770 FXUS61 KPHI 240522 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1222 AM EST Sun Jan 24 2021 .SYNOPSIS... High pressure will shift southeastward from the Great Lakes region overnight to the western Atlantic on Monday. A surface low will slowly weaken as it progresses from the southern Plains on Sunday night to the Carolina coast on Tuesday. A much stronger low will develop near the Carolinas and southern Mid- Atlantic Wednesday night and Thursday, shifting quickly eastward offshore Thursday night and Friday. Strong high pressure will build into the eastern U.S. Friday and Saturday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... As of 1215 AM, a tight pressure gradient remains in place and given lingering cold air advection some mixing remains. This is resulting in a notable breeze in many areas, although this is less than what was observed during the day Saturday. This combined with temperatures mostly in the 20s is resulting in wind chill values well down into the teens (below zero in the higher terrain of the Poconos). Cloudiness remains trapped beneath an inversion and extends into portions of our western zones within the northwest flow. Meanwhile, warm air advection is increasing up the Ohio Valley and this combined with fast westerly flow aloft is starting to toss some cirrus our way. Made some adjustments to the wind and wind gust grids (increased a bit) and sky cover, with some mainly minor tweaks to the temperatures and dew points. A very dry air mass is in place with dew points in many areas down into the single digits. && .SHORT TERM /6 AM THIS MORNING THROUGH TUESDAY NIGHT/... High pressure overhead to start the day on Sunday will gradually shift southward off the Southeast coast by overnight, while additional high pressure remains well to our north over southern Canada. This will lead to cool but quiet conditions Sunday, with temperatures rebounding a few degrees over Saturday. The winds will also drop off as the pressure gradient slackens. Mainly clear skies early will likely give way to increasing mid and high level clouds later in the day and especially overnight as a weakening surface wave approaches out of the Midwest. Can't rule out a flurry overnight, but large amounts of dry air in the lower levels will likely keep the area dry as that wave dries up entirely in the face of high pressure. Lows Sunday night mainly in the 20s. A weather system is still expected to impact the area mainly Monday night and Tuesday. However, what was already looking like a minor system is looking even less impressive today. Monday continues to look mainly dry, with a lowering overcast and cool temperatures but precipitation likely holding off until after dark in most areas. This system will be fighting multiple headwinds as it arrives. In the mid levels, a shortwave ejecting out of the south-central Plains on Monday will be driving a surface low just out ahead of it. Fairly stout ridging will be in place from the Southeast and extending up into the Great Lakes. As a result, what starts as a potent shortwave will deamplify quite rapidly as it lifts off to the east-northeast. This will cause the surface low to weaken as well, and to become elongated as a weak secondary wave attempts to develop near or south of Delmarva. All in all, poor mid and upper level support for this system. In addition, there will be a lot of dry air in the boundary layer at the onset, which will take time to erode especially to the north, where precipitation may even struggle to begin on Monday night. Much of the 23.12z guidance suggests the steadiest precip to the north will occur during the daytime Tuesday as what is left of the primary low passes overhead. Trended PoPs to reflect an all around slower timing. There has not been much change in the thinking for precipitation type. The primary low center passing to our west will cause warm advection aloft. However, the weakening of that low and development of the coastal wave will keep the warm advection on the weak side, and high pressure to the north will keep colder and drier air in place near the surface. Consequently, expect frozen ptypes to be dominant, outside of the southernmost zones where more in the way of plain rain is likely. All snow is most likely across the far north, but sleet and light freezing rain or drizzle may be fairly extensive across the central portion of the area. Expectations for QPF, which were not high to begin with, have lowered further today due to the limiting factors described above. It now appears liquid equivalents will mainly be on either side of 0.25 inches, more across the south where it will fall as mostly rain. Consequently, forecast snow totals are mostly in the 1 to 3 inch range and those meager amounts may come over a period of several hours. While snow totals will not be impressive, there are still a couple of factors that would support some travel impacts. First, the cold weather over the weekend will keep the ground cold and allow for snow to stick, especially with the steadiest of it moving in during the overnight in the urban corridor. Also, am concerned that light freezing rain or drizzle could be widespread with this event. As we move into Tuesday, lift weakens further and the probability of ice present in the cloud layer decreases, a good setup for freezing drizzle. While accretion would likely be limited, a few hundredths to a tenth of an inch of glaze ice on top of a couple inches of snow could make Tuesday a treacherous travel day especially over eastern PA and much of NJ. So overall, a minor system but one we'll continue to watch, and still suspect advisories may be needed for much of the area mainly due to the combined snow and ice potential. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... Main focus for this period remains the developing storm moving through the eastern U.S. on Thursday. The large-scale pattern on Wednesday should feature two northern-stream upper lows of interest. The first will be in central Canada, and the second will be shifting southward of Greenland. In between, a high-amplitude ridge will be in place, which will lead to continued confluent upper flow to our northeast. Meanwhile, a southern-stream vort max will be ejecting quickly from the central Plains and is expected to strengthen rapidly as it becomes favorably positioned to the south/southwest of a digging vort max in the Canadian Maritimes. This process will contribute to a coupled upper jet structure in the western Atlantic, which models depict will result in explosive cyclogenesis Wednesday night and Thursday. The aforementioned confluent upper-level flow to our northeast, though, is likely to keep the intensifying surface low south of our CWA. The 12z model suite is fairly agreeable on a track from the southern Mid-Atlantic eastward or east-northeastward from the VA/NC coast in the 12z Thursday to 00z Friday time frame. Though some variability model run to model run is expected to continue given the complex ambient upper-level flow pattern, confidence is a little bit higher than normal that a track considerably farther to the north than current guidance is unlikely. Confidence is much lower, however, with the timing of the rapid cyclogenesis. The 12z GFS is fastest with this, which produces a slower moving low a little bit north and west of consensus. The 12z ECMWF remains the most suppressed and progressive, with the CMC not too far from the GFS by 00z Friday (just delayed with the strengthening). With the appearance of converging solutions, think a consensus blend approach is appropriate for this period and have incorporated into the latest forecast. The bottom line in terms of forecast changes: (1) Increased PoPs on Wednesday night and Thursday in the CWA, with the highest PoPs remaining south of Philadelphia and generally slight-chance PoPs north of I-78. Precipitation type looks to be generally snow for areas north of the I-76 corridor and more of a mix to the south (with subtle warming on Thursday). (2) Kept forecast temperatures a little colder than consensus on Thursday, as models tend to be too eager to warm the boundary layer on the north side of progressive/developing surface lows. (3) Increased winds markedly Thursday and Thursday night, especially with the approaching (intense) surface high in southern Canada and the Great Lakes region. A general summary of the forecast: After a brief period of mainly tranquil weather Wednesday and Wednesday evening as transient/amplifying midlevel ridging moves through the region, the developing surface low in vicinity of the Carolinas on Thursday night and substantial large-scale ascent downstream of the pivoting midlevel vort max in the Ohio Valley will lead to widespread development of precipitation Wednesday night. Temperatures look to be cold enough for snow in the CWA should precipitation start here overnight. However, there may be lingering effects from the aforementioned transient ridging as well as some critical low-level moistening to complete before precipitation onset, so it may take most of the night for more notable precipitation to develop. By Thursday, steadier/heavier precipitation should commence, though likely with the heaviest to the south of the CWA. Nevertheless, some snow is probable in the southern half to two-thirds of the CWA (possibly becoming more of a mix with any diurnal warming that can occur). Winds will be picking up during the day and may become quite strong by Thursday afternoon/night -- later forecasts will likely see a continued upward trend in forecast winds. Precipitation is expected to shift offshore Thursday night, but stronger winds will remain through Friday as cold advection sets in. High pressure will approach Friday night, with some of the coldest air of the season possible. However, the high will be quickly migrating, so some recovery is anticipated on Saturday via return near-surface flow. && .AVIATION /05Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Overnight...VFR, however some clouds at 4000-5000 feet are around especially in the ABE and RDG areas. Northwesterly winds 8-12 knots with local gusts to 20 knots at times. Moderate confidence. Sunday...VFR with increasing clouds. Northwest winds 10-15 knots with gusts up to 20 knots, diminishing some by late afternoon. Moderate confidence. Sunday night...VFR ceilings for a time. Northwest winds diminishing to around 5 knots, then becoming locally light and variable. High confidence. Outlook... Monday... Mainly VFR, with BKN to OVC CIGs lowering through the afternoon. Light winds, gradually shifting from northerly to easterly but with speeds only around 5 kt. Moderate to high confidence. Monday night-Tuesday... Mainly MVFR or IFR, though VFR conditions may continue through a portion of Monday night especially to the north. Mixed precipitation is expected, with mainly snow for RDG, ABE, and TTN, and a wintry mix elsewhere. Precipitation should begin early Monday night and linger through the day Tuesday, but may not begin until Tuesday morning near RDG, ABE, and TTN. Precipitation is expected to be mainly light intensity. East-northeast wind around 10 kt, with gusts to 20 kt possible near the coast. Moderate confidence. Tuesday night... Lingering restrictions are possible especially early, with improvement more likely by later in the night as precipitation tapers off. Winds shifting from northeast to north at 5 to 10 kt. Low confidence. Wednesday...Most likely, VFR conditions will prevail, though cannot rule out restrictions if any clouds/precipitation from Tuesday/Tuesday night linger(s). Northeast winds 5 to 10 kt. Low confidence. Wednesday night and Thursday...Restrictions probable as snow develops (highest chances south of PHL). Northeast winds 5 to 15 kt Wednesday night becoming north to northwest on Thursday and increasing to 10 to 20 kt with higher gusts. && .MARINE... Winds continue to diminish, so will cancel the Gale Warning that was in effect and convert it to a Small Craft Advisory that will run through Sunday. The SCA will end from south to north throughout the day. Fair weather will continue, and there is a chance for freezing spray through the overnight. Outlook... Sunday night-Monday night... Sub-SCA conditions expected. Northwest winds Sunday night will gradually shift to northeast on Monday, with gusts mainly 20 kt or less. Seas 1 to 3 ft. Tuesday-Tuesday night... On the Atlantic coastal waters, SCA conditions may develop later Tuesday and through Tuesday night as seas build to near 5 ft and northeast winds gust close to 25 kt. On Delaware Bay, sub-SCA conditions should continue. Wednesday...North to northeast winds 10 to 20 kt with potential for higher gusts. Seas of 4 to 6 feet on the Atlantic waters. Wednesday night...Northeast winds increasing to 15 to 25 kt with higher gusts. Seas increasing to 5 to 8 feet by night's end with potential for developing snow or a rain/snow mix. Thursday...Gales possible, if not likely. A chance of rain and/or snow with visibility restrictions. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 1 PM EST this afternoon for ANZ452- 453. Small Craft Advisory until 6 PM EST this evening for ANZ450- 451. Small Craft Advisory until 10 AM EST this morning for ANZ430- 431-454-455. && $$ Synopsis...CMS Near Term...Gorse Short Term...O'Brien Long Term...CMS Aviation...CMS/Gorse/MPS/O'Brien/O'Hara Marine...CMS/MPS/O'Brien/O'Hara