387 FXUS61 KPHI 191650 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1250 PM EDT Tue Oct 19 2021 .SYNOPSIS... High pressure will drift slowly eastward through the southern and eastern U.S. through Thursday. A cold front will move through the Mid-Atlantic Thursday night and Friday, with low pressure developing offshore Friday night and Saturday. High pressure will build back into the eastern U.S. by the beginning of next week. && .NEAR TERM /THROUGH TONIGHT/... 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 THURSDAY NIGHT/... The forecast for our region looks quite nice for this period, with the Mid-Atlantic remaining on the northern periphery of a broad surface high in the Southeast into the adjacent western Atlantic. The upper-level pattern will feature two separate perturbations that will modestly interact as they progress eastward from central to eastern portions of North America during the period. The northern perturbation, a strong upper low digging south-southeastward through Ontario Wednesday and Wednesday night, will pivot eastward into Quebec on Thursday. The second perturbation will migrate eastward from the central Plains to the Saint Lawrence Valley through the period. This second perturbation will weaken as it impinges on downstream ridging in the western Atlantic (a common evolution seen the past couple of months). Thus, the attendant cold front extending south-southwestward from the progressive surface low near/north of the Great Lakes region will undergo frontolysis as it approaches the local area Thursday night. QPF model to model looks light/scattered and primarily stays to the north and west of the CWA. The only exception appears to be the Poconos, Lehigh Valley, and northwest New Jersey, where some slight-chance to chance PoPs are placed Thursday afternoon/night as the southern perturbation glances the region. With downstream ridging in place from the aforementioned systems, Wednesday and Thursday should be warm, with highs and lows several degrees above seasonal averages. It is possible the current forecast is a little conservative, especially for Thursday, with highs approaching 80 not out of the question in portions of the urban corridor and in Delmarva. && .LONG TERM /FRIDAY THROUGH TUESDAY/... Models continue to struggle with the large-scale evolution during this period, with large run-to-run changes still noted with the deterministic simulations. Though the overall forecast continues to look fairly tame for our region during this period, confidence in the details is lower than average. The main sticking point to forecast predictability appears to be tied to the languishing upper low in southeast Canada and its potential interactions with fast-moving southern-stream perturbations across the northern/central U.S. These southern- stream perturbations, in particular, feature little run-to-run consistency, and given their origins in the notoriously challenging northeast Pacific, suspect model variability will continue to be quite high in the Days 4-7 period for the next several days given that deep troughing will likely persist there. The overall setup appears favorable for our region to be relatively unscathed, however, since the upper low will be close enough to prevent strong cyclogenesis from occurring in close proximity to the area (the more favorable area being in the western Atlantic). Thus, as one perturbation moves through the Northeast Friday night and Saturday, most of the sensible- weather impacts will likely be to our east. Nevertheless, there will likely be a period of time in which at least light precipitation occurs in our CWA, most probably some time in this Friday night to Saturday time window. This is when PoPs are highest during the medium-range period, with conditions expected to dry out Saturday night and Sunday. However, we will need to watch the evolution of the upper low in southeast Canada as well, since it may pivot far enough southward to bring another bout of light precipitation by early next week. Confidence in this scenario is low, given the widely disparate model solutions. Notably, however, there would be limited impacts from this system to our area given the moisture-starved appearance to its proximate environment. The more noticeable impact from the evolving pattern is expected to be a significant cooldown, with cold high pressure eventually settling into the region. Though the cooldown will likely be gradual (i.e., a few degrees each day), highs on Monday are forecast to be about 20 degrees lower than highs on the previous Thursday. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. This afternoon...VFR with west to northwest winds around 10 kt with gusts up to 20 kt possible at times. High confidence. Tonight...VFR with light west winds. High confidence. Wednesday...VFR with west winds around 10 kt with occasional gusts to 20 kt possible. High confidence. Outlook... Wednesday night through Thursday night...Mainly VFR with light and variable winds Wednesday night becoming southwest 5 to 10 kt on Thursday. Some showers possible northwest of the Philly terminals Thursday night. Moderate confidence. Friday...Mainly VFR with west or southwest winds 5 to 10 kt. Moderate confidence. Friday night and Saturday...Restrictions possible with a chance of showers. Winds generally light northwesterly. Low confidence. && .MARINE... The Small Craft Advisory for the Atlantic coastal waters continues through tonight as winds should ramp up again later this afternoon into this evening. For Delaware Bay, conditions should generally be just below advisory criteria through tonight, though a few gusts to 25 kt may occur at times. Outlook... Wednesday and Wednesday night...Sub-advisory winds/seas expected. Thursday and Thursday night...Southwest winds approaching advisory criteria Thursday afternoon and night, with seas building to around 5 feet. Friday through Saturday...Sub-advisory conditions expected. A chance of showers. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ450>455. && $$ Synopsis...CMS Near Term...CMS/Fitzsimmons Short Term...CMS Long Term...CMS Aviation...CMS/Fitzsimmons Marine...CMS