Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 231419 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1019 AM EDT Sun Apr 23 2017 .SYNOPSIS... Low pressure will deepen along an stationary front over the Southeast U.S. while high pressure builds into the Mid Atlantic today. Low pressure will reach the Southeast coast Monday and then track slowly up the eastern seaboard Monday through Wednesday. A cold front will approach from the west Thursday before moving through the region Thursday night or Friday. This boundary may return back northward as a warm front early next weekend while high pressure builds to our north. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Not a whole lot to change with the forecast. Weak high pressure will briefly build across the area through today, then offshore later today. Dry air in the low-mid levels is will continue to move southward across much of the area today. Portions of southern New Jersey and central/southern Delmarva may hold on to the mid-level moisture through the day and keep more widespread cloudiness. The rest of the area should only see high level clouds through this afternoon. Only made minor changes to the forecast highs today. Where more sunshine occurs across the northern two thirds of the area, temperatures are expected to warm more than the southern third where more cloud cover will linger into the afternoon. Either way, a nice day is expected. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... The upper low digs farther southeast into Georgia on Sunday night while acquiring a slight negative tilt. An attendant surface low will progress to near the Southeast Coast by 12Z Monday, with rapid downstream ridge amplification continuing in highly difluent flow. With a downstream jet streak off the coast of the Mid-Atlantic, upper-level divergence will generate a region of broad, deep ascent along and east of the Appalachians. Areas of rain/showers will develop and slowly move northeastward, impinging upon the Delmarva Peninsula during the night. The process will be slow given the snail`s pace of the upper low, but current thinking is precipitation should begin moving into the Eastern Shore of Maryland and Delaware near or after midnight. However, there will be a pronounced dry layer in the low and midlevels that will need to become saturated before steadier precipitation begins, so measurable rainfall will likely be slow to materialize during the night. For now, kept PoPs fairly low or unmentionable north of the Mason-Dixon Line with a sharp increase upward to the far southern portions of Delaware and Maryland. QPF during this time should remain around or below a tenth of an inch. Low temperatures will generally be in the 40s, with the lowest readings likely in the north where nocturnal radiational cooling will be most pronounced. && .LONG TERM /MONDAY THROUGH SATURDAY/... A cutoff low will drift eastward over the Southeast states on Monday. A downstream ridge block over the western Atlantic Ocean should steer the low northward up the eastern seaboard through midweek. The predictability challenges that are commonplace with the formation and decay of cutoff lows are at play with this upcoming one. Models show varying solutions regarding how quickly the cutoff low opens up, which will influence the track (both location and timing) of the low as it turns northward up the coast. Although the first half of the week is shaping up to be damp for much of the Mid Atlantic, there are still timing details related to the onset, duration and end of rain that cannot be predicted with very high confidence at this juncture. A subtle trend has been seen in the guidance over the past few days that corresponds to keeping the low cutoff a bit longer on Monday along the Southeast coast. This trend signals a slower northward expansion of the precipitation shield. NE PA and N NJ could potentially remain dry with the surface ridge in place during the day on Monday when locations in SE PA, S NJ and Delmarva see light rain. Rain is currently forecast to be greatest in coverage (60 POP in far NW zones to 90 POP in SE zones) and intensity (0.5-1.0 inch) from overnight Monday night through Tuesday evening, when our region becomes positioned near the nose of a cyclonically-curved low-level jet around the northern side of the low circulation. Assuming the low remains offshore, rainfall rates will be modest since the region would be situated within the stable, cool sector of the system. However, higher rainfall rates and locally moderate to perhaps heavy rainfall would be a possibility if the low winds up tracking inland, allowing a coastal front to progress onshore. High temperatures will be below normal both Monday and Tuesday owing to the persistent onshore flow and overcast skies. However, temperatures north of I-78 may be able to get very close to normal on Monday since these northern zones will likely see a later arrival of the denser cloud shield. We will continue to advertise conditions drying out during the day Wednesday but this is subject to change depending on how quickly the low moving up the eastern seaboard reaches our latitude. There is a slight concern that the low continues to trend slower, extending the rain into Wednesday. Since it doesn`t seem to be the most likely scenario at this point, we will also continue to forecast clearing from SW to NE during the afternoon and temperatures reaching the lower 70s in Delmarva and mid to upper 60s in E PA and NJ. Low pressure is forecast to cut well to our NW across the Great Lakes, placing the Mid Atlantic in the warm sector ahead of a cold front for the end of the week. Subsidence underneath the upper ridge over the area will favor strong daytime heating that should allow high temperatures to rise into the 80s across most of the area each afternoon Thursday and Friday. An onshore component to the flow will yield notably cooler conditions (60s-lower 70s) at the shore. Models have trended later with the arrival of the cold front compared to runs 24 hours ago. Accordingly, a mention of showers and thunderstorms with the front was delayed until Thursday night but PoPs were kept low for Thursday night since timing of convection does not coincide favorably with diurnal cycle. Slight chance PoPs were extended into Friday with the possibility of the cold front still in the process of moving thru. Yet another storm system is forecast to organize over the Great Plains heading into next weekend. Southerly flow ahead of this system may allow the cold front that previously moved through the region to return back northward as a warm front. However, the front may struggle to advance northward with high pressure anchored to our north. Temperatures and our sensible weather for Saturday will greatly depend on the position of the warm front. Highs in the 80s would be possible on Saturday if the front lifts north of the region but temperatures could wind up being 20 degrees or more lower if the warm front owing to onshore flow on the cool side of the boundary. Blended our previous forecast with 00Z MEX, which hedged between both extremes. && .AVIATION /14Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. General VFR conditions through the period. Partial to almost full clearing will occur north of the Mason-Dixon Line with more considerable mid and high cloudiness to the south through the day. CIGs will lower gradually by late afternoon/early evening to the south of KPHL but should remain VFR at the terminals through the night. Winds will veer from northerly to easterly or southeasterly during the day, remaining generally below 10 kts. Rain is expected to progress slowly northward across the Delmarva Peninsula late Sunday night and may begin to affect KILG, KMIV, and KACY by daybreak Monday with conditions deteriorating to sub-VFR once the rain moves in. OUTLOOK... Monday...Rain expands northward across Delmarva, SE PA and S NJ. CIGs likely to lower to MVFR from PHL southward and possibly IFR at ACY/MIV later in the day. E-NE winds 10-15 kt Phila terminals, east and 5-10 kt farther N/W. Monday night through Tuesday night...Periods of rain. Restrictions may start out in MVFR earlier Monday night, but will eventually lower to IFR from S to N later Monday night. IFR will be commonplace Tuesday and Tuesday night. E-NE winds 10-15 kt with higher gusts of 20-25 kt in the coastal plain. Wednesday...IFR at the start will improve to MVFR and then VFR as rain ends from SW to NE. Wednesday night and Thursday...VFR and light winds. && .MARINE... Winds are expected to stay below advisory thresholds through at least this evening and likely for much of the night across the coastal waters of New Jersey and Delaware, with directions switching from northerly to more easterly during the day. However, wind speeds should begin increasing late tonight off the Delaware and southern New Jersey coasts, with advisory-level conditions expected by around daybreak Monday. Seas will generally remain just below advisory criteria through tonight, but are expected to begin rising to 5 feet or higher near daybreak Monday. Seas especially will need to be monitored closely tonight to determine if small craft advisory conditions will be met earlier than expected, as with east/northeast wind regimes, seas are usually higher than model simulations project. OUTLOOK... Monday through Tuesday night...Confidence in both winds and seas (just winds for the DE Bay) meeting criteria high enough to issue a third and fourth period SCA. The SCA will need to be extended into Tuesday and Tuesday night as we get closer. E-NE winds will strengthen over our waters as an area of low pressure slowly moves up the Southeast coast. Expect winds and seas to ramp up to SCA levels rather quickly in our southern coastal waters Monday morning, then farther north along the coast and in the DE Bay Monday afternoon. Both the 00Z NAM and GFS indicate a brief surge in winds during the late afternoon and evening with the potential for gusts near gale force in the coastal waters adjacent to S NJ and DE. Winds may briefly increase close to gale force again 24 hours Tuesday afternoon/early evening. There is currently much higher confidence in SCA than GLW but will still warrants a mention of "gales possible" in the HWO. Seas look to peak Tuesday afternoon and night to 7-9 ft in our coastal zones. Wednesday and Thursday...Winds will diminish but seas may still remain elevated at or above 5 ft, necessitating a SCA. && .TIDES/COASTAL FLOODING... Positive tidal anomalies are expected with persistent onshore flow occuring 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 from 6 AM Monday to 6 AM EDT Tuesday for ANZ452>455. Small Craft Advisory from noon Monday to 6 AM EDT Tuesday for ANZ430-431-450-451. && $$ Synopsis...Klein Near Term...CMS/Robertson Short Term...CMS Long Term...Klein Aviation...CMS/Klein Marine...Klein Tides/Coastal Flooding...

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