Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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062 FXUS61 KPHI 251327 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 927 AM EDT Tue Apr 25 2017 .SYNOPSIS... Low pressure along the coast of the Carolinas will slowly progress northward along the Eastern Seaboard through Wednesday before progressing eastward into the northwest Atlantic late this week. A strong subtropical high develops in the western Atlantic Thursday through the weekend. A front will attempt to approach the area late this week but will likely stall in the northern Mid-Atlantic this weekend. A strong cold front will sweep through the eastern U.S. early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Nearly vertically stacked 1000 mb surface low remains positioned near Cape Hatteras, and will slowly drift to the north today. The first bands of precip are spreading from southwest to northeast into DE, southern NJ, and southeast PA. Expect the leading edge of the more steady precip will reach the Delmarva zones by late morning, Phila area around midday and I-78/I-80 corridors in northeastern PA/northwestern NJ mid to late afternoon. Models have been in very good agreement with this timing. Kept a slight chance for thunderstorms S/E of I-95, where hi-res CAM guidance was highlighting a tongue of modest elevated instability mainly this afternoon. There is a risk for locally heavy rainfall. A heavy rain band that was occurring upstream over NC was juxtaposed with the TROWAL and easterly low-level jet on the northern side of the low as well as an inverted surface trough that extends northwestward from the surface low. Accordingly, these features were used to prognosticate the timing and location of the heaviest rain for our area later today. These features appear to provide a favorable setup for heavy rainfall mainly this afternoon in the coastal plain. Model spread for QPF has also narrowed with the latest iteration of guidance, adding to forecast confidence. Official rainfall forecast was a blend between WPC and the wetter NAM/HRRR/GEM to take into account the potential for convection to enhance rates this afternoon in eastern portions of DE and NJ. The highest amounts in these locations were 1.5-2.0". && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... Low pressure near the Outer Banks late this afternoon will track north-northeastward and just off the Delmarva coast tonight. The widespread rain should exit the region toward sunset as both the low-level jet and TROWAL will progress downstream of us. However, the threat of locally heavy rainfall will persist through the evening in the coastal plain as the coastal front sets up near the NJ coast. Elevated instability associated with the cold pool aloft under the upper low could support a few thunderstorms in E NJ. Expect showers to end from SW to NE this evening in E MD and overnight elsewhere. Moisture trapped beneath a low-level inversion will keep stratus and fog over the area tonight. There is a potential for fog to become dense in spots, especially near the coast. The combination of persistent onshore flow and overcast conditions should prevent temperatures from falling much more than a few degrees tonight. Lows range from the upper 40s in the Poconos to the upper 50s in S DE. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... By 12Z Wednesday, the slow-moving surface low should be positioned just off the coast of Delaware or New Jersey, with models already showing quite a bit of spread in its placement and movement thereafter. Fortunately, the implications of these model differences are relatively minor for our area, as precipitation will be winding down in our region by this point. Nevertheless, some residual showers are likely to persist on the northwest side of the low, potentially aided by a coastal front that will be slow to dissipate and/or move away from the northern/central NJ coast during the day. As midlevel subsidence and attendant drying move in during the day, precipitation will become lighter/spottier, with most of the showers likely out of the area Wednesday night. Regardless of the coverage of showers, low-level clouds are likely to stick around for much of the day, especially north/east of the Delmarva Peninsula. There may even be some fog on Wednesday morning and again Wednesday night. Most guidance looked too warm given the expected persistence of the cloud cover, so went just above the colder MET guidance for max temperatures, though this may still be too warm. Midlevel ridging will amplify quickly upstream of the weakening low, as a potent shortwave trough progresses into central North America. Models are showing a considerable amount of disagreement with the small-scale phenomena associated with this feature, with the 00Z GFS in particular being problematic owing to what appears to be a substantial amount of convective feedback. Placed little or no weight on the GFS solution for the rest of the long-term forecast as a result. However, confidence is not much better with the rest of the model suite given the complex interactions associated with the smaller-scale perturbations revolving around the larger-scale cyclonic flow. My suspicion is that the models are likely a bit too progressive with the system in general, particularly given the retrograding subtropical ridge in the western Atlantic, which will aid in the amplification of the midlevel East Coast ridging. This is a blocky pattern, and models tend to progress waves too readily in such a regime. A surface low is forecast to progress NNE from the central U.S. into the Great Lakes/southern Canada Wednesday and Thursday. The associated cold front will struggle to move eastward as blocky ridging takes hold. Despite weakening large-scale lift as the surface low and attendant midlevel vort max move well north of the region late this week, frontal ascent in a moistening warm sector should be enough to generate isolated to scattered convection along the front as it progresses into the eastern Ohio Valley and adjacent Appalachians Thursday night. The front makes its run for the CWA Thursday night through Friday night, with the highest chances for precipitation generally north of I-78. There will be sufficient instability for thunderstorms. GFS forecast soundings do hint at a somewhat favorable CAPE- shear parameter space for severe storms on Friday during peak heating, but the displacement from the main vort max seems to be a primary limiting factor. Nevertheless, this will be something to monitor as the end of the week approaches. The main question in the long-term period is what happens to this front over the weekend. The strong subtropical ridge in the western Atlantic and the displacement northward of the vort max into far eastern Canada suggests the front will likely stall somewhere in the Northeast. Unfortunately, this could be relatively close to our region, which sets up this forecast for major potential for error over the weekend. For now, it seems that most of the operational mid-range models place the front north of the area. This would make for a warm and humid weekend as moist southerly/southeasterly flow entrenches itself equatorward of the remnant front. Perturbations in the west-southwesterly midlevel flow would move generally parallel to this boundary, bringing occasional chances for convection near it. Because of the uncertainty in the placement of this boundary, its general proximity to the region, and added uncertainty associated with the timing of these perturbations, felt a broadbrush of slight chance to low-end chance PoPs were justified through the weekend across the area. Kept temperatures close to the previous forecast (i.e., on the warm side) given the higher likelihood of the front not quite making it through the area. Note that this is not an indication that it will storm the whole weekend or even very much of it; instead, it is an indication that there could be some storms at some point this weekend. Importantly, the weekend forecast is low confidence. If the front ends up being closer to or even through the region, temperatures will be considerably colder than forecast. However, the trend has been to position this boundary farther north (i.e., the amplified ridging has been forecast to be stronger) with time, which makes sense given the blocky nature of the pattern, so this seems like a fairly low probability outcome at this point. One other caveat - the closer this boundary is to the area - the better the chances for more precipitation this weekend. By early next week, a deep, negatively-tilted trough approaches the eastern U.S. with an attendant strong cold front moving through the region. Widespread/strong convection will likely develop along the front as it sweeps through the Northeast on Monday (if the timing of the model guidance is to be believed). Pattern recognition suggests the potential for a more pronounced severe threat as this boundary moves through. However, given the large spread in simulations by this point, too early to get into the finer details. && .AVIATION /13Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Widespread IFR conditions in place across the TAF sites for the rest of today, and moderate to heavy rain will spread in from the south going into this afternoon. Local/temporary LIFR restrictions likely in the heaviest rain. E-NE winds 10-15 kt with gusts to 20 kt expected at most terminals today. Stronger winds near the coast will affect ACY where gusts to 30 kt are possible into this afternoon. Steady rain ends from SW to NE between 22-03Z. However, do not anticipate much improvement in the CIGs behind it with low clouds trapped over the region. Expect IFR to LIFR conditions for most of tonight. Some of the guidance is indicating dense fog tonight but this may be a bit overdone. Nonetheless, restrictions from VSBY are also expected. OUTLOOK... Wednesday...Scattered showers, especially in NJ. Residual low clouds/fog possible in the morning, but conditions should primarily be VFR by afternoon. Winds north or northeast 5 to 15 kts. Confidence below average. Wednesday night...Low clouds and fog (MVFR/IFR) again possible with light winds. Confidence below average. Thursday...Winds becoming southeast and increasing to 5 to 15 kts. Potential for some gusts near the coast. Slight chance for thunderstorms late, mainly north/west of KPHL. Generally VFR, except locally sub-VFR conditions near any storms. Confidence average. Thursday night through Saturday...Generally VFR with south/southwest winds 5 to 15 kts. Locally sub-VFR conditions with isolated/scattered storms, especially Thursday night and Friday. Best chances for storms near/north of KPHL. Confidence average. && .MARINE... A Gale Warning remains in effect for the coastal waters and the lower Delaware Bay today. Do not expect a long duration of gales today with the period generally confined to 2-4 hours (longer in the DE coastal waters). It looks to be an increasingly marginal setup the farther north you go along the coast with it being highly uncertain if winds get to gale force in our northern coastal waters of NJ. Seas will quickly build to 7-10 ft today in concert with the surge of gale-force winds. A SCA remains in effect for the upper DE Bay though the ending time of the SCA was pushed ahead to this evening with winds quickly ramping down as the coastal low approaches. A SCA will be needed for the coastal waters for tonight after the GLW expires this evening or is cancelled beforehand. A Marine Dense Fog Advisory may be needed for tonight with a very moist airmass advecting over the relatively cool waters. The potential for dense fog would be favored after the rain ends this evening. OUTLOOK... Wednesday through Friday...Sub-advisory level winds expected. However, seas will likely remain near or above 5 feet through the period. A chance of showers on Wednesday, and a chance of storms late Thursday night through Friday. There may be some restrictions to visibility Wednesday and Wednesday night from patchy fog. Friday night...Sub-advisory conditions expected. A chance of thunderstorms persists, with locally gusty/erratic winds near any storms. Saturday...Some potential for advisory-level winds by afternoon. A low chance of storms. && .TIDES/COASTAL FLOODING... A Coastal Flood Advisory was issued for tonight for coastal zones with minor tidal flooding expected along the DE and NJ shores, in the DE Bay and on the far lower part of the DE river. The minor flooding should also affect the back bays and the estuaries. This tidal flooding episode is a result of the cumulative effects of onshore flow and the approaching new moon. Overwhelming majority of guidance (ESTOFS, ETSS and ensembles from Stevens Institute) continues to predict widespread minor coastal flooding at all of our tidal forecast points this evening along oceanfront and DE Bay. The surge on top of the base astronomical tides is expected to be a foot to a foot and a half at that time. With all that said, we will have to monitor the potential need to upgrade to a warning if ESTOFS verifies, which predicts Cape May, Atlantic City and Lewes will reach moderate flooding. This appears to be an outlier solution with the setup not particularly primed for a moderate coastal flood event. It appears as though the tidal Delaware River above the Commodore Barry Bridge area may just reach the minor flooding threshold. However, the impacts should not be widespread enough there to warrant a Coastal Flood Advisory. We are not anticipating any coastal flooding along the upper eastern shore of Chesapeake Bay. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...Coastal Flood Advisory from 6 PM this evening to midnight EDT tonight for NJZ012>014-020>027. Coastal Flood Advisory from 9 PM this evening to 1 AM EDT Wednesday for NJZ016. DE...Coastal Flood Advisory from 6 PM this evening to midnight EDT tonight for DEZ002>004. Coastal Flood Advisory from 9 PM this evening to 1 AM EDT Wednesday for DEZ001. MD...None. MARINE...Gale Warning until 6 PM EDT this evening for ANZ431-450>455. Small Craft Advisory until 9 PM EDT this evening for ANZ430. && $$ Synopsis...CMS Near Term...Klein/MPS Short Term...Klein Long Term...CMS/Klein Aviation...CMS/Klein/MPS Marine...CMS/Klein/MPS Tides/Coastal Flooding...

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