000 FXUS61 KPHI 191420 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 920 AM EST Tue Feb 19 2019 .SYNOPSIS... High pressure over the Northern Plains will progress eastward towards the area today and then through the Northeast on Wednesday before departing. A complex low pressure system will organize over the Ohio Valley on Wednesday, then move through the region Wednesday night before departing on Thursday. High pressure returns to close out the work week. Low pressure returns to impact the area this weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... No major changes with the 930 AM update. Only did a minor update for temperatures, dew points, and sky cover based on current trends. Previous discussion: Quasi-zonal midlevel flow will be in place today across the Mid- Atlantic, with a surface high moving into New York and Pennsylvania by afternoon. Weak midlevel perturbations will be focused to the south of the region, with large-scale descent leading to a quiet and mostly sunny day. Northerly to northwesterly surface winds will gradually weaken as the surface ridge drifts closer to the area. Cold advection with relatively unfavorable trajectories for downsloping will keep maximum temperatures in check today, and dry advection will lead to rather low dew points this afternoon. Trended maximum temperatures down a little bit given the latest statistical guidance, combined with the pronounced cold advection (and lack of aid from more substantive downsloping effects). Highs are expected to be in the upper 20s to around 30 in the Poconos and far northwest New Jersey, and in the 30s to lower 40s elsewhere. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... A highly amplified trough in the central U.S. and attendant surface low developing in the southern plains will aid in the development of widespread precipitation in the middle/lower Mississippi Valley and Ohio Valley tonight. Difluent/divergent upper-level flow (aided by the right-entrance region of an anticyclonically-curved 250-mb jet streak in New England) will allow for large-scale ascent to spread rapidly eastward into the Appalachians and Mid-Atlantic during the night. However, the surface high to our north will be in close proximity to the CWA. This should keep low-level flow from the north to northeast. These trajectories would be cold and dry, and when approaching the higher terrain of the Appalachians, should allow for fairly pronounced cold-air damming to develop. Even though warm advection aloft will be increasing overnight, the dry low levels will be a challenge for any precipitation to overcome. Models are hinting at a rather slow progression northeastward of the precipitation shield to our southwest, with most indicating completely dry conditions through 09z before precipitation begins to seep into the south/west CWA by daybreak. With these trends (and models' tendencies to overcome retreating surface highs too quickly anyway...refer to last week's event for an excellent example of that bias), decided to lower PoPs considerably during the tonight period. Kept mentionable PoPs mainly confined to Kent County MD and DE southward, with initial precipitation type as mainly snow given projected low-level thermal profiles. The biggest impacts sensible-weather wise should be after the tonight period. Lows tonight may be slow to fall given increasing cloud cover, though the northern CWA may get quite cold (where cloud cover will take the longest to develop). Would not be surprised to see readings approaching 10 degrees in far northern New Jersey. Elsewhere, readings in the 20s should be expected, and these lows may occur fairly early followed by nearly-steady or even slowly-rising temperatures overnight. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Big Picture: The large-scale synoptic pattern will be more or less static through the long term period with a high amplitude ridge over the Gulf of Alaska, longwave troughing over the western US, and a mid-lvl ridge over the sub-tropical Atlantic. Various shortwave disturbances will eject from the western trough and generally track north and west of the area along the western periphery of the aforementioned mean ridge. An upper level jet will generally be centered just north of the area however it will meander at times into favorable locations to enhance ascent over the forecast area. The sfc.low track to the northwest of the area will result in precipitation events being largely driven by ejecting shortwaves interacting with moist isentropic lift to the SE of lows. There is some indication in the global ensemble means that the mean Atlantic ridge will flatten and the western longwave trough will propagate eastward and settle north of the area over Hudson Bay by the end of the month. However, as always with long-range forecasts, confidence is low. Wednesday/Thursday storm system: 930 AM Update: No changes made yet to the forecast for this period, as we are waiting to see more of the 12Z guidance. However, it is interesting to note that the current precipitation shield extends further east (across the Carolinas) than what the models had depicted. I am not sure this will result in any significant changes for our area as models already had the precip extending across our region by late tomorrow. We will have a complete update on this system with the 4 PM update. Previous discussion: The most impactful system of the long term period will come at the very beginning, as high pressure over New England moves offshore and an area of low pressure deepens & moves northeastward over the Central US. Guidance remains in general agreement that moisture advection combined with fairly vigorous H7-850 isentropic lift, and some modest jet support will result in widespread light to moderate precipitation spreading S-N into the area starting Wed AM. Currently thinking that the onset of impactful wintry precipitation will be a bit after sunrise into mid morning in Delmarva and Chester/Berks counties, around mid to late morning in the PHI-urban corridor, and finally reaching the I-80 corridor by early Wed afternoon. As is typical in these overrunning setups precipitation type will become more of a challenge as the event unfolds. At present the column looks cold enough that any precipitation should be snow for the entire forecast area Wed morning outside of perhaps southern Delmarva where some rain or sleet could mix in by late morning. The situation gets more complicated into Wednesday afternoon as warm advection aloft increases. Ptype should be plain rain for most of Delmarva and far southern NJ by mid-afternoon Wed, with mixed precipitation spreading into parts of the Delaware Valley/urban corridor around this time as well. Finally by Wednesday evening rain will likely be the dominant ptype south and east of the fall line, while some combination of sleet and freezing rain will prevail north and west of the fall line. Heading into Wednesday night/Thursday AM the rain line will continue to creep northward with most of the area transitioning to all rain by Thursday AM, although some of the colder spots in the Poconos/NW NJ could hold onto freezing rain through most if not all of the event. Generally did not make many changes to snow amounts with this forecast package outside of raising totals slightly in coastal NJ, and also in the Lehigh Valley/I-78 corridor where guidance is slower with the onset of mixed precipitation than previous cycles. This results in generally 2-5 inches across most of the area and consequently going watches look good (as we get within 24 hours of the event with the day shift, these will either go to warnings or advisories depending on trends/location). Once again the range of possible accumulation values is high primarily due to the chance that warm aloft arrives earlier (later) than forecast cutting (raising) snow amounts. Thursday afternoon-Saturday morning: Once the cold front passes Thursday PM the airmass will dry considerably ending any lingering precipitation and little sensible weather is expected towards the end of the week as high pressure moves toward the area from the Midwest. Temperatures should be seasonal after a warm start to Thursday (since the area is in the warm sector). Saturday afternoon-Sunday evening: Another storm system will track over the Central US/Great Lakes this weekend bringing another round of precipitation for the area. Given the surface low track north and west of the area and lack of entrenched low-lvl cold air, expect the majority of the precipitation to fall as rain (although some initial snow or mixed precipitation might be possible over eastern PA Saturday night ahead of the sfc warm front). Another cold front crosses the area Sunday PM drying things out behind it. The weekendsfc.low has a bit better upper support than the Wed system and thus deepens a bit more so both prefrontal and postfrontal winds could be a bit higher. && .AVIATION /14Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR with north to northwest winds around 10 kts. Some gusts to 20 kts or so cannot be ruled out. High confidence. Tonight...VFR expected, but increasing/lowering clouds from south to north. Light winds, with perhaps a predominant northerly component. Moderate confidence. Outlook... Wednesday..Deteriorating conditions from SW-NE Wednesday morning, with all sites likely down to IFR or even LIFR conditions in snow by noon. Snow will change to sleet/freezing rain Wednesday afternoon starting at ILG/MIV/ACY and transition to mixed precipitation or rain at all terminals outside of ABE and RDG by 00Z. Winds generally E/SE less than 10 knots, outside of ACY where winds may be stronger in the evening. High confidence in IFR conditions, low confidence in ptype. Thursday...MVFR or lower conditions in rain in the morning, improving to VFR by the afternoon. Winds shifting to the W/NW by the afternoon between 10-15 knots with some gustiness. High confidence. Friday...VFR conditions with northwesterly winds under 10 knots. High confidence. Saturday...VFR conditions with lowered flight categories possible in the afternoon winds shifting to light south/southeast. Low confidence in lowered flight categories in the afternoon. && .MARINE... 615 am update: Winds and seas are diminishing/subsiding, and the small craft advisory will be allowed to expire as scheduled at 7 am. Previous discussion... Small craft advisory conditions continue on the waters early this morning, but seas are beginning to subside fairly quickly at buoy 44009 now. Buoys 44091 and 44025 should follow suit in the next few hours as winds also slowly diminish below criteria. The small craft advisory continues through 7 am, but thereafter, conditions should remain sub-advisory through tonight. Northwest winds today will slowly transition to north and northeast tonight, with speeds generally 5 to 15 kts late this afternoon onward. Seas are expected to lower to 3 feet or less by late this afternoon. Outlook... Wednesday through Wednesday night...Increasing easterly winds Wed PM with gusts around 25 knots possible by Wed evening. Transition from snow, wintry mix to rain expected Wed afternoon into the evening. Thursday...Seas building to SCA levels by Thursday AM on ocean waters with winds shifting to the West/NW by Thursday afternoon. Friday/Saturday...Seas/winds dipping below SCA by early Friday morning with sub-SCA conditions prevailing through Saturday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Winter Storm Watch from Wednesday morning through Thursday morning for PAZ054-055-060>062. Winter Storm Watch from Wednesday morning through late Wednesday night for PAZ070-071-101>106. NJ...Winter Storm Watch from Wednesday morning through Thursday morning for NJZ001-007-008. Winter Storm Watch from Wednesday morning through late Wednesday night for NJZ009-010-012-015. Winter Storm Watch from Wednesday morning through Wednesday evening for NJZ016>019-021. DE...Winter Storm Watch from Wednesday morning through Wednesday evening for DEZ001>003. MD...Winter Storm Watch from Wednesday morning through Wednesday evening for MDZ008-012-015-019-020. MARINE...None. && $$ Synopsis...Carr Near Term...CMS/Johnson Short Term...CMS Long Term...Carr/Johnson Aviation...CMS/Carr Marine...CMS/Carr