000 FXUS61 KPHI 161832 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 232 PM EDT Sat Oct 16 2021 .SYNOPSIS... A cold front will move across the area today and shift offshore of the east coast by this evening. Steady west to northwest flow will remain across the northeast region for much of the upcoming week with low pressure located to the north across Canada Maritimes and high pressure across the deep south and southeast states. Another cold front is forecast to move across the east coast late in the week. && .NEAR TERM /THROUGH SUNDAY/... The strong cold front we have been tracking is now entering the region as low pressure tracks northeastward through Quebec. Along and just ahead of the front, a broken line of convective showers has been developing and has entered our western zones. Little to no lightning has been observed with this activity due to marginal instability profiles, and this trend is expected to continue. However, brief gusty winds in excess of 40 mph have been observed to our west with this line, and we are likely to see that translate into our region as well. The line will continue to move through our region through early this evening. The overall severe weather threat is fairly limited, especially in terms of getting wind gusts to the technical severe criteria (58 mph). However, we often see winds of only around 40-45 mph cause at least isolated tree and utility damage in the mid-Atlantic, and such gusts are certainly possible as the convective line moves through. We still cannot entirely rule out a brief tornado on any kinks in the line due to good low level SRH, but that is definitely a low risk. The front will also be taking on more of an anafrontal structure as it moves through, which will help limit the severe threat but will also allow lighter, non-hazardous showers to stick around for a few hours behind the line, through a good portion of the evening. Still no hydro concerns with this systems with most areas expected to receive less than half an inch of rain, with some localized modestly higher amounts possible. Otherwise, there will be plenty of other markers of change as this front comes through. Southerly winds have been gusty ahead of the front today. As it moves through, a sharp shift to west/west- northwest flow is expected. While winds may remain a little breezy behind the front, this will not be the type of front that has a very strong push of northwest winds behind it. If anything, wind speeds should trend downward behind the front. Temperatures and dew points will drop rapidly behind the front as cooler and drier air pours in, with temperatures in the 70s and 80s quickly being replaced by 60s. Heading into tonight, lows will fall into the upper 40s and low 50s, though the steady winds will prevent ideal radiational cooling and prevent us from dropping any further. Once the lingering showers this evening push offshore, likely by around 11PM, dry weather is expected for the overnight. For Sunday, we will be squarely within the northwest flow, cold advection regime. It will be a much more fall-like day with highs only rising into the low 60s in most areas, with a steady breeze and much drier air. A fairly robust shortwave embedded within the larger scale trough moves through Sunday afternoon. This will likely yield a good deal of diurnal stratocumulus, and also a chance for scattered showers mainly to the north and west. In the winter, this would be a favorable setup for snow squalls. In this case, it'll just be a few scattered showers which may contain brief gusty winds or a bit of graupel, and again should favor northwestern areas like the Poconos, Lehigh Valley, and northern New Jersey. For the most part though, it will be an uneventful and refreshing day after the recent summer-like stretch. && .SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/... For Sunday night through Monday night, a steady west to northwest flow will be in place across the Mid-Atlantic and northeast states. Within this northwest flow, a couple of weak surface troughs will move across the area, while a couple of short wave/vorticity impulses move across the area. There will be enough moisture to lead to a period of stratocumulus across the area, but may also be enough moisture and lift for a slight chance of showers on Monday across the Poconos and northwest New Jersey. Wind gusts will be in the 20- 30 mph range. && .LONG TERM /TUESDAY THROUGH FRIDAY/... The long term forecast looks on track per the latest guidance. The steady west to northwest flow will remain across the region for the first half of the long term for Tuesday into Wednesday, before return flow develops by Thursday. Tuesday and Wednesday are expected to remain dry as northwest flow aloft will keep dry air across the area. Winds for Tuesday and Wednesday will continue to be gusty out of the west to northwest 20-25 mph. By Thursday, return flow develops as the high pressure to our south shifts offshore ahead of the next approaching frontal system which is expected to move across the area Thursday night into Friday. There are some differences between model guidance in how much moisture and precipitation will be associated with this frontal passage. Nevertheless, there will be a 20 to 30 percent chance of showers for the Thursday night through Friday time period. Northwest flow behind the front will bring mainly dry weather on Saturday with more clouds and perhaps a shower across southeastern Pennsylvania into northwestern New Jersey. && .AVIATION /19Z SATURDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Through 03z... The overall thinking for this afternoon and evening remains similar. An approaching cold front will bring a band of showers and isolated thunder to the region from about 19z to 02z, from west to east. The only chance for thunder will be right along the leading edge of this activity, and it would be brief. For PHL and points south and east, while thunder cannot be entirely ruled out, the probability no longer appears high enough to warrant a mention in the TAF, and instead will use a TEMPO +SHRA group to cover the convective line for PHL. A wind shift from southerly to west-northwesterly is expected behind the line. Southerly or south- southwest winds ahead of the precipitation will continue to gust around 20 to 25 kt. Behind the front, west-northwest winds will be around 10 to 15 kt with some gusts to 20 kt possible through the evening. It now appears more likely that there will be a few hours of light rain behind the initial convective line. Have introduced a high-end MVFR group in the TAF at all of the terminals to cover this, with both CIGs and VSBYs possibly being borderline MVFR/VFR. Rain will gradually push away from west to east through this evening. High confidence in the overall evolution, moderate confidence in the timing details and the extent of MVFR conditions late this afternoon and evening. From 03z Tonight through 12z Sunday... VFR. West-northwest wind around 10 kt. High confidence. Sunday...Mainly VFR. An afternoon rain shower is possible mainly near RDG and ABE. West to northwest winds increasing to 10-15 knots, gusting 20-25 knots. Some briefly higher gusts are possible in any showers. High confidence. Outlook... Sunday night-Wednesday...VFR conditions expected. west to northwest winds 10-15 knots during the day with gusts 15-25 knots; gusts dropping off overnight and speeds diminishing to 5-10 knots overnight. Thursday...VFR as clouds increase ahead of a cold front. Southwest wind 10 to 15 knots. Medium confidence. && .MARINE... Through early this evening... A Small Craft Advisory remains in effect for southerly winds of around 15 kt gusting 25 to 30 kt. Seas 4 to 6 ft. Later this evening and tonight... A Small Craft Advisory will remain in effect for the Atlantic coastal waters of New Jersey and Delaware, as well as Delware Bay. A frontal passage will bring a band of showers and possibly a rumble of thunder during the evening. As this front and its associated rainfall moves through, winds will shift from south to west-northwest, with speeds remaining around 15 kt with gusts to 25 kt or a little higher, especially near the mouth of Delaware Bay due to channeling effects. Some brief gusts to near gale force are possible with the frontal passage, and we may cover these with short fused marine warnings if necessary. Seas 4 to 5 ft. Sunday... Currently, the Small Craft Advisory is scheduled to expire at 6AM Sunday. It will remain breezy on Sunday, with west-northwest winds around 15 kt gusting close to 25 kt. It is marginal as to whether we will reach advisory criteria for most of the day. The best chance for more consistent gusts around 25 kt will be for the waters of southern New Jersey, Delaware, and lower Delaware Bay. It's also fairly likely there will be a period of somewhat lower winds in the morning, after the current SCA expires, before a modest increase by afternoon. For now, have declined to extend the SCA due to lack of confidence in the wind speeds, but an extension or a new advisory may be needed tomorrow. Seas 3 to 4 ft. Outlook... Sunday night-Monday...Small Craft Advisory levels expected. Monday night-Tuesday...Winds may fall below advisory levels for a period overnight Monday into Tuesday, before winds increase again later on Tuesday. Tuesday night-Wednesday...Winds may begin Tuesday night above advisory levels, before lowering below advisory levels overnight and continuing through Wednesday. Thursday...Winds and seas below small craft criteria. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EDT Sunday for ANZ430-431- 450>455. && $$ Synopsis...Robertson Near Term...O'Brien Short Term...Franklin/Robertson Long Term...Franklin/Robertson Aviation...O'Brien/Robertson Marine...O'Brien/Robertson