Late Week Nor’Easter

Quiet, mild weather is closing out the month of February. Temperatures will climb to the upper 40s and lower 50s today under mainly sunny skies. Busy southwest winds will offset the incoming warmth a bit but overall our very mild February will finish out in pleasant fashion.

As we flip the calendar to March, all weather eyes in the region will be on a late week ocean storm which is destined to bring us several days of unsettled weather, beginning late Thursday night and persisting right into Sunday morning.

Upper level energy digging through the Southwestern United States today will eject out into the Great Plains on Thursday and slide east-northeastward thereafter. At the same time, a disturbance embedded in the northern stream branch of the jet stream will move out of Canada and dig southward through the Midwest and Great Lakes. These two features will merge over the Ohio Valley and Mid-Atlantic on Friday and push eastward into the waters south of New England Saturday and Sunday. Together, these two pockets of jet stream energy will yield a formidable storm center over the Northeast – one that will be rather slow to depart the region and will bring us a prolonged stretch of nasty early-Spring weather.

While model guidance is in good agreement on the overall evolution of the upcoming storm, there are still-to-be-ironed-out-differences in outputs that will have sizable effects on exactly how the system plays out in our neck of the woods.

Even so…as of right now, it is fairly safe to say that the primary threats for our region will be heavy – perhaps flooding – rains, strong – likely damaging in spots – onshore winds, and significant coastal flooding and beach erosion.

Rainfall: Model charts indicate some classic signals for anomalous rainfall across Southern New England – including a 2 to 3 standard deviation -u flow at 250mb on the GFS and GFS ensembles…indicative of easterly winds extending tens of thousands of feet up into the atmosphere – or basically a reversal of the standard westerly jet stream flow we usually see. This upper level easterly flow not only signals a slow-moving storm system but it also implies a very deep layer of moisture rotating around the storm system and moving westward off of the Atlantic Ocean. This is a pretty classic set-up for flooding rainfall across Southern New England and it’s not surprising to see GFS ensemble mean probabilities of >2″ of rain in 36 hours at well over 50 percent covering a large area. Some operational guidance has been yielding total precipitation in excess of 4 inches in spots. Barring a sizable shift in guidance, it is likely that much of Southern New England will be subject to 1 to 3 inches of rainfall with spot (to-be-determined) totals going notably higher. Should it materialize, the widespread nature of the heavy rainfall would lead to flood concerns.

925 mb winds

Wind: While there are variations in the placement and timing of the core of strongest winds, nearly all model products show a powerful low level jet taking shape and holding across coastal New England Friday and Saturday. 925 mb winds on many products exceed 80 knots during the peak of the storm’s intensity sometime Friday or Friday night…with only a slow abatement of winds through the day on Saturday. Whether this core of wind is focused over coastal Massachusetts or a bit further north remains to be seen but in either case wind gusts over 50 mph will be common up and down the coast of Massachusetts for an extended period of time. Whether wind speeds go significantly higher than that will depend upon the exact placement and evolution of the aforementioned wind field which will become more clear as we push through today and Thursday. Regardless, some limb and tree damage and resultant power disruptions are possible especially given the saturated ground.

Coastal Flooding and Erosion: With the oncoming full moon, tides will be running at their highest levels of the month late this week. A large, slow-moving storm superimposed over astronomically high tides is a classic set-up for significant issues across our east and northeast-facing coastal communities. Unlike the early January storm, which was fast-moving and only impacted one high tide cycle, the upcoming storm will impact at least 3 high tides (Friday midday, Friday night, Saturday midday) and possibly as many as 4 or 5 (Friday midday, Friday night, Saturday midday, Saturday night, Sunday midday). Likewise, the large, slow-moving circulation will result in a “different kind” of coastal flood event versus the early January storm. While the January 4th event featured a dramatic water rise, offshore wave heights and offshore wave energy were limited due to a modest duration and shorter fetch. In this case, the extensive circulation and prolonged easterly and northeasterly flow will yield much greater wave heights and much more substantial wave energy heading toward the east coast of Massachusetts. So while surge values and total water levels could fall short of the January event, this system has the potential to cause greater and more widespread problems should everything come together “just right”. In either case, beach erosion will be very substantial along east and northeast-facing shores.

All of this will come into better focus over the next 24 hours but odds, overall, of a moderate to high impact weather event are quite high and it’s likely we will not escape the Friday-Saturday period without some impacts. As a side note to all of this…another aspect to watch is the potential for a transition from heavy rain to very heavy wet snow over the interior of Southern New England…especially the higher terrain of Massachusetts but possibly extending down into Connecticut.

Post a Comment

Your email address will not be published. Required fields are marked *