FXUS61 KPHI 310230 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 930 PM EST Mon Jan 30 2023 .SYNOPSIS... A weak cold front moves across the region tonight, then becomes nearly stationary over the Mid-Atlantic. Several waves of low pressure will develop on and track along that front through the mid-week period. High pressure settles over the northern half of the forecast area otherwise during this time. A strong cold front moves through the region on Friday, followed by Arctic high pressure for the weekend. && .NEAR TERM /THROUGH TUESDAY/... Clouds diminished earlier this evening across the Delaware Valley, south NJ and much of Delmarva providing some pleasant sunshine for the close of the late-January day. Clouds held across the Poconos, north NJ and the Lehigh Valley. Now, clouds from the NW are slowly moving back across the CWA as the front approaches. Sky and temperatures were adjusted in the grids for the evening. As the boundary drifts across our region overnight, hi res guidance is suggesting light precipitation will be associated with its passage. Precip will start as light rain before transitioning briefly to a rain/snow mix or all snow showers for our highest elevations across the Pocono Plateau and northern Lehigh Valley immediately behind the front. Forecast soundings indicate most of the moisture and lift below the DGZ, so snow crystal growth will be inefficient and any rain/snow amounts with this front will be light with lacking upper support and weak convergence along the boundary; QPF should only be up to around a tenth of an inch with at most a dusting of snow. No impacts are expected as road temperatures are well into the 50s as of this afternoon. Temps will linger in the low to mid 40s ahead of the front before dropping to the 30s (20s in the Poconos) behind the front overnight. For Tuesday, the boundary will continue to slowly push to the south and stall south of our forecast area. Some ripples of vorticity/weak low pressure will ride along it and keep precipitation chances in the forecast for the Delmarva area and perhaps into southern New Jersey. Temperatures should be well above freezing so expecting just plain rain with totals around another tenth to two tenths of an inch possible. Northwesterly winds around 5-10 mph may gust in the 15-20 mph range behind the front as cold advection ensues. The day will be cloudy but dry for the rest of the area with highs in the upper 30s north and west of the Fall Line to near 40 to the south. && .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/... Weak low pressure develops on a stalled frontal boundary over the Mid-Atlantic and passes mainly south of the region from after midnight Tuesday night through midday or so Wednesday. There seems to be a slight shift north to the track of this low, and this looks to bring a shot of light accumulating snow towards the I-95 corridor. Models also indicating a strong shortwave passing north of the region, and this may enhance the snow across the northern half of the forecast area. QPF amounts will be light, generally less than 1/10 inch across Delmarva and southern New Jersey with only several hundredths of an inch up to the I-95 corridor. As a result, snowfall accumulation will overall be less than an inch, with the "highest" amounts from southern Delaware to extreme southeast New Jersey. Most locations north and west of that area will pick up a dusting to as much as 1/4 inch snowfall. Snow tapers off Wednesday morning. This may be just enough snow to end the streak of consecutive days without measurable snow in Philadelphia. Prior RGEM model runs showed a swath of 2 to 4 inches of snow, but 12Z/30 RGEM has backed off on that and has come in line with less than an inch of snow. A colder airmass overspreads the area. Highs on Wednesday will top off in the mid to upper 30s, then lows Wednesday night will be in the 20s. Another weak low passes south of the region Thursday, and may bring some wintry precip into southern Delmarva and possibly into extreme southeast New Jersey by afternoon. Again, QPF looks to be light. Highs will be in the upper 30s to low 40s. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... A short-lived blast of Arctic air will spread into the region Friday through Saturday night. The local area will be caught between two systems Thursday night. Low pressure will track along a stalled frontal boundary, passing south of Delmarva Thursday night through Friday morning. Any precip should stay south of the region by Thursday night. Meanwhile, a strong upper trough with strong shortwave energy will pass north of the region, with the strongest shortwave energy passing through central New York and the Pocono Mountains. Although the bulk of the snow showers will stay north of Carbon and Monroe counties, cannot rule out a few snow showers late Thursday night, and will carry slight chance PoPs for the southern Poconos. A strong cold front ahead of an Arctic airmass then blasts through the region Friday afternoon. 850 mb temperatures fall to -15C to - 22C by Friday evening. The daily high will be sometime Friday morning, and then temperatures fall to the teens and 20 through the afternoon. Cold and dry conditions then on tap from Friday night through Saturday night. A tight northwest pressure gradient will develop over the area between high pressure centered over central Canada north of the Great Lakes and deepening low pressure over Newfoundland and Labrador, and winds increase to 15 to 25 mph with 25 to 35 mph gusts Friday afternoon and Friday night, then winds diminish by Saturday. Lows will drop to as low as -5 in the southern Poconos and northern New Jersey, the single digits otherwise, and in the low teens in Delmarva. Wind chills will be as low as -15 across much of southeast Pennsylvania and New Jersey, but from -20 to -25 in the southern Poconos and far northern New Jersey. Highs on Saturday will range from 15 to 25 throughout, and these temperatures will be around 20 degrees below normal. High pressure slides over the area Sunday night, and then will move offshore Sunday. Return flow sets up, and the Arctic airmass departs. Temperatures warm back up to normal on Sunday. Uncertain forecast for Sunday. Arctic high pressure will be offshore, but a deep trough will approach from the west and pass through the East Coast. 12Z/30 GFS has surface low pressure developing ahead of it, and this would bring some precip to the area during the day. 12Z/30 ECMWF and 12Z/30 CMC are dry. However, models are indicating a coastal low developing off the Southeast coast. For now, that system itself is too far south, but will see how this evolves over the week. High pressure returns by Monday. && .AVIATION /02Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR ceilings with a period of MVFR ceilings possible late as a weak frontal boundary moves through. A few showers possible especially around ABE/RDG. Some light snow showers may mix in after around 09z. Southwest winds around 5 knots, becoming northwest 5 to 10 knots after midnight. Moderate confidence. Tuesday...MVFR ceilings possible for terminals along I-95 and KMIV/KACY with rain showers. KRDG/KABE expected to remain predominantly VFR. Northwesterly winds 5-10 knots with gusts 15-20 knots. Moderate confidence. Outlook... Tuesday night...Potential for sub-VFR conditions, especially at KMIV/KACY in low CIGs and rain/snow, changing to snow after midnight. Potential for those sub-VFR conditions to spread towards the I-95 corridor terminals KTTN-KPNE-KPHL-KILG. LGT/VRB winds, becoming NW 5 to 10 kt late. Low confidence. Wednesday through Wednesday night...Improving conditions by 18Z. W winds 5 to 10 kt. Moderate confidence. Thursday through Thursday night...Potential for sub-VFR conditions at KMIV/KACY late Thursday/Thursday night, and potential for sub-VFR conditions at KRDG/KABE. VFR otherwise. SW winds 5 to 10 kt, becoming W 10 to 15 kt with gusts to 20 kt Thursday night. Low confidence. Friday through Friday night...VFR. NW winds 15 to 20 kt with 25 to 35 kt gusts. Moderate confidence. Saturday...VFR. N winds 10 to 15 kt with gusts to 20 kt. High confidence. && .MARINE... South to south-southwest winds are anticipated to remain in the 10 knot range through the evening with seas 2-3 feet. A weak frontal boundary pushes through after midnight, shifting winds to the north- northwest and increasing to 10-15 knots with gusts around 20 knots through the day on Tuesday. Seas will remain around 2-3 feet on Tuesday. No marine headlines are expected. Outlook... Tuesday night...Sub-SCA conditions. VSBY restrictions in rain/snow. Wednesday through Wednesday night...Sub-SCA conditions. Thursday through Thursday night...Sub-SCA conditions Thursday, then SCA conditions develop Thursday night. Friday through Friday night...Strong SCA conditions with potential for gale force winds. A chance for freezing spray starting Friday afternoon. Saturday...SCA conditions come to an end. A chance for freezing spray. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ SYNOPSIS...MPS NEAR TERM...Brudy/PO SHORT TERM...MPS LONG TERM...MPS AVIATION...Brudy/MPS/PO MARINE...Brudy/MPS/PO