818 FXUS61 KPHI 221021 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 621 AM EDT Sun May 22 2022 .SYNOPSIS... Bermuda high pressure remains off the Mid-Atlantic coast Sunday. A strong cold front will move through Sunday night as low pressure passes to the northwest, with the front slowly settling offshore on Monday. High pressure builds in for Tuesday and Wednesday, then a frontal system may approach the area later next week. && .NEAR TERM /THROUGH TONIGHT/... 6AM Update... A few showers have developed over portions of eastern Pennsylvania, possibly in response to the passage of a gravity wave and/or the presence of a weak MCV from convection to our south last night. Have added an isolated shower mention for the next couple hours in the Lehigh Valley and Poconos. This activity should lift northward and out of our area through the morning. Previous discussion follows... It is an unseasonably warm early morning, with temperatures overnight struggling to drop below 70 in much of the area and in some cases staying well up in the 70s. Unlike yesterday morning, fog has not been much of a factor, though some highly localized visibility restrictions have been observed mainly up north. There is also some higher level cloudiness over the area, the remains of multiple areas of convection to our west and south yesterday evening. Another hot day is expected today, with little change in the air mass from yesterday and with southwest flow continuing. Ahead of a cold front, we do see some very modest height falls today. As mentioned above, there is also some mid and high level cloudiness around this morning which may persist to some extent for much of the day. This combination should keep highs a little cooler today than Saturday, mainly in the low 90s. However, with dew point values in the mid 60s to near 70, it will be fairly humid, and max heat index values will favor the mid 90s. As a result, maintained the Heat Advisory for the urban corridor, though it is on the marginal side. We also continue to have a risk for convection today. The arrival of the cold front will provide sufficient forcing for scattered to widespread showers and thunderstorms to develop in the warm sector. Based on its timing, western and northern portions of the area will stand the best chance for rainfall, though some activity may survive to the coast or at least close to it. Storms may develop over the western zones as early as around 2PM, but the bulk of activity should be concentrated from about 4 to 9PM, with the axis shifting east with time. Regarding potential threats today, the SPC maintains a marginal risk for most of the area and a slight risk over the far northwestern zones. This seems reasonable to me. The setup today is not overly exciting from a dynamics perspective, with moderate to high instability and weak to borderline moderate shear. The main positive for today's storms appears to be a high equilibrium level (EL) of 40 to 45 kft, which will favor tall thunderstorms capable of producing locally strong to severe downdraft winds (with decent DCAPE values expected) and possibly some hail. Shear vectors also turn a little more perpendicular to the frontal boundary by late today which could enhance the wind threat if any transient bowing segments are able to organize. But mid-level lapse rates are not impressive, and while CAPE values may approach or exceed 3000 J/kg due to the high EL, the profiles are still rather tall and skinny. And with bulk shear only about 20 kt, it will be difficult for storms to organize beyond single cells and transient multicells, which would quickly become outflow dominant as they outrun frontal forcing and ingest mid-level dry air. Overall, this is a fairly typical summertime thunderstorm setup, and while I expect we'll get a couple of instances of strong to damaging winds, this event does not have high end severe weather potential. The storm threat will wane into tonight. With the warm air mass in place, this won't be a case where convection dies right at sunset. However, as showers and storms progress east, a combination of loss of diurnal heating, weakening forcing, and eventually the marine influence will lead to a steady decrease in storm coverage and intensity. Remaining convection will have moved offshore and/or dissipated by around midnight. As the front moves offshore, winds will shift from southwest to northwest, drawing cooler air into the area. There won't be a whole lot of time for cold advection to occur overnight, but the frontal timing does look a little bit faster than it had the past couple nights. As a result, most areas should drop into the upper 50s and low 60s, with some cooler readings possible further north and west. This will bring an end to the early season heat event. && .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/... Surface high pressure will be centered over the Great Lakes region Monday morning as a broad upper level trough remains over southern Canada. The wave that brought the showers and thunderstorms to the region Sunday will be exiting northeast with multiple perturbations behind this wave following in its wake. This fractured PV pattern has really given model guidance a tough time on whether or not the entire area will be dry Monday and Tuesday. In particular, to the south the sub-tropical ridge will be surpressed with multiple packets of PV streaming northeast over the ridge (or over central PA and NJ). The means that even though the surface cold front pushes well south of the area, the low level 850 MB front stalls over southern DE. Enough low level dry air exists over NJ and PA that a dry forecast looks likely Monday into Tuesday. Towards southern DE, enough moisture is close enough to the border that slight chance PoPs were maintained. Sounding profiles never completely dry out though which means Monday and Tuesday will be mostly cloudy to overcast. Skies will try to clear across northern NJ on Tuesday, but the push south will quickly stall. Expect near normal temperatures on Monday and Tuesday with high temperatures slightly below normal (low to mid 70s) both days and low temperatures in the low to mid 50s. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... The rather chaotic mid and upper level pattern Wednesday will give way to a more organized and amplified pattern Thursday through Saturday. The surface high pressure that was in control of the weather Monday into Tuesday will translate east towards the Canadian Maritimes on Wednesday with surface winds veering from the east. The persistent onshore flow will allow dewpoints to moderate into the 50s while also keeping high temperatures only in the low 70s. Mostly cloudy to overcast skies will persist with most of the region remaining dry. Thursday into Friday an upper level low that formed over the western United States will eject northeast towards the Great Lakes region. Surface low pressure will deepen as it heads towards Lake Superior with multiple pieces of PV advecting over the CWA. As this occurs, the chances of precipitation will increase through the day Thursday. On Friday, a surface cold front will sweep through the region as the upper level low starts to fill. This means precipitation will exit the coast Friday evening with clearing expected Saturday morning as the dry conveyor belt moves overhead. Behind the cold front, temperatures will return to near normal for this time of year with highs in the upper 70s for Saturday. && .AVIATION /12Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Through 12z... VFR. Some very localized vsby restrictions with fog are possible, but it appears unlikely any of the terminals will be affected. Winds mostly light and variable, but favoring a light southwest flow especially from PHL southward. High confidence. Today... Prevailing VFR. A cold front will approach the region late this afternoon and evening. Ahead of it, scattered thunderstorm development is expected in the warm and humid air mass. Scattered storms will begin moving in from west to east around 18z and especially after 20z. The best chance for tstms will likely be in western portions of the area such as near RDG and ABE, but a tstm threat will extend to PHL as well by very late afternoon or early evening. Southwest winds increasing to around 10 kt with some gusts of 15 to 18 kt possible. Gusty and erratic winds possible in stronger tstms. Moderate confidence both in timing and extent of tstm coverage. Future TAF updates will better refine the timing and placement of potential tstms. Tonight... Lingering showers and storms possible through about 03z, with their coverage and intensity steadily diminishing after dark and with the focus of them shifting eastward. Localized restrictions are possible, but prevailing VFR conditions will continue, and any remaining precipitation should be offshore by around midnight. As the cold front crosses the region, winds will shift from southwest to northwest, with speeds mainly less than 10 kt. High confidence. Outlook... Monday...Some possible MVFR restrictions possible in the morning with a recovery to VFR expected later in the morning. Northwest winds 5 to 10 kts gradually shifting to northeast overnight. Moderate confidence. Tuesday...VFR. Northeast to east wind 5 to 10 kts, a little breezier at the coast. Moderate confidence. Wednesday...Mainly VFR. East to southeast wind 5 to 10 kts. Moderate confidence. Thursday...VFR except MVFR in scattered showers and thunderstorms. South wind 10 kts. Moderate confidence. && .MARINE... Through tonight... Sub-SCA conditions are expected. Southwest wind 10 to 15 kt, shifting to northwest around midnight tonight behind a cold front. Some southwesterly gusts near to above 20 kt are possible this afternoon and evening. Seas 3 to 4 ft. Showers and tstms are possible mainly near and after sunset, however, activity will likely be weakening as it moves over the waters. Outlook... Monday-Wednesday...No marine headlines anticipated. Offshore winds early on Monday, becoming onshore later Monday into Tuesday and through midweek. Seas of 2 to 4 ft expected. Thursday...Winds and waves below small craft advisory levels with scattered showers and thunderstorms. Seas 3 to 4 ft. Rip Currents... There is a LOW risk for the development of dangerous and life threatening rip currents at NJ and DE beaches today. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Heat Advisory until 6 PM EDT this evening for PAZ070-071-102- 104-106. NJ...Heat Advisory until 6 PM EDT this evening for NJZ008-010-012- 015-017>019. DE...Heat Advisory until 6 PM EDT this evening for DEZ001. MD...None. MARINE...None. && $$ Synopsis...Haines/O'Brien Near Term...O'Brien Short Term...Haines Long Term...Haines Aviation...Haines/O'Brien Marine...Haines/O'Brien