Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 122347 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 647 PM EST Fri Jan 12 2018 .SYNOPSIS... A strong cold front will cross the region late tonight through Saturday morning as an area of low pressure moves along it. High pressure will build in late Saturday through Sunday before sliding to the north of the area on Monday. An area of low pressure will cross the region on Tuesday into Wednesday. High pressure will return for the latter part of the week. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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Main focus will be broad area of low pressure and associated strong front located to our west that will move through the area tonight. Ahead of this front, deep southerly flow has brought PWATs near 1.5 to 1.7 inches in warm, moist airmass with temperatures and dew points into the 50s and 60s. Several rounds of locally heavy rain have already moved through and while much of the area ins a lull as of this writing, additional rounds of showers will move through this evening and again could be heavy at times. Pops were updated with the 6:30 pm update to hone in on the exact timing of the showers throughout the night. There will continue to be a concern for hydro issues, particularly for poor drainage and low lying areas that see heavier rain. For further details on this, please see our hydro section below. Otherwise, as we head into the overnight, a sharp cold front will begin to cross the area NW to SE beginning across the southern Poconos around 6z. This will occur as precipitation becomes steadier as an area of low pressure moves north and east along the aformentioned front. A winter Weather Advisory remains in effect as we still expect rain to change to freezing rain and sleet as the low levels see a sharp temperature drop....from the 50s to below freezing within just a a few hours. This should occur around the 6-7z time frame for the southern Poconos and shortly there after across the Lehigh Valley. Ice accretion should remain under a quarter inch so don`t expect widespread power outages but do expect surfaces to quickly become slick and ice covered due to the freezing rain and freezing of any already wet surfaces. The front will not reach the I-95 corridor and places like Trenton and Philadelphia until very late tonight just as the precip is exiting so we don`t expect wintry precipitation for these areas. The other concern for tonight will be areas of fog, especially near the coast and Delaware Bay due to the moist southerly flow off the waters. This may be an issue until the front crosses early Saturday.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/... Early Saturday, precipitation will be quickly exiting as the low moves from a position right over the area quickly NE into New England. The strong cold front will also be transversing the coastal plain right around 12z and as a result daytime highs will occur early with temperatures falling through the day. Around sunrise, temperatures may still be above freezing from I-95 S/E but late morning expect all areas to be in the 20s to low 30s or even a bit colder in the far north...so a very sharp cool down! As a result, even through we don`t expect freezing rain or wintry precip along the I-95 corridor there won`t be a lot of time for some areas between when the rain ends and temperatures drop to freezing so any standing water and wet surfaces will quickly freeze and become ice covered. Otherwise, freezing rain/sleet occuring farther north will exit by late morning with the remainder of the day seeing dry but cold and gusty conditions with steady or falling temps. && .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/... Saturday night through Monday...As the low pressure continues to move through the Canadian Maritimes, we will start to see high pressure building into the region by late Saturday. The pressure gradient will remain fairly tight across the Saturday night through at least at least mid-day Sunday before starting to relax. This will help to keep the winds up across the region. With a northerly flow, cold air will filter into the area and it will feel markedly colder than recent days. With the winds remaining up overnight, we will also deal with some wind chills falling into the single digits across much of the area and into the negatives across the southern Poconos and higher terrain of northwest New Jersey. High temperatures will remain cold through Sunday and Monday. Although some moderating of the temps on Monday is expected as the high shifts to the north of our area and we see more of return flow develop across our area. Highs on Sunday will remain in the upper teens across the north to the mid to upper 20s across the rest of the area. Monday is expected to be around 5 degrees warmer than Sunday. Conditions through this period are expected to remain dry. Monday Night Through Friday... Overview... A southern stream shortwave skirts by to our south off the Carolinas Monday night. A trough across the eastern US amplifies and cuts off Wednesday into Thursday, with large model disagreement on where this occurs. Meanwhile, high amplitude ridge over western North America gradually weakens along with a lack of high latitude blocking over the Davis Strait. These features, along with additional shortwave energy feeding into the northeast US, suggest the cut off low will gradually fill and lift northeast of the Middle Atlantic by Fri. For sensible weather, an Alberta Clipper gives two opportunities for precipitation Monday night into Thursday. Mon night through Tue night...With the approach of the Alberta Clipper low, the first chance for light wintry precipitation is possible. The ECMWF keeps a surface ridge to the east in place longer compared to the GFS. Thus, the GFS begins precip as early as Mon night while the ECMWF holds off until Tue. The GEFS mean is more conservative compared to the GFS, with precip beginning across the southern Poconos and Lehigh Valley later Tue night. We continue to confine precip to the aforementioned areas for Tue night, becoming more widespread across the remainder of our region into Wed. As for precipitation type, a consensus of the guidance maintains 2-meter temperatures below freezing during this period. Model soundings indicate temperatures below freezing through the column, in addition to the mid and upper levels being saturated with respect to ice. While soundings indicate a veering low-level wind profile, suggestive of warm air advection, soundings do not indicate much in the way of a warm nose. Therefore, expect predominantly snow, with little if any mixed precip at this point. Given modeled QPF is relatively light, do not expect more than a dusting to a couple inches of snowfall potential, with the highest amounts focused northwest of the fall-line at this time. With sub-freezing temperatures, this would make roadways slick. Temperatures near seasonal levels. Wed thru Fri...Alberta Clipper moving through the lower Great Lakes with eventual redevelopment further east, but the tough question is exactly: where? The latest suite of 12Z/12 operational models continue to diverge, with little run to run consistency. The GFS develops low pressure off the NC coast Wed night and moves it northeast along the northern wall of the Gulf Stream, well south of the 40N/70W benchmark, into Thursday. The UKMET is similar. These scenarios would have little impact on our region, with the storm system remaining well offshore. Meanwhile, the ECMWF is much further west, developing low pressure east of the Virginia Capes Wed night, then northward through Cape Cod on Thu. It`s expansive precip shield on the western side of the system (a bit of a model bias at times) is also suspect. Nevertheless, this solution has greater impacts on our region in terms of wintry weather. The respective ensembles are similar, the GEFS further east and the EPS further west. But there is a lot of spread noted in the mid- level height fields of both the GEFS and EPS regarding the placement of the cutoff low, which translates into uncertainty with the location of the attendant surface system, e.g., storm track. WPC favors a blend of the GEFS and EPS ensembles, which places emphasis on more extensive redevelopment further north and east of our region, which would limit impacts. This scenario would favor a return to generally fair weather Thursday into Friday, and we continue that thinking with this forecast, with caveat that confidence is on the lower side given the uncertainty. Therefore, impactful weather is still possible, and we advise paying attention to future forecasts. Otherwise, it will be unseasonably cold Wednesday into Friday, with temperatures 5 to 10 degrees below normal. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...Widespread IFR conditions or lower expected, for both ceilings and visibility due to precipitation and areas of mist and fog. Tempos have been added as an attempt to counter hour to hour wide variations in conditions at most TAF sites. An abrupt shift from southwesterly to northwesterly winds is expected around 06Z and later as a cold front moves through the region NW to SE. Wind gusts to 25 kt are possible behind the front. Marginal conditions are possible for LLWS as you get close to the coast overnight, right now this was left out of the 00Z TAFS. Average confidence. Saturday...MVFR or lower is possible to start the day conditions improving to VFR by 18z. Northwest winds gusting as high as 25 kt. Average confidence. Outlook... Saturday night...VFR conditions expected. Northwest winds around 10 to 15 knots. Confidence: High. Sunday...VFR conditions expected. Northwest winds around 10 to 15 knots, becoming more northerly overnight. Confidence: High. Monday...Mainly VFR conditions expected. Light north to northeast winds. Increasing mid-high clouds late. Confidence: Moderate. Monday night...Periods of light snow and IFR conditions are possible late, mainly at ABE and RDG. Light northwest winds. Tuesday through Wednesday...Periods of light snow and IFR conditions are possible across all the terminals, with the greatest potential in terms of intensity and duration at ABE and RDG. Northwest winds 5 to 10 knots. Overall, lower confidence prevails.
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&& .MARINE... Periods of rain and fog will continue to result in visibility restrictions tonight. In addition, expect SCA conditions to persist through Saturday and beyond. Winds will be SW through tonight then abruptly shift to NW Saturday as a cold front crosses the waters early in the day and brings and and to fog and rain. Outlook... Saturday night...VFR conditions expected. Northwest winds around 10 to 15 knots. Confidence: High. Sunday...VFR conditions expected. Northwest winds around 10 to 15 knots, becoming more northerly overnight. Confidence: High. Monday...Mainly VFR conditions expected. Light north to northeast winds. Increasing mid-high clouds late. Confidence: Moderate. Monday night...Periods of light snow and IFR conditions are possible late, mainly at ABE and RDG. Light northwest winds. Tuesday through Wednesday...Periods of light snow and IFR conditions are possible across all the terminals, with the greatest potential in terms of intensity and duration at ABE and RDG. Northwest winds 5 to 10 knots. Overall, lower confidence prevails. && .HYDROLOGY... Rainfall...Total rain amounts of 1 to 2 inches can be expected through early Saturday. The heaviest rainfall, in terms of accumulation, is generally expected across northern NJ. Forecast Points...Many of our 48 hydrological forecast points will see rises in the 2 to 6 foot range. For the most part, these rises will remain well within bankful with the exception of the Passaic basin. A few gages are now forecast to reach Caution Stage. Outside of our 48 forecast points, poor drainage and/or low lying flooding is possible through late tonight. Specifically... The Delaware basin will see rises up to 1/2 bankful. The Schuylkill basin will see rises up to 1/2-3/4 bankful. The Raritan basin will see rises up to 3/4 bankful. The Passaic basin will see rises to 3/4 bankful to near bankful. The NAEFS and SREF ensembles indicate that widespread 2.50 inch basin averages would be needed to bring crests up to minor across portions of the Raritan and Passaic basins. We are not expecting this to occur at this time. A review of snow on the ground indicates that much has melted across the HSA. Mainly only trace amounts remain. Consequently, much of the SWE is gone as well. The exception is across northern NJ and is one of the reasons why the hydrologic models are 3/4 bankful or higher across portions of the Passaic and Raritan basins. River ice...As expected, the build up of ice on local rivers and streams have taken a step backwards the last 36 hours. Observations and images we are seeing show a melt has occurred. 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 for ice jams outside of the actual hydrology. Results and impacts will be very different if rivers and streams are running high or are 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, and going into this event, the region had more available channel than average. As mentioned, 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 below average flows, and the width of the channel itself (e.g. Delaware, Passaic, Raritan, Schuylkill), the threat of flash flooding continues to be 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 during Saturday and beyond. This will reverse our current two day melting trend. A build up of ice can be expected once again next week. && .CLIMATE... For today, 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 *Preliminary Record Event Reports (RERs) were issued this morning for ABE, RDG, and MPO, for record highs that were either tied or broken. Temperatures may fluctuate further, and final reports will be issued later today. 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 && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Winter Weather Advisory from 4 AM to 10 AM EST Saturday for PAZ060>062. Winter Weather Advisory from 1 AM to 10 AM EST Saturday for PAZ054-055. NJ...Winter Weather Advisory from 4 AM to 10 AM EST Saturday for NJZ001-007. DE...None. MD...None. MARINE...Dense Fog Advisory until midnight EST tonight for ANZ431- 450>455. Small Craft Advisory until 6 PM EST Sunday for ANZ450>455. Small Craft Advisory until 6 PM EST Saturday for ANZ430-431. && $$ Synopsis...Meola Near Term...Fitzsimmons/Gaines Short Term...Fitzsimmons Long Term...Meola Aviation...Fitzsimmons/Gaines/Meola Marine...Fitzsimmons/Meola Hydrology... Climate...

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