Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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551 FXUS61 KPHI 102113 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 413 PM EST Sat Dec 10 2016 .SYNOPSIS... Large high pressure in the Ohio Valley moves to the mid Atlantic states by Sunday morning. Low pressure crossing the Great Lakes region Sunday night pushes a warm front into our area. This will be followed by a cold frontal passage Monday afternoon as the low moves to Maine. A strengthening cold front is expected Wednesday. Strong high pressure builds east through the end of the work week bringing our coldest air of the season so far Thursday and Friday. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... Snow squalls generated by cold air moving over the relatively warmer waters of the Great Lakes will continue through the late afternoon hours over parts of the southern Poconos...Lehigh Valley...and northwest New Jersey. Most areas that get any snow will experience a dusting to less than one-half inch...but this will be enough to cause slippery conditions on untreated roadways. W TO NW winds are expected to diminish quickly around sunset as mixing subsides and as high pressure becomes centered over the mid- Atlantic region tonight. As the northwest flow begins to shut down, snow streamers from the Great Lakes will also shut down, bringing an end to lake effect snows over the northern portions of our CWA. Under clear to partly cloudy skies...radiational cooling will bring temperatures down to the low teens in the north and low to mid 20s south. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... A warm front associated with a low pressure system moving north through the Great Lakes region...coupled with cold air still in place from a high pressure system moving off the mid-Atlantic coast...is expected to produce snow over the northern third of the CWA beginning Sunday afternoon. Temperatures are expected to be cold enough for all snow in the Poconos...Lehigh Valley...and northwest New Jersey. However...snowfall totals through 6 PM Sunday should be limited to around one inch or less given the limited moisture at the front end of this event. A Winter Weather Advisory has been issued for snow and ice accumulations expected during the overnight hours Sunday night into Monday morning...generally north of the I-78 corridor. A few snow showers...perhaps mixing with some light rain...could occur as far south as the Philadelphia metro area...with little to no impact expected. Winds are expected to be light and variable to start the day...but a southerly flow in the 5 to 10 mph range is expected to develop during the afternoon hours. High temperatures will range from the mid to upper 20s in the far north to around 40 on the Delmarva. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... **Potential for three snow/ice events between now and next Saturday for e PA and the north half of NJ** The La Nina Ohio valley storm track appears to be in place and this winter gets off to a good start over northeast PA and northern NJ. Snow cover should be expanding nicely across the northern tier of the USA the next week or two and that probably means down into a portion of our area as well... which will also allow for the easier southward advance of sub normal cold. This December looks to be far different than last years extreme record warmth as a more realistic winter develops in our area. Hazards: The main story (aside from any remaining snow squall streamers into the I-78 -I80 region tonight), will be a snow and ice advisory hazard for portions of e Pa and northwest NJ Sunday afternoon into early Monday. There is less certainty regarding another wintry event Tuesday night or Wednesday, and then there should be a warm air advection snow or ice event (to rain) developing next weekend. Impacts: Sunday afternoon-evening and/or Monday morning commute. slower travel due to snow or the results of mixed pcpn. 500 mb: A nice little short wave moves from the Great Lakes Sunday night to Maine by late Monday. A very cold for the season vortex in central Canada (-3 sd) moves to eastern Canada by Friday, then strong ridging develops along the east coast next weekend as a longitudinally large positive tilt trough develops in the western USA, permitting an extensive warm air advection pattern (aloft) to occur in the mid Atlantic states. Temperatures: Calendar day averages Monday will average about 5 degrees above normal, near or slightly above normal Tuesday, cooling below normal Wednesday with calendar mins possibly occuring at 1159PM, then average around 10 degrees below normal Thursday and Friday, and still a few degrees below normal next Saturday. Temps could run continuously at or below freezing along and northwest of I-95, including New Brunswick, Trenton, Philadelphia and Wilmington from approximately 10 PM EST Wednesday through 10 AM Saturday. Forecast basis: Unless otherwise noted, a 50 50 blend of the 12z/10 GFS/NAM MOS guidance Sunday night-Monday night, thereafter the 12z/10 GFS MEXMOS Tuesday, and then the 15z/10 WPC guidance Tuesday night- Saturday, at times blended with the 12z/10 GFS MEXMOS guidance or the long term temps were adjusted because of the 12z/10 ECMWF 18z 2m temp fields. The dailies... Sunday night-Monday morning event: Mixed wintry pcpn much of I78 north Sunday night (periods of snow) into early Monday morning (ice), then ends as above freezing temps rain showers forenoon. South of I-78 there is quite a bit of uncertainty regarding amounts of any precipitation when temperatures and thermal profiles would allow wintry precipitation. These patterns without a blocking high to the north, usually yield some snow and ice but not likely to have warning amounts, except if more than .50 inches of water equivalent frozen-freezing pcpn can occur. The Great Lakes short wave induces a strengthening southwesterly 850MB jet in the Ohio Valley and also Va Sunday evening that begins merging into a sw 60 jet core near Baltimore MD around 06z/12 and then Monday morning is just east of NJ. By then, the wintry pcpn damage is done. Confidence: This forecast is still very dependent on how quickly the warm air will move in, so there is still uncertainty with the exact timing of the change over, and consequently snow and ice amounts. We felt it was in everyones best interest to issue an advisory where confidence for mixed freezing precipitation was 80 percent. The advisory may yet need to be adjusted based on the updated model guidance and forecaster expectations. Monday afternoon/evening: dry air advection is expected on the heels of the cold front, so precipitation should come to an end from west to east behind the cold front which should be off shore by the evening hours. Its still possible a couple of showers could linger in se DE till around 8 PM Monday. Tuesday...Dry during the day with weak high pressure interlude and a decent amount of sunshine through some mid and high clouds. A good day to catch up on wrapping up outdoor decorations etc before a southward expansion of frozen ground develops at the end of the week. Tuesday night...increasing clouds. Wednesday...Multi models disagree on the northward extent of a wintry pcpn event, which would probably be snow here...if it precipitates. Nothing major foreseen but still worthy to keep an eye on, especially with sub freezing temperatures immediately following. Thursday-Friday...Brrr... the real deal cold and I still dont believe this is pure arctic. I think of this as strong polar outbreak, with a secondary cold frontal passage early Friday that may be accompanied by flurries or a snow shower. Saturday... a bit of snow to ice and then rain along and southeast of I-95, but a little longer period of snow and ice possible along and north of I-78. && .AVIATION /21Z SATURDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR conditions are expected through Sunday morning as high pressure crests over the area tonight...then moves off the mid-Atlantic coast on Sunday. W to NW winds gusting to 20 knots or so late Saturday afternoon will subside quickly this evening...then become light and variable for most of the night. A few snow flurries are possible in the far north late this afternoon and very early this evening...but are not expected to impact any TAF sites. Clouds will lower starting Sunday morning across the region, with precipitation in the form of snow showers moving into northern areas during the afternoon...perhaps impacting both RDG and ABE after 18Z with sub-VFR conditions. The other TAF sites are expected to remain VFR through a good part of the afternoon. Light and variable winds in the morning are expected to become southerly in the 5 to 10 knot range during the afternoon. Outlook... Sunday night and Monday...Widespread MVFR and even IFR conditions at times, especially 06z-18z/12 with low ceilings and reduced visibilities. Precipitation could begin as snow and may be a wintry mix for most TAF sites KRDG -KABE and KTTN Sunday afternoon or night. All precip should change over to rain no later than 14Z Monday. Gusty west winds will be possible behind the cold front on Monday afternoon with conditions probably becoming VFR by late in the day. Monday night...VFR. Breezy northwesterly winds possible during the evening hours. Tuesday...VFR with sct-bkn clouds aoa 8000 ft. Wednesday...MVFR or even IFR conditions are possible if wintry precip moves into the region. Gusty northwesterly winds by night. Thursday...VFR sct-bkn aoa 4000 ft. northwest wind gusts to 25 or 30 kt. && .MARINE... Winds along the northern New Jersey coast have subsided enough to allow the SCA to expire. There could still be a few gusts approaching 25 knots in this area into the evening hours, but the trend in down. W to NW winds under 10 knots are expected tonight...becoming south around 10 knots later Sunday afternoon. Seas are expected to remain well below 5 feet during the same period. Outlook... SCA probable for the Atlantic waters late Sun night-Monday for seas and probably a period of 25-30 kt waa southerly gusts shifting to west by afternoon. However, transfer is poor and gales, if any, should be restricted to the outer coastal waters, if there. We probably wont know since 44009 conts ots into Feb 2017 and the stronger wind fields are expected to remain south of 44091 (seas sensor) and 44065 (wind and seas). Tuesday...winds and sea should drop below SCA criteria early in the day, and sub SCA conditions should continue for the remainder of the day. Wednesday...Mostly Sub-SCA conditions are expected through the day, then SCA winds probable Wednesday night behind a cold front with potential for gale force gusts in the northern Nj waters. Thursday...At a minimum, widespread northwest flow caa SCA and probably a gale event of intermittent 34-40 kt gusts all waters Thursday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Winter Weather Advisory from 1 PM Sunday to 9 AM EST Monday for PAZ054-055-062. Winter Weather Advisory from 1 PM Sunday to 6 AM EST Monday for PAZ060-061-105. NJ...Winter Weather Advisory from 1 PM Sunday to 9 AM EST Monday for NJZ001-007-008. Winter Weather Advisory from 1 PM Sunday to 6 AM EST Monday for NJZ009-010. DE...None. MD...None. MARINE...None. && $$ Synopsis...Drag 412 Near Term...Miketta Short Term...Miketta Long Term...Drag 412 Aviation...Drag/Miketta 412 Marine...Drag/Johnson/Miketta 412

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