Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 102023 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 323 PM EST Sat Dec 10 2016 .SYNOPSIS... A large surface high builds towards the region through tonight. By late tomorrow, the high will shift off shore as a low pressure system crosses the Great Lakes region, bringing to our region a warm front late Sunday night followed quickly by a cold front on Monday. A strong cold front is expected Wednesday. High pressure builds east through the end of the work week. && .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/... previous long term discussion below. This will be updated by 5 pm. Main story through the long term remains the Sunday/Monday wintry mix precipitation event. Changes from previous forecast: latest model runs have trended faster with the arrival of the warm front early Monday morning. Additionally, both the GFS and ECMWF show a dry slot over Delmarva and far southern NJ and far SE PA ahead of the arrival of the warm front. If both of these trends continue, the I95 corridor and locations south and east should see mostly (if not all) rain. For locations northwest of the I95 corridor (especially along and north of I78), this may unfortunately mean that they will have a longer period in the transition zone, meaning a longer period of freezing rain and sleet. As a result, the forecast snow amounts have decreased slightly, while storm total ice forecast for I78 and further north have increased. Hazards/impacts: At this point, while it looks like we might fall short of warning criteria (either for ice storm or winter storm), it will likely be a high end advisory event for I78 and north that could result in treacherous travel conditions especially Sunday night into at least the first half of the Monday morning commute. Additionally, there may be enough ice accumulation to result in some power outages in these locations. For the rest of the region, if these trends hold, the main impacts would be slippery conditions on area roads through the first half of Sunday night (if the precip moves in that early). However, by the time of the morning commute, precip should be all rain. Confidence: This forecast is still very dependent on how quickly the warm air will move in, so there is still considerable uncertainty with the exact timing of the change over, and consequently snow and ice amounts. Monday afternoon/evening: dry air advection is expected on the heels of the cold front, so precipitation should quickly come to an end behind the cold front which should be off shore by the evening hours. The one possible exception is that with the strong northwesterly flow behind the front, there may be some lake effect snow showers, which may get as far south and east as the southern Poconos. Tuesday through Saturday: Stayed close to model consensus to focus on both the snow showers this morning and the Sunday/Monday event. A brief period of snow is possible behind the next (and stronger cold front on Wednesday. && .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... previous outlook discussion below...this will be updated by 5pm... Sunday night and Monday...Widespread MVFR and even IFR conditions at times with low ceilings and reduced visibilities. Precipitation could begin as snow and may be a wintry mix for most TAF sites Sunday night (with possible exception of KMIV and KACY, which may have all rain). All precip should change over to rain no later than 18Z Monday. Windy conditions will be possible behind the cold front on Monday. Monday night...conditions should gradually improve to VFR. Breezy northwesterly winds possible during the evening hours. Tuesday...VFR conditions expected. Wednesday...MVFR or even IFR conditions are possible if wintry precip moves into the region. Breezy northwesterly winds possible. && .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... previous outlook discussion below...this will be updated by 5 pm... Monday...Prolonged SCA conditions for both winds and seas possible first with southerly winds behind a warm front early in the day, then an abrupt shift to breezy northwesterly winds is expected late in the day with a cold front. Gale force gusts are possible especially on the ocean waters. A gale watch may be issued if confidence remains this high as we get closer. Tuesday...winds 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, though a brief period of gusts above 25 kt will be possible behind a cold front. && .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 Near Term...Miketta Short Term...Miketta Long Term...Drag Aviation...Drag/Miketta Marine...Drag/Johnson/Miketta

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