Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 182241 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 641 PM EDT Wed Oct 18 2017 .SYNOPSIS... High pressure remains in control of the weather for much of the rest of the week into the weekend. A dry frontal boundary may move into the area Thursday into Friday. A front and low pressure moving along it will bring unsettled conditions for later Monday into Wednesday. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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630 PM ESTF: Minor changes to the evening temps and dewpoints, otrw no changes. High pressure remains over the mid-Atlantic region. However, this high will undergo gradual weakening tonight as an upstream shortwave trough approaches from the Ohio Valley. This upper shortwave trough will be associated with increasing high clouds responsible prior to its arrival. Do not anticipate these clouds to have a significant impact on radiational cooling given the high base of the cirrus. Accordingly, temperatures should drop rather quickly this evening once the boundary layer decouples and winds become calm. Compared to recent nights, higher dewpoints tonight will (1) raise the floor for how low temperatures can bottom out, (2) inhibit frost formation and (3) increase the risk for radiational fog. At this point, am skeptical to think fog will be much more than patchy in coverage and shallow in depth (mainly ground fog). Forecast lows range from the upper 30s/lower 40s in the sheltered valleys of NE PA/NW NJ and the NJ Pine Barrens to the lower 50s for the urban I-95 cities and along the coast.
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/... High pressure sinks southward toward the Southeast U.S. on Thursday. Modest pressure (height) falls at the surface (aloft) occurs across the northern mid-Atlantic region as a weak upper shortwave trough and surface cold front progress eastward. The aforementioned cold front will trail the upper trough enough to remain upstream of the forecast area through the period. The setup will allow us to remain situated in the pre-frontal warm sector and for this warming trend to continue into tomorrow. Sided closer to the warmer MAV guidance for temperatures, yielding forecast highs in the lower to middle 70s (except cooler in the higher elevations of NE PA-NW NJ). Filtered sunshine with high clouds during the morning will give way to sunny skies by the afternoon. Meanwhile, SW winds 5-10 mph in the morning are forecast to increase to 10-15 mph by afternoon. && .LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/... Thursday night thru Monday...High pressure at the surface and aloft will be in control of the weather with fair weather expected. Temperatures generally 5 to 10 degrees above normal. Monday night thru Wednesday...A pattern change with a sharp h5 trough moving into the ern part of the country. Low pressure and a slow moving front associated with the upper trough will affect the area with occasional rains. Locally heavy rains are possible with an upper trough beginning to take a negative tilt as it crosses the area. The 12Z NA models are showing QPF totals generally in the 1 to 3 inch range across the area for the period Mon_Wed. The higher totals are across the wrn counties. Much colder temperatures will be ushered in behind the system with below normal temperatures for the middle part of next week. && .AVIATION /00Z THURSDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Generally VFR through Thursday. However, patchy ground fog may develop at the fog-prone, rural TAF sites between 05Z-11Z Thursday which could result in a brief period of MVFR visibility. Confidence in fog development was too low to mention in 18Z TAFs except at MIV. Light southwest winds tonight, increasing too 8-15 kt in the afternoon. OUTLOOK... Thu night thru Monday...VFR expected thru the period.
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&& .MARINE...
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S-SW winds 5-10 kt will continue tonight and Thursday morning, then increase 10-15 kt across the coastal Atlantic waters during the afternoon. Wind field may become locally enhanced to 15-20 kt off the coast of Ocean and Monmouth Counties via Ambrose jet late in the day. Held off on a SCA at this juncture since poor mixing profiles should inhibit higher winds aloft to reach the surface. Accordingly, capped gusts below 25 kt. Seas will generally be around 3 ft in the coastal waters and 2 ft or less in the Delaware Bay...mostly a se swell of 9 or 10 seconds. OUTLOOK... Thu night...Near SCA winds across the nrn NJ coastal waters Thu evening, but not confid enough for a flag attm. Fair. Friday thru Sunday...Sub- SCA conditions thru the period with fair weather.
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&& .CLIMATE...
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**Top 5 warmest October on record for PHL and ABE appears likely and its virtually certain a top 10 warmest October for all of our forecast area** Todays max/min plus the Mount Holly 330 PM fcst for the next 7 days, adding on day8-11 from FTPRHA GFS 2m max/min temps and then normal the last 3 days of the month results in the following. PHL 63.5 degrees or 6 degrees above the October 30 year avg of 57.5. This again pegs PHL for a tie as #2 warmest October on record. 1. 64.5 2007 2. 63.5 1971 (2017?) 3. 62.7 1947 To drop out of the top 10 warmest the PHL avg would have to lower to 61.2 degrees (a 2.4 degree drop from the current projection). Based on the 12z/18 ensembles both GEFS/NAEFS and ECEFS...this will not happen unless todays ensembles completely miss the last 3 days of the month. So while a record is probably out of reach top 5 is likely. ABE 59.5 or 7 degrees above the 30 year avg of 52.5. This also ties for #2. 1. 60.8 2007 2. 59.5 1984 and possibly 2017? 3. 59.3 1947 4. 58.8 1971 For Allentown to drop out of the top 10, the projected avg would have to lower to 57.2 or a drop of 2.3 degrees from the current projection. Again, thats highly unlikely unless something very unusual and not currently ensembled occurs the last 3 days of the month. It should be pointed out that the D8-14 fcst from the Climate Prediction Center (CPC) .. is for above normal temperatures for the last week of the month. We are on the edge of the transition to cold that will be quite noticeable in the nations midsection next week. Uphill (southwest flow at 500MB and the trough axis constantly Appalachians or west) makes it difficult for cold air to cross the mountains into our area. Now a moment to share some information from a new tool recently available to us... The last two years in PHL 10/15-10/17/17 is the warmest in the period of record...averaging 3 degrees above normal. Ditto Allentown,,averaging 3.7 degrees above normal! Ditto ILG...averaging 2.5 above normal. We will look at all of this again either Saturday or Sunday morning.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...O`Hara Near Term...Drag/Klein 640 Short Term...Klein Long Term...O`Hara Aviation...Drag/Klein/O`Hara 640 Marine...Drag/Klein/O`Hara 640 Climate...

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