026 FXUS61 KPHI 251736 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1236 PM EST Tue Feb 25 2020 .SYNOPSIS... A warm front located to our south will stall to our south today, then several waves of low pressure will move along this boundary to our south Tuesday into Wednesday. A stronger area of low pressure will lift across the Great Lakes region Wednesday into Wednesday night, pulling a cold front across the east coast. Breezy northwest flow will be in place for Thursday, Friday, and Saturday, before shifting to the north Sunday. && .NEAR TERM /THROUGH TONIGHT/... A wet and dreary period is setting up for the next 24-36 hours across the region. A shortwave trough will pass across the region today. At the surface, weak low pressure will move into the region throughout the day then move offshore overnight in response to the upper disturbance. Cloudy skies in place today, and another wave of light rain is passing through Delmarva, the Delaware valley, and southern New Jersey. Rain may be a bit more persistent in Delmarva, and generally up to 1/10 inch QPF possible with as much as 1/4 inch QPF possible in Delmarva into this afternoon. Winds will generally be light (5 mph or less) but favoring a southerly direction initially. Winds will shift to the east into the afternoon then around to the northeast as the low moves offshore this evening. Temperatures will warm slowly throughout the day to around 50 degrees. As the main round of rain today moves out of the forecast area later this afternoon and evening, areas of fog and drizzle will likely remain owing to a near surface inversion persisting through the overnight period. Have maintained the mention of this in the forecast. With lack of any forcing aloft, it's unlikely to have any significant rainfall outside of a few showers during this period. Temperatures will fall slowly to around 40 degrees overnight. Winds will be north to northeast around 5 mph or less. && .SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/... Synoptic overview: On Wednesday, a longwave trough will make its way from the central US during the day eastward toward the East Coast into the overnight. The associated jet streak will pivot across the southeastern US leaving our region in an area of strong divergence aloft (left-exit region of the jet). At the surface, an area of low pressure will move quickly from the Ohio Valley toward western PA/NY as it intensifies (sub-1000 mb) and begins occluding. The NAM and other hi-res guidance indicate a notable secondary low developing across the Mid- Atlantic Wednesday evening and passing over the forecast area overnight. However, this appears to be overdone somewhat resulting some sort of convective feedback in the jet aloft. The global guidance isn't nearly as robust with the secondary low. This has potential to impact resulting surface temperatures, dewpoints, and winds. Nonetheless, a progressive and strongly forced frontal boundary will sweep quickly across the forecast area Wednesday night. Details: Low clouds along with lingering fog, drizzle, and light rain showers can be expected through much of the day. Winds will turn easterly and increase to around 10 mph throughout the day Wednesday with the main surface low strengthening to our west. Temperatures will warm to within a few degrees of 50. As the main system approaches Wednesday evening, chances for showers will increase. Showers may be accompanied by moderate to locally heavy rain. There will be at least some elevated instability ahead of the main frontal boundary, so have continued mention of thunder in the forecast. There is a highly conditional, but non-zero threat for strong wind gusts with deeper convection (showers/thunderstorms) that develops. This is dependent on the surface low structure and whether or not the secondary low stays west of the forecast area or is much weaker. If it does, the warm front to our south may lift north into the area before the main cold front arrives. But again, this is highly conditional. As the system lifts north, a dry slot will quickly end precip toward the south as a frontogenesis forced band of precip impacts the northwestern portion of the forecast area. As colder air filters in toward daybreak Thursday, rain is likely to change over to snow across the Pocono plateau where a quick 1-2" of snow is forecast. If this transition occurs faster or the system trends farther east, higher snow totals may occur. Winds will increase and shift westerly behind the front. && .LONG TERM /THURSDAY THROUGH MONDAY/... On Thursday, the two low pressure systems, the Great Lakes low and the triple point low, will lift to our northeast and begin to merge into one low Thursday night into Friday. As this happens, our area will remain under strong west to northwest flow. This low will remain to our northeast through the end of the week before it dissipates. This will keep the strong flow across the area through the weekend, although winds will shift toward the north by Sunday. Winds will remain gusty with gusts reaching at least 20-30 mph, although Thursday will be the windiest day with gusts reaching 30- 40 mph. With the northwest flow, a few weak surface troughs, along with short wave/vorticity impulses aloft could move across the area Thursday through the weekend. With each one of these short wave and trough passages, some lake effect snow showers could affect portions of northwest Pennsylvania and northwest New Jersey. It would not be surprising to see a brief moderate to heavy snow shower or squall Thursday and Friday afternoon across northeast Pennsylvania and northwest New Jersey as there could be a combination of steep low- mid level lapse rates and enhanced low-mid level moisture. A weak clipper system may skirt just to our south late Friday into Friday night, which could bring a brief period of light rain or snow to far southern Delaware and Maryland. Sunday is expected to be precipitation free and winds begin to relax some as high pressure tries to briefly build across the region. However, as we go into Monday, guidance begins to diverge. The GFS has a weak frontal system affecting the area as an area of low pressure moves across southern Canada bringing some precipitation to the area, while the ECMWF keeps dry weather in place with high pressure. This is a week away, so we will have just a chance pops with rain or snow. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Thru evening...Plenty of IFR conditions around the terminals with some LIFR already at a few (KILG KMIV). The lower CIGS will arrive across KABE shortly. The low CIGS and vsbys will continue into the evening. Rains will become less widespread, but ocnl DZ is expected to develop. Light SE to E winds thru the period. Moderate-High confid. Tonight...IFR/LIFR conditions expected. Ocnl drizzle for the first part of the night. Light E to NE winds expected. Moderate confidence. Wednesday...Widespread low CIGS expected with low VSBYS at times in fog early. Ocnl DZ possible. Slow improvement in conditions beginning during the mid-morning with MVFR possible by the afternoon. East winds less than 5 knots early then 5 to 10 knots by afternoon. Low-moderate confid. Outlook... Wednesday night...IFR initially, improving to MVFR or VFR by 12Z Thursday. Moderate rain showers before 09Z with associated visibility restrictions. Southeasterly winds becoming westerly around 10 kts. Moderate confidence. Thursday-Thursday night...Mostly VFR, possible MVFR at times early in the day. West to northwest winds 10-20 knots, with gusts 25-35 knots. Moderate-high confidence. Friday-Saturday...Mostly VFR. West to northwest winds 10-20 knots, with gusts 20-30 knots. High confidence. && .MARINE... Today-tonight...Conditions expected to be below advisory levels with a warm front stalling to our south. OUTLOOK... Wednesday...Winds likely to remain below advisory levels, however, seas may approach 5 feet late in the day. Wednesday night...Winds and waves both increase above advisory levels Wednesday night. Thursday-Thursday night...Winds will be at least Small Craft Advisory levels, however, gale force conditions are becoming more likely. Friday-Saturday...Winds lower back to advisory levels. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Robertson Near Term...MPS/Staarmann Short Term...Staarmann Long Term...Robertson Aviation...O'Hara/Robertson/Staarmann Marine...Robertson/Staarmann