490 FXUS61 KPHI 031045 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 645 AM EDT Wed Apr 3 2019 .SYNOPSIS... A strong low off the Mid-Atlantic coast will race northeastward to the Canadian Maritimes tonight. High pressure will build into the Northeast on Thursday. A weak, slow-moving system will move through the eastern U.S. Friday and Saturday. High pressure will briefly build into the region late this weekend, but another system will affect the Mid-Atlantic early next week. && .NEAR TERM /THROUGH TONIGHT/... The ocean storm continues to churn northeastward well off the Mid-Atlantic coast early this morning. Precipitation amounts last evening were generally a tenth of an inch or less in eastern NJ and central Delmarva, with nothing measurable occurring to the northwest. Unfortunately, this sets the stage for a rather concerning fire weather day across the area, since wetting rains did not really occur virtually anywhere. As the ocean storm moves northeastward, high pressure will build in from the west. A strong pressure gradient will develop in between, with rather deep mixing (likely to 800-750 mb) expected to develop during the late morning and afternoon hours. Winds aloft are reasonably strong (~35-45 kts in this layer), so surface gusts of 25-35 kts should be readily occurring by early afternoon. Dew points will plunge as the deeper mixing sets in, so enhanced fire weather potential is expected to develop. See the fire weather section for more details. Did consider the need for a wind advisory, but BUFKIT soundings look too marginal at this time. However, a few gusts to 40 kts are possible, and should models be underplaying the efficiency of downward momentum transfer, a short-fused advisory may be required. Regardless, it will be a breezy day across the area. Trajectories are favorable for downsloping, so temperatures are expected to be warm. Guidance (statistical and raw model) numbers are in strong agreement, so used a consensus blend for max temps today. Results are quite close to continuity. The pressure gradient should slowly relax this evening and overnight, but lingering gusts of 20-25 kts may occur into the evening hours. With time, winds should lessen. Decent radiational cooling is expected with the slackening winds and mostly clear skies. However, did not go below guidance numbers for tonight, as I suspect winds will take time to diminish, there tends to be a cold bias with models on the initial night of west/northwest surface flow, and increasing cloud cover may occur late as a weak perturbation moves toward the area. && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... Main concern for this period is precipitation Thursday night through Friday night across the area. A surface ridge will progress through the Northeast on Thursday, with dry conditions. Clouds will be on the increase as weak perturbations move through the region in progressive midlevel flow. Additionally, winds will be weaker owing to the surface high. As a result, temperatures will probably be a few degrees cooler than Wednesday. Did not stray too far from the previous forecast. Models continue to struggle with the southern-stream system affecting the region Friday and Friday night. The ECMWF continues its slower/wetter bias, with the CMC/UKMET considerably weaker/drier. The GFS is somewhere in between with the QPF but certainly on the faster side, probably too much so given its systematic bias. Meanwhile, the NAM is closer to the ECMWF solution (actually even wetter). I leaned toward the wetter solutions here, given the southern origins of the system (and ample moisture supply from the Gulf) and the slower movement of the system. I raised QPF considerably for Friday/Friday night (generally by a factor of two), with the caveat that drier solutions still exist. However, the CMC has seemed too dry with southern-stream systems the past several months, and given this plus the always-too-progressive GFS, think the model biases are pronounced enough here to weigh the forecast more heavily on certain deterministic solutions rather than a general blend. With the above in mind, thinking the potential for wintry precipitation in the far northern CWA Thursday night is not particularly high. The GFS looks way too aggressive in bringing precipitation into the area (probably by 6+ hours). Did not want to reduce PoPs too much Thursday night, but my suspicion is the forecast is a little overdone still during this time frame. Suspect the steadiest precipitation will be Friday and Friday night, just in advance of the attendant vort max in a region of differential cyclonic vorticity advection and favorable upper- level divergence in the left-exit region of a digging jet streak (with residual effects from the right-entrance region of a departing jet streak). A little concerned that the models are too aggressive in warming the higher elevations in the Poconos and northwest New Jersey on Friday, especially given NAM BUFKIT soundings (which tend to perform relatively well in this area). Maintained some slight-chance freezing rain mention in these areas through Friday morning, but the colder guidance suggests this may occur for a longer portion of the day. Will need to watch this potential closely. Rain should begin to diminish and/or move east of the area late Friday night as the perturbation sluggishly moves offshore. Temperatures on Friday are subject to large errors, as effects of precipitation will combine with potential for some cold-air damming to prevent temperatures from climbing much at all in northern portions of the CWA. Additionally, not expecting much cooling Friday night as cloudy/rainy conditions linger. Expect some edits, potentially major ones, to max/min temps during this period in later forecasts. && .LONG TERM /SATURDAY THROUGH TUESDAY/... Models continue to exhibit large differences in the medium-range period, making this quite the challenging forecast. Suspect most of Saturday and Saturday night will be dry as the perturbation responsible for the precipitation Friday and Friday night will be well to the east. However, there are some indications weak/upstream vorticity maxima may move through the Northeast during this period, so cannot rule out the possibility of a few showers, especially during the day. Temperatures look warm, as the origins of the antecedent system are not cold, and there may be some downsloping in the system's wake. Midlevel ridging amplifies to close the weekend, so expecting the warm conditions to continue. However, models continue to struggle with the handling of the upstream/amplified shortwave in the plains. The GFS maintains a much faster progression into the eastern U.S., with effects reaching the area by Sunday night. There is not a whole lot of support for this from the CMC/ECMWF, though the ECMWF is noticeably faster than last night's 00z simulation. There remain large synoptic-scale differences with the associated vort max/trough, with the GFS much more amplified than the ECMWF (the CMC being somewhere in between). With these discrepancies in mind, a consensus blend seemed reasonable, supporting mentionable PoPs Sunday night through Tuesday, though highest Monday afternoon through Tuesday morning (where consensus is strongest). This system has the potential to bring some decent QPF to the region, and would be dynamic/strong enough to generate some instability in the pre- frontal warm sector. Included/maintained some mention of thunder Monday and Tuesday afternoons. Given the uncertainty in system timing, felt that the warmest guidance was a little too aggressive for the Sunday through Tuesday period. Lowered max temps on Monday and Tuesday in particular a few degrees, as some cloud cover/precip may limit the warming potential to some degree. Given the large model differences, not a lot of confidence with these specifics, however. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Mostly clear with west winds increasing to 15 to 25 kts with gusts 30+ kts late this morning and this afternoon. High confidence. Tonight...VFR with west to northwest winds diminishing gradually to around/below 10 kts by late evening. High confidence. Outlook... Thursday...VFR with west winds around or below 10 kts. High confidence. Thursday night through Friday night...Periods of sub-VFR likely with rain/showers. Winds becoming east or southeast 5 to 15 kts, with stronger gusts possible near the coast. Winds may switch to southerly/southwesterly Friday night. Moderate confidence. Saturday through Sunday...Becoming VFR with northwest winds around 10 kts on Saturday becoming north to northeast Saturday night and east to southeast on Sunday. Increasing cloudiness on Sunday. Low confidence. && .MARINE... Small craft advisory conditions will continue today on the Atlantic waters, even as winds slacken through the morning as directions become more westerly. This is because seas will take quite a while to subside from their current values, likely remaining above 5 feet into at least the evening. Meanwhile, the near-shore waters will likely see advisory-level gusts as strong mixing occurs on land during the day. Thus, Delaware Bay remains in the small craft advisory through 8 pm tonight, with the caveat that winds may not reach criteria on the main channel. Outlook... Thursday through Sunday: Mostly sub-advisory winds/seas expected. Rain likely Friday and Friday night, with visibility restrictions possible. && .FIRE WEATHER... A red flag warning is in effect today for eastern Pennsylvania, New Jersey, Delaware, and the Eastern Shore of Maryland. West winds will increase to 15 to 25 mph with gusts 30 to 40 mph common. Relative humidity will drop to below 30 percent by late morning or early afternoon, allowing for several hours of critical fire weather conditions. With a lack of wetting rains overnight, fuels are dry, and the meteorological environment is favorable for the enhanced spread of any fires that develop. Conditions should slowly improve during the evening, as winds are expected to diminish as a surface high approaches the area. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Red Flag Warning from 10 AM this morning to 9 PM EDT this evening for PAZ054-055-060>062-070-071-101>106. NJ...Red Flag Warning from 10 AM this morning to 9 PM EDT this evening for NJZ001-007>010-012>027. DE...Red Flag Warning from 10 AM this morning to 9 PM EDT this evening for DEZ001>004. MD...Red Flag Warning from 10 AM this morning to 9 PM EDT this evening for MDZ008-012-015-019-020. MARINE...Small Craft Advisory until 11 PM EDT this evening for ANZ450>455. Small Craft Advisory until 8 PM EDT this evening for ANZ430- 431. && $$ Synopsis...CMS Near Term...CMS Short Term...CMS Long Term...CMS Aviation...CMS Marine...CMS Fire Weather...CMS