Here's a breakdown on certainties/uncertainties as our storm moves in. The most impactful parameters are fairly certain:
WHAT IS CERTAIN:
- HISTORIC, MAJOR OCTOBER NOR'EASTER that will deliver heavy, wet snow to most of the interior, resulting in widespread power outages for many and record snow for so early in the year, even into Boston, Hartford and Providence by the storm's end.
- POWER OUTAGE THRESHOLD: Where leaves are on the trees, accumulations of 3" or greater will cause scattered outages. Accumulations of 5" and greater will cause widespread outages.
- HEAVY RAIN EAST: Approximately 1.5" to 2.25" of rain will fall in the Boston to Providence corridor, and many areas along and inside of Interstate 95, with similar but slightly lesser amounts on Cape Cod. This will be enough for some wet basements, but moreover, for big puddles and hydroplaning concerns with a few streams coming above their banks during the heaviest rainfall Saturday evening and night.
- COASTAL WIND: Coastal winds will gust over 45 mph. Cape Cod gusts will exceed 60 mph at times. Wind will come from the northeast, then shift to whip from the north Sunday morning.
- MINOR COASTAL FLOODING: Minor coastal flooding will result from the combination of astronomically elevated tides with 10-15 foot waves, particularly at vulnerable northeast-facing shorelines
WHAT IS UNCERTAIN, AND WHAT IT MEANS TO YOU:
- EXACT SNOWFALL AMOUNTS: Exact snowfall amounts have inherent uncertainty, but especially in a situation like this, due to a combination of rain/snow precipitation type, elevation dependency with colder temperatures in higher terrain, but also the important factor of snow to water ratio. That is, temperatures throughout the entire cloud determine the TYPE of snow that falls - most spots will be heavy and wet, but variance of only a few degrees aloft can make a huge difference in how much water equals how much snow. The impact to you and I is not significant: as you look at my accumulation map, yes, there will be some variance of one, two or even three inches in some spots, especially when elevation comes into play, but the bottom line is IF YOU ARE IN A REGION I'M FORECASTING TO GET 2" or more of snow, you are close to the 3" threshold for scattered outages I mentioned above, and should be PREPARED FOR SCATTERED OUTAGES. IF YOU ARE IN A REGION I'M FORECASTING TO GET 4" or more of snow, you are close to the 5" threshold for WIDESPREAD POWER OUTAGES.
- DURATION OF POWER OUTAGES: It is always nearly impossible to accurately forecast duration of outages, but many are asking if this will be akin to the ice storm of a couple winters ago. For most, the answer is no, but extended outages are likely for some. The ice storm featured damage to tree limbs and lines, PLUS the need to chip ice away from downed lines to get them off the ground and back in service. This storm will bring the damage to tree limbs and lines, but no ice encasement. On Cape Cod, some outages will result from purely wind alone, but those outages should be fairly quick to restore.
- HOW MUCH SNOW FALLS ALONG ROUTE 495 TO THE COAST IN EASTERN MA: The rain/snow line will collapse all the way to the coast late Saturday night through Sunday morning, from northwest to southeast. This will occur as wind shifts to blow from the north, after previously blowing from the northeast, and will mean a period of wind-whipped snow for the even the coastal plain Sunday morning, which may actually look like a blizzard for a few, but likely will not be of long enough duration (3 hours or longer, continuously) to qualify. There is some question as to how intense that "backlash" snow will be, but it's my estimation that is should bring at least a couple of inches to most spots, with less on Cape, and that's what you see in my accumulation map. If the backlash snow moves quicker, amounts on the South Shore of MA, in particular, will be less. If it lingers, amounts may be a bit higher.