Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 220342 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1142 PM EDT Tue Mar 21 2017 .SYNOPSIS... A cold front will slide through the area tonight. A large area of high pressure then builds in from the northwest and stays over our region through Thursday before shifting off shore. A warm front is forecast to lift through our region on Friday. Low pressure is expected to approach from the west over the weekend. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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Cold front through the northern Great Lakes, and into western NY/PA. Front will work its way through the region during the early morning hours, generally starting around 3am, and working its way through the rest of the CWA through daybreak Wednesday. NW winds will increase behind the frontal boundary, and by daybreak, will range from 10-20 MPH with gusts up to 30 MPH. Will adjust hourly grids based on latest surface obs, but will not make many significant changes, as forecast remains on track.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
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Strong high pressure will build towards the area Wednesday. A strong and gusty NW wind with dry air will be across the area through the day. Skies will be clear and temperatures will be below normal. Highs will only be in the low/mid 40s over Delmarva and the 30s elsewhere. Wind chill values will will only be in the 20s in many areas for the afternoon. Winds will gust 30-40 mph at times.
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&& .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/... The long term will start off tranquil, but get complicated quickly! Thursday...With the center of the high passing almost directly over the region, Thursday should be a quiet and tranquil day albeit colder than normal. Temperatures will generally be 10 to 20 degrees below normal, with some record lows possible Thursday morning (see climate section below). Thursday night into Friday...This is where the forecast starts getting complicated. A warm front is expected to lift through the region during the day on Friday (a bit slower than what most models were depicting yesterday). At the same time, a broad upper level ridge will be approaching from the west. So we could potentially have mesoscale lift (associated with the front) in the lower levels, while some subsidence is possible in the upper levels. At this point, do not expect that as the ridge appears to be trailing far enough behind the front that the two will not be juxtaposed. Further complicating matters is the fact that the boundary layer will be quite dry leading into this event, with dew points likely starting in the teens and lower 20s across much of the region early Friday morning. If we see enough lift out ahead of the front, we could see a brief period of freezing rain (and possibly some sleet) generally north and west of the I95 corridor. Having said that, there have been a few events this winter where models were too fast with warm air advection across the Poconos and NW NJ, so they have the highest risk for multiple hours of freezing/frozen precipitation, but still considerable uncertainty. Saturday...Warm air advection continues. The one possible negating factor could be persistent cloud cover through the day. However, will continue to go on the high side of guidance as most guidance has had a consistent cold bias with southerly flow patterns. Saturday night into Monday...Good agreement that a low pressure system will be sliding northeast out of the southern Plains into the Great Lakes region through this time. That is about the extent of what I have high confidence in. Less confidence, though there appears to be better model agreement today, that the low will be weakening/filling as it propagates northeast. What I have very low confidence in, a front is expected to slide south as a cold front Saturday night, then stall, and lift north as a warm front Sunday night. How far south this front will get before stalling could have a big impact on temperatures (especially with the highs on Sunday) and precip type Sunday night into Monday morning. At this point, most model solutions show the front stalling somewhere between Philly and the Delaware Water Gap. For locations that stay south of the front through the entire event, expect another very warm day on Sunday, and all rain. Locations north of the front can expect temperatures as much as 15 degrees below normal on Sunday, and in the higher terrain, a wintry mix is possible Sunday night into Monday morning. For now have stayed close to a mean of model solutions given the high uncertainty. Tuesday...Another low may develop along what is left of the previously mentioned front. Models depict little upper level support and it should fill and weaken as well as it propagates east. && .AVIATION /04Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. For the 00Z TAFs...VFR through the period. BKN-OVC CIGs will develop/lower to around 10 kft this evening through the early overnight before a strong cold front pushes through the region between 08Z and 12Z, after which skies will rapidly clear. Winds will sharply increase and begin gusting after frontal passage, with directions ranging from 300 to 350 most of the day tomorrow. Speeds of 15-25 kts with gusts to 35 kts are possible after frontal passage through late afternoon Wednesday. Outlook... Thursday...Mostly VFR conditions are expected. Friday...MVFR or lower conditions are possible with precipitation generally along and northwest of the Delaware Valley (including PHL, ILG, PNE, TTN, ABE, and RDG). For ABE, RDG, and TTN, there may be a brief period of a wintry mix Friday morning. Friday night through Saturday...mostly VFR conditions are expected. Saturday night and Sunday...MVFR or lower conditions are possible with rain across the region. && .MARINE... No changes to the marine forecast this evening. Expecting frontal passage in the 4-8 am time frame, with strong/gusty northwest winds picking up immediately after passage. Gale-force gusts expected most of the day Wednesday, especially during the afternoon hours. Previous discussion... Nice conditions on the waters this afternoon will continue through much of the overnight. A sharp cold front will cross the waters well after midnight and cause winds to shift to NW and increase. Gale warnings are in effect for the period after this frontal passage as winds will probably gust 35 to 40 knots at times. We expanded the Gale to cover Delaware Bay and the Delaware coastal waters. Cold air and decent flow aloft should have no problem mixing down. No precipitation is expected tonight or Wednesday. Outlook... Wednesday night...winds and seas will diminish through the overnight hours. Thursday...Once wind gusts drop below 25 kt early in the day, sub-SCA conditions are expected. Friday...winds may approach 25 kt leading to SCA conditions Saturday...sub-SCA conditions expected. Sunday...there is a chance wind gusts could build above 25 kt late in the day. && .FIRE WEATHER... Relative humidity levels will drop behind the front Wednesday. Gusty winds will cause near Red Flag conditions during the afternoon. After calling the fire weather partners, it seems that soil moisture is still rather high in many areas. The Delmarva is the only exception where conditions may be rather marginal for RFW. We will probably issue an SPS overnight to highlight these dry windy conditions. && .CLIMATE... More record events possible this week. One or 2 of the following are vulnerable record equaling lows Thursday morning March 23, primarily ABE. Others are listed for reference ACY-13 1875 PHL-14 1885, 1875 ILG-14 1934 ABE-13 1934 TTN-13 1934 GED-16 1959 RDG-14 1906 MPO- minus 8 1912 && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Warning from 6 AM to 11 PM EDT Wednesday for ANZ430-431- 450>455. && $$ Synopsis...Johnson Near Term...CMS/O`Hara Short Term...O`Hara Long Term...Johnson Aviation...CMS/Johnson/O`Hara Marine...CMS/Johnson/O`Hara Fire Weather... Climate...

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