737 FXUS61 KPHI 190528 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 128 AM EDT Tue Oct 19 2021 .SYNOPSIS... Our region will remain in west to northwest flow through Wednesday as high pressure remains centered across the Southeast with low pressure over the Canadian maritimes. A weak area of low pressure will track through the Great Lakes on Thursday, causing a cold front to move through the region Thursday night. A couple of upper level disturbances may bring some reinforcing colder air by next weekend. && .NEAR TERM /THROUGH TONIGHT/... Temperatures are chilly across the area early this morning, with values on average a few degrees below consensus. Seeing some upper 30s in some surface observations in sheltered valley locations near/northwest of the Fall Line and in the Pine Barrens. Not out of the question that some patchy frost is observed in these areas around daybreak, though coverage/duration should be low/brief. An upper low over the Canadian Maritimes will move only slowly eastward during the next 24 hours, with a broad surface high shifting slowly southeastward through much of the southern and eastern U.S. The northern Mid-Atlantic will remain at least on the fringes of steady west-to-northwest surface flow today, but with midlevel ridging impinging on the region as well as a subtle backing of the winds today versus yesterday, conditions will be noticeably different today. The biggest change will be to the low-level advection pattern, with only modest cold advection becoming mostly neutral by late in the day. The surface flow will also become more favorable for downsloping. Combined with increasing heights/thicknesses from the approaching ridge, temperatures should be noticeably warmer today, likely aided by less persistent cloud cover as well. Forecast highs are around ten degrees higher than those seen yesterday. Given the expected persistent pressure gradient today, west to northwest winds may gust to 20 to 25 mph or so late this morning and this afternoon. A subtle perturbation will progress east-southeastward within broad midlevel ridging approaching the eastern U.S. tonight. Subtle large-scale lift in advance of the perturbation combined with weak low-level warm advection should prevent substantial cooling tonight. Lows are forecast to be about ten degrees or so warmer than this morning's readings. One caveat, however, is if winds decouple quickly during the evening hours (and before increased cloud cover occurs). Should this happen, temperatures may drop rapidly near/after sunset, especially in the Pine Barrens. && .SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/... Very quiet weather heading through the midweek period. Strong high pressure will remain centered to our south, slowly shifting east with time. Dry weather is expected. We get some warm advection on Wednesday as ridging over the Great Lakes region shifts east. Associated height rises and a shift to more westerly flow with plenty of sunshine and dry air will bump temperatures back up into the lower 70s. It will be a beautiful day. Otherwise, overnight lows will mostly favor the lower 50s. This is above average, mainly due to winds remaining a bit elevated at night as a decent pressure gradient remains in place between the high to the south and slow- moving low pressure over Atlantic Canada. && .LONG TERM /THURSDAY THROUGH MONDAY/... Overview... A relatively benign pattern continues through the weekend, though we will deal with the usual ups and downs of autumn. At upper levels, the main feature to watch will be a developing trough over the Great Lakes region. After a midweek warm-up, this trough is likely to bring a period of cooler weather by the end of this week and the weekend, but there remains uncertainty on how robust this trough will be. The overall appearance of the guidance for the late week and weekend is not as cold as it was, say, 24 hours ago, due to the trough being held back to the west a little more and keeping our flow more zonal. It does still look like we will trend cooler into the weekend, but how much of a cooldown remains in question with models still exhibiting considerable run-to-run variation. So some additional changes to the temperature forecast are very possible. Otherwise, our recent run of drier than normal weather looks to continue, as while there may be a couple of chances for showers, it does not look like we will have any widespread soaking rain events this period. Dailies... Thursday-Thursday night... As surface high pressure shifts offshore of the Southeast, fast-moving low pressure will track into the Great Lakes. This combination will allow steady southwest flow to develop, advecting additional warmer air into the region. Thursday looks like the warmest day of the week, with highs returning to the mid-70s. A cold front associated with the low pressure will start to approach by late afternoon or more likely the evening. Due to the unfavorable diurnal timing, meager dynamics, and very limited moisture, this front is likely to produce little if any rainfall. There will be just a slgt chc heading through the evening and overnight, with somewhat better odds across the northern zones. Friday-Friday night... Most indications are for a dry day behind the cold frontal passage. A few degrees of cooling are likely behind the front, but the deeper troughing and associated colder air hangs back to the west, so it will still be fairly mild for the season. Highs in the upper 60s and lower 70s are currently forecast. Some guidance has also suggested a wave of low pressure riding along the offshore front may bring some rain at least to the southern portion of the area overnight. This does not look like a big deal and it may miss entirely, but have maintained some slgt chc to chc PoPs for the possibility, greatest to the south. Saturday-Monday... As mentioned, the details of the forecast here are quite uncertain. Troughing will become more established over the Great Lakes, with a reservoir of colder air building up over the Lakes and southern Canada. That troughing should make at least some progress south and east with time, but models are wavering on how deep the trough will become. It is likely we will see a continued cooling trend. The latest consensus guidance suggests only a trend towards seasonable values, but there is still some potential for colder outcomes, which could result in frost/freeze concerns if they materialize. Confidence regarding precipitation chances is a little higher. While some spotty showers here or there cannot be ruled out due to a couple of shortwaves in the flow, especially on Saturday, there are no heavy precipitation events expected, and much of the time should be dry. && .AVIATION /05Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of tonight...VFR with light west to northwest winds. High confidence. Tuesday...VFR with west to northwest winds around 10 kt with gusts up to 20 kt possible at times. High confidence. Tuesday night...VFR with light west winds. High confidence. Outlook... Wednesday-Wednesday night... VFR. Westerly winds 5 to 10 kt, with some gusts near 20 kt Wednesday afternoon, then becoming light and variable Wednesday night. High confidence. Thursday-Thursday night... VFR. Slight chance of a shower in the afternoon or evening mainly north and west of PHL. Southwest wind 5 to 10 kt. High confidence. Friday-Friday night... Mainly VFR, with a chance for showers during the evening and overnight. Westerly wind 5 to 10 kt becoming light and variable at night. Moderate confidence. Saturday-Saturday night... Mainly VFR, but showers are possible which may bring restrictions. Northwest wind 5 to 10 kt. Moderate confidence. && .MARINE... The small craft advisory has expired for Delaware Bay but continues for the Atlantic coastal waters through 6 am. Winds and seas should generally be just below criteria by late this morning into the early afternoon. However, guidance suggests that west to northwest winds will increase again later today, potentially approaching advisory criteria again by evening. Another advisory may be required for this time frame, but we will hold off on issuance until the current advisory expires. Outlook... Wednesday-Wednesday night... Sub-SCA conditions expected. West wind 10 to 15 kt with gusts to 20 kt, becoming southwest overnight. Seas around 2 ft. Thursday-Thursday night... A period of SCA conditions is possible for late Thursday and Thursday night as southwest winds gust near 25 kt and seas build to 4 to 5 ft. Friday-Saturday... No marine headlines are currently anticipated. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EDT early this morning for ANZ450>455. && $$ Synopsis...O'Brien Near Term...CMS Short Term...O'Brien Long Term...O'Brien Aviation...CMS/O'Brien Marine...CMS/O'Brien