Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 230108 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 808 PM EST Wed Feb 22 2017 .SYNOPSIS... High pressure will remain centered east of the Mid Atlantic coast through the end of the week. A cold front will approach from the west Thursday then stall at night, before lifting northward as a warm front Friday. A strengthening low pressure system will develop over the Plains and track across the Great Lakes Saturday, with a strong cold front sweeping across our area Saturday afternoon. High pressure then builds to our south later Sunday into Monday. The next cold front arrives during Monday before stalling to our south, then it should move northward as a warm front Wednesday. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... 630 PM ESTF: Minor changes to the temp/dews fcst. SPS may be issued around 915 PM this evening for much of the area for patchy dense fog by dawn. Expect favorable radiational cooling conditions this evening under variable clearing cirrus with a light south wind. Models indicate stratus forming to our south toward the mouth of the Chesapeake Bay and lower Delmarva this evening, then advecting/expanding to the north- northeast overnight across the coastal plain. Farther north into eastern PA and NJ, a better setup for radiational fog looks to exist since the stratus should arrive later in the night or possibly not until early morning. While locally dense fog is a possibility toward daybreak, the current thinking is the stratus will limit the extent of it. Differences between models in timing and areal coverage of the fog/stratus contribute to some uncertainty with this aspect of the forecast. Most of the region should remain dry tonight but a weak disturbance tracking over Delmarva may be accompanied by an isolated shower or patchy drizzle overnight. Forecast low temperatures range from the upper 30s in the sheltered valleys in northwestern NJ to the upper 40s in the the cities and mid Delmarva region. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/... High pressure will remain anchored off the Mid-Atlantic coast on Thursday. Return flow around the high will allow the spring-like warmth with build into tomorrow. Near-record high temperatures are forecast from about the Philadelphia region southward. It is important to note some uncertainty with the temperatures exists with the forecast for tomorrow. How warm we will get will depend on how quickly the morning fog and stratus mixes out. Stratus occasionally is stubborn to erode this time of year due to the low sun angle. Where stratus persists into the afternoon, highs will be in the 60s vs the 70s. Forecast soundings and RH fields show lower clouds eroding from south to north during the late morning and afternoon. Accordingly, highs in the low to mid 70s are favored in Delmarva, southeastern PA and southern NJ with 60s farther north. A weakening cold front will approach from the west during the afternoon. Models generally show enough lift to support isolated to scattered convection in our far western zones of eastern PA and northwestern NJ during the mid to late afternoon. Forecast soundings show a weak mid-level cap in place. Added a slight chance for thunderstorms out toward Reading and in the southern Poconos, where a relative min in CIN indicates a low potential for deeper convective to develop. && .LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/... Summary...Potential for near record warmth through late week, followed by a strong cold front Saturday afternoon and evening. The synoptic setup is comprised of a digging trough across the Plains and Midwest Thursday night and Friday, then this moves across the Great Lakes Friday night and Saturday before lifting across the Northeast and eastern Canada. The flow may then turn more zonal for awhile allowing for some moderation in the temperatures once again after a brief shot of cooler air. We used a model blend for Thursday night through Friday night, then blended in the 12z WPC Guidance thereafter. Some adjustments were then made following additional collaboration with our neighboring offices. For Thursday night and Friday...A frontal boundary near our northern zones Thursday night into Friday morning should shift northward as a warm front. This is due to increasing flow and warm air advection ahead of a organizing/strengthening storm in the Northern Plains. There is not a lot of focused lift, however some showers could result. Some fog and low clouds should develop each night as the low- level moisture increases. Despite Friday starting out with more clouds and some fog, enough clearing should take place to allow for ample boundary layer warming and there is the potential for record highs. It appears that any showers with the warm front Friday should be shifting north of our area. Some thunder may occur near the northward moving front especially to our west, however this may tend to weaken with an eastward extent. For Saturday and Sunday...A strong upper-level trough will move across the Great Lakes and Northeast Saturday and Saturday night, sweeping a strong cold front across our area. Some low clouds and fog may be around to start Saturday. We should be within the warm sector and the synoptic pattern is favorable for at least some convection to move through the area ahead of or with the cold front. Forecast soundings show a strengthening wind field ahead of the cold front Saturday with some instability (high shear-low CAPE scenario). While the instability may be on the low side, frontal/synoptic forcing and moisture advection could result in a low-topped convective line. As a result, a mention of thunder is maintained in the forecast ahead of the cold front however the strength of the convection is less certain at this time. While it will be another warm day on Saturday, it appears that high temperatures fall short of records. Once the cold front sweeps through later Saturday, cold air advection and much drier air moves in for Saturday night and Sunday. Given the front offshore Sunday, dry and much cooler on Sunday along with a gusty northwesterly wind. For Monday through Wednesday...High pressure shifts off the Southeast coast to start Monday, with a cold front moving through our area. This looks to be on the weaker side however cannot rule out a few rain/snow showers. As another storm system ramps up across the Plains, the front should stall just to our south into Tuesday before starting to move northward as a warm front Wednesday. Low pressure looks to track once again across the Great Lakes with a driving warm sector northbound Wednesday. We went with a chance of showers (rain/snow showers Monday night) Tuesday ahead of the warm front then with some higher PoPs on Wednesday as the cold front nears. && .AVIATION /01Z THURSDAY THROUGH MONDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR with some cirrus and light south wind. IFR st/fog forms between 03z-08z depending how fast saturation is reached... generally MIV and ILG first. Widespread IFR conds expected by 10z except possibly parts of s NJ near KMIV where it could be MVFR CIGS/VSBY. Patches of embedded LIFR st/fog expected, especially KILG/KPHL/KPNE/KABE/KTTN toward 10z. Confidence in quite high that cigs between 500 and 1500 feet will form most TAF sites between 06z and 10z. But timing of the vsby below 1 mile is below normal. Thursday...IFR/LIFR conds in st/fog most TAF sites and maybe even a little mist will all be slow to erode/lift from south to north but should reach MVFR or VFR vsby by 17z with cigs at that time between 1500-3500 and rising/thinning. Please see TAFS for details. Light south to southwest winds in the morning increase to around 10 kt with gusts 15-20 kt during mid afternoon. OUTLOOK... Thursday night and Friday...VFR ceilings may lower to MVFR/IFR Thursday night into Friday morning along with MVFR/IFR visibility due to fog. Conditions should improve during Friday, however some low clouds and fog may then redevelop Friday night. Some showers are possible mainly Thursday night and Friday morning with a warm front lifting northward. Saturday...There is the potential for MVFR/IFR conditions at times with showers and potential thunderstorms with a cold front. Conditions should improve at night in the wake of the cold front. Southerly winds ahead of the cold front potentially gusty, turn west and northwest at night. Sunday and Monday...VFR overall. West-northwest wind gusts up to 30 knots possible Sunday, then diminishing at night and Monday. && .MARINE... An MWS for FOG or even MWW DENSE FOG Advisory for dense fog may be needed for late tonight and Thursday morning. Winds and seas below SCA levels tonight and Thursday. Expect fog to develop tonight and may be locally dense in spots late tonight and Thursday morning as dewpoints rise into the mid/upper 40s and maybe near 50F south. OUTLOOK... A frontal zone Thursday night will lift north as a warm front through Friday, which should become located to our north during Friday. An increase in warmth and dew points may result in some dense fog developing on the waters especially Friday through early Saturday. A strong cold front arrives later Saturday, with gusty southerly winds becoming west-northwest in its wake. Small Craft Advisory conditions may develop mainly Saturday and Sunday. Much warmer air over cooler waters though should limit the mixing Saturday, then improved mixing Saturday night and Sunday as cooler air arrives. There is a chance for a period of low-end gale force gusts Sunday. The conditions should then be below Small Craft Advisory criteria Monday. && .CLIMATE...
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For those who refer to the RTPPHI: it hasn`t run correctly on its 715 AM/PM cron since the 20th but we were able to produce a version around 8 PM tonight. Not all the trace amounts of rain are in there. This will have to suffice until an improved fix hopefully is implemented Thursday evening. Thank you. For reference, here are the high temperature records for Thursday, Friday and Saturday. Location Thursday 2/23 Friday 2/24 Saturday 2/25 ACY 72-1985 75-1985 77-1930 PHL 75-1874 74-1985 79-1930 ILG 72-1985 78-1985 78-1930 ABE 71-1985 76-1985 74-1930 TTN 74-1874 74-1985 76-1930 GED 67-1985/1990 72-1961 76-1975 RDG 72/1932/1922 77-1985 77-1930 MPO 60-1977 60-1984 70-1930
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Gorse Near Term...Drag/Klein Short Term...Klein Long Term...Franck/Gorse Aviation...Drag/Gorse/Klein Marine...Drag/Gorse/Klein Climate...808PM RTP reference

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