996 FXUS61 KPHI 300525 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1225 AM EST Wed Dec 30 2020 .SYNOPSIS... Strong high pressure builds across much of the East Coast tonight before shifting offshore during Wednesday. A cold front moves across our area Thursday, then stalls to our south Thursday night before returning northward late Friday as a warm front as low pressure moves across the Great Lakes. A cold front will move across our region Saturday with another low pressure system possibly developing and moving north along this front for Sunday. High pressure returns early next week. && .NEAR TERM /THROUGH TODAY/... No significant changes with this update. Another shortwave trough will pass across the Great Lakes region Wednesday. At the surface, high pressure will continue building across the area into the overnight with the center settling directly over the area into the early morning hours on Wednesday. The high will shift offshore through the remainder of the day as a cold front approaches the region from the northwest. Temperatures have dropped rather quickly this evening into the mid to even low 20s as winds have diminished. A cold night in store with lows in in the upper teens to right around 20. With clear skies initially tonight, conditions will bode well for observing the last full moon of 2020, the Cold Moon (appropriately named). High clouds will increase into the pre- dawn hours as return flow around the high pumps in upper-level moisture overhead. Temperatures will likely warm a few degrees by late morning if cloud cover becomes thick enough. With high pressure moving offshore throughout the day on Wednesday, southerly winds will increase resulting in temperatures recovering to around seasonal averages despite the fairly strong cold high pressure during the morning hours. The exiting mid-level wave should result in decreasing cloud coverage by the afternoon, but then mid- level clouds will be on the increase from the west with the system across the Great Lakes. The forecast will remain dry through Wednesday. && .SHORT TERM /TONIGHT THROUGH FRIDAY/... The forecast gets a bit more complicated for this period as a complex system affects the area in two pieces. To start the period Wednesday evening, low pressure associated with northern stream energy will move from Ontario into Quebec pushing a cold front southward into the area. This looks to bring some light precipitation arriving from north to south by later Wednesday evening into the overnight. Temperatures should actually warm up through the night due to the SW flow ahead of the front so precip should mostly just be light rain however there could be a bit of freezing rain at the onset over northern zones through the southern Poconos. Lows should be mostly in the 30s but again, these lows should occur in the evening with temps rising into the overnight. For Thursday, the aformentioned cold front pushes south across the area through the course of the day as low pressure moves from Quebec into Atlantic Canada. This will bring some continuing light rain and showers into Thursday morning, especially in the general vicinity of the I-95 corridor, before precipitation should taper off through the afternoon as the front moves south. Otherwise it will be a mostly cloudy day with highs mostly in the 40s except some low 50s over southern NJ and Delmarva. Complex forecast continues to evolve Thursday night into Friday as cold, dry high pressure builds in north of the area through Thursday night in the cold fronts wake. Meanwhile the cold front stalls near or just south of the region as low pressure associated with the southern stream develops near Texas. It should be mostly dry across the area Thursday night as colder, drier air filters in from the N/NE. Lows by Friday morning should be near freezing near the I-95 corridor with mid to upper 30s farther south and 20s to the north where there could even be some partial clearing. For Friday, expect precipitation to develop from south to north associated with the southern stream low as it moves north toward the Great Lakes. There are some timing differences in when this precip arrives with the NAM being the slowest and the GFS being the fastest bringing precip in very early in the day. We trended towards a middle ground solution as advertised by the GEM Regional as precip should generally break out SW to NE starting around mid/late morning and continuing into the afternoon. As it looks now precip should arrive near or just before midday around Philly. The big challenge will be precip types. With the low taking a track well to our west this will help bring in warmer air aloft however with the cold high to our north this will mean cold air will be stubborn to move out in the lowest levels. In fact the latest indications are that wintry precipitation (most likely in the form of sleet or freezing rain) could affect areas as far south as the I-95 corridor at the onset before a change to rain occurs from south to north. In fact the dry air in the low level may aid in further cooling as the precip begins. As mentioned, the precip should change to rain from south to north but there could be a fair amount of icing before this occurs over our northern zones near and north of I-80. Our early forecasts have around one to two tenths of an inch of ice accretion over the southern Poconos for Friday. && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... Temperatures continue to warm Friday night as low pressure moves north into the lower Great Lakes pushing a warm front through the area. As a result, all areas should see an eventual change to rain before precipitation winds down overnight. Total Rain amounts are not expected to be as significant as last week's system but could still add up to around an inch or so. This shouldn't be enough to cause any hydro issues though. Saturday will be quite mild as the low passes north of the area as it moves from the lower Great Lakes towards Maine. This will eventually drag a cold front through the area but before this occurs highs will be mainly in the 50s to low 60s. There could be a few spotty showers around but otherwise it should be dry with highs in the 50s to low 60s. For Sunday and Monday...the initial upper-level trough gradually lifts out of the Northeast, however additional energy may drop into the East carving out another trough. This feature would result in offshore low pressure development, and the amplification may allow some precipitation to extend back into at least some of our area for a time Sunday. This is of lower confidence. Eventually, upper-level ridging gradually moves across our area Sunday night into Monday. && .AVIATION /05Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Through 12Z...VFR conditions expected with increasing high cirrus clouds. Light and variable winds. High confidence. Today...VFR conditions expected. Some mid level clouds around 5000 ft possible, but mostly high cirrus clouds. Winds will start light and variable but settle out of the south and southwest at 5-10 knots by midday. Gusts up to 20 kt possible. High confidence. Tonight...Starting VFR, but MVFR ceilings are expected to build in between 03 and 09Z. Brief visibility restrictions to MVFR are possible with rain showers, primarily after 06Z. Southerly and southwesterly winds starting at 5 to 10 kt, but could increase after 06Z. A southwesterly low level jet is expected to develop and progress over the region between 03 and 09Z. For the most part, expect surface winds to stay up enough to preclude significant Low level wind shear. However LLWS will be possible at KRDG and KABE between 03 and 06Z, and a brief period of LLWS is possible for the Delaware Valley TAF sites centered around 06Z. Moderate confidence on most aspects of the forecast, but low confidence on LLWS. Outlook... Thursday...Starting with widespread MVFR ceilings and some visibility restrictions in rain showers. However, expect conditions to improve from north to south during the later half of the day. Winds will be westerly becoming northwesterly with speeds of 5 to 15 knots or less. Moderate confidence on the overall pattern, but low confidence on the timing of improving conditions. Friday...Sub-VFR conditions probable with a period of rain. A wintry mix is likely at RDG and ABE at the onset and possibly even as far south/east as KPHL/KPNE/KTTN. East/northeast winds 10-15 knots, becoming east to southeast at night. A period of low- level wind shear possible Friday night. Moderate confidence. Saturday...Conditions return to VFR in the morning. Westerly winds 10-20 knots. Moderate confidence. Saturday night through Sunday...Sub-VFR conditions could return due to another potential low pressure system developing along the coast. Low confidence. && .MARINE... Winds and seas have dropped below advisory criteria this evening and will continue to remain calm through the overnight as gusts drop to around 10 knots and seas drop to 1-3 feet. However, winds and seas will begin to build heading into Wednesday afternoon. A Small Craft Advisory will likely be needed again by tomorrow evening for southerly winds gusting up to 25 kts and seas building up to 5 feet, especially for the New Jersey coastal waters. Outlook... Wednesday night...Small Craft Advisory conditions anticipated. Thursday and Friday...Small Craft Advisory conditions probable much of this period. Winds will shift to northerly late Thursday into Friday, then turn east to southeast or south by late Friday. Saturday through Sunday...SCA conditions may linger into Friday night but conditions should then be mainly below SCA levels for Saturday and Sunday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Fitzsimmons/Gorse Near Term...Davis/Johnson/Staarmann Short Term...Fitzsimmons Long Term...Fitzsimmons Aviation...Fitzsimmons/Johnson Marine...Fitzsimmons/Staarmann