Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 210910 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 510 AM EDT WED SEP 21 2016 .SYNOPSIS... High pressure builds into our area through tonight, while low pressure near the North Carolina coast will weaken as it slowly shifts eastward through Thursday. High pressure will then shift offshore Thursday, and a cold front slides across our area from the north later Friday into early Saturday. High pressure from Canada builds into our region later Saturday through Monday before shifting offshore. A warm front should then lift to our north late Monday with a cold front arriving during Tuesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Early this morning...considerable and mostly high cloudiness with patchy dense fog. dewy. calm wind becomes light north to northeast. low temperatures well above normal. Today...Sunshine through considerable and variably thick high cloudiness. northeast wind 5 to 15 mph. Max temps about 10 degrees above normal...a bit warmer than yesterday. Forecast basis: a 50 50 blend of the 00z/21 gfs/nam mos. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/... Considerable high cloudiness slowly thins southeastward late at night. Patchy fog possible late but with lower dewpoints, the risk of fog appears less likely. lows a few degrees cooler than that of this Wednesday morning but still around 5 degrees above normal. Forecast basis: a 50 50 blend of the 00z/21 gfs/nam mos. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Summary...Warm days into the weekend, then a taste of Fall Sunday and Monday before some warming occurs. Not much chance for additional rainfall through early next week. The synoptic setup is comprised of zonal flow initially Thursday that then amplifies Friday and right into early next week. This occurs as a strong trough migrates across the Rockies Friday and Saturday with downstream ridging, however a trough also amplifies across eastern Canada and New England. This pattern change to a much more amplified one will offer a much cooler airmass into our region, however the details are a little less certain although the overall pattern locally looks to be mostly dry. We used a model blend approach Thursday into Saturday, then blended the 00z WPC Guidance into continuity thereafter. Some adjustments were then made following additional collaboration with our neighboring offices. For Thursday and Friday...An upper-level low near the eastern Carolinas will weaken as an initial zonal flow will start to transition to an amplifying trough across New England. The latter takes place mostly on Friday. This will result in high pressure, centered to our northeast, weakening as it shifts offshore. A generally light northeasterly flow Thursday will keep temperatures a bit cooler near and along the coast, however inland with 850 MB temperatures near +15C afternoon temperatures Thursday are expected to top out in the 80-85 degree range for many areas. As a cold front arrives later Friday, the flow should turn more westerly with slight warming. The timing of the cold front Friday varies some in the guidance, however the main push of cooler air looks to arrive Friday night and especially beyond. There is not much moisture along and ahead of the cold front, and forcing looks to be unorganized and weak. As a result, just carried slight chance POPs across the far north Friday night as weakening showers are forecast to be tracking southeastward. For Saturday and Sunday...A strong upper-level trough slides across the Northeast Saturday with some additional amplification possible Sunday. This will drive a cold front to our south Saturday as surface high pressure, situated north of the Great Lakes to start the day, builds southeastward. Decent cold air advection is forecast to occur under a north-northeast surface wind. While the airmass should be rather dry, there could be stratocumulus initially with the onset of the cold air advection particularly over the warmer ocean waters. There should be a zone of overrunning from near the Ohio Valley westward, however any precipitation induced by this should remain there instead of tracking east with the southbound cold front. Given high pressure building down, we kept a dry forecast going over the weekend. It may turn a bit breezy for a time over the weekend especially along the coast as the high builds down and the pressure gradient tightens some along with cold air advection. Given not much clouds anticipating and some lowering of the wind especially more inland, both nights should be cool to chilly. For Monday and Tuesday...A much more amplified pattern is forecast with a trough in New England and another strong trough moving out of the West and across the Plains. In between, a ridge builds and arrives in our area later Monday into Tuesday. This will drive surface high pressure right over our area Monday before shifting offshore into Tuesday. Some quick return flow may occur later Monday as the high slips to the east. Given some warm air advection, a warm front may lift to our north Monday night. A cold front then moves through during Tuesday, however its strength is less certain as some guidance develops a closed low from the Great Lakes to the Ohio Valley. If this occurs, the front should slow eastward along with better moisture return ahead of it. Given the uncertainty, went with slight chance POPs for many areas Tuesday. Some warming is expected ahead of the aforementioned cold front. && .AVIATION /09Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Early this morning...VFR cirrus but patches of LIFR fog is possible at some of the countryside TAF sites though an increasing northeast wind toward 10z should mix out the lower visibility to vfr. Today after 12z...VFR cirrus. East-northeast around 10 knots, perhaps a wind gust around 15 knots in a couple of spots in the late morning and early afternoon hours. Tonight...VFR with cirrus thinning southeastward late. patchy MVFR fog possible late but less likely than the occurrences of early this morning. light northeast wind. OUTLOOK... Thursday and Friday...VFR overall. Early morning fog is possible at a few of the rural airports. East-northeast winds less than 10 knots, becoming light Thursday night then turning west to northwest during Friday and Friday night. Saturday and Sunday...VFR overall as high pressure gradually arrives from the northwest. North-northeast winds 10-15 knots, especially during the day Sunday. && .MARINE...
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Water temperatures have reverted to significantly above normal. Seas on the Atlantic waters will remain steady around three feet today and rise to 4 feet tonight with a possible need for SCA seas vicinity DE tonight. Wind gusts from 15-20 knots are possible on the ocean but are expected to remain below SCA criteria. OUTLOOK... An east-northeast wind Thursday into Friday should increase some for a time, however it appears that gusts should remain below 25 knots. The seas however especially off the Delaware coast could build to 5 feet for a time. A cold front settles through late Friday into early Saturday with a surge of much cooler air in its wake. This may result in northerly wind gusts nearing 25 knots for a time Saturday and Sunday. RIP CURRENTS... Today...low enhanced risk NNJ and Moderate risk s NJ and DE for the formation of dangerous rip currents, in part dependent on a 1 ft 12 second ESE swell from Tropical Cyclone Karl arriving and in part dependent on the strength of the onshore flow. Water temperatures have reverted to significantly above normal and so while swimming off the NJ and DE beaches these next several warm days will be fun... be aware that rip currents can be dangerous to your health. There were rescues this past weekend and a beach drowning with the cause as yet, unknown. The point: odds favor being safest swimming within sight of a life guard. Thursday and Friday...A generally low or low enhanced risk is forecast. Weekend...There is a pretty good chance of a moderate risk for the formation of dangerous rip currents on at least one of the weekend days as a 4 to 5 foot ESE swell from Tropical Cyclone Karl arrives along with a gusty northeast wind.
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&& .CLIMATE... September is well on its way to a top 10 warmest September through most of our forecast area, and...for the 3rd consecutive month! So while there may be a 2 or at most, 3 day string of below normal temperatures between Sunday and Tuesday, that is not likely to be enough to prevent a third consecutive top ten warmest month for much of the NWS Mount Holly forecast area. In this case a probable second consecutive top 5 warmest month in the period of record for PHL and ABE and possibly ACY too. We`ve run the actual numbers through the 20th, our forecast from today through the 28th and then the GFS2m max/min as seen on the FTPRHA. The following are the predictive sampling of reliable long term climate sites. Philadelphia: Projects a 74 degree average or about 5 degrees above the monthly normal of 69.1. This will probably be a #4 or #5 warmest September on record for Philadelphia. Philadelphia September average temperature rankings 75.4 -1881 74.5 -2015 74.1 -1931 74.1 -1930 74.0 -2016 #4 73.8 -2005 72.9 -2010 72.9 -1921 72.4 -1900 Philly ranked #7 warmest July followed by a warmest ever August (in the POR dating back to 1874). Atlantic City: where there can be greater variability due to the sea breeze cooling during the afternoon and the radiative cooling at night, is still projecting around a 71.5 degree monthly average temperature, or 4+ degrees above the monthly normal of 67.2. Odds are locking into a top 4 to 8 warmest September in the period of record dating back to 1874. Atlantic City recorded a #7 warmest July and warmest ever August. 73.3 - 1961 72.8 - 1881 72.3 - 1931 and 1930 71.7 - 1921 71.6 - 2015 71.5 - 2016 and 1933 71.3 - 2005 Allentown: projects to a monthly average of near 69.9 degrees, or almost 6 degrees above the monthly normal of 63.9 and a highly probable 3rd to 5th warmest September on record. Allentown ranked 8th warmest July and #2 warmest August. 70.8 - 1961 70.3 - 1980 69.9 - 2016 69.7 - 2015 69.4 - 1931 Allentown and Philadelphia have so far recorded only 1 day below normal through the first 20 days of September, TTN only 2 days below normal and RDG 3 days below normal. Rainfall: The rains of earlier this week were welcome. Still only ACY and GED of our long term climate sites are above the monthly normal. Multiple weather forecast models indicate little or no rain here for at least the next 7 days. The only good news, is that evaporative rates will be a little smaller due to the climatology of lower temperatures and shortening days. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Gorse Near Term...Drag Short Term...Drag Long Term...Gorse Aviation...Drag/Gorse Marine...Drag/Gorse 510a Climate... is the U.S. government's official web portal to all federal, state and local government web resources and services.