Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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346 FXUS61 KPHI 240806 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 406 AM EDT Mon Apr 24 2017 .SYNOPSIS... An area of low pressure will deepen along the Southeast coast through tonight and then move slowly up eastern seaboard Tuesday and Wednesday. A cold front will approach from the west late Thursday into Friday. This boundary may return back northward as a warm front during the weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Water vapor imagery indicates an upper low making very slow progress southeastward into northern Georgia early this morning, with an elongated surface low extending from the SC/GA border SSE into the Atlantic waters east of Florida. The cutoff upper low is now nearly neutrally tilted and is expected to become slightly negatively tilted today as a jet streak rotates around the upper low. Highly difluent flow will develop just downstream in the Coastal Carolinas and southern Mid-Atlantic. In conjunction with right-rear quadrant jet dynamics from a departing New England jet streak, broad/deep ascent will continue/intensify today along much of the Eastern Seaboard. This will foster a gradual intensification of the surface low as it slowly pivots north-northeast along the Southeast Coast. Strong low-level onshore flow will develop north of the low, with isentropic ascent combining with upslope flow to generate widespread rain and embedded convection along and east of the central/southern Appalachians today. Our region will be on the far northern fringe of the stronger ascent today, but the upper low will cease its southeastward push today as it acquires the negative tilt. Subsequent to this, the stronger ascent will begin its approach to the area. This should result in a gradual uptick in precipitation in the Delmarva Peninsula today with precipitation starting out light/spotty owing to relatively weak ascent this far north of the low and the relatively dry low levels that will necessitate moistening/saturation for steadier precipitation to develop. Recent simulations of higher-resolution guidance (notably the HRRR/WRF-ARW) are bringing in precipitation just a little bit faster (18Z-21Z in the Delmarva Peninsula; 21Z-00Z south of I-78/I-195) as a perturbation ejects northward from the upper low this afternoon. However, given my aforementioned concerns with the low-level air being fairly dry, kept PoPs in the slight chance to chance range for much of these areas today, with good chance to likely PoPs confined mostly south of the Mason-Dixon Line from 3 pm to 6 pm as model agreement improves regarding precipitation coverage. Precipitation during this period will be light, but will also act to put a quick cap on temperatures, which should hold fairly steady in the central/southern CWA this afternoon as cloud heights lower and surface diabatic heating is dampened. Highs today are forecast once again to be highest northwest of the Fall Line (mid 60s), where at least partial sunshine will remain this morning before overcast conditions move in, with temperatures in the upper 50s to lower 60s south/east (lowest near the coast). An increasing east/northeast wind should be expected (generally 10 to 20 mph by afternoon - lighter northwest of the Fall Line). && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/... The surface low will make little progress tonight, likely remaining in the vicinity of the Coastal Carolinas as the attendant upper low pivots slowly northeastward to near Myrtle Beach by 12Z Tuesday. Most operational models depict a predecessor vorticity maximum ejecting northward from the upper low tonight, moving through the northern Mid-Atlantic region between 03Z and 09Z. Ascent just downstream (north) of this perturbation will generate steadier/more widespread precipitation across the area this evening and tonight with some potential for a brief drier period as the perturbation departs late tonight. However, accurately timing these perturbations is a challenge, particularly given the already low-predictability pattern in place. Thus, although there is some agreement regarding the timing of the steadier precipitation (generally in the 21Z Monday to 09Z Tuesday time frame), confidence is simply too low to delineate a drier period PoPs-wise. Therefore, generally broadbrushed PoPs during the tonight period, with good chance to likely PoPs across the area through the night - with the caveat that it probably will not rain the entire night in any one location. BUFKIT soundings and convection-allowing models are providing some indication of elevated instability and more convective precipitation near/off the coast tonight. Though much of the ascent in our region tonight looks large-scale (generally stratiform in nature), the thin- CAPE profile from parcels rooted near 900-850 mb would permit the development of scattered convection, which the latest SPC-WRF, WRF- ARW, and HRRR simulations all appear to be depicting this evening. Have included a slight chance of thunder roughly from Stevensville, MD, to Atlantic City for the tonight period. QPF should generally be light -- a tenth to a third of an inch in general, but amounts may be locally higher with any convection that develops in the southern portions of the area. Temperatures are forecast to be rather steady overnight with the expected overcast conditions and precipitation. Steady east/northeast winds should continue to increase as the proximity to the slowly intensifying surface low gradually increases. Winds of 10 to 20 mph are likely, with some gusts to 30 mph or so possible near the coast. && .LONG TERM /TUESDAY THROUGH SUNDAY/... A vertically-stacked low over the eastern Carolinas Tuesday morning is expected to weaken as it track slowly up the Mid-Atlantic coast Tuesday and Wednesday. As discussed yesterday, a westward shift in the low track has been noted recently in the models. There still remains modest spread in the low track among the difference ops models and ensemble guidance. Rain looks to become widespread across the area Tuesday-Tuesday night. QPF amounts vary considerably from model to model, which could be tied to the seemingly minor differences in storm track, storm structure (especially the low- level jet and TROWAL on the eastern and northern side of the cyclone) and mesoscale boundaries (e.g., coastal front). There is a potential for a swath of enhanced rain along and just on the cool side of the coastal front so monitoring the position of this boundary will be important. Some of the wetter solutions (e.g., NAM, CMC) bring the coastal front inland (close to I-95) and show more pronounced lift within the TROWAL and at the nose of the low- level jet, resulting in a local max rainfall band of 2-3". Meanwhile, there is another mode in the precip distribution among the guidance (e.g., GFS, NAM nest, WRF ARW and ECMWF) that indicates the heavier rain confined to coastal areas as the coastal front remains just offshore. The latter camp of guidance generally advertises QPF of 1.0-1.5" within this heavier band, with less amounts farther inland. Forecast reflects a compromise of both scenarios with the heaviest amounts (QPF 1-2") falling over the coastal plain. A slight chance of thunderstorms was introduced to the forecast in Delmarva and S NJ where elevated instability may exist assuming the coastal front advances this far northwest. Temperatures on Tuesday will also be dependent of the position of the coastal front with highs in the 50s (60s) northwest (southeast) of the boundary. Models disagree about what happens with the coastal low by the time it reaches our latitude late Wednesday-Thursday with some solutions curving the storm out to sea, some keep in an a northward track into New England and while others dissipate it just off the northern Mid- Atlantic coast. Either way we should dry out sometime during the day Wednesday and remain precip free Wednesday night and Thursday. Forecast confidence for high temperatures on Wednesday are below normal as it will depend on how quickly skies clear out behind the low. The current thinking is there will be a notable temperature gradient across the area Wednesday afternoon with SW zones (Delmarva/SE PA) having the best opportunity to see sunshine arriving in time for afternoon heating than locations farther N/E. The SREF/CMC matched this idea fairly well with highs in the 70s S/W of Philly and 60s to the N/E. Warmer air aloft arrives on Thursday owing to strengthening subsidence underneath a shortwave ridge. However, the forecast has trended cooler for Thursday with E-SE winds in the boundary layer advecting cooler marine air inland (which is backed compared to yesterday`s runs). Low pressure is expected to pass well to our NW through the Great Lakes and into E Canada late this week. This track will allow the pre-frontal warm sector to further become established Thursday night and Friday. Highs in the 80s are anticipated for Friday, except cooler along the coast due to a sea breeze. Friday also contains a chance for showers and storms with a cold front approaching from the west. The aforementioned front should weaken and stall over the region Friday nigh and into the weekend as it encounters downstream ridge blocking over the western Atlantic. The position of this boundary will greatly influence temperatures and chances for rain this weekend. Models advertise a strong temperature gradient near this boundary with highs in the 80s (maybe even close to 90) in the warm sector south of the front and 60s (maybe even 50s) on the northern side. The forecast reflects the idea that the warm front should be able to lift north of the area on Saturday. By Sunday, the boundary may start to sag southward (as a backdoor cold front) into our northern zones in response to high pressure building southward from Canada. The best chance for showers and storms will reside close the front. With uncertainty in the boundary position, PoPs were kept low for the upcoming weekend (20-30 percent). && .AVIATION /08Z MONDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR conditions are expected at the terminals through at least early afternoon, but CIGs will gradually approach MVFR by late afternoon or early evening from south to north. Rain is expected to move in to KILG/KMIV/KACY around or after 21Z and from KPHL northward between 00Z and 06Z. Conditions will continue to deteriorate through the evening, likely becoming IFR near or after 06Z Tuesday. Light and variable winds will become primarily E or ENE by late this morning, slowly increasing to 10 to 15 kts after 15Z, with potential for occasional gusts to 20 kts or so, especially at KMIV/KACY. OUTLOOK... Tuesday and Tuesday night...High confidence for IFR with periods of rain. E-NE winds 10-15 kt with gusts up to 25 kt, especially near the coast. Wednesday...A gradual improvement from IFR to MVFR from SW to NE is anticipated during the morning. MVFR CIGs may linger thru much of the afternoon, particularly from PHL, N and E. Light winds Wednesday night and Thursday morning...Low clouds may remain trapped beneath a low-level inversion. This could result in continued MVFR CIGs and even expansion of MVFR back farther to the S/W. Lower confidence in IFR but it`s still a possibility, especially late Wednesday night-early Thursday. Thursday afternoon through Friday...Predominately VFR. SEly winds on Thursday become Sly Friday. && .MARINE... East to northeast winds will increase to small craft advisory levels early this morning in the Delaware coastal waters, with these stronger winds pushing northward into Delaware Bay and the New Jersey coastal waters during the afternoon. Winds are expected to be strongest late this afternoon and early this evening, with gusts approaching but likely staying predominantly below gale-force during this time. There may be a subtle decrease in winds late tonight, but conditions should remain above advisory thresholds. Seas will rise above 5 feet today and will continue to build through the night. Rain is expected to progress south to north through the coastal waters late this afternoon through tonight. Some restrictions in visibility are expected, especially tonight. Isolated thunderstorms may occur in the marine zones tonight. OUTLOOK... Tuesday and Tuesday night...SCA was extended to cover this period with high confidence for winds and seas to meet advisory criteria. There is a potential for easterly winds to gust to near gale force briefly during the afternoon. Confidence of gales is not particularly high with the strongest pressure falls confined south of our waters and with stable mixing profiles. Will continue to mention the possibility of gales in the HWO. Seas look to build to 7- 9 ft in our coastal zones. Wednesday through Friday...Winds will become light. However, seas will likely still remain elevated at or above 5 ft through Thursday night or even Friday morning. && .TIDES/COASTAL FLOODING... Positive tidal anomalies are expected with onshore flow persisting through Tuesday night. Ensembles included in the Stevens Flood Advisory System indicate a high probability for minor coastal flooding at our tidal points along the oceanfront and DE Bay with the Tuesday afternoon-evening high tide. Coastal Flood Advisories may eventually be needed as tidal departures of around a foot above astronomical tide would be sufficient to produce minor coastal flooding with the new moon on Wednesday. A few of the outlier ensemble members in the Stevens tidal guidance predict moderate coastal flooding, particularly at Reedy Point and Lewes. However, there is a much higher likelihood that the magnitude of flooding would be limited to minor since onshore flow will not be particularly strong (predominately be below gale force) and the low pressure center will be relatively weak (MSLP above 1000 mb) at the time it moves in close proximity to our region. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ452>455. Small Craft Advisory from noon today to 6 AM EDT Wednesday for ANZ430-431-450-451. && $$ Synopsis...Klein Near Term...CMS Short Term...CMS Long Term...Klein Aviation...CMS/Klein Marine...CMS/Klein Tides/Coastal Flooding...Klein

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