textproduct: Gray - Portland

WHAT HAS CHANGED

Snowfall amounts have come into better focus for Friday afternoon into Saturday morning.

KEY MESSAGES

1. Difficult travel is looking increasingly likely across much of New Hampshire for the Friday afternoon and evening commute as snow pushes in from the southwest. Snow will continue into western Maine in time for Friday evening commute with several inches possible by Saturday morning.

2. We continue to monitor trends of a coastal low pressure system that may bring a chance of snow to the area Sunday night and Monday. Current trends in guidance are favoring a less impactful solution, but nothing can be completely discounted at this time range.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... High pressure will retreat to the north Friday morning as a compact upper low rolls east across the Great Lakes region. The primary surface low will occlude over Michigan Friday afternoon with a secondary low forming near Long Island, NY Friday evening. The latest suite of mesoscale models are in good agreement that a push of WAA from the primary low will result in moderate Fgen forcing at 700 mb advancing SW to NE across the forecast area Friday. This will produce a band of moderate to locally heavy snowfall with rates around 0.5 to 1 inch per hour that translates across south-central NH Friday afternoon and into SW Maine Friday evening. Before snow breaks out surface temperatures will likely climb above freezing for all areas south of the mountains. While initial snowfall may not effectively accumulate, snowfall rates will likely compensate with accumulating snow impacting travel across southern and central NH by the afternoon and SW Maine by Friday evening. A warm nose aloft will creep into southern NH late Friday evening that may allow for snow to mix with or change to a period of sleet.

The initial band of snow will weaken as pushes northeast through central Maine Friday evening. The compact upper low will slide overhead Friday night into Saturday morning while the secondary low tracks east to a position south of Nova Scotia. The upper low will continue to provide a source of lift while good moisture remains within the dendritic growth zone. Most models suggest that a surface trough will linger over far western Maine into NH through Saturday morning that will allow for continued light snowfall. Much of the accumulating snowfall will likely end NE to SW by Saturday afternoon. The going forecast brings 4 to 7 inches across much of the area with lesser amounts near the Canadian Border and north of the Capital District of Maine. Amounts across southern NH are closer to 4 inches due to the mixing with sleet, while amounts here could increase if precipitation stays all snow.

KEY MESSAGE 2 DESCRIPTION... A trough begins digging into the Mid-Atlantic Sunday sparking cyclogenesis off the coast of the Carolinas. Meanwhile, high pressure builds over the northeast making for a mostly dry day with seasonable high temperatures. The aforementioned low begins moving northeast up the coast with many ensemble solutions suggesting high pressure keeps it mostly at sea. However, there are some notable recent trends in the ensemble guidance. The wave of interest that ultimately kick starts this is just coming into range of the RAOB network and really doesn't enter the Pacific Northwest until later today. That being said with what limited recon data the models have been able to incorporate, they have trended the ridge in the western CONUS more westerly. This would ultimately allow the low to end up closer to the coast and result in a greater chance for a more impactful snowfall for the Northeast. While the majority of ensemble members are clustered well to the south and east of the benchmark, there are still more than a few members nearby. It would be unwise to write this one off just yet until it can be sampled further. Probabilities for greater than an inch of snow on the New Hampshire Seacoast from the GEFS have trended up to a 30-40% chance. Even the Euro ensemble, which has wanted nothing to do with this storm from the beginning, is painting some 20% probabilities now in the New Hampshire Seacoast. NBM probabilities in the 19/01Z run have not budged giving the Seacoast less than a 10% chance of greater than an inch and less than 50% for measurable snow. This seems less realistic to me given what has been discussed. The reason I focus on the Seacoast is because that is the spatial extent of the probabilities for now, emphasizing the majority of ensemble members still out to sea. However, if a western trend in the 500 mb pattern continues to hold I wouldn't be surprised to see that trickle down to a westward trend in the low track. Plenty to watch over the next couple of days.

High pressure looks to either stay locked in or fill back in following the early week system, so should see mostly dry weather and seasonable temperatures heading into the middle of next week. A signal for a clipper system is starting to emerge as early as Wedensday night, but there is a lot riding on how things evolve with the first system so best to just take it one at a time.

AVIATION /12Z THURSDAY THROUGH MONDAY/

Through 00Z Friday...VFR prevails through the period with light winds.

Outlook...

Thursday night...VFR prevails through the night with clouds thickening and lowering across southern NH towards Friday morning.

Friday-Friday night. Ceilings will lower from SW to NE with snow overspreading the area Friday afternoon. IFR/LIFR is likely across southern NH by Friday afternoon with remaining TAF sites dropping to IFR/LIFR Friday evening. Snow continues Friday night.

Saturday...Snowfall rates decrease Saturday morning with some improvement by Saturday afternoon.

Saturday night - Sunday: Snow tapers off Saturday night, bringing gradual improvement to VFR, which should then prevail through the day Sunday.

Sunday night and Monday: Confidence is low on expected conditions for this time period. A coastal low pressure system may bring snow to the area, but it also may track out to sea leaving the area dry.

MARINE

7am Update to allow SCAs to expire on schedule as winds and waves continue to subside.

Previous discussion below... North winds gusting 20-25 kts and seas around 4-5 feet will continue through day break. High pressure builds in from the north today for quiet conditions over the waters into Friday morning. Low pressure tracks south of Cape Cod Friday night with NE winds gusting around 30 kts into Saturday morning with light freezing spray. Winds shift northerly Saturday with gusts around 20 kts while seas linger around 5 feet.

Conditions in the extended forecast period are going to depend highly on the track of strong low pressure moving up the east coast. At least small craft advisories are likely as wave heights build to greater than 5ft Sunday night and remain elevated into midweek. Wind gusts are going to be more track dependent. I would expect to see at least gusts 25-30kts as it makes its pass, but if it makes a closer pass gales are possible. Elevated wind gusts would taper off Tuesday morning.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...None.