textproduct: Gray - Portland

WHAT HAS CHANGED

Confidence continues to increase for plowable snowfall across southern NH on Wednesday night.

KEY MESSAGES

1. A narrow axis of snowfall will develop from west to east across New England Wednesday through Wednesday night. Light snowfall accumulations will bring potential for slick travel Wednesday evening and night across southern NH. Some north or south shifts in the axis of snowfall remain possible.

2. The next chance of widespread precipitation looks to be in the Friday/Saturday time period. While conditions look supportive of mostly snow, there remains a wide spread of solutions for snow magnitude and where it falls.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... A frontal system will setup over southern New England on Wednesday into Wednesday night as an area of low pressure develops and moves along this boundary. To the north, high pressure over Canada combined with northerly flow will result in drier air over much of the region with a transition zone across southern NH. As a result, a relatively narrow band of accumulating snowfall is expected with the vast majority of the snow accumulation to be confined to southern NH and towards the Massachusetts state line. Latest 12Z forecast guidance shows total QPF of generally between 0.10-0.20" across southern NH with closer to 1/3rd of an inch towards the Monadnocks with the assistance of some terrain enhancement. Forecast soundings show a rather high DGZ (~600 hpa) with the greatest forcing for ascent below this layer. In addition, there will likely be around 20 kts of wind aloft, which will help to break up the dendrites some. Therefore, snow-liquid Ratios (SLR) should be generally <=13:1. This results in overall 1-3" of accumulation from the Concord region and points south with 3-4" possible in the Monadnocks. A coating to an inch is possible as far north as the Lakes Regions of NH and ME with little to no snow north of this. There is some potential for things to shift north/south within the next 24 hrs or so, which would result in changes to expected snowfall amounts.

Timing wise, snow may begin as early as mid afternoon across southern NH but sfc temperatures will be relatively mild into the middle to upper 30s and this combined with the increasing sun angle should limit impacts to most area roads until closer to sunset. Snow will then end within an hour or two either side of midnight.

KEY MESSAGE 2 DESCRIPTION... Next widespread precipitation system arrives late week in the Fri/Sat time frame. Current pattern supports continued active forecast as strong jet bisects the CONUS.

Key features associated with this event will be low pressure moving north into the Great Lakes Friday morning, remnant low pressure across the Upper Midwest, and commanding high pressure over Hudson Bay. This overall setup appears fairly similar to the one taking place Wednesday of this week, but current guidance has a further north consensus.

As the low advances into the Great Lake, SW to NE jet will shear the system east with a northerly push of IVT. Broad high pressure spilling across northern/central Quebec and Ontario will hold, preventing much of the energy progressing north and instead advect east across New England.

There is fairly good agreement on positioning, but differences in magnitude and mesoscale features will continue to vary QPF and potential surface impacts. Looking at ensemble clusters, two scenarios encompass roughly 66% of members between the GEFS, ENS, and GEPS. The first, heavily weighed towards the ENS, is a more amplified pattern in the eastern CONUS with slightly lower QPF across the forecast area compared to the second cluster. The second, weighed heavier in the GEFS and GEPS, features a more zonal pattern to the south of the region with greater QPF. Why the lower QPF despite more amplification? Perhaps QPF for this system will be more dependent on longevity in the precip shield vs. any one surface forcing feature. Thus a more consistent west to east flow aligns in a precip axis. For now, resulting QPF would support a couple inches of snow for much of the forecast area, but with the spread in guidance, there is still time to refine specifics.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

Through 18z Wednesday...MVFR ceilings will persist through at least 06Z Wednesday and some patchy FG may bring visibility restrictions. CIGs will increase later tonight through 12Z Wed with mainly VFR conditions through 18Z Wed. CIGs will then begin to lower again after 18Z Wed across southern NH TAF sites (KMHT, KCON, KLEB, and KPSM) with -SN developing. Some restrictions are possible as far north as KPWM but forecast confidence for this is low.

Outlook...

Wednesday Night: MVFR cigs may remain across far southern NH, trending to VFR overnight.

Thursday and Thursday night: VFR w/ no sig wx.

Friday and Friday night: MVFR cigs approach from the west across NH with SN. IFR vis possible in SN.

Saturday: Uncertainty how quickly SN exits east along with ceiling restrictions.

Saturday night and Sunday: Trend should be VFR with high pressure to the north.

MARINE

Seas outside of the bays will increase to 3-5 ft late tonight with 1-3 ft in the bays. Northerly winds will increase Wed night with gusts up to 25 kts possible across the outer waters but otherwise winds and seas will largely remain below SCA criteria.

A period of SCA winds will be possible Friday night and Saturday morning with gusts up to 30 kts. This will accompany low pressure as it slides east across New England. At the same time, low pressure may redevelop off the Mid-Atlantic coast. In NE winds, wave heights should respond, building 5 to 6 ft off the NH seacoast and southern ME coast.

GYX WATCHES/WARNINGS/ADVISORIES

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