Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 111958 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 258 PM EST Thu Jan 11 2018 .SYNOPSIS... High pressure remains centered to our east through tonight. A cold front moves into our area Friday night. Meanwhile, low pressure tracks along the front and across our region later Friday night into early Saturday which pulls the cold front offshore Saturday morning. Arctic high pressure then builds into our area Sunday into Monday. Low pressure and a cold front moves through our area Tuesday, followed by high pressure during Wednesday. && .NEAR TERM /THROUGH TONIGHT/...
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Near record warmth has occurred today at Georgetown today. 66 was their high at 152PM, 1F shy of record. We have further delayed the start of the waa rain but we still think that most of our area will have measurable on-going by dawn Friday. We have isolated thunder in the fcst around 09z over the southern two thirds of the fcst area. Most wont see thunder but the instability burst implies rain to be briefly heavy. Temps should rise into the lower 60s along and east of the I95 corridor except along the chill waters of DE Bay. This is warmer than MOS. It also implies that we will likely set numerous record warm low temps for Friday (just like a year ago). Again, how things transpire this evening (briefly radiating after sundown in NJ) should even out as winds increase late at night.
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&& .SHORT TERM /FRIDAY/...
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Record warmth expected in parts of our area with periods of heavy rain, causing periods of poor drainage street flooding (several FLSPHI issuances expected) and elevated stream flows where melting and runoff coincide. No flood watch since we think there wont be enough snowmelt runoff and rainfall combined to threaten small stream flooding and that the ice while it may start moving, wont be enough to cause any major backup of stream flow. Widespread .7 to 1.3 inches of rain anticipated Friday during the day. Thunder is possible and we have it as isolated in the grids. No MLCape attm for svr but we`ll have a wind advy posted to cover this eventuality as well. As far as wind advy criteria...Not an overly strong flow at 925-850 but the bl winds are near 27 kt for a time Friday and the hrrrx and hrrre were fcstg 40 kt gusts. Temps in the 60s.
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&& .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Summary...Near record warm Friday evening, then colder air arrives later Friday night and Saturday with an icing potential for parts of eastern PA and northwest NJ, followed by even colder air Sunday through early next week; some snow possible Tuesday. Synoptic Overview...A sharp upper-level trough arrives in the East later Friday. A strong southern stream short wave rolls northeastward Friday night into Saturday and allows the trough axis to take on a negative tilt, then the entire upper-level trough amplifies into the East through the weekend. While the trough may weaken for a time early next week, its presence should be maintained in the East. Low pressure tracks across our area early Saturday. A clipper system Tuesday may be potent enough to produce an area of snow. For Friday night and Saturday...A dynamic setup as southern stream energy rolls northeastward and supports a surface low along a strengthening baroclinic zone. This front will be arriving into our western areas Friday night then it shifts eastward as the surface low rolls northeastward. The next round arrives Friday night and Saturday morning as surface low pressure tracks over our area. Large scale ascent becomes maximized Friday night and early Saturday morning as the upper- level trough takes on a negative tilt, and with a precipitable water surge to about 1.5 inches areas of heavier rain should occur. Some guidance shows weak elevated instability developing Friday night ahead of the surface low and cold front, and for now included a slight chance of thunder mainly for our southeastern areas. Total rainfall amounts still look to be on the order of 1-2 inches across the region that basically comes in two rounds. For more on on this, see the hydrology section below. Fog should be around although this may be kept in check for most areas by stronger winds aloft. Based on the latest forecast some record highs will be challenged Friday and this warmth will linger for a time Friday night. Now for the more challenging part of the forecast, ice potential. The model guidance overall shows colder low-level air advecting southeastward later Friday night as a strengthening baroclinic zone is on the move. Some guidance is much more robust with this than others, however the initial push of colder air is forecast to be shallow with warmer air aloft taking longer to cool. This sets the stage for freezing rain and/or sleet for portions of eastern PA and northwestern NJ. Of particular concern is the southern Poconos into far northwestern NJ where cold low-level air could result in a decent amount of icing. Portions of the southern Poconos could see ice accumulations of up to one-quarter of an inch. This however could be more sleet if the low-level cold layer ends up being a little deeper. As of now, we are coming up with more freezing rain but did include a mention of sleet. This changeover occurs after midnight Friday night and especially closer to daybreak Saturday. Temperatures should be the warmest in the very early morning hours (or just after midnight in some cases) then temperatures fall as the low-level cold air advection surges southeastward. The southern and eastern extent of this will depend on the speed of the surface low. The overall precipitation should be ending in the morning with the bulk of the comma-head precipitation shifting to our northwest and north. It will be much colder Saturday night, with wind chills down to around -10F in portions of the southern Poconos. For Sunday and Monday...Low pressure moves across the Canadian Maritimes Sunday with strong cold air advection across our region. It will be much colder as a result with high temperatures pretty much below freezing area-wide both days. There will be a breeze Sunday resulting in a wind chill, then high pressure builds into the area later Sunday and Monday allowing for less wind. A clipper system tracking near or just south of the Great Lakes Monday will approach our area Monday night with some increase in clouds. For Tuesday and Wednesday...A clipper system is forecast to pivot across the area Tuesday, with the initial weak surface low near the eastern Great Lakes with probable redevelopment occurring near or east of the New Jersey coast as strong short wave energy arrives. At least some guidance continues to show an area of stronger lift which then results in an expanding area of snow but where this occurs is less certain. For now, continued with lower chance PoPs for much of the area Tuesday then these lowering some at night. High pressure gradually arrives from the west during Wednesday and it looks like cold conditions continue. && .AVIATION /20Z THURSDAY THROUGH TUESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Remainder of this afternoon...VFR cigs except MVFR CIGS vcnty KRDG KABE. Light mostly south wind. Tonight...Low ceilings, either MVFR or IFR, are expected to develop overnight. Additionally, visibility restrictions in fog and/or rain are possible at all TAF sites after 06Z. Another concern will be the potential for low level wind shear. While surface winds will remain southeasterly (starting at less than 10kt but increasing late tonight), a low level southwesterly jet is expected to move east with wind speeds of 40 to 50 kt likely. LLWS will likely develop first over KABE and KRDG after 00Z, then develop over the rest of the TAF sites 03Z or later (as the low level jet moves further northeast). Friday...MVFR variable IFR conditions, to even brief LIFR conditions due to low clouds, fog and periods of rain. Most of the IFR and LIFR conditions should occur at KRDG and KABE. Gusty south wind briefly 28-38 kt vcnty KPHL KILG KTTN and around 40 kt vcnty in parts of DE. Outlook... Friday night and Saturday...MVFR/IFR to even LIFR conditions due to low clouds, fog and periods of rain. The rain ends toward midday Saturday, potentially as freezing rain and/or sleet at RDG and ABE, with conditions improving as a cold front moves offshore. Southerly winds around 10 knots Friday shifts to northwest 10-20 knots Saturday. A southwesterly low-level jet to around 50 knots at 2000 feet moves through during a portion of Friday, and this may lead to low-level wind shear. Sunday...VFR overall. Northwest winds mainly around 10 knots. Monday...VFR. Light northerly winds, becoming light/variable or calm.
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&& .MARINE...
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SCA coming late tonight and Friday. Isolated gusts near 35 kt possible but cold water in the 30s, should limit this possibility. However, don`t be surprised if we issue a couple of short fuse Special Marine Warnings, especially if thunder develops or there is a well defined short line of heavy showers. Showery rains and patchy fog should result in visibility restrictions on Friday. Confidence is above average. In addition, significant ice melting and breakup on bays and along the coast will continue through this time. Temperatures are expected to stay above freezing through the overnight period. Outlook... Friday night and Saturday...Small Craft Advisory may need to be extended into Friday night or Saturday. Gusty southerly winds quickly shift to northwesterly Saturday afternoon. Since the airmass is much warmer ahead of a cold front, the stronger winds are not anticipated to mix down and therefore gusts should remain below gale force. Fog may be an issue for navigation through Friday night. Sunday...The winds are anticipated to be below advisory criteria, however seas may linger near 5 feet for a time on the ocean. Freezing spray develops. Monday...The conditions are anticipated to be below Small Craft Advisory criteria.
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&& .HYDROLOGY...
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The 10 AM Friday forecast for the Delaware is a 3 to 6 foot rise by Saturday morning of natural flow due to snowmelt and rainfall runoff... WELL within bankful. The nearly 2 weeks straight of sub-freezing temperatures has created a build up ice on local rivers and streams. We wanted to address the ice and the prospects of ice jams. Unfortunately, neither the formation or dissipation of an ice jam can be predicted with certainty, although we know what can influence them. The best approach is awareness and to take notice of day to day changes on a river or stream of concern. Ice jams can develop near river bends, near the confluence of tributaries, where slope changes, and around bridges. Solid ice cover is not an ice jam. There needs to be a restriction with flow. Even with solid ice cover, water can flow freely under the ice. A combination of well above normal temperatures for several days, heavy rainfall, and additional runoff from snowmelt is a worst case scenario outside of the actual hydrology. Results and impacts will be very different if rivers and streams were running high or were above normal or as they are now, running low or below normal. The more capacity a channel has, the more that channel can withstand rises (absorb the water within bankful). For the Thursday night through Saturday timeframe, one to two inches of rainfall is predicted across the region. Daytime temperatures are expected to peak in the 50s or 60s for a couple of days. Those two factors alone will begin to move some ice or at least begin the melting process. But, while it`s been cold, there hasn`t been much water equivalent snow. Across our inland basins, there is really only 1 to 5 inches on the ground where there is snow. Snow water equivalent or SWE, is no more than an 1/2 inch. In addition, our river levels are running below normal for this time year. In many cases, on the Delaware for instance, levels are 1/4 bankfull or less. So there is a lot of capacity within banks to withstand the volume of increased runoff rises. As mentioned, for the next several days, awareness is the best approach. The high impact ice jams we tend to hear about are those that break and cause flash flooding downstream of a broken jam. Again, thanks to the high capacity of channel we have, due to low flows, and the width of the channel itself (e.g. Delaware, Passaic, Raritan, Schuylkill), the threat of flash flooding is very low. What has been more common in our Hydrologic Service Area (HSA), is water building up behind a restriction or ice jam. Again, while this cannot be precisely forecast, the process is a slow one. It could take a day or days for water to build up behind a jam, rather than hours. Preparation time is usually available. Outside of our mainstem rivers, where the threat of flash flooding is minimal, the flash flood threat (or flood threat) can increase for smaller creeks or streams due to less available volume in the channel. But on the flip-side, there is usually less water in these tributaries to cause widespread impacts. Temperatures get cold once again later in the weekend through next week and will temporarily lessen the chance for any ice movement.
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&& .CLIMATE... For Friday, January 12th: Location Record High Temp Record Warm Minimum Temp -------- ---------------- ------------------------ ACY 67 in 2017 49 in 2017 PHL 72 in 1890 49 in 2017 ILG 68 in 2017 46 in 2017 ABE 60 in 1932 43 in 2017 TTN 68 in 1890 47 in 2017 GED 70 in 1975 N/A RDG 63 in 2017 47 in 2017 MPO 56 in 1975 N/A Record Daily Rainfall Location (values under 1.8 inches) -------- ------------------------- RDG 1.70 in 1915 ACY 1.19 in 1915 ILG 1.26 in 1900 GED 1.61 in 1991 && .EQUIPMENT... Pottstown Temp conts ots, ufn. We`ll try to keep everyone posted. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Wind Advisory from 5 AM to noon EST Friday for PAZ070-071. NJ...Wind Advisory from 5 AM to noon EST Friday for NJZ013-017>020- 022-027. DE...Wind Advisory from 5 AM to noon EST Friday for DEZ001>003. MD...Wind Advisory from 5 AM to noon EST Friday for MDZ008-012-015- 019-020. MARINE...Small Craft Advisory from midnight tonight to 6 AM EST Saturday for ANZ431-450>455. Small Craft Advisory from midnight tonight to 1 PM EST Friday for ANZ430.
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&& $$ Synopsis...Drag/Gorse Near Term...Drag 258 Short Term...Drag 258 Long Term...Gorse Aviation...Drag/Gorse 258 Marine...Drag/Gorse 258 Hydrology... Climate... Equipment...

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