Good writeup from GYX
Snow begins to enter southwestern New Hampshire in the 8-9 am window Tuesday morning as surface low pressure approaches the region from the northwest, dragging a warm front up from the
southwest. Snow will then gradually overspread the area from southwest to northeast entering southern Maine, closer to the 1pm hour. Rates are expected to be generally light as forecast soundings now suggest minimal negative Omega values within the DGZ. However, with profiles also well below freezing the snow
character is expected to be on the fluffier side so it likely accumulates efficiently. This may lead to some travel impacts,particularly in New Hampshire, as early as late morning and early afternoon. Timing for impacts in Maine start later, buteveryone should expect a less then ideal evening commute. We are
generally looking at 2-5 inches across the area, with locally higher amounts, 6-8 inches, on the central Maine coastal plain. The lower end amounts look to be in southern and western New Hampshire as these locations end up on the outer fringes of the deepening low.
On the note of the locally higher amounts, guidance has had a consistently strong signal for the low deepening and closing off as it moves off the coast setting up an inverted trough back
across Maine. This has the look of a Norlun trough setup as onshore winds will funnel moisture off the ocean into the trough creating a locally higher band of QPF somewhere along the Maine coast. Models have been trending up with this locally higher QPF footprint in the range of 0.5-0.75 inches, and with snow ratio on the order of 12- 15:1 this could easily produce warning level snowfall.
Norlun troughs are notoriously finicky withtheir footprint, sometimes being as much as 40 miles wide, but on the other extreme they have been as small as just impacting a few towns. They are also notorious for really putting down the
snow where they do form, so it is not out of the question for someone to end up with snowfall nearing a foot or more, but
again this would likely be isolated.
Due to the uncertainty, I`m not eager to upgrade the watch this forecast cycle, but the footprint is good. Hi-res models are locking on to the formation of a heavier snowband moving up the
coast with soundings showing much more negative Omega through the DGZ, that could push rates 1-2 inches per hour. The
fortunate thing is this looks to occur mostly late Tuesday night which should help keep travel impacts to a minimum and allow snow clearing operations to be unimpeded. The take away should
be 6-8 inches with isolated higher amounts near a foot possible within the current Watch area. I did go ahead with the Advisories as this is higher confidence and didn`t want to ignore the rest of the area due to the uncertainty along the
coast.