Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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399 FXUS61 KPHI 150601 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 201 AM EDT Sun Oct 15 2017 .SYNOPSIS... High pressure is centered well east of the Mid-Atlantic coast overnight, however it extends to over our region. A strong cold front crosses our region Sunday night, then high pressure builds just to our south by Tuesday. High pressure will remain nearly stationary for the rest of the week and into next weekend. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... The disturbance off the coast will continue to dissipate tonight while weak high pressure builds towards our area. Overall, there is not enough mixing or dry air to scour out the low clouds over our area. It`s expected that drizzle and fog will return to many areas overnight. We have continued with the fcst much as it was before. Low tonight will drop only into the upper 50s across the north and low/mid 60s over Philadelphia/srn NJ and Delmarva. Winds will be light and variable overnight. Updated the forecast to better reflect the current temperatures and trends. && .SHORT TERM /SUNDAY/... High pressure will move across the area while a strong cold front approaches from the west. The pressure gradient improves through the day, resulting in increasing SW winds through the day. The mixing will scour out the low clouds during the morning and sunny skies should occur by late morning and last into the afternoon. Temperatures will soar to much above normal readings. Highs should top out in the upper 70s north and low 80s south and central areas. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... Hazards: Frost advisory likely portions of northeast PA and nw NJ north of I-78 early Tuesday. First period issuance at this time and just too far in advance for a seasonably normal and somewhat marginal expected occurrence due to late developing decoupling. Summary...Breezy and mild Sunday night with Mondays max temps possibly at 1 am Monday at which time a strong cold front moves through with brief but significant cooling early next week, then steadily warmer mid week into next weekend as high pressure parks nearly overhead. 500MB: A -1sd trough crossing the Great Lakes Sunday night reaches New England Monday. Another trough passes by, further north across Quebec Tuesday, then very strong ridging (several standard deviations above normal) develops across the Great Lakes and northeast USA later in the week into next weekend. Temperatures... October has averaged 8 to 10 degrees above normal for the first 13 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 Monday should be 2 to 5 degrees above normal with the min occurring at 1259 AM Tuesday, then Tuesdays calendar day average should be a degree or two below normal, warming Wednesday to nearly 5 degrees above normal, and eventually to daily averages 10 degrees or more above normal Friday and Saturday. 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. Forecast basis...unless otherwise noted a 50 50 blend of the 12z/14 GFS/NAM MOS Sunday-Monday night, the 12z/14 GFS MEXMOS Tuesday and thereafter the D4-8 WPC 15z/14 12 hourly max/min T and POP, and the 6 hourly td, wind and sky. The dailies... Sunday change from the early morning thinking and here is that portion of the discussion. An upper-level trough moving across the Great Lakes and Ohio Valley will drive surface low pressure well to our north Sunday night. This will sweep a strong cold front across our area during Sunday night, with a sharp wind shift to the northwest with its passage. The guidance shows that the bulk of the large scale forcing shifts to our north, however enough convergence with the cold front along with some instability should allow for a band of showers to arrive into our western zones. The question is how much does this weaken with an eastern extent. The organization of the showers should be on the decline with an eastern extent given the timing and little instability in place. As a result, no thunder and continued with the highest PoPs (likely) from near the I-95 corridor westward, tapering to chance (scattered) farther south and east. Much of the model guidance hints that the showers occur behind the cold front/wind shift, and this characteristic could allow some more organized showers to continue farther east. There should also be a period gusty northwest winds 20-30 mph toward dawn Monday. For Monday...Partly to mostly sunny, breezy and much cooler behind the cold front (afternoon temps around 20 degrees cooler than the day before) with a gusty northwest wind to 25 mph. The calendar day max temp is expected to occur near 1 am Monday, and be on the order of 5 degrees warmer than the Monday afternoon high temperature. One note for Monday afternoon...Right now our Min RH is forecast above 30%, BUT if our forecast ends up drying RH`s out below 30%, and if minuscule less than .05 inch amounts of showers occur 12 hours previous, then our partners may wish us to post an SPS for an elevated FWX concern. No action needed at this time since its a marginal risk, benefiting from review of future model guidance and partner input. Monday night... A frost advisory will likely be needed and some places may see freezing temps of 30-32F. This falls sort of in the normal range of first freeze, ultimately depending on what actually occurs. Gusty north to northwest winds early to around 20 mph decrease - decouple inland late at night but gusty north to northwest winds may continue along the immediate couple of miles of the coast all night. Tuesday...Sunny and seasonably cool. Northwest wind backs to west or southwest. The coolest day of the week. Wednesday through Saturday...Looks fantastic for mid October. As a gradually building ridge approaches from the west with rising heights, surface high pressure is forecast to be nearly stationary right over our area through Saturday. This will translate to slow moderation in the temperature department during this timeframe. Larger than normal diurnals are expected. Since this looks to be primarily a west or northwest pressure gradient wind flow pattern, very little late night fog is anticipated...just a bit too dry. This pattern could hold Sunday into Monday before troughing just to our west complicates the weather forecast beginning the 22nd or 23rd. && .AVIATION /06Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Through today...MVFR/IFR to locally LIFR ceilings early this morning are expected to improve by late morning, then to mostly VFR this afternoon. MVFR to IFR visibility due to fog and even local drizzle is expected to improve by mid morning. The timing of the improvements is of lower confidence. Light winds, becoming southwest and increasing to 10-15 knots with local gusts to about 20 knots. While confidence is high regarding the increase in winds, especially this afternoon, there is lower confidence regarding the extent of the gusts. Tonight...VFR overall, although some showers move through later this evening and overnight (local MVFR possible) as a strong cold front arrives. Southwest winds around 10 knots, however gusts up to 25 knots are possible at times especially with a wind shift to the northwest behind a cold front. OUTLOOK... Monday...VFR. Northwest winds 10-15 knots with gusts 20-25 knots diminishing toward midnight Monday night. Tuesday...VFR. light northwest wind to start becomes west southwest during the afternoon then northwest at night. Gusts under 20 kt. Wednesday...VFR. Light northwesterly winds becoming variable at night. Thursday...VFR. Light wind. && .MARINE... We have cancelled the SCA for the northern NJ coastal zones as sea at buoy 44065 have been below 5 feet for several hours. We will continue with the SCA for the remaining coastal zones of NJ and DE through Sunday morning. Mostly cloudy conditions with patchy fog and drizzle expected tonight and Sunday morning. More SCA conditions expected later Sunday as winds and seas increase ahead of the approaching cold front. After some morning fog and drizzle, fair weather is expected Sunday. Winds and seas are expected to increase through Sunday afternoon and the SCA may need to be extended. OUTLOOK... Sunday night...SCA possible ahead of and certainly behind the cold front with gusts to near 30 kt by dawn Monday, induced by surface pressure rises and cold air advection. Monday...Conditions will be close to Small Craft Advisory criteria through Monday night...possibly strongest winds around 25 kt at sunrise and sundown. Tuesday...Winds and seas diminish and no marine hazard anticipated after daybreak. Light north becoming west during the day then northwest late at night. Wednesday...The conditions are anticipated to be below Small Craft Advisory criteria with a northwest wind becoming light and variable at night. Thursday...No marine hazards. Light wind. && .CLIMATE... Much warmer air arrives for Sunday. Some places could be close to record high temperatures. Below are the October 15th record high temperatures for the official climate locations that are currently forecast to be within 3 degrees of their record. Allentown, PA 82 in 1947 Reading, PA 83 in 1989 Trenton, NJ 84 in 1975 Georgetown, DE 84 in 1975 October projected positive departures continue to indicate a top 10 warmest October is likely to occur in our forecast area, in some places possibly top three. On October 9th we projected the PHL October avg temp to be 62.2 or 4.7 degrees above normal...or 7th warmest on record. Today`s projections using this mornings 330 am D1-7 max/min temps, the 00z/14 FTPRHA GFS model 2meter max/min temps for D8-11 and then daily normal temps the 25-31st yields an avg of 62.7 degrees or 5 degrees above normal and a rank of #3 warmest on record (official records back to 1874). Confidence is very high that the month of October will be a top 15 warmest in PHL and other climate sites in our forecast area,, above average for top 10, and possibly vying for the warmest ever October. In PHL that warmest ever October was a 64.5 degree avg in 2007, followed by 63.5 in 1971 and then 62.7 in 1947. NAEFS odds somewhat favor an ultimately warmer average than that which is projected above, as the NAEFS has us in its coolest period with regard to normal, this coming week of the 16th-20th. The NAEFS probabilities warm us, relative to normal, the last 10 days of the month. The caution here... the GEFS is trending to several stronger and stronger cold troughs trying to dig down into the Ohio Valley late in the month so the NAEFS is not necessarily a lock yet on anything warmer than what we`ve projected today. For Allentown PA...using the same process as for Philadelphia, Allentown projects the 2nd warmest October on record (back to 1922). The warmest was 60.8 degrees in 2007, followed by possibly 2017 at 59.7, October 1984 was the next cooler October, averaging 59.5 degrees. IF...IF these positive departures are equal or greater than what is projected above... then for both PHL and ABE this would make it the 4th month this year with a monthly positive departure of 5 degrees or greater (J,F,A,O) with no month this year having any average negative departure greater than 2 degrees. In fact only 3 months of the first 10 will have averaged cooler than normal...March, May and August. These average temperature projections will change, but I dont think by more than a degree or two. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EDT early this morning for ANZ452>455. && $$ Synopsis...Drag Near Term...Meola/O`Hara Short Term...O`Hara Long Term...Drag Aviation...Drag/Gorse Marine...Drag/Meola/O`Hara Climate... is the U.S. government's official web portal to all federal, state and local government web resources and services.