Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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687 FXUS61 KPHI 300136 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 936 PM EDT Wed Mar 29 2017 .SYNOPSIS... High pressure builds in from the north tonight and Thursday before moving offshore Thursday night. Low pressure in the southern Plains will move northeastward to the Ohio Valley by Thursday night, then off the Mid Atlantic coast by Saturday morning. High pressure will return for the end of the weekend. Another low pressure system will arrive later Monday night and Tuesday, then high pressure briefly returns on Wednesday. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... An upper air analysis this evening shows a trough in northern New England, a ridge from the Tennessee Valley northward to the Great Lakes, then a robust trough in the central to southern Plains. The latter is helping to build the downstream ridge, placing strong surface high pressure near Hudson Bay. There is also considerable 850 mb warm air advection indicated into portions of the southern Midwest this evening. High pressure centered near Hudson Bay this evening will continue to build southward into the northern Mid Atlantic through the overnight. This along with a cooling boundary layer will allow for less wind and cooler temperatures. Meanwhile, the aforementioned warm air advection in combination with a 250 mb jet positioned to our north will help pull some high level clouds across our area especially late. This may thin eastward for awhile as the ridge axis remains to our west. We still indicate some cloud cover arriving through the overnight from west to east. For the 930 PM update, the hourly temperature, dew point and wind grids were adjusted based on the latest observations. Some locales are experiencing a quicker drop in temperature thus far, therefore some adjustments were needed. There is drier low-level air advecting in from the north, therefore the dew points for many areas were lowered a little faster. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/... The center of the high tracks into eastern Canada on Thursday, pulling an upper level ridge onto the Eastern Seaboard late in the day. Meanwhile, low pressure continues to organize over the Central Plains and Midwest. The upper ridge should keep precip at bay for the daytime hours, but high level clouds initially will then lower and thicken throughout the day. Temperatures should be just shy of normal, topping off in the mid 40s up north to the mid and upper 50s elsewhere. && .LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/... Long term looks wet, with potential for at least two systems producing substantial precipitation accumulations for the region. The first system looks to affect the region beginning Thursday night. Potent vort max in the Ozarks region 00Z Friday moves into the Ohio Valley by 12Z, acquiring a negative tilt. Impressive difluence downstream and upper-level jet coupling from a retreating anticyclonic jet streak in New England and a southern-stream cyclonic jet streak nosing into the Southeast will provide a prolonged duration of strong/deep ascent along much of the Eastern Seaboard. Warm-air advection precipitation should break out Thursday night across the Mid-Atlantic. Residual northeast flow from the departing surface high in southeast Canada and New England should allow a cold wedge of near-surface air to seep well south into the CWA east of the Appalachians. Mesoscale model guidance is consistently showing a freezing rain signature in the southern Poconos Thursday night. A pronounced warm nose is present above the near-surface cold(er) layer, with little change in the thermal profile from Thursday night through Friday evening. Temperatures will be flirting with the freezing mark much of this time, and models often are too warm in such regimes, especially in the southern Poconos (it seems). There is large uncertainty remaining, however, given the marginal cold air near the surface. Given the expected QPF (more on that below), there is potential for quite a bit of ice accumulation, especially on elevated surfaces, near Mount Pocono and in adjacent Sussex County, NJ. Then again, very little may occur at all if the boundary layer stays on the warmer side. Instinct is telling me that at least an advisory will be warranted for Carbon, Monroe, and Sussex Counties during this time frame. However, plenty of time to figure out the details, as this remains 30-60 hours out. Regarding the rest of the area, the dynamical nature of the vort max and the sustained southerly fetch downstream of it imply widespread and substantial QPF are likely in the CWA Friday. A SSE 50+ kt low- level jet will advect a considerable amount of moisture (PWATs well in excess of 1 inch) into the Mid-Atlantic during the day Friday. With the vort max approaching the region during the afternoon, substantial differential cyclonic vorticity advection combined with low-level isentropic lift along a pre-existing baroclinic zone suggests widespread moderate to heavy rainfall for an 18-hour window (generally 12Z Friday to 06Z Saturday) across the entire region. There remain some discrepancies among the model guidance, particularly regarding the degree of moist advection (the NAM being noticeably drier) and the locations with maximum QPF (consensus being in a corridor from SE PA to northern/central NJ), but the strength of the system and the associated lift give relatively high confidence in 1+ inch QPF across the region, with potential for localized 2-3 inch totals. To this point, instability looks limited/negligible across the area, so kept thunder out of the grids. However, some hydrologic issues may occur if the stronger- seeming model simulations verify. Will allude to this potential in the HWO. Residual wraparound showers may occur in PA/NJ through Saturday morning, but the surface low should be well offshore by this point. Winds will switch to northerly, but the southern origins of the system suggest temperatures will fall little after system passage. Additionally, there may be some downsloping impeding any cold air advection that may be present. High pressure builds in Sunday and Monday, and this period should generally be dry. Temperatures will warm to seasonal or slightly above seasonal values. Clouds will once again be on the increase Monday as the next southern-stream system advances to the East Coast. The track of this system continues to look more southwest to northeast (from the Mid- South to the Ohio Valley), which suggests a warmer scenario compared to the Friday/Saturday low. Another substantial fetch of moist air will precede the system, and the closer proximity of the warm sector suggests a higher probability for convection. Timing/track uncertainties remain, with the 12Z GFS a noticeable flat/north outlier compared to the CMC/ECMWF. Both of these latter models produce high amounts of QPF across the southern CWA (given the southward displacement of the surface low track), but there remain discrepancies between these two models regarding the nature of the precipitation (with the CMC providing a prolonged period of isentropic lift along a zonally-oriented warm front, whereas the ECMWF indicating more influence from pre-cold frontal convection, at least in Delmarva). Meanwhile, the GFS definitely has more of a warm-sector precipitation scenario. My suspicion is that the southern solutions make more sense, but too far out and too much run-to-run variability to feel very confident. Did raise PoPs across the region given the strong signal with precip/timing among the model suite. As with the end-of-week system, substantial QPF looks possible with this next low. After a brief dry period Wednesday, models suggest another system affecting the region by the end of next week. && .AVIATION /02Z THURSDAY THROUGH MONDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR. Northerly winds near 10 knots, becoming light and variable later this evening and overnight at most terminals. Thursday...VFR. A ceiling at or above 10000 feet will develop mainly in the afternoon. Variable winds near 5 knots becoming northeast in the morning, then turning southeasterly. OUTLOOK... Thursday night...VFR ceilings lowering, potential to MVFR toward daybreak Friday as some rain arrives. East or southeast winds less than 10 knots. Friday and Friday night...Sub-VFR CIGs/VSBYs and rain likely. East to southeast winds 10 to 20 kts with higher gusts near the coast becoming more northerly late Friday night. Confidence average. Saturday through Sunday night...Mainly VFR with mostly north or northwest winds at or below 10 kts. Exception will be Saturday, with speeds up to 20 kts (gusts up to 25 kts) possible, especially near the coast. Confidence above average. Monday...Generally VFR with winds around 10 kts becoming more easterly. Increasing cloudiness likely. Confidence above average. && .MARINE... High pressure builds across the waters tonight through Thursday. N winds may gust to 20 KT on the ocean waters late tonight through Thursday morning, but Small Craft Advisories should not be needed. OUTLOOK... Friday and Friday night...Advisory-level winds/seas likely. Gale- force gusts possible, especially off the New Jersey coast. Rain and visibility restrictions likely. Saturday and Saturday night...Advisory winds/seas likely. A chance of rain early on Saturday. Sunday and Monday...Sub-SCA winds expected. Seas may remain somewhat elevated early in the day Sunday. && .TIDES/COASTAL FLOODING... Though astronomical tides will be gradually diminishing through the week now that we are past the new moon, the threat of minor tidal flooding along the NJ and DE Atlantic coasts increases by Friday. We currently have high tides about 0.5 feet above the astronomical tide. We expect this to continue through Thursday. Beginning Thursday night, a low pressure system will bring a prolonged period of onshore flow, further increasing the surge. The tide of most concern is still the high tide Friday evening and Friday night, particularly along the northern and central New Jersey Shore. By this tide cycle, it will take a surge of 0.8 to 1.0 feet to reach minor flooding thresholds, which is likely. However, not sure yet if we will have another 0.3 feet surge to reach advisory thresholds. The exact magnitude of the surge will be dependent on how quickly the onshore flow develops and how strong it will be by Friday. ETSS shows water levels at Lewes and Sandy Hook touching minor flooding thresholds with the high tide cycles tonight and Thursday night. This is unlikely though as with the expected wind direction, we should not see a surge any higher than what we currently have at least through Thursday night. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...CMS/Gorse Near Term...Gorse Short Term...MPS Long Term...CMS Aviation...CMS/Gorse Marine...CMS/MPS Tides/Coastal Flooding...Johnson is the U.S. government's official web portal to all federal, state and local government web resources and services.