Weekend Storm To Bring Rain, Snow, Wind, Chilly Air To New England

Precipitation will begin Saturday, likely from southeast to northwest


Two days ago I shared with you my ideas on a storm set to develop just off the New England coastline this upcoming weekend, strengthening quickly as it moves southeast of Nantucket and into Nova Scotia.  Earlier in the week, there was a great deal of uncertainty on the details...now, there actually is a pretty decent amount of agreement among available guidance on the overall track of the storm, and here are the points I'll detail in this post:

  •  Precipitation begins Saturday, likely from southeast to northwest
  • Heaviest amounts east, falling overnight Saturday night
  • Cold air turns some rain to snow
  • Significant accumulation possible in Northern New England - particularly Maine
  • Gusty wind and chilly air Sunday - wintry wind chills expected

Check out the average sea level pressure (barometric pressure) forecast, showing low pressure (storms) and high pressure (fair weather) systems, from 21 different American guidance products (GFS Ensemble) below, and 51 European guidance products (ECMWF Ensemble) just below that - note the close agreement, valid Saturday evening:

21-member GFS Ensemble:


51-member ECMWF Ensemble:


The agreement shows a quickly strengthening storm passing southeast of Nantucket as it trucks northeast, bringing a wide swath of gusty wind to New England, and an onshore northeast wind flow, which probably will turn "ageostrophic" - blowing toward the center of the storm, from the north-northeast, then the north - when the storm undergoes its quickest strengthening phase.  Strongest winds actually should come behind the storm, on Sunday, as the low pressure departs and high pressure readies its eastward move into New England. Below is the wind forecast, valid Sunday afternoon, from that same GFS Ensemble, at about 2200 feet in altitude, which is often a good indicator of gust strength.  Note the average wind direction and speed is northwest at up to 40 knots - indicating potential gusts of 45 mph:


With cold air in Southern Canada, these winds will be the perfect pull to bring that chilly air into New England - the coldest air of the young season - with highs only in the 40s...30s north...with wind chill values around 30-35 Sunday afternoon!  Though cold of this nature has little real impact, surely it will be a shock to the body and feel awfully chilly, particularly for those tailgating at the Patriots game or spending the day outside!  Below are projected temperatures (left) and wind gusts (right) for Sunday early afternoon:


As for total precipitation, with the storm at its strongest phase east of New England, highest amounts should be in eastern areas, accordingly.  Though still some disagreement on exact details, the general idea is rain developing Saturday morning through midday from southeast to northwest in Southern and Central New England, developing latest and remaining lightest in northwest New England, lasting until early Sunday morning before shutting down last in Maine.  The precipitation forecast below, from the ECMWF guidance, may be a good starting point, though higher amounts very well may fall for interior Eastern New England than depicted here:


 Finally, there is the important issue of snow.  With cold air streaming into the backside of the storm, accumulating snow certainly seems likely for some of Northern New England - the key question on this is how quickly does the cold air stream in, and how far west does the substantial moisture field extend?  As of Wednesday, I've aired the following thoughts on NECN with regard to snow:


In the southeast area of "rain to some flakes," the idea is this thing may end as some flurries or snow showers.  In the mountains of Northern New England in this zone, a couple of inches of snow would be possible.  The area highlighted for potential plowable snow is where the moisture and cold air have the best chance of intersecting, given the evolving pattern, and the guidance is starting to pick up on that.  Though their amounts for any given area are very different, compare the European and American deterministic guidance, and the GFS Ensemble guidance, below.  All are different, but all show the same thing - highest axis of snow in the area I've outlined, and both deterministic guidance show amounts of over 18" in the highest totals!  Though too early to assure such a scenario will play out, I felt it wasn't too early to sound the alarm on a plowable snow possible in these areas:

 European (ECMWF) guidance - over a foot and a half in North-Central Maine:


American (GFS) guidance - over a foot and a half just east of Maine:\


American 21-member guidance average - highest amounts in Maine:


Obviously still some details to work out with placement and amounts of snow...but surely there is a much clearer picture than there was a couple of days ago.

I'll keep you posted on NECN and online.


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