Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 231411 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1011 AM EDT Fri Mar 23 2018 .SYNOPSIS... Low pressure will linger today in the vicinity of the Canadian Maritimes. Strong high pressure builds to our north over the weekend, then extends southward over our area early next week. Meanwhile, a weak low pressure system slides well to our south Saturday night into Sunday. A cold front is then forecast to arrive toward later next week. && .NEAR TERM /UNTIL 10 PM THIS EVENING/... Mid morning water vapor satellite imagery shows an upper low near Lake Erie-Lake Ontario. This disturbance is forecast to dive southeastward thru the northern Mid-Atlantic region late this afternoon and early evening. There is a very weak surface reflection marked by a surface trough (wind shift) over the eastern Great Lakes-Adirondacks-northern New England. This surface trough will progress southward through eastern PA and NJ this afternoon and evening. Daytime mixing combined combined with lift ahead of the shortwave disturbance will lead to the development of high- based stratocu and increase in mid-level cloud deck from northwest to southeast across the region this afternoon. Hourly sky grids were updated to account for timing. Hourly PoPs were also updated with 20-30 PoPs pushed back about 2-3 hours later compared to previous forecast (matched up better with anticipated timing of lift ahead of the trough and with the latest hi-res CAM reflectivity guidance). 18Z is the first mention of showers along and north of I-80. Convection should be isolated later this afternoon and early this evening (perhaps widely scattered in the southern Poconos and northern NJ). With highs in the 40s across most of the region this afternoon, thermal profiles are warm enough for mainly rain. However, given the significant dry layer beneath the LCL, strong evaporative cooling could allow for temps to briefly drop into the 30s and for some snow to mix in (assuming we have ice crystals). For the higher elevations (above 700 ft) in the NE PA and NW NJ, where temperatures will only be in the 30s this afternoon, snow would be the preferred ptype given that ice crystals will be available for snow growth (it may not be). For these higher elevations, BTV snow squall parameter (per GFS, NAM, RAP) indicates a marginally supportive environment for squalls in the southern Poconos this afternoon, possibly in far NW NJ and Lehigh Valley toward sunset. A deeper look at the forecast soundings, reveals a lack of of robust lift, moisture, and stronger winds in the boundary layer. Accordingly, the setup does not appear to be favorable for heavier snow showers or squalls today. Convection should wane after sunset with the loss of daytime instability but isolated sprinkles/flurries were kept in the forecast after dark this evening for southeastern PA, southern/central NJ, and northern Delmarva as the activity pushes southward. && .SHORT TERM /10 PM THIS EVENING THROUGH 6 AM SATURDAY/... As the mid and upper level low propagates off shore, should see clouds clear out late this evening and overnight. This, combined with relatively light winds, will set the stage for efficient radiative cooling. Therefore, expect temperatures to be 5 to 10 degrees below normal, with lows generally in the 20s across the region. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Summary...Weak low pressure slides well to our south over the weekend with chilly air in place, then warming is forecast to arrive as next week progresses. Synoptic Overview...A highly amplified pattern over the weekend and to start next week, with a closed low in the western Atlantic while a ridge builds into the East. An upper-level trough is forecast to become established from central Canada to the southern Plains next week, and a cutoff low in the western Atlantic may slow the eastward movement of the Plains trough. This should result in a ridge aloft into the East, which probably narrows with time and also surface high pressure that gradually weakens. A cold front is forecast to gradually approach from the west late Thursday. For Saturday and Sunday...A potent upper-level trough, at least the western portion of it, slides across the Northeast and Mid-Atlantic region Saturday night and Sunday. This along with strong high pressure that builds across portions of eastern Canada will steer a weakening surface low well to our south Saturday night into Sunday. While no precipitation is anticipated with the southern system, potent short wave energy moving across the area with some enhanced low/mid level moisture and low/mid level lapse rates steepening during Sunday, may lead to some rain/snow showers. This does not look all that widespread if it were to occur, therefore kept PoPs on the low side across portions of the area. Otherwise, it will be chilly and breezy at times especially Sunday as strong high pressure well to our north has more influence and strengthens a northeast wind. Therefore, much more of a breeze and even cooler conditions along the coast as opposed to farther inland. For Monday and Tuesday...High pressure builds southwestward across much of the East Coast, while a potential storm is located well offshore. This will maintain a northeasterly flow which should weaken during Tuesday. Given some ridging aloft and stronger high pressure building southwestward at the surface, dry conditions are anticipated across the area. Temperatures remain on the chilly side, although some warming is expected on Tuesday as the northeast flow wanes. It will likely still be cooler though at the coast. For Wednesday and Thursday...An upper-level ridge along the East Coast Wednesday is forecast to narrow and weaken with time as a trough amplifies across the Midwest and Plains. The eastward progression of the trough may be slowed due to a cutoff low in the western Atlantic. This will have an impact on the timing of an incoming cold front. As of now, looks like the frontal zone slows Wednesday and especially Thursday as the trough sharpens some more. There should be some waves of low pressure developing along the front as additional short wave energy swings through the base of the amplifying trough and interacts with the baroclinic zone. Given a more southwesterly flow ahead of the main trough aloft and surface cold front, warm air advection should start to boost the temperatures during this time frame. We even have temperatures around 60 degrees forecast for parts of the area Thursday afternoon. The degree of warming will depend on cloud cover and the timing of the cold front and any showers with it. For now given the warming, kept the precipitation type as rain for much of the region along with low PoPs as the front may end up being slower. && .AVIATION /14Z FRIDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Mostly VFR conditions expected. There is a slightly chance of rain/snow showers along and north of a KRDG to KPNE line. However, the chance is too low to include in the TAFs at this time, and even if showers develop near these TAF sites, it is unlikely that conditions will lower below VFR. Northwesterly wind will continue around 10KT, gusts could approach 20 KT between 15 and 21Z, primarily for KPHL, KILG, and KACY. High confidence on the flight category, moderate confidence that the entire period will be dry. Tonight...VFR conditions with only high clouds (at or above 15000 ft AGL). Winds will remain northwesterly but wind speeds should drop below 10 KT. High confidence on all aspects of the forecast in this period. Outlook... Saturday...VFR, with a ceiling between about 3000-5000 feet possible late at night especially for ACY and MIV. Northwest winds around 10 knots, becoming north-northeast at night. Sunday...Mostly VFR. Isolated snow/rain showers possible during the day. Northeast winds 10-18 knots with gusts up to 25 knots, with the highest winds closer to the coast. Monday and Tuesday...VFR overall. Northeast winds around 10 knots with gusts up to 20 knots Monday especially closer to the coast, then becoming light east or southeast during Tuesday. && .MARINE... Winds and seas on the coastal waters have dropped below small craft advisory criteria. Therefore, we have cancelled the remaining SCA. For today and tonight, winds and seas are likely to stay below SCA criteria on all the waters. Gusts near or above 20 KT are expected late this evening and overnight. There is a small chance of gusts at or above 25 KT in that same period on the coastal waters, but chance is too low to issue another SCA at this time. Outlook... Saturday...The conditions are anticipated to be below Small Craft Advisory criteria. Sunday and Monday...Small Craft Advisory conditions are anticipated due to gusty northeast winds (around 30 knots) and building seas. The winds should start diminishing during Monday, however seas look to remain elevated. Tuesday...Winds are expected to be below Small Craft Advisory criteria, however seas on the ocean may remain elevated especially due to a storm well offshore. && .CLIMATE... It has been a snowy March this year, and the four climate sites with snowfall data are now in their top 5 snowiest March`s on record. Snowiest March`s on Record... Philadelphia - Currently tied for second. 1) 17.7" in 1941 2) 15.2" in 2018 2) 15.2" in 1914 4) 13.4" in 1958 5) 12.4" in 1993 Allentown - Currently at fourth. 1) 30.5" in 1958 2) 21.6" in 1993 3) 21.3" in 1960 4) 20.7" in 2018 5) 18.3" in 2017 Wilmington - Currently at second. 1) 20.3" in 1958 2) 14.2" in 2018 3) 13.9" in 1993 4) 13.4" in 1960 5) 13.3" in 1914 Atlantic City - Currently at fifth. 1) 17.6" in 1969 2) 15.6" in 2014 3) 13.4" in 1960 4) 12.7" in 1956 5) 9.6" in 2018 1-Day Snowfall Records in March... Atlantic City, 5.7 inches on 3/21/18 is the 7th highest. Wilmington, 6.7 inches on 3/21/18 is the 6th highest (2-day total of 8.1 inches is tied for the 7th highest). Allentown, 13.2 inches on 3/21/18 is the 4th highest behind 16.7 inches on 3/13/93, 16.5 inches on 3/20/58 and 13.8 inches on 3/3/60. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Gorse Near Term...Klein Short Term...Johnson Long Term...Gorse Aviation...Gorse/Johnson Marine...Gorse/Johnson Climate... is the U.S. government's official web portal to all federal, state and local government web resources and services.