Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 212328 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 628 PM EST Mon Jan 21 2019 .SYNOPSIS... A strong area of low pressure across the Canadian Maritimes will gradually shift eastward, while strong arctic high pressure builds over our area during Tuesday. As the high shifts offshore Tuesday night, low pressure moves across the Great Lakes region Wednesday then into interior New England Thursday. A cold front will move across our region Thursday, with a secondary cold front arriving on Friday. A coastal storm may develop Sunday and Monday. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... Deep low pressure currently located over Atlantic Canada with a 1036 mb surface high nosing into the midwest. This is maintaining a tight pressure gradient over the area with a strong NW flow driving down arctic air. Winds continue to gust 30 to 40 mph and with temperatures in the single digits and teens for most areas this is producing subzero wind chills. Wind Chill Advisory remains in effect for Carbon and Monroe until 6 AM Tuesday but for remaining zones we won`t quite be at criteria but it will still be quite cold. Heading into tonight, low pressure continues to drift northward through Atlantic Canada with high pressure moving east toward the region. This will result in the pressure gradient finally relaxing with winds diminishing with time through the evening and overnight under mainly clear skies. Expect lows generally in the single digits except low teens over the southern Delmarva and the Philly metro area. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/... High pressure crests over the area Tuesday with temperatures moderating as the coldest part of the airmass moves out. Skies will be mainly sunny with lighter winds and highs reaching the 20s to low 30s...coldest north and mildest south. These temperatures are still a good 8 to 10 degrees below average. && .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/... Summary...Milder air returns Wednesday and Thursday with some wintry precipitation at the onset, followed by colder air Friday and into the weekend; A coastal storm is possible during the Sunday and Monday time frame. Synoptic Overview...An elongated upper-level trough is forecast to be from near Hudson Bay Canada all the way down into the southern Plains Tuesday night and Wednesday. This feature shifts eastward through Thursday and looks to weaken with time. This drives surface low pressure to our west drawing in warmer air (especially compared to the current arctic air in place), however plenty of energy diving southward into the Midwest amplifies a new upper-level trough from the Midwest and Great Lakes during the end of the week. The pattern becomes more uncertain as the positioning of a closed low or part of the polar vortex varies, but especially as does the short wave strength in the southern portion of the upper-level trough in the Deep South. While the guidance agrees that an overall trough will be in the East, the depth and timing of its axis carries more uncertainty. This has important implications on the placement of a surface low, which could be a coastal storm for the Mid-Atlantic and Northeast later in the weekend and early next week. For Tuesday night...Another positive tilted trough shifts eastward from the Midwest and Plains. This results in downstream ridging also shifting eastward, taking surface high pressure over our region to start Tuesday evening and then offshore. As the return flow becomes more established, low-level warm air advection will begin to increase. The winds are expected to decouple overall especially in the evening, and with just some high level clouds to start the temperatures should drop off fairly quickly in the evening. An increase in high level clouds through the night however will determine the extent of the temperature drop off. Overall a cold night, however nowhere near as cold as just recently. For Wednesday through Friday...The guidance is in good agreement with a long wave upper-level trough from the Midwest to the southern Plains shifting eastward. This drives surface low pressure up across the Great Lakes during Wednesday then into New England Thursday. An accompanying cold front moves through our area Thursday especially the first half of the day, then a secondary cold front arrives on Friday. The timing of the warm air advection precipitation will be key given the thermal column warming with time. The guidance shows a more pronounced warming of the 800-850 mb layer. This occurs faster than at the surface especially along the I-80 corridor. As a result, some sleet may occur before going over to freezing rain. If enough of a dry layer is present as precipitation arrives, then evaporative cooling at the onset could result in some snow in the far north. Overall, looks more like a period of some freezing rain before going over to all rain. The main area of freezing rain looks to be in the western part of Berks County, the Lehigh Valley and especially the Poconos into northwestern New Jersey. Given the surface high to our east and the storm track to our west and north, sufficient warming is expected to change any icing over to plain rain Wednesday afternoon and evening. Guidance overall shows a decent moisture plume ahead of and along the cold front, and this may result in some times of heavier rain. The system should be moving along therefore the potential for flooding looks to be on the lower side at this point. The rain ends from west to east during Thursday as the cold front shifts offshore, then cold air advection kicks in along with an increasing northwesterly breeze. The rain should end before sufficient cooling arrives limiting the chance for a change to snow before ending. This sets up a colder and drier Friday, although some lake effect snow showers may try and make a run at the southern Poconos. For Saturday through Monday...A shift to a more amplified pattern aloft stems from a closed low (a portion of the polar vortex) dropping southward from near Hudson Bay, and energy in the southern part of the trough in the Deep South and Gulf of Mexico. It is the latter energy especially that the guidance is handling differently, and this has important implications on a potential coastal storm later in the weekend and early next week. The GFS continues to be weaker with the southern energy and it then just takes it out to sea, however the GFS-FV3 is stronger with the surface low development but is more offshore compared to the ECMWF. The Canadian on the other hand significantly amplifies the energy into one and places a potent storm right over our area. The potential is there for a storm, however once again the challenge is the handling of energy in the northern and southern streams. No significant changes were made given the uncertainty especially at this time range, and we blended in the 12z WPC guidance. && .AVIATION /00Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR conditions. Northwest winds gradually diminishing to 5-10 knots by the overnight period. Tuesday...VFR with light and variable winds around 5 knots. Outlook... Tuesday night...VFR with some increase in clouds. Southerly winds near 5 knots, becoming light and variable. Wednesday and Thursday...MVFR to IFR conditions developing Wednesday afternoon and continuing through at last Thursday morning. Periods of rain expected, which may start as some sleet and freezing rain at RDG and ABE. South to southwest winds around 10 knots, becoming west to northwest Thursday. The conditions should improve to VFR later Thursday. Friday and Saturday...VFR is anticipated. && .MARINE... Gale force gusts are still expected to continue at least into this evening. Through the evening we should see winds begin to decrease and by late this evening, winds may be below gale force on the Delaware Bay. On the Atlantic coastal waters, gale force gusts are likely to continue for much of the night. The first significant freezing spray event of the season is still ongoing. Moderate freezing spray occurring with even locally heavy freezing spray. On the upper Delaware Bay, heavy freezing spray has occurred and a warning continues with advisories for remaining waters. The freezing spray conditions will improve when the winds diminish tonight into early Tuesday. Sub SCA conditions can be expected by mid morning Tuesday continuing through the remainder of the day as high pressure moves in. Finally, low water levels with the low tide will continue until 10 PM for the Delaware Bay and until 11 PM for the tidal Delaware River. Outlook.. Tuesday night through Thursday...Southerly flow and the seas increase and Small Craft Advisory criteria is probable, then decreasing later Thursday with a wind shift to the west and northwest. Friday and Saturday...Small Craft Advisory level winds may develop for a time Friday behind a secondary cold front, otherwise seas on the ocean may remain around 5 feet. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Wind Chill Advisory until 6 AM EST Tuesday for PAZ054-055. NJ...None. DE...None. MD...None. MARINE...Freezing Spray Advisory until 4 AM EST Tuesday for ANZ450>455. Gale Warning until 4 AM EST Tuesday for ANZ450>455. Heavy Freezing Spray Warning until midnight EST tonight for ANZ430. Low Water Advisory until 10 PM EST this evening for ANZ430-431. Gale Warning until midnight EST tonight for ANZ430-431. Freezing Spray Advisory until midnight EST tonight for ANZ431. && $$ Synopsis...Gorse Near Term...Fitzsimmons Short Term...Fitzsimmons Long Term...Gorse Aviation...Fitzsimmons/Gorse Marine...Fitzsimmons/Gorse

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