Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 220128 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 928 PM EDT Sun May 21 2017 .SYNOPSIS... Low pressure tracks across southeastern Canada tonight through Monday. An associated cold front will move through our area later Monday, then stall just offshore through Tuesday. An area of low pressure moves across our area Wednesday morning, followed by a cold front late Wednesday night and Thursday morning. High pressure will build in from the southwest late Friday into Saturday, then it shifts offshore into Sunday. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... 800 pm update: Model agreement remains below desirable levels this close to the event. The remaining discrepancies involve three main forecast concerns: one in the near term and two in the short term. The near-term forecast issue is onset of precipitation. The latest runs of the HRRR have sped up onset somewhat and are in considerably better agreement with other convection- allowing/mesoscale guidance (e.g., the WRF- ARW, RAP13, RGEM, etc.). Nevertheless, the midlevel perturbation associated with this initial batch of showery precipitation is somewhat on the weak side, which may explain some of the remaining variability among the model simulations. The 18Z NAM, e.g., remains on the slower side with the perturbation, though it is notably stronger with this first perturbation than most higher-resolution guidance, at least in terms of precipitation- producing lift. Given the remaining discrepancies in onset and rate of speed in which the precipitation spreads eastward or northeastward, reduced PoPs during the 03Z-06Z time frame but kept them similar after 08Z, when most models agree that sufficient lift will be present. However, one caveat to this is that there appears to be two periods of steadier precipitation with this event. The first is with the aforementioned perturbation tonight. The second is with a cold front moving through the region tomorrow and a frontal wave that moves rapidly northeastward through the Mid-Atlantic. The insinuation here is that there may be a period of relatively little or more scattered/showery precipitation in between the two sources. Previous discussion... Rest of this afternoon...Clouds generally increase and thicken. Max temps generally 1 to 4 degrees below normal except 5 to 10 below normal far s NJ and the DELMARVA. East southeast winds continue gusting 15-20 mph. Tonight...models have been differing timing of onset of rain tonight. Am still unsure as we go to press, but since the HRRR is still very very late tonight, have followed. no certainty of .01 in ne pa so kept pops generally as previous but arrived at those pops a little slower. Looks like with the PWAT advection that the heavier showers may be occuring the Delmarva prior to sunrise. 09z/21 SREF modeled PWAT increases to 1.7" by morning Delmarva. Thunder remains in the fcst but low confidence of occurrence on the Delmarva and i could more easily easily see it not occur. Forecast basis was a 50 50 blend of the 12z/ 21 GFS/NAM MOS. LOWS normal to 5 degrees above. && .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/... 800 pm update: As mentioned in the near-term discussion, two forecast concerns remain for the short-term period, largely based on continuing model disagreement. The first concern is QPF. A review of the 12Z and 18Z simulations of the NAM and GFS, the latest higher-resolution guidance, and comparison to 12Z non-NCEP models continues to paint a rather unclear picture of how much precipitation falls with this event. The 18Z NAM looks awfully dry northwest of the NJ coastal plain. The latest simulations of the RAP are also quite dry. However, the HRRR and to lesser degrees the 12Z ECMWF, WRF-ARW, and WRF-NMM are more suggestive of heavier precipitation in these areas. With the 18Z GFS somewhat on the drier side northwest of I-95, as well as strong suggestion of a sharp gradient between heavy precipitation and very low totals in much of the higher-resolution guidance, I was inclined to reduce QPF northwest of the urban corridor. I also narrowed the axis of higher QPF a tad, based on stronger agreement regarding locations of maximum QPF. However, confidence in magnitude of QPF is very low. It is certainly possible my modifications produce storm totals that are too low south/east of I-95. Expect more fine-tuning through the overnight as the latest guidance comes in. For now, totals are generally 0.25-0.75 inches northwest of the urban corridor, 0.75-1.25 inches in the urban corridor, and 1-1.5 inches southeast of I-295 (with locally higher amounts almost certain to occur). The second concern is presence of any elevated instability. Latest model soundings continue to suggest meager parcel buoyancy during the event, but it will not take much to generate fairly deep lift, especially given the deep, moist profiles expected. With PWs around 1.75 inches by Monday afternoon, any convection will likely produce locally heavy rain rates. For now, kept thunder confined to coastal NJ and Delmarva for Monday, but this remains low confidence. If buoyancy appears to be stronger than progged, fast rain rates are probable during portions of the event, which would lead to somewhat greater hydro concerns. Main changes to the grids (besides to QPF) were lingering higher PoPs longer in the daytime period, given somewhat slower consensus of precipitation timing and increased chances of instability showers subsequent to the main precipitation shield moving through during the morning. Previous discussion... Monday...A wet morning-early afternoon is in store for the region as a cold front approaches and moves into our area during the afternoon. A little thunder maintained far south portion of the forecast area but not probable and so a low confidence part of the fcst. Model consensus is for the heavier rain to occur southeast of I95, near the path of any warm frontal wave of low pressure and closer to the better instability. PWAT 1.5 to 1.75 inches so a period or two of heavier showers and associated brief poor drainage flooding possible. This rainfall should lower flood guidance thresholds for mid week events. The DELMARVA and southern NJ should be able to handle 2 inches of rain in 6 hours Monday morning. Am expecting that nuisance poor drainage flood statements that impact travel will need to be issued sometime between 10z and 20z se of I-95. The afternoon may see very little rain along and west of I-95 but showers could break out again late in the day if it warms enough. Fog should develop in the light and moistened boundary flow off the ocean late in the day along the Atlantic coasts. Forecast basis was a 50 50 blend of the 12z/21 GFS/NAM MOS. These high temps may be several degrees too warm depending on on the persistence of showers through the afternoon. If it quits sooner, it would warm to our fcst values. For now the 330 Pm fcst temps indicate about 5 to 8 degrees below normal, 10 below near and north of I80 and only 2 below normal southern DE. Uncertainty regarding temps and how persistent the afternoon rain. && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/... Summary...Times of wet weather, however warmer temperatures look to occur especially later this week and next weekend. Synoptic Setup...An amplifying upper-level trough is forecast to gradually shift eastward from the Midwest and Plains Tuesday night and Wednesday. This should move out of the East Friday as some ridging arrives and establishes high pressure off the East Coast during next weekend. At the surface, an area of potentially disorganized low pressure looks to move through Tuesday night and Wednesday. If short wave energy can consolidate more ahead of the aforementioned trough, then surface low pressure would be able to strengthen and organize. While the pattern for awhile looks unsettled, there is less certainty with the details especially for the Tuesday through Wednesday time frame. We used a model/continuity blend for Monday night through Tuesday night, then blended in the 12z WPC Guidance thereafter. Some adjustments were then made following additional collaboration with our neighboring offices. For Monday night...An upper-level trough/closed low should be moving well to our north and taking surface low pressure with it. An associated cold front will slide across our area but it should then stall just offshore. The associated warm advection and related isentropic lift is forecast to be shifting north and east of our area during the evening. This will result in any showers diminishing and shifting offshore. A weak surface high then builds in overnight and there might be just enough drying and subsidence for a time to erode the lower cloud cover. If the clouds thin out enough, fog may develop late especially across northern New Jersey and eastern Pennsylvania as these locations will be more removed from a developing northeast low-level wind. The highest PoPs were kept during the evening hours. For Tuesday and Wednesday...An upper-level trough is forecast to migrate eastward from the Midwest and Mississippi Valley. This will allow for some downstream riding, however the model guidance shows several embedded short waves within the southwesterly flow aloft. These should result in surface low development into Mid Atlantic region, although the parent surface low is currently forecast to track into the Great Lakes Wednesday night. The initial surface low should develop on the stalled frontal boundary, however timing is less certain as some guidance is faster with this feature. It appears the main forcing may arrive Tuesday afternoon and night, then some drying tries to work in during Wednesday. While there is less certainty with the details, went ahead and made some adjustments to the PoPs with the highest values mainly from about I-95 on south and east later Tuesday into early Wednesday. For Thursday and Friday...As an upper-level trough shifts eastward from the Great Lakes and Ohio Valley Thursday, surface low pressure is forecast to move from the eastern Great Lakes toward northern New England. Its trailing cold front should cross our area mostly Thursday morning, however there may be a weak surface low near the triple point as it crosses our region. Some showers are expected with this system Thursday and enough instability should be realized to generate some thunderstorms and therefore a chance of thunder is included for much of the area mainly Thursday afternoon/early evening. As the trough begins to lift out Friday, a weak cold front or surface trough moves through. This in combination with cyclonic flow and some instability may generate some showers especially across the western/northern zones. The PoPs were kept on the lower side at this time. A westerly wind will increase Friday as low pressure strengthens across New England. There does not appear to be much cooling though in the wake of this system. For Saturday and Sunday...Some guidance is slower in removing the upper-level trough from the Northeast Saturday, however overall a narrow ridge should arrive as the next trough moves across the Plains and Midwest. The arrival of the ridge allows for surface high pressure to build in from the southwest before shifting offshore into Sunday. There may be a warm front lifting to our north Saturday night into early Sunday, however the opportunity for convection with it is less certain this far out especially given the main energy is currently forecast to be over the Great Lakes to the Central Plains. As of now, included a slight chance PoP for most areas Sunday and Sunday night. && .AVIATION /02Z MONDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. For the 00Z TAFs...BKN CIGs around 5-7 kft early this evening will gradually lower to MVFR and potentially IFR by daybreak as showers move into the region. Heavier showers look to move through the terminals from 12Z to 18Z, but chances for rain exist before and after this time frame. IFR should settle in during the daytime hours and remain for the rest of the TAF period. Winds generally at or below 10 kts from the S or SE. Small chance of thunder, but probabilities are too low for TAF mention at present. OUTLOOK... Monday night...IFR/MVFR conditions should improve to VFR from west to east in the evening. Showers end in the evening, then local fog is possible. Light south to southwest winds, becoming light north-northwest or light and variable. Tuesday...VFR ceilings lower to MVFR or IFR as showers develop, especially from about PHL south and east. Lower confidence on the timing. Northeast or east winds mostly 10 knots or less. Wednesday...Times of MVFR/IFR conditions along with some showers. A few thunderstorms are possible late in the day or evening. East- southeast winds 5-15 knots. Thursday and Friday...Some showers and thunderstorms around Thursday resulting in times of MVFR/IFR conditions, improving Thursday night or Friday. South to southwest winds Thursday, becoming westerly and potentially gusty on Friday. && .MARINE... 930 pm update: Small craft advisory for the Delaware coastal waters has been cancelled. Seas at buoy 44009 have remained below 5 feet all evening, and winds are well below thresholds. Though seas will be close to the 5-ft threshold for much of the night, do not think there is much justification for keeping an advisory at this time. Other main change was to add a slight chance of thunderstorms to all of the marine zones tomorrow. No change to the rip current forecast from the discussion earlier this afternoon. Previous discussion... Probably extending the SCA hazard grids for the Atlc DE waters through midnight and then it may need a further extension thereafter. Its a marginal SCA. Elsewhere, isolated southeast wind gusts to 25 kt late this afternoon should subside this evening. Southeast wind tomorrow but wind and seas just below SCA criteria. OUTLOOK... Monday night and Tuesday...Fog should occur for a time, especially on the ocean zones, Monday night as dew points around 60 degrees move over the cooler waters. This may dissipate later at night though as a northeast wind and clouds increase. Winds may gust to near 25 knots late Tuesday across the southern zones with seas building to around 5 feet. There is lower confidence with the details given timing differences with low pressure along a stalled front. Wednesday through Friday...The winds should be below Small Craft Advisory criteria, however seas may reach or just exceed 5 feet at times on the ocean zones. Rip Currents: Onshore wind and continuing easterly swell projects a moderate risk for the formation of dangerous rip currents this afternoon/evening. Water temps this Sunday afternoon were variable along the coast...upper 50s to mid 60s. Tomorrow tentatively: Please review the 830 PM update for tomorrows forecast. We`re looking at low risk with a slightly smaller primary easterly swell and a little less onshore wind. Its close to moderate in a few spots. The weather Monday should be pretty shabby by midday as showers low clouds develop with fog later in the afternoon on the beaches. The safety message: during this pre Memorial Day week... if you`re a weak swimmer, swim with a strong swimmer and a flotation device nearby. Do not swim near piers and jetties where any rip currents tend to be stronger. Respect the power of water, and do not overestimate your swimming ability. Ocean surf swimming is quite different than swimming in the pool or lake. When departing the surf aware of incoming waves. Sometimes a much larger than expected wave can knock down a surf zone swimmer/walker, especially if your back is turned away from the ocean. The wave knock down-face plant can cause disabling upper extremity injury. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Gorse Near Term...CMS/Drag Short Term...CMS/Drag Long Term...Gorse Aviation...CMS/Drag/Gorse Marine...CMS/Drag/Gorse is the U.S. government's official web portal to all federal, state and local government web resources and services.