Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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956 FXUS61 KPHI 181110 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 610 AM EST Sat Nov 18 2017 .SYNOPSIS... High pressure will move off the East Coast this morning while an area of low pressure tracks across the Midwest. The low will move northeastward and strengthen across the Great Lakes region later today and tonight. A strong cold front is expected to move through the Mid-Atlantic region early Sunday morning while the low progresses through the Saint Lawrence Valley on Sunday. High pressure will build to our south Monday into Tuesday, before a cold front moves across the area Tuesday night or Wednesday morning. High pressure builds into the area Wednesday night and Thursday. The high weakens to our south by Friday while another area of low pressure approaches. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Transient midlevel ridge moving over the region at this time will be followed by a series of midlevel perturbations downstream of a potent shortwave trough presently moving out of the Rockies into the central plains. A surface low in eastern Kansas this morning will deepen and move rapidly northeastward into the Great Lakes region by late this afternoon, as the shortwave trough pivots into a more neutral tilt via a strong 500-mb jet streak impinging upon the low- level warm sector in the Ohio/Tennessee Valleys. Downstream difluent/divergent flow will aid in deep large-scale ascent. Widespread precipitation will occur downstream of the surface low and poleward of a developing baroclinic zone stretching from northern Illinois to the northern Mid-Atlantic region. Though the strongest ascent should stay to our west during this period, in closer proximity to the main vort max and associated surface low, the predecessor midlevel perturbations, in conjunction with strong low-level isentropic ascent, will permit widespread showers to progress eastward through Pennsylvania and adjacent states today. High-resolution models vary on the southward extent of the precipitation, with the HRRR sometimes indicating coverage extending through much of Delmarva and sometimes keeping the showers confined north of the Mason-Dixon Line. The drier simulations are corroborated by the 00Z NAM/NAM Nest and the 00Z GFS, whereas the wetter simulations have support from the SREF and the Hi-Res WRF- NMM. Given the discrepancies, used a blend of the guidance, which naturally weighs the drier solutions a little higher (i.e., there are more of them). General timing looks to begin 14Z-17Z, with potential for several rounds through the course of the day, especially in southeast PA. Instability is minimal this far east, with model soundings showing a deep near-surface stable layer and moist-adiabatic profiles above - in other words, no thunder mention in the forecast. QPF through the day may be minimal in the far southern CWA up to a half inch or so in the southern Poconos/Lehigh Valley, with confidence in totals around average. As the surface low to the west deepens and approaches, expect the south/southwest winds to start increasing late in the day, though they should be far from out-of-hand given the fairly stable near- surface layer and the still-modest surface pressure gradient through day`s end. Forecast temps are a blend of ECS/MET/MAV MOS as well as input from HRRR for potential modification from rain. Though agreement is generally excellent among the guidance, expect some unexpected deviations from the hourly/max temperatures owing to impacts from showers. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... The surface low in the Great Lakes region early this evening will continue deepening and moving northeastward through the St. Lawrence Valley as a strong cold front approaches the Eastern Seaboard late tonight. A synoptically-induced low-level jet will be screaming along and just downstream of the cold front. With substantial low- level isentropic lift combined with differential cyclonic vorticity advection downstream of the main vort max, deep large-scale ascent will accompany the front, allowing for a band of widespread showers to develop. General quarter to half inch QPF is expected in the northern part of the area (where lift will be stronger), but most everyone should see at least some precip as the front approaches/moves through the area. Water loading will likely aid in downward transport of the stronger winds aloft, so strong wind gusts may occur in proximity to showers. There are some indications of shallow/thin instability in close proximity to the front, so a rumble of thunder is not impossible. However, the more likely threat is contributions to surface wind gust potential via convectively induced downdrafts. The timing of the frontal band(s) of showers looks to be after midnight, with potential for a relatively dry period earlier in the evening. As the front races eastward, winds will become westerly and northwesterly and rapidly increase as cold air advection commences and strong mixing occurs. The wind threat may be enhanced with the showers but will continue well after the front has passed (see forecast discussion for Sunday below). Temperatures are likely to stay elevated tonight in advance of the front, with little or no decrease in temperatures through the evening/early overnight hours. After frontal passage, the temperature plunge begins. Notably, precipitation should be all rain for the southern Poconos through 6 am Sunday, as temperatures should be just starting to make the drop by this point. && .LONG TERM /SUNDAY THROUGH FRIDAY/... The period starts out with a strong cold frontal passage early Sunday morning. A brisk westerly wind behind the front can be expected on Sunday. The combination of CAA and daytime heating will lead to steepening boundary-layer lapse rate and favorable mixing profiles shortly after sunrise. BUFKIT momentum transfer from NAM/GFS profiles favor frequent wind gusts of at least 40 mph thru about mid afternoon. It appears that gusts across the area will be on the cusp of a Wind Advisory (greater than 45 mph), especially from about mid morning thru midday, if the strongest CAA arrives in concert with the greatest pressure rises/(cold conveyor belt) low- level jet. The models have trended 1-2 mb weaker with the surface low (982-984 mb) as it passes to our north during the morning, yield a Wind Advisory setup that is marginal enough to hold off at this time. Temperatures are forecast to fall during the morning, then struggle to rise much during the afternoon as CAA offsets surface heating. There is a potential for lake-effect snow streamers to reach the higher elevations of northeastern PA (Poconos). It now appears that the bulk of these snow showers should hold off until after sunrise Sunday evening. Although snowfall accumulations are expected to be minor, a brief snow squall could potentially lead to locally hazardous travel in the Poconos if convection can be sustained this far downstream (low confidence at this point). W-NW winds will remain breezy Sunday night with low pressure continuing to strengthen over southeastern Canada and high pressure building to our southwest. A cold night is in store with continued CAA over the Mid Atlantic but the winds should keep the boundary layer well mixed, preventing temperatures from dropping too much overnight. Lows range from the mid 20s in the Poconos to mid 30s in the cities and along the coast. High pressure builds across the Southeast U.S. Monday. Cool temperatures (10 degrees below normal) and dry weather can be expected to start the work week. High pressure moves offshore on Tuesday. Southerly return flow around the high will yield a moderating trend with temperatures returning to near normal by Tuesday afternoon. A cold front will likely move thru the area late Tuesday night or Wednesday morning. The previous forecast was blended with WPC Guidance and low chances for rain was introduced to the forecast as a result. Models generally show an area of low pressure developing along the front which would enhance rainfall. However, with large spread in the models regarding the location and timing of secondary low development, confidence in the forecast is lower than normal for the Tuesday night-Wednesday morning period. Cool and dry weather is anticipated for Thursday and Friday with high pressure build in. Forecast high generally range between 40-45 degrees on Thursday and 45-50 degrees Friday. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Conditions should remain VFR this morning with showers moving in from the west around or after 15Z. CIGs will gradually lower to MVFR by late this afternoon or early this evening; however, sub-VFR conditions cannot be ruled out, at least temporarily, before this time, especially at KRDG/KABE. It is possible KACY/KMIV see little or no rainfall today. Winds should be generally southerly around 10 kts, with potential for gusts up to 20 kts, especially at KMIV/KACY. Confidence in CIGs/VSBYs is somewhat below average with winds well above average. Tonight...Conditions should gradually deteriorate at all sites to sub-VFR (primarily CIGs), with a band of showers likely to move through between 06Z and 12Z. Gusty/erratic winds may occur in proximity to showers. Outside of showers, southwest winds 10 to 15 kts with gusts to 25 kts are possible, with quick switch to west or northwest after cold frontal passage late tonight or early Sunday. Winds will likely become stronger quickly after the front passes. Confidence in overall evolution is above average, but in timing of CIGs/wind switch is below average. Outlook... Sunday...CIGs quickly rise to VFR in most areas Sunday morning although stratocu behind a cold front could keep high-end MVFR CIGs lingering into the late morning in a few spots. Breezy W winds 15-20 kt will gust to around 35 kt (possibly higher gusts to near 40 kt). Moderate confidence on CIGs in the morning; high confidence otherwise. Sunday night through Tuesday...VFR. W-NW winds gusting 15 to 25 kt. High confidence. Monday night...VFR and light winds. High confidence. Tuesday...VFR. W-SW winds gust 15-20 kt during the afternoon. Moderate confidence. Tuesday night and Wednesday...Potential for rain to move in late Tuesday night-Wednesday morning (greatest chance is along the coast). Sub-VFR CIGs possible. Low confidence. && .MARINE... Gale warning has been issued for Delaware Bay starting at 1 am Sunday (with small craft advisory conditions likely beginning today), and a gale warning remains in effect for the Atlantic waters beginning at 1 pm today. Strong south to southwest winds will begin rapidly this afternoon, with occasional gusts to gale force likely over the Atlantic waters. As a cold front approaches late tonight, strong gusts may occur in proximity to showers before a switch to west/northwest winds late tonight (or early Sunday), with frequent gale-force gusts likely thereafter for the entire marine area. Seas in the Atlantic should rapidly build to 5-8 feet by this evening and potentially higher overnight. Showers are also possible today, especially for Delaware Bay and the New Jersey coastal waters. OUTLOOK... Sunday night...Gale Warning was extended into Sunday night for the waters. There may be a brief lull in the winds early Sunday evening before restrengthening a bit late in the evening and/or overnight as the low deepens well to our north and the pressure gradient tightens overhead. Monday through Tuesday night...SCA will be needed, initially for winds and seas on Monday, just for seas Monday night, then for winds and seas on Tuesday. Wednesday...Winds and seas likely to drop below SCA criteria. && .TIDES/COASTAL FLOODING... There is a potential for blow out tides on Sunday, especially in the Delaware Bay and the tidal portion of the Delaware River, where strong W-NW winds will effectively drain water out of the bay. The low tide Sunday afternoon/evening seems to be the most susceptible for low water concerns that could impact navigation in the area. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Warning from 1 PM this afternoon to 5 AM EST Monday for ANZ450>455. Gale Warning from 1 AM Sunday to 5 AM EST Monday for ANZ430- 431. && $$ Synopsis...Klein Near Term...CMS Short Term...CMS Long Term...Klein Aviation...CMS/Klein Marine...CMS/Klein Tides/Coastal Flooding...Klein is the U.S. government's official web portal to all federal, state and local government web resources and services.