overnight Sunday into Monday, so we do expect a very windy overnight Sunday into Monday period. We can expect at least 40-50 mph winds, especially for south and eastern New Jersey, and central and southern Delaware and Maryland where the strongest winds are expected. The front will have moved offshore by Monday afternoon/Monday evening. Gusty winds will continue night into Tuesday, before beginning to relax overnight Tuesday. There will be a slight chance of showers Tuesday as a couple of short waves/vorticity impulse move across the area. There continues to be some disagreement with the long term models in how they handle a potential coastal storm developing along the old cold front offshore of the east coast Tuesday night into Wednesday.. The ECMWF is much more aggressive strengthening the low moving it offshore of the Mid- Atlantic and bringing rain or even snow to portions of the area. The GFS and Canadian suppresses the system and keep rain away from the area. We will have a chance of showers due the uncertainty, but no more than 40 to 50 percent across southern New Jersey and southern Delaware and Maryland. There is better agreement with a possible clipper type system affecting the area around Wednesday night into Thursday. This could bring a chance of rain, or even snow showers to portions of the area Wednesday night into Thursday. High pressure is forecast to briefly build into the area late Thursday, before another potential storm at the end of the week. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Mainly VFR, though CIGs of 3500-6000 feet will likely encompass much of the area by midday. West to northwest winds 15 to 25 kt with gusts 30-40+ kt likely. Moderate confidence. Tonight...VFR with west to northwest winds 10 to 20 kt with gusts up to 30 kt, especially during the evening. High confidence. Outlook... Saturday...VFR with west winds 10 to 20 kt with gusts up to 30 kt, likely diminishing slowly by late afternoon. High confidence. Saturday night...Mainly VFR with winds becoming light southerly or southeasterly. Moderate confidence. Sunday...Increasing cloudiness with south winds increasing to 10 to 20 kt with gusts up to 30 kt by afternoon. Mainly VFR, but conditions may approach MVFR late. Low confidence. Sunday night-Monday...A frontal system will move across the area leading to periods of moderate to heavy rain, along with MVFR to IFR conditions likely, improving during the day Monday. Very wind conditions are likely to develop overnight Sunday into Monday, with gusts potentially reaching 35-45 knots. Moderate confidence. Monday night-Tuesday...VFR conditions expected. West to northwest winds gusting to 20-30 knots. Moderate confidence. && .MARINE... Gales continue on the Atlantic waters and Delaware Bay today. West to northwest winds of 20 to 35 kt with gusts to 45 kt are likely. Conditions should slowly improve tonight but will likely exceed gale criteria through most of the night. The gale warning continues through 6 am Saturday. Seas will generally range from 3 to 6 feet. Outlook... Saturday...Advisory conditions likely with west winds 10 to 20 kt with gusts to 30 kt. However, conditions will improve with time and possibly become sub-advisory by evening. Saturday night...Sub-advisory winds/seas anticipated. Sunday...South winds will increase rapidly, likely reaching advisory criteria by afternoon. Seas are expected to approach/exceed 5 feet late in the day as well. Sunday night-Monday...A frontal system will move across the area, leading to very strong winds across the waters. At least Gale force gusts are expected, with potentially Storm force winds possible late Sunday night into Monday. Monday night-Tuesday...Gale force winds could continue into early Monday night, before lowering to Small Craft Advisory levels overnight into Tuesday. && .FIRE WEATHER... Windy conditions will occur again today. This will allow the low levels to mix readily, with dew points likely dropping to the upper teens to lower 20s. This will result in relative humidity values dropping to near or below 30 percent in most locations outside the southern Poconos. As a result, the threat for the rapid spread of fires is quite high today, and a Special Weather Statement has been issued in coordination with our fire-weather partners. Conditions will not improve on Saturday. Although winds will likely be lower, relative humidity values will also likely be lower than values seen today. As a result, the threat for the rapid spread of fires will be high again. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Freeze Watch from late tonight through Saturday morning for PAZ060>062-101-103-105. Wind Advisory until 7 PM EDT this evening for PAZ054-055- 060>062-070-071-101>106. NJ...Freeze Watch from late tonight through Saturday morning for NJZ001-007>010. Wind Advisory until 7 PM EDT this evening for NJZ001-007>010- 012>027. DE...Wind Advisory until 7 PM EDT this evening for DEZ001>004. MD...Wind Advisory until 7 PM EDT this evening for MDZ008-012-015- 019-020. MARINE...Gale Warning until 6 AM EDT Saturday for ANZ430-431-450>455. && $$ Synopsis...Robertson Near Term...CMS Short Term...CMS Long Term...Robertson Aviation...CMS/Robertson Marine...CMS/Robertson Fire Weather...CMS is the U.S. government's official web portal to all federal, state and local government web resources and services.