266 FXUS61 KPHI 100803 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 403 AM EDT Wed Oct 10 2018 .SYNOPSIS... High pressure will remain centered offshore today into tonight. A strong cold front will then arrive later Thursday, which will interact with the remnants of Hurricane Michael as it accelerates northeastward and off the Mid-Atlantic coast later Thursday night and Friday morning. High pressure then builds in later Saturday before shifting offshore during Sunday. The next cold front should arrive later Monday followed by weak high pressure on Tuesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... After a period of partial clearing this evening, nocturnal decoupling and substantial low-level moisture have allowed fog to form in much of central/southern New Jersey into portions of New Castle County, DE. With surface observations indicating increasingly widespread quarter-mile visibilities and webcams indicating very low visibilities in similar areas, a dense fog advisory appears to be required through the morning rush for these areas. It is possible the fog will spread into the Philadelphia metro area, but at this point, it looks like the main threat for denser fog is in the more rural areas to the south and east. Will continue to monitor and expand/contract the advisory as necessary. Meanwhile, the large-scale pattern continues fairly similar to yesterday, with a surface high in the western Atlantic nosing into the eastern U.S., with midlevel ridging continuing and even amplifying somewhat poleward of very dangerous Hurricane Michael. Latent heat release from Michael and considerable poleward low-level flow downstream of a deep trough in the central U.S. will aid in ridge amplification today. Nevertheless, moisture will continue to stream northward in advance of Michael, sustaining a very moist/tropical thermodynamic profile today. Similar to yesterday, think that once the low clouds/fog dissipate (which may be gradual), little lift will be required to generate some spotty showers. The GFS/NAM are a little less bullish today with this, perhaps owing to a lack of obvious larger-scale lift as the ridge amplifies. However, there are indications in some of the guidance of a low-amplitude vort max moving into the northern Mid-Atlantic this afternoon. This may be enough to generate sufficient instability for some convective showers, especially near any low-level foci (e.g., differential heating boundaries from dissipating clouds or orography). Have spread low PoPs across the CWA this afternoon from south to north, though capped them at slight chance. Given the high-octane air in place, any showers that develop will be quite capable of producing downpours. Temperatures are once again a challenge today. Given the somewhat increased ridging aloft today, suspect it may be a little warmer than yesterday, especially if the clouds diminish fairly early in the day. However, the cooler guidance fared a little better yesterday, so I used consensus pretty much as is for highs across the area. This ends up being about 1-2 degrees warmer than yesterday's max temps, in general. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/... Hurricane Michael is expected to make landfall this evening in the Florida Panhandle, while the large-scale trough to our west begins its push eastward into the Upper Midwest/Great Lakes. Attendant cold front will move into the eastern Ohio Valley by 12Z Thursday, with considerable warm/moist advection downstream in the Appalachians and East Coast. High-resolution models show fairly widespread precipitation moving into the area by late evening and overnight. Model soundings indicate increasingly buoyant (though marginally so) profiles entering into the western CWA during the night. Inserted slight chances of thunder generally west of the Delaware River. Atmospheric profiles are anomalously moist as Michael aids in substantial moisture transport into the region. PWs reach 2+ inches after midnight, and with very warm moist adiabatic profiles, precipitation processes will be primarily of the warm-cloud variety. That spells heavy rain potential, especially if stronger convective cores develop. Cell motions look to be fairly fast, but regenerative convective processes may occur owing to the increasing low-level flow. Inserted heavy-rain wording in the grids, generally northwest of the Fall Line (in closer proximity to the approaching trough). Temperatures will be very warm, probably near or above 70 in the urban corridor and southeastward and not far from these values elsewhere. May again see some patchy fog across the area as winds decouple and dew points continue rising, especially in areas that remain at least partially clear well into the evening and generally before the showers move in. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Summary...Tropical moisture enhanced by the remnants of Hurricane Michael tracking to our south and east Thursday into Friday and it interacting with an incoming strong cold front. This will bring some heavy rain to parts of our region along with a local flood threat; Colder air arrives for the weekend into early next week. Synoptic Overview...The western Atlantic ridge weakens some and shifts south and east late this week as the energy from current Hurricane Michael accelerates northeastward. In addition, an upper- level trough from the Northern Plains and Great Lakes to adjacent Canada will drive a strong cold front across the East later Thursday. This front will interact with Michael and induce baroclinic forcing as if shifts offshore of the Mid-Atlantic coast early Friday. The storm will continue to accelerate out to sea during Friday. A potentially strong short wave looks to amplify the trough aloft eastward during Saturday as it quickly crosses the Northeast. The flow then turns more zonal as surface high pressure builds across our region for later Saturday into Sunday, however the next trough shifts across the Great Lakes pulling a cold front across our area later Monday. For Thursday and Friday...Showers are expected to become more numerous Thursday and Thursday night with some embedded thunderstorms due to the combined effects of moisture streaming north from current Hurricane Michael ahead of the cold front, as well as forcing from an upper disturbance moving through in the southwesterly flow ahead of the trough. Precipitable water values are forecast to exceed 2.25 inches. There is some indication from the forecast soundings that some training or backbuilding may occur for a time ahead of the front due to deep south to southwest flow. This is a favorable setup for very heavy rain with an associated local flash flood threat. Held off on a Flash Flood Watch as the heaviest widespread rain looks to fall in generally a less flood prone area of southeastern New Jersey and parts of Delmarva, and any flooding may be more localized. The downpours are anticipated to increase through the day Thursday as forcing strengthens and instability increases some. As we go through Thursday night into early Friday, the cold front settles across our area however it will interact with the remnants of Hurricane Michael. This will result in baroclinic forcing that will strengthen the low pressure center of Michael as it accelerates northeastward and tracks to our south and east. As this occurs, showers should collapse into our eastern and southern areas and will probably congeal into an area of steadier rain for a time. This is where the heaviest rainfall amounts are expected with a swath of 2-3 inches quite possible across our far eastern and southern zones. The guidance shows this quite well with the precipitation shield becoming more organized and the most focused along and northwest of the surface low track. This is typically the case with a tropical system transitioning to a non-tropical one. Areas near and north/west of the I-95 corridor will see showers/rain diminish by the overnight with the passage of the cold front. By late Friday morning, post-tropical system Michael will be moving away from our area with any heavy rain along the coast ending. A period of gusty winds are anticipated with the strongest closer to the coast during the first half of Friday. This takes place as the winds shift to the north and northwest and cold air advection surges in allowing for better downward momentum transfer as low pressure deepens as it moves further away. Based on the forecast soundings, wind gusts look to be in the 25-35 mph range on Friday as the colder and drier air surges in. The winds diminish some Friday night with temperatures dropping into the 40s for much of the area. For Saturday through Tuesday...Colder air settles in for Saturday within a northwest flow regime. The guidance is trending stronger with a short wave amplifying a trough aloft Saturday as it quickly crosses the Northeast. This is also resulting in much of the guidance showing some showers moving through at least the northern areas Saturday. The airmass will be notably drier therefore it may limit the shower activity, however for now added in slight chance PoPs during the day Saturday for parts of the northern zones. Cloud cover may need to be increased farther south as well for a time Saturday. Surface high pressure then builds in later Saturday. A chilly Saturday night is expected with even some 30s in northwest New Jersey, the Poconos and parts of the Lehigh Valley where patchy frost is possible. As we go into the latter part of the weekend into early next week, a progressive pattern continues with the next system looking to approach during this time though there are some model differences regarding the timing and evolution of this system. At this point, Sunday looks to stay dry with shower chances increasing some early next week. Some warming should occur Monday ahead of the cold front, but overall a cooler pattern looks to remain. && .AVIATION /08Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of tonight...Highly variable conditions expected at the terminals, with CIGs/VSBYs oscillating between VFR and LIFR as shallow fog and/or scattered low CIGs develop and dissipate over the next few hours. Conditions should generally worsen for longer durations through daybreak, especially at PHL/PNE/ILG/MIV/ACY. Nearly calm winds. Overall confidence is moderate, but very low regarding the timing of category changes. Wednesday...Gradual improvement to sustained VFR is expected by late morning, though timing this transition is a challenge. Current indications are this process should occur between 13Z and 16Z, though models have not been stellar with similar scenarios in recent days. Winds becoming south around 5 to 10 kts. Low confidence. Wednesday night...Conditions are expected to be VFR during the evening but will gradually deteriorate overnight to MVFR or worse. Showers are likely near/after midnight, with rapid changes in CIG/VSBY in their proximity. Winds light southerly. Moderate confidence. Outlook... Thursday and Friday...MVFR/IFR conditions with showers becoming more widespread along with a few thunderstorms Thursday ahead of a cold front. A period of steadier/heavier rain resulting in IFR conditions should evolve Thursday night into Friday morning near and south/east of PHL as the remnants of Hurricane Michael tracks to our south and east. The conditions should improve to VFR during Friday from west to east. South-southwest winds around 10 knots, become northwest later Thursday night into Friday behind the cold front with gusts to around 25 knots at times. The winds diminish some Friday night. Saturday and Sunday...VFR overall. Northwest winds around 10 knots Saturday become light and variable at night, then becoming light southwesterly during Sunday. && .MARINE... Winds/seas should remain below advisory criteria through this evening, with generally fair weather expected. There may be a few showers on Delaware Bay and the Delaware Atlantic coastal waters, but any of these would be spotty. Winds will generally be southerly or southeasterly 5 to 15 kts, with seas near or below 4 feet. After midnight, winds will begin to increase and may approach advisory criteria by daybreak, especially over the Delaware Atlantic waters. Seas will be increasing as well and may approach 5 feet by morning. Showers will increase, especially after midnight, with visibility restrictions likely in their proximity. Outlook... Thursday and Friday...Small Craft Advisory conditions are expected with initially southerly winds Thursday, then north to northwest by Friday. Rain/showers are expected with even some thunderstorms possible. There is increasing potential that a period of gales occurs from early Friday morning through early Friday afternoon mainly from the Atlantic coastal waters from about Atlantic City southward including much of Delaware Bay. These details will depend on the track of the remnants of Hurricane Michael as it transitions into a strengthen offshore storm during Friday. Small Craft Advisory conditions may linger behind the system through Friday night. Saturday and Sunday...The conditions should be below Small Craft Advisory criteria as high pressure gradually builds in. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...Dense Fog Advisory until 9 AM EDT this morning for NJZ013-014- 016>022-025>027. DE...Dense Fog Advisory until 9 AM EDT this morning for DEZ001. MD...None. MARINE...None. && $$ Synopsis...Gorse Near Term...CMS Short Term...CMS Long Term...Fitzsimmons/Gorse Aviation...CMS/Gorse Marine...CMS/Gorse