One of yesterday’s forecast posts outlined the potential for some weekend snow and lo and behold model guidance has increased precipitation outputs across the region significantly in the last 24 hours. In fact, several available tools point to another late season snow- and wind-storm across far E and SE MA and out onto the Cape and Islands.
Big Picture: Surface low pressure pressing eastward through the far Northern Plains today will move into the Western Great Lakes tonight and then dig southeastward to the Ohio Valley and Mid-Atlantic on Saturday. The system will then shift into the waters east of the Mid-Atlantic and south of New England Saturday night, attempting to make a quick and harmless escape out to sea.
However, a trailing upper level circulation pressing into the Mid-Atlantic on Saturday will catch up to the surface low pressure, slowing it down and eventually causing it to stall southeast of Nantucket Sunday and Sunday Night – as the entire system becomes “vertically stacked”. As a result, surface low pressure will wobble in place (perhaps even retrograde westward?) southeast of the region Sunday, Sunday Night and into Monday morning, before gradually heading eastward late Monday and into Tuesday.
What Does This Mean?: The evolution outlined above is a fairly classic set-up for a long duration “surprise” Cape Cod/Nantucket nor’easter and surface charts from model guidance appear to be trending that way.
Most products now show an extended period (36 hours in some cases) of strong NE winds beginning Sunday morning and persisting into Monday over the Cape and Islands. The ultimate strength of those winds remain in question, but history would suggest gusts in excess of 50 mph are on the table for at least some of the event. Likewise, numerous available models now show precipitation breaking out around the Cape Sunday morning, with steady (and perhaps heavy) precipitation persisting through the day and into the nighttime hours, before gradually tapering down and becoming more intermittent Sunday night and Monday.
Will it Snow? The atmosphere will remain fairly cold for this time of year through the upcoming weekend and will be supportive of snow for at least a portion of the time. However, the arctic cold over the region has maxed out over the last 24 hours and the air mass itself is beginning to grow a bit stale and “old”. Temperatures – while certainly still cold – at the surface and aloft will continue to gradually moderate through the weekend as the worst of this week’s chill dissipates. With that in mind, a strong, persistent northeast flow of air could allow for marginally “warm” marine air to eventually get involved over the region – meaning a mix or flip to liquid precipitation is certainly possible at some point. The exact details are to-be-determined but an all snow event is not locked in. That said, the odds for 4 or more inches of snow are climbing.
Concerns: If we assume the storm is coming and snow and wind are in the future, the main issue may very well be the combination of gusty winds and a heavy wet snowfall causing tree and limb damage and resultant power related issues. As mentioned above, the air mass is losing its polar / arctic characteristics and will become increasingly “marginal” as the weekend progresses. While the atmosphere should remain cold enough to support snow on Sunday, we may end up with a thick layer of air not far from the freezing mark – suggesting that any snow that falls will be of the heavy / wet variety. And given winds to 40 to 50 mph are easily within the realm of expectations, this raises concerns.
Unfortunately, this is also where the finer details of the forecast become so important. Frequently in late season snow events such as this, precipitation intensity will help determine the surface temperature, which helps determine how easily snow sticks and accumulates…which obviously shapes how the system impacts the region. If snow falls steadily and at a moderate to heavy clip, it tends to hold the surface temperature down a couple of degrees. The near 32F temperature results in snow easily accumulating and sticking and clinging to all surfaces. However, if the precipitation is more light and/or intermittent (and we have a strong wind blowing in off of the 40F ocean) we often see temperatures tick up a degree or two – sometimes several degrees above freezing – and the snow has a much more difficult time accumulating. In those cases, we are left with a wind-driven sloppy wet snow (sometimes mixed with rain) which doesn’t accumulate much and proves to be a cold, raw, depressing Spring nuisance more than anything.
With all of that in mind: The next 24 hours will be spent evaluating the incoming model data and trying to get a better handle on the finer details (timing, precipitation intensity, duration etc) of the upcoming weekend forecast. It’s a good idea to check back and see how the forecast has evolved and if you are tired of winter, just hope the system cuts off and stalls out far enough offshore to spare us a snowstorm.