Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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205 FXUS61 KPHI 162309 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 709 PM EDT Mon Apr 16 2018 .SYNOPSIS... Low pressure was located over northeastern Pennsylvania this afternoon. A cold front will move off the Mid-Atlantic coast this evening as the low lifts into upstate New York. The low will stall over southern Quebec late tonight through Tuesday night. Another area of low pressure organizing over the Midwest will track eastward into the Northeast U.S. Wednesday night and Thursday. Meanwhile, a warm front with this system will try to move northward into our region before the trailing cold front moves through late Thursday. High pressure gradually builds into the region late this week and into the weekend. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... 600 pm update: Lowered temperatures and dew points considerably across the area as colder air has filtered in behind the cold front (along with a shift to westerly winds). Used a combination of statistical/hi-res 2-m temp/dew guidance for the rest of the night, as the hi-res guidance seemed to have a somewhat better handle regarding the expected nocturnal cooling evolution. Current PoPs/Wx grids look reasonable at this time, so made no changes to these. Previous discussion... An area of low pressure moving across northeastern Pennsylvania will continue lifting northward tonight, before combining with another low pressure moving out of the Great Lakes region well to our north. There will continue to be some scattered showers on the backside of this low into the late afternoon and evening hours as the mid-level trough and associated short waves/vorticity impulses move into the area. The showers will not occur everywhere. However, most shower activity will dissipate later this evening as the sun sets. The two areas that have the better chance of showers this evening will be northeastern Pennsylvania and northern New Jersey which will be closer to the departing low, as well as southern portions of Delmarva which will be closer to the base of the short wave/vorticity impulse. Temperatures will be cooling off through the night, so if any showers to continue across the higher elevations, some snow could mix in at times later this evening and overnight. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/... The aforementioned low to our north will continue to spin across southeastern Canada through the day Tuesday, which will keep our area under west to northwest flow. With this northwest flow, we expect cloud cover to remain across much of the area through much of the day. The weather should start out dry, and likely remain dry for most places. However, as the day progresses, low-mid level lapse rates increase, while there remains some enhanced low-mid level moisture and a short wave/vorticity impulse moves across the area as well. This is forecast to lead to a chance of showers by the afternoon. Temperatures are forecast to warm to make all precipitation rain, except possibly the highest elevations across the Poconos where some snow showers may occur. Otherwise, a partly to mostly cloudy day, with winds becoming gusty later in the morning into the afternoon can be expected. && .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/... Tuesday night...A upper low will remain cutoff over the Saint Lawrence Valley with the northern Mid-Atlantic region positioned near the base of the trough. Westerly flow will maintain an seasonably cold airmass across our region. Isolated rain/snow showers (snow for the higher elevations) possible during the evening with the passage of an upper shortwave disturbance, especially along and north of I-80. Temperatures will generally fall into the mid 30s overnight (slightly cooler in the southern Poconos and warmer in the cities and near the coast). Do not anticipate the need for Frost/Freeze headlines at the moment with lows forecast to be just above freezing in areas where the growing season has begun in SE PA, C/S NJ and Delmarva. Additionally, a 5-15 mph westerly wind overnight will not be favorable for frost formation. Wednesday...The cutoff low to our north will finally weaken and eject downstream as a kicker system digs around the backside of the trough. This upstream low will track eastward across the Midwest on Wednesday. Strengthening southwesterly flow ahead of the low will help advect draw a warm front northeastward toward our region during the afternoon. There is still considerable uncertainty regarding if the warm front makes it into our region. Given a lack of thick stratus and some type of CAD wedge north of the boundary, the forecast reflects the warm front reaching the Delmarva. Highs will likely be in the mid 60s south of the front and only low to mid 50s to the north. Wednesday night and Thursday...Low pressure tracks eastward into the Northeast states. Models are still in disagreement regarding the track of the low with one camp of solutions favoring a track to our north across upstate NY and southern New England while others take the low right across the middle of the forecast area. Temperatures will depend on the exact track of the low with temperatures generally in the 50s north of the low and attendant warm front and 60s to the south. Regardless, temperatures should start to fall during the afternoon on Thursday with strong occurring CAA behind a cold front. PoPs were limited to likely north of I-80, to slight chance in the Delaware Bay region and to chance everywhere in between due to uncertainty in the low track. Thursday night...Low pressure is expected to move offshore. An isolated shower or high-elevation snow shower is possible during the evening. Northwesterly flow will advect colder/drier air into the region, aiding the keeping the boundary layer mixed during the night. Breezy winds gusting to 25-35 mph are expected. Forecast lows range from around freezing at Mount Pocono to around 40 degrees in the I-95 corridor and coastal plain. Friday: Surface low will lift into the Canadian Maritimes, but the cold/deep trough aloft will remain over the Northeast. Additionally, with a surface high building into the Midwest/Ohio Valley during the day, the surface pressure gradient across the Mid-Atlantic should remain elevated, allowing for breezy northwest winds to continue through the day. Kept highs well below seasonal averages, and increased winds/gusts given the expected surface/upper-level pattern. Finally, there are some indications that weak perturbations in the northwest flow will move through the northern Mid-Atlantic and Northeast, and this may be enough to generate some sprinkles or light showers in eastern PA and northern NJ. Added some slight- chance PoPs to the grids for this period. Friday night and Saturday: CMC/ECMWF seem to indicate that the trough axis will move to the east of the region during this period, but the 12Z GFS indicates an upstream perturbation that is considerably stronger and slower. So far, this solution looks somewhat anomalous, so I was not inclined to be more pessimistic in the northern CWA on Saturday (when the GFS-simulated vort max glances the area). However, will keep an eye on this, as it may require another day of slight-chance PoPs for light showers. With northwest midlevel flow remaining (and resultant relatively low heights), Friday night and Saturday will likely remain below seasonal averages temp-wise. Winds will be diminishing Friday night, so expect temperatures to be a little bit colder than the previous night, but Saturday may be a little warmer than Friday as the trough is anticipated to begin its move out of the area. Saturday night through Sunday night: Models show another weak wave in the northwest flow moving through the region during this period, but are in quite a bit of disagreement regarding the precipitation this perturbation generates. ECMWF appears to be the most aggressive, but the GFS is shunted well south (in closer proximity to an approaching upper-level low). The CMC generates little precipitation with the northwest-flow vort max, suggesting a more moisture-starved system. That seems a little suspect given the fairly potent look to the vort max and the implications on synoptic- scale lift. Do think the best chances for showers are near/south of the Mason-Dixon Line, where slight-chance to chance PoPs reside Sunday and Sunday night. Expecting a slight warmup during this period given the development of light return flow and subtle warm advection during this period; however, kept max temperatures on the low side of consensus given potential for increased cloud cover and precipitation. Monday: Models begin to diverge with the upper low moving into the Southeast, but all seem to indicate the Mid-Atlantic will be on the upstream side of a surface high. Even so, I am not sold on the dry look of the models with the approach of a system into the eastern U.S., particularly given the large discrepancies in the simulations. Kept some mention of slight-chance PoPs this period (though my suspicion is that Monday will be dry). Whatever pattern ends up occurring, it seems that heights/thicknesses will be somewhat higher than previous days, so nudged temperatures a little bit upward from Sunday. && .AVIATION /23Z MONDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...Becoming VFR, though residual MVFR conditions may continue through about 02Z, especially north and west of PHL. West winds diminishing to around 10 kts after 02Z. Moderate confidence. Tuesday...Predominantly VFR, though BKN-OVC CIGs between 3500 and 6000 feet will likely develop during the day, perhaps nearing MVFR thresholds at RDG/ABE. Spotty showers will develop, with coverage likely a little bit greater near RDG/ABE/TTN. Elsewhere, confidence is not high enough for TAF inclusion. West winds 10 to 20 kts with gusts to 25 kts or so. Moderate confidence. Outlook... Tuesday night and Wednesday...VFR. W winds 5-15 kt gusting 20-25 kt. Forecast confidence: High Wednesday night and Thursday...There is high uncertainty regarding how far north the warm front makes it which is important for aviation as the greater chance for MVFR or even briefly IFR conditions and rain will reside north of the boundary. Forecast confidence: Low Thursday night...Improving to VFR everywhere. NW winds will become gusty. Forecast confidence: High Friday: Predominantly VFR, though there may be some sprinkles or light showers northwest of PHL. Northwest winds 10 to 15 kts with gusts to 25 kts. Moderate confidence. Friday night and Saturday: VFR with northwest winds diminishing. Moderate confidence.
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&& .MARINE...
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700 pm update: Gale warning was allowed to expire for the northern NJ coastal waters, as stronger gusts via cold air advection have not materialized thus far. A small craft advisory has been issued in its place through Tuesday night. Previous discussion... We have cancelled the Gale Warning for the southern New Jersey and Delaware Atlantic coastal waters as well as Delaware Bay, and replaced them with a Small Craft Advisory. There will likely be a lull in winds later this evening, but the winds are expected to increase again later in the overnight and continue through the day Tuesday. Even without the wind gusts, seas will remain elevated on the ocean through tonight and into Tuesday. Outlook... Tuesday night...SCA was issued for the DE Bay and the S NJ and DE coastal waters with W winds 15-20 kt gusting 25-30 kt. This SCA will eventually need to be expanded up the coast to include the rest of the NJ coastal waters once the current Gale Warning expires. Wednesday and Wednesday night...SCA conditions may linger into the first part of the morning with sub-SCA conditions to follow. Thursday and Thursday night...SCA conditions likely, first with SWly flow ahead of a cold front on Thursday, then with W-NW winds behind the fropa Thursday night. There is a brief potential for gales immediately behind the front Thursday night. Friday: Advisory-level northwest winds are likely, but conditions should be dry. Friday night and Saturday: Sub-advisory winds/seas expected.
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&& .TIDES/COASTAL FLOODING... 600 pm update: Allowed coastal flood warning for the tidal Delaware River to expire, as gauges were receding as expected. However, residual closures from earlier flooding will likely continue for the next several hours. Additionally, issued a coastal flood advisory for Middlesex, Monmouth, and Ocean Counties based on latest guidance showing another round of coastal flooding with this evening`s high tide. From observations to the south (where winds have switched to westerly), the surge has not lowered appreciably, and this suggests the threat of flooding is high enough for advisory issuance. Meanwhile, will sift through guidance again later this evening to determine if the tidal Delaware River will require another advisory with tomorrow morning`s high tide. Current thinking is that this will not be necessary, but models have done quite poorly with the event so far, so I am far from convinced. 245 PM Update...Upgraded the tidal portion of the Delaware River (including Philadelphia and Trenton) to a Coastal Flood Warning as positive tidal departures that were initially in the 3-4 ft range this morning have been slow to decrease this afternoon likely due to a slight delay in the cold fropa and associated wind shift from southerly to westerly. The Philadelphia tidal gauge had already reached moderate criteria just after 2 PM. High tide at Philadelphia will occur in the next few minutes but will take about 60-80 minutes later to occur farther upstream toward Trenton. 130 PM Update: Coastal Flood Warnings have been allowed to expire for coastal NJ and DE. Most tidal gauges captured minor tidal flooding. However, the severity of flooding was exacerbated by freshwater runoff from the heavy rain that occurred with the morning high tide. A Coastal Flood Advisory was issued for the eastern and northeastern shore of the Maryland Chesapeake Bay for this afternoon and evening. Positive tidal anomalies about both Tolchester and Cambridge tidal gauges have been increasing steadily over the past few hours and has not been particularly well forecast by models. Expect these surge values to remain steady or even start to decrease heading toward the next high tide (just after 5 PM at Cambridge and just after 8 PM at Tolchester Beach) in conjunction with a wind shift from Sly to Wly. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...Coastal Flood Advisory until 2 AM EDT Tuesday for NJZ012>014- 020-026. DE...None. MD...Coastal Flood Advisory until 11 PM EDT this evening for MDZ008- 012. Coastal Flood Advisory until 9 PM EDT this evening for MDZ015- 019-020. MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ430-431- 450>455.
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&& $$ Synopsis...Klein Near Term...CMS/Robertson Short Term...Robertson Long Term...CMS/Klein Aviation...CMS/Klein/Robertson Marine...CMS/Klein/Robertson Tides/Coastal Flooding...CMS/Klein/Miketta

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