Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 170620 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 220 AM EDT Tue Apr 17 2018 .SYNOPSIS... Low pressure was located over the Adirondacks this evening. 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 /THROUGH TODAY/... 2 AM...A cooler airmass continues to advect into the region in the wake of yesterday`s system. The low has moved off to the north and east of the area with brisk west winds in its wake with the system`s broad upper level low remaining situated over the Great Lakes extending E/SE into western NY and PA. Short waves pivoting around this feature continue to bring considerable cloud cover along with chances for scattered to isolated showers across parts of the area. For the remainder of the overnight, shortwave moving through may bring a few isolated showers across the southern Delmarva with a few showers as well over the southern Poconos where precip has already mixed with and changed to snow showers in higher elevation areas such as Mount Pocono. Heading into the day Tuesday, as the initial shortwave detailed above moves out, some partial clearing may occur in the morning over the southern half of the forecast area. However the combination of diurnal heating and another shortwave will lead to clouds quickly rebuilding by the late morning into the afternoon with some scattered showers developing. Best chances for showers will be over eastern PA across parts of central and northern NJ with lower chances farther south. Forecast soundings indicate deep mixed layer to at least 800 mb so this will support gusty winds being brought down to the surface with gusts to at least 20 to 25 mph likely. The airmass will be quite cool for this time of year and we lowered high temps with this update. From around the I-95 corridor south and east generally expect highs around 50 with highs in the 40s farther north and west except upper 30s across the southern Poconos. && .SHORT TERM /TONIGHT/... Scattered showers quickly diminish in the evening with the passage of the shortwave and loss of heating. Also, as the center of the upper low pushes slightly to the north and east away from the area this will help lead to a general clearing trend across southern portions of the forecast area. Temps will be chilly...lows generally in the mid 30s from the I-95 corridor S/E with upper 20s to low 30s farther N/W...coldest across the southern Poconos. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... 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 /06Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Through 12z...Mainly VFR with a broken deck of clouds generally 7-10 kft. Winds west near 10 knots with a few gusts to around 20 knots. Tuesday...Mainly VFR with broken deck 4-5 kft. Some scattered showers will be around in the afternoon with the best chances for these affecting any TAF sites being at KRDG, KABE. Winds west 10-15 knots gusting up to 25 knots. Tuesday night...VFR with clouds diminishing south to north. Winds diminish to around 10 knots in the evening. Outlook... 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. && .MARINE... 2AM...SCA remains in effect...west winds will ramp back up again heading into the day Tuesday as the pressure gradient strengthens with SCA conditions expected for all the waters. Seas will come down a bit from current wave heights but will remain elevated at 4 to 6 feet through Tuesday. For Tuesday night, SCA remains in effect through 10z Wednesday as gusts to near 25 knots continue through the night...especially over the ocean waters. Outlook... 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. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ430-431- 450>455. && $$ Synopsis...Klein Near Term...Fitzsimmons Short Term...Fitzsimmons Long Term...CMS/Klein Aviation...Fitzsimmons/Klein/Robertson Marine...Fitzsimmons/Klein/Robertson

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