Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 170137 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 937 PM EDT Mon Oct 16 2017 .SYNOPSIS... High pressure will build into the mid Atlantic states tonight through Thursday and then remain nearly stationary until moving east of New England on Sunday. A cold front should approach from the west next Monday. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... Strong shortwave trough/vort max moving through the Northeast at this time with the Mid-Atlantic well upstream of the cold front that moved through the area last night. Skies have cleared as dry air and subsidence have moved in. Objective surface analysis at 01Z indicates a 1026-mb surface high in far eastern portions of the Ohio Valley. This has reduced the surface pressure gradient across the region considerably this evening. Combined with nocturnal decoupling, winds have diminished quickly. Updated the grids to lower the winds for the rest of the evening. So far, temperatures required little modification, but with near-ideal radiational cooling anticipated, may see some errors with time as localized effects not handled well by guidance become more and more dominant. With the above in mind, current frost advisory looks OK. There is a pretty good chance of patchy frost in the Pine Barrens as well, but confidence is not high enough regarding coverage to justify issuing an advisory, especially since models are handling temperatures well there so far this evening. Dew points, on the other hand, have generally been too low, so raised these a few degrees across the board through the night. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/... Wonderful weather thru Tuesday. High pressure building in from the west with dry and cool air in place. After early morning lows in the 30s/low 40s in many areas, highs will climb into the upper 50s to low 60s most areas. These readings will be 5 to 10 degrees below normal for mid-October. winds will be light mostly from the W or NW. && .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/... Hazards: Frost as per near term. A new frost hazard for the Pine Barrens was issued for early Wednesday morning. Rainfall: none expected Tuesday night through early Monday then increasing chances with an approaching cold front from the west. 500MB: Ridging developing over the northeast is briefly suppressed a bit on Thursday by a strong trough passing through eastern Canada. Ridging resumes in earnest through the weekend...several deviations above normal...then begins its departure to the Maritimes early next week as a deepening trough develops from the Great Lakes region into the Ohio Valley. Temperatures... October has averaged 8 to 10 degrees above normal for the first 15 days of the month, excluding the ACY Marina where departures from normal are less due to the tempering influence of the nearby ocean (both warm and cold). Calendar day averages Wednesday should be 3 to 5 degrees above normal, eventually warming to daily averages near 10 degrees above normal Friday Saturday Sunday and then 10 to 15 degrees above normal Monday. The daily diurnals will be larger than usual due to the dry airmass in place and light wind fields, nighttime lows probably a little colder than statistically modeled in the non urban areas until Sunday night. Forecast basis...This forecast was a 50 50 blend of the 12z/16 GFS/NAM MOS Tuesday night-Wednesday night, the 12z/16 GFS MEXMOS Thursday and thereafter the D4-8 WPC 15z/16 12 hourly max/min T and POP, and the 6 hourly td, wind and sky. WPC max temps in the D4-8 time frame were blended 50 50 with the warmer MEX which is now being supported by the warmer 12z/16 EC 2m temps at 18z. WPC D4-8 min temps were blended 50 50 with the cooler 12z/16 MEX temps which tend to better reflect the countryside radiation values. Decoupling is anticipated most of these forecast nights. The dailies... Tuesday night...Clear and still chilly in the suburbs where frost is possible. Light west wind or calm. Ideal radiation conditions southern NJ with BL winds less than 15 kt. We do see some elevations with BL winds 17-20kt n of I-78 so we dont think quite as cool in the north Tues night as down south and that`s where phase two of this frost- freeze episode is forecast in the NPW via a frost advisory for the Pine Barrens. Normally this freeze-frost includes the region of far southern Monmouth County into Ocean County near Toms River. Doing all the natural modification of the blended temps did not allow for an advisory there but my climatological sense would lean toward expanding the advisory there. Wednesday...Sunny. The warming trend quickly begins. Max temps about 8 to 10F warmer than Tuesday afternoon. Light wind becomes south to southwest in the afternoon. Wednesday night...Clear. Ideal radiational cooling conditions everywhere and lowered the countryside guidance temps by 2-3F and might need to do more lowering. Still, milder than Tuesday night, in part because its coming off a warmer Wednesday. Thursday...Sunny. Max temps about the same as Wednesday. Southwest wind. Thursday night...latest ensemble guidance has minimized any possible cooling and dropped the mention of a weak-minor dry CFP from the synopsis. Friday-Sunday...Sunny days and clear nights. Highs should favor the 70s with lows mostly in the 40s except 50s in the urban centers on Friday, Saturday and Sunday. Northwest wind Friday, light southeast to south wind Saturday, south wind Sunday. It`s conceivable a little patchy fog may develop Saturday night or Sunday night. As comparison information: Normal temperatures for the coming week and weekend in southeastern Pennsylvania, New Jersey, Delaware and northeastern Maryland are generally highs in the 60s and lows in the 40s. Normal highs in the elevated terrain of the Poconos and northern New Jersey are mostly in the middle and upper 50s with normal lows in the middle and upper 30s. Next Monday...begins a transition to several fast moving cold fronts for the end of the month and greater day to day variability with possibly non diurnal max/min temps for a couple of days. Southwest wind Monday. && .AVIATION /02Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR through the TAF period. Light north to northwest winds may become light/variable or calm tonight, especially northwest of KPHL, then become re-established tomorrow morning before backing more to a southwest direction during the afternoon. OUTLOOK... Tuesday through Saturday...Mainly VFR. && .MARINE... 930 pm update: So far, the advisory looks OK. The ending time may be a little soon for the southern NJ and DE coastal waters (and maybe even Lower Delaware Bay), but confidence is too low to make a 2-3 hour adjustment to the expiration time at this juncture. Main question is if advisory criteria will occur on Upper Delaware Bay this evening. So far, the trend has been downward, not upward. With northwest flow, do not want to be premature in expiring the advisory given potential for enhanced channel flow to occur. However, my suspicion is that the advisory will be dropped early here. Previous discussion below... We will keep the SCA flag in place overnight with the colder air arriving across the waters. A minor adjustment to the ending time Tue is that we extended the ending time out 2 hrs (now at 12Z). Gusts around 25 knots will continue into the early overnight hours before decreasing. Seas will be mostly 4-5 ft tonight before lowering a bit Tuesday. Fair weather tonight and Tuesday. OUTLOOK... Tuesday through Saturday...No marine headlines are anticipated. && .CLIMATE... The following section will probably be discarded later tonight. Calendar day averages have averaged above normal daily from October 3rd...and will today as well, at most locales due to the warm 1 am temps (mid aftn temps were 7 to 15 degrees cooler than the 1 am high temp reading for today-Monday the 16th). Tuesday should be our first below normal day since the first of the month. Wednesday the 18th begins another streak of above normal calendar day averages, probably through at least the 24th. Thereafter, the last week of the month should average near normal as per the NAEFS and ECEFS (with two or three below normal days but also 2 or 3 above, due to several fast moving frontal transitions in a positive PNA pattern). The 06z and 12z/16 GEFS 500MB fields have a couple of days of significantly colder than normal weather in the 10/25-27 time frame but the GEFS has lost the trough for the last 4 days of the month, having changed to zonal flow and looking like the NAEFS/ECEFS. We dont know if that substantial change to zonal is going to hold. So...the first 15 days have averaged 8 to 10 degrees above normal (except the ACY Marina whose departures should be less due to the nearby tempering influence of the ocean). Taking this mornings max at 1 am, and todays min that will occur just after midnight, and then the 330 am PHI max/mins Tuesday through Sunday the 22nd, followed by the FTPRHA GFS 2m max/min temps for the 23rd-26th and closing with daily normals the 27th-31st; results in the following for October. ABE still projects top 5... currently projected ranking at #4.. 59.1 having slipped slightly primarily due to yesterdays cloud capped max temp that was significantly less than fcst. IF, our forecast ends up two degrees too warm over these last 16 days of the month ABE would still rank #11 warmest October. PHL projected rank #2 warmest at 63.7 (I miscalculated the avg two days ago)... and similar to ABE, if the projected average slips 2 degrees from what we calculated this morning, PHL would still rank #9 warmest October. Will review again late Wednesday afternoon. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Frost Advisory from 2 AM to 9 AM EDT Tuesday for PAZ054-055. Frost Advisory from 4 AM to 9 AM EDT Tuesday for PAZ060>062- 101-103-105. NJ...Frost Advisory from 2 AM to 9 AM EDT Tuesday for NJZ001-007- 008. Frost Advisory from 4 AM to 9 AM EDT Tuesday for NJZ009-010. Frost Advisory from 1 AM to 9 AM EDT Wednesday for NJZ021-022- 027. DE...None. MD...None. MARINE...Small Craft Advisory until 8 AM EDT Tuesday for ANZ450>455. Small Craft Advisory until 6 AM EDT Tuesday for ANZ430-431. && $$ Synopsis...Drag Near Term...CMS Short Term...O`Hara Long Term...Drag Aviation...CMS/Drag Marine...CMS/Drag/O`Hara Climate...Drag

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