Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 170327 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1027 PM EST Tue Jan 16 2018 .SYNOPSIS... A ridge of high pressure will weaken across the area today. A low pressure system and front will approach from the Great Lakes region tonight. Another low will develop off the coast and affect the weather tonight and early Wednesday. More high pressure will build towards the area late this week and remain for the weekend. Low pressure and a cold front will move towards the Middle Atlantic region for the start of next week. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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950 PM Update...Periods of light snow continue to affect our western zones from Berks County north into the southern Poconos. The precip/snow has been a little slower to push east so we adjusted POPs to reflect this. Also, based on latest model guidance and trends, we adjusted snow amounts down a bit...mainly for the southern Poconos. Otherwise, no major changes. Still expect the snow to pick up in intensity as precip expands east overnight. To all snow observers including media... please send your amount reports to our skywarn account, or our NWS FB or twitter accts. fwiw...you`re probably aware of 4-6" in parts of central PA now with mdt snowfall rates - 1/2sm vsby. Even 5" in Carbon County now. Thank you in advance. wd/phi 551P. Afternoon update...Surface analysis shows an inverted trough located off the southern Mid-Atlantic coast. This trough extends northwestward across the Delaware Valley and toward the eastern Great Lakes where the remains of a clipper low exists. A secondary low will organize off the Mid-Atlantic coast tonight. The coastal low is expected to track about 100 miles east of the DE and NJ coastline late tonight into early Wednesday morning. A complete update to the forecast was published late this morning. A new storm total snow graphic and experimental probabilistic graphics were sent to the web as well. These updates required headline changes. The key changes in the forecast are listed below: (1) Upgraded to a Winter Storm Warning for Carbon and Monroe Counties in PA and Sussex County in NJ with 5-7" of snowfall (including the 1" or less that has fallen so far) expected for this event. Hi-res models are keying in on strong mesoscale banding on the northwestern side of the developing coastal low late this evening and overnight over the southern Poconos and continuing into the Wednesday morning commute for northwestern NJ. (2) Tweaked the start/end times for the Winter Weather Advisories with an emphasis on delaying the onset of accumulating snow to the S/E of the Lehigh Valley, southern Poconos and far NW NJ (where light snow is ongoing). See the experimental onset timing graphic on our winter webpage for specifics. (3) Introduced more mixing E of the Fall Line, especially early on in the event. Temps this afternoon in the 40s along/E of I-95 and mid to upper 30s just to the west. Just about all of the guidance shows this residual warm boundary layer present at the onset of precipitation tonight, resulting in mainly rain along and E of I-95 at the onset and a rain-snow mix just to the N/W. CAA and dynamical cooling will allow for a changeover to snow from NW to SE toward early morning but expect riming and melting at the surface to result in lower snow ratios and thus snowfall amounts near and especially south of I-78 as well as east of I-287. (4) Emphasized window for heavy snow with snowfall rates of 1-2" per hour late tonight across NE PA and N NJ. The heavy snow potential extends into the first half of Wednesday morning along and east of the DE Valley. This means significant impacts for the morning commute. Even though snowfall amounts are generally lower than warning criteria for Warren and Morris Counties in NJ (3-6"), we will have to consider upgrading to a warning based on impact on the morning rush.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/... The accumulating snow should be over by midday for just about the entire area. However, very light snow or flurries could linger or redevelop during the afternoon near the coast as the upper low and associated upper-level jet streak approaches from the southwest. High temperatures range from the mid 20s in northeastern PA to mid 30s in central/southern NJ, Philadelphia, and Delmarva. && .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/... Main focus of the long term is the warmup late this week through this weekend and a storm system moving through the region early next week. Most of the focus today was on the first 36 hours of the forecast, so overall made few changes from the previous shift. Trough axis swings through the Eastern Seaboard Wednesday night, with a broad surface high building into the Mid-Atlantic and Southeast for the rest of the week into the weekend. After a small- scale perturbation moves through the Northeast Thursday night (which should be dry), midlevel zonal flow will be present Friday through the weekend, allowing the surface high to slide eastward and promoting southwesterly surface flow to develop east of the Appalachians this weekend. This pattern will result in a lengthy dry period and a distinct warming trend after a chilly Thursday (when heights/thicknesses will be lowest, owing to the proximity of the trough). Forecast highs are in the 50s for Philly this weekend, based on a consensus blend of statistical guidance...though I leaned a little on the higher side of consensus given the tendency for MOS to be on the low side in warming patterns. Operational models develop a strong surface cyclone in the central plains this weekend (with downstream ridging further contributing to warmth Sunday and Monday in our area) before lifting it rapidly northeastward into the Great Lakes and adjacent southeast Canada early next week. A strong cold front will progress through the East Monday and Tuesday, with showery precipitation developing near the front. Temperatures will likely be warm enough for liquid in most locations for the event, with the exception being the far northwest CWA during the precip`s waning stages. Regarding timing, models generally agree on Monday afternoon through Monday night. Given such good agreement this far out, boosted PoPs to high chance or likely during this time frame. The greater uncertainty lies with the intensity of the precipitation (given the southern origins of the system and the warmth it advects downstream, there is some potential for decent precipitation totals, especially if embedded convection can develop), extent of cold air on the upstream side of the system (i.e., precipitation type issues as the system departs), and the speed with which the precipitation moves out before the colder air moves in. As previously mentioned, did not change the details during this time frame too much, but my initial thoughts were to go on the warm side and on the quick side given typical model biases with northeast- progressing inland surface lows originating from the southern/southwestern U.S. By midweek, temperatures will be slightly cooler, and conditions should dry out. && .AVIATION /04Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...Deteriorating conditions expected, with sub-VFR CIGs spreading east through the Philly terminals as well as KMIV/KACY with time as precipitation spreads east from the Poconos/Lehigh Valley through the overnight hours. Temperatures will likely be warm enough for the precip to be rain to start with at the Philly terminals (with transition to snow expected near/after 09Z) and may be predominantly rain at MIV/ACY (at least through 12Z). Sub-VFR VSBYs will develop during snow. Winds will be light and variable, though transition to northwesterly should occur by daybreak. Confidence in precipitation type/timing is below average. Confidence in CIGs/VSBYs is slightly below average. Confidence in winds is above average. Wednesday...MVFR/IFR CIGs/VSBYs during the early morning hours will rapidly improve to VFR as precipitation moves out from west to east. Precip will likely end as snow at all terminals. There is a chance for lingering light snow through much of the day at MIV/ACY, though confidence is too low for mention in the TAFs at this time. Winds will become northwesterly and increase to 7 to 15 kts with potential for some gusts to 20 kts or so during the late morning and afternoon. Overall confidence is slightly below average. OUTLOOK... Wednesday night...VFR with northwest winds 5 to 15 kts. High confidence. Thursday through Sunday...VFR with light west or southwest winds. High confidence. && .MARINE... Tonight...Seas at our nearby offshore buoys continue to remain elevated between 4-6 ft this afternoon despite a light onshore wind. We extended the SCA into tonight with the latest guidance keeping seas around 5 ft through tonight. Wednesday...NW winds will strengthen in response to a deepening coastal low that tracks just to the east of our waters. Expect wind gusts to increase from S to N across the region during the day with gusts around 25 kt. Seas will be steady state between 4 and 6 ft. The SCA continues for the coastal waters. Additional SCA was issued for the lower DE Bay for winds. OUTLOOK... Wednesday night and Thursday: Freezing spray possible near the coast and in Delaware Bay. Small craft advisory conditions for Lower Delaware Bay and the Atlantic waters, with gusty northwest winds and seas near 5 feet slowly diminishing by Thursday afternoon. Thursday night through Sunday: Sub-advisory conditions and fair weather expected. && .EQUIPMENT...
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Sudlersville NWR conts off the air overnight. RDG temp turned off and our ET staff hopefully can resolve tomorrow. Data went bad in a gradual fashion beginning ard 2325z. PTW and GED have FAA comms issues.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...Winter Storm Warning until 11 AM EST Wednesday for PAZ054-055. Winter Weather Advisory until 11 AM EST Wednesday for PAZ062. Winter Weather Advisory until 7 AM EST Wednesday for PAZ060- 061-101-103-105. NJ...Winter Weather Advisory until noon EST Wednesday for NJZ008>010. Winter Storm Warning until 11 AM EST Wednesday for NJZ001. Winter Weather Advisory until 11 AM EST Wednesday for NJZ007. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EST Thursday for ANZ450>455. Small Craft Advisory from 9 AM Wednesday to 6 AM EST Thursday for ANZ431. && $$ Synopsis...CMS Near Term...Fitzsimmons/Klein Short Term...Klein Long Term...CMS Aviation...CMS/Fitzsimmons Marine...CMS/Klein Equipment...Drag 1026PM.

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