Major Winter Storm Coming Saturday

 The weather community was on a proverbial roller coaster ride this week. Some of the weather models produced an epic blizzard; others showed little to no snow. I wanted to post this blog last night but I was shook when I reviewed the data. 24 hours later things are much more clear and my reservations are mostly gone. In this blog I'll go over the weather headlines, the evolution of the storm, a timeline, and last but not least...a snowfall forecast. 

Weather Headlines

The National Weather Service Boston office (located in Norton) has issued a *Winter Storm Watch* from late Friday night through Saturday Evening 

NWS Boston Headlines 

These will be upgraded to Winter Storm or Blizzard Warnings tonight or tomorrow morning. This storm has the look of a textbook Southern New England blockbuster snowstorm. I'm going to nerd out for a minute to break down the genesis of the storm. If you only care about the when and how much, skip down a section. 

General Overview

To start, lets head up to ~18,000 feet to see what's going on at 500 mb

18z GFS 500 mb Height, Vorticity valid Thur 7 PM (image Tropical Tidbits)

You can see the shortwave in the southern stream moving through the Four Corners. The northern stream energy is more subtle but it is diving across the Dakotas toward Minnesota and Iowa. Perhaps you can pick it up better on the Mid-Level Water Vapor Loop (courtesy of College of Dupage Weather Center)

At the surface it looks like...not much of anything at all. There are some light snow showers in Colorado and a few flakes in South Dakota/Nebraska. 

18z GFS MSLP and Rain/Snow (Image Tropical Tidbits)

So how can something so tame turn into a blizzard? 

12z NAM 500 mb heights/vort (Image Tropical Tidbits)

The two shortwaves meet near the east coast as the southern and northern jet streams phase. Anytime you see a vorticity maximum at the base of a trough, as we see in the last frame of the GIF, meteorologists will look immediately to right front quadrant for a storm. When you get a phase of the jet stream, said storm is able to get very strong. Very strong storms produce more intense precipitation, stronger winds, and larger waves. There will be a very strong storm; so where will it track?

18z GFS MSLP and Radar Saturday 7 am to Sunday 2 am (image Tropical Tidbits)

The  American GFS is furthest south and east. Eastern MA would still get slammed but this would be more manageable in Central Massachusetts. I'll note that at the upper levels the GFS isn't all that different and has slowly nudged back west today. It still has plenty of wind and moisture and it bombs the low to 964 mb. 

18z ECMWF (Euro) MSLP/Precipitation Type valid Saturday 1 am to Sunday 1 am

We pick up the EURO at 1 AM Saturday. As you can see, its already snowing. The EURO is closer to the coast and also deepens the storm to around 964 mb. The EURO would be a blizzard for Eastern Connecticut, Rhode Island, Central and Eastern Massachusetts. I mentioned in the opening I was mostly confident. If the jet stream phase is late, or incomplete, the energy could move more east and shut out the Western parts of New England. 

18z NAM Hires Simulated Radar Saturday 1 AM-Sunday 1 AM (Image WeatherModels.com)

I think this is too far east, but it would be foolish to completely dismiss given the windshield wiper ways of the weather models with this storm.

Timeline

I will need to refine this but I leaning towards snow breaking out after midnight on Friday night, picking up in intensity by 8 AM. Snow will then fall, heavy at times, if not the entire time, until 7-8 PM when the intensity should begin to wind down. During this time, there will be some areas of enhanced precipitation. These places may see 2-4"/hr snowfall rates. Right now, I think the best chance of that happening is in Essex County on the North Shore and Plymouth County to the South Shore. We can't rule that out in Worcester County. That will be the focus of tomorrow's blog. The low pulls away around midnight and we are left to shovel on Sunday. 

Snowfall Totals

Most communities in Southeastern New England should prepare for 1-2 feet of snow. Yes, feet. I will make a more specific prediction tomorrow. I'll refine it to 12-18", 18-24", or perhaps...24"+. 

I'll leave you with the National Weather Service forecast. It's highly possible (probable) that several communities approach 3 feet. If the 2-4"/hr snowfall bands sit over any one area for a few hours, they will have a shot at 30". 

More tomorrow including a look at any coastal flooding and the winds. Thank you for reading!