Mild Thursday Followed By Two Potential Storms of Snow and Rain
Saturday Mix of Snow, Sleet and Rain To Cause Travel Delays For Many, Winter Storm Conditions Central & North

As an Active Weather Pattern Takes Aim Before Christmas, The Atmosphere May Try to Pull Some Tricks

10_DAY_ACTIVE 20211215One to three inches of mountain snow tonight ushers in 60° weather for some Thursday...then accumulating snow returns for some of New England Saturday, with a First Alert for Northern New England, watching rain/snow line farther south. Christmas Eve was on the AM 10-day! LKS_SNOW_SPECIFIC_NEWENG_ACTIVE (6)

After a dry Wednesday, rain and snow showers arrive from west to east in New England, early evening (west) to late evening (east). Mostly rain southern half of New England, snowy roads in the accumulating snow area. After early AM showers Thursday depart for all but far northern Maine, temperatures will rise as a southwest wind increases. Little additional rainfall is expected aside from a wandering sprinkle/light shower Thursday, with a break or 2 of PM sun possible. Gusty, not damaging. 

LKS_SNOW_RANGE_NEWENG_ACTIVE (5)

Here's our latest NBC Forecast System snow prediction for Saturday's storm. Remember: the storm is nearly guaranteed to happen, we've had 95-100% chance of precipitation last couple of days. Big question is where does rain/snow line set up . Interestingly, both the operational American (GFS) and European (ECMWF) guidance indicated the potential of a sizeable snow event Saturday as far south as the MA Turnpike corridor. I'm certainly not arrogant enough to write that off, I just find it hard to believe in this setup. Our forecast high temp Friday is 55-60° in Southern New England so you need to drive in new cold by Sat mid/late AM when storm arrives - we can do that with a dropping dew point (meaning air can cool as storm moves in) in Northern NewEng, just not sure we can do it south. The north wind Friday night into Saturday at this point is forecast to be fairly light, not screaming in, so it's a cold air drain that will reach a peak Sat AM, then turn around as the wind turns to blow from the east and southeast later Saturday. All these factors together tell me chance of accumulating snow Northern half of New England is quite high, with Southern NH/Northern MA southward much less certain on how much snow - if I were a betting man, I'd lean wet not white there. But, humble approach, will keep watch.

MSLP_6hr_GEFS MSLP_6hr_GEFSSo, there's the issue of the disappearing storm next Tue (photo 1: yesterday's Tue forecast, 2: today's), I don't entirely trust that. 1st, this goes down as one of the most *rapid* changes from storm to no storm in my recent memory. How does such major change happen? The difference is evident not only at the surface, but also hugely apparent in the middle/upper levels of the atmosphere. Check out the atmospheric energy (vorticity) forecast valid next Tuesday from yesterday, then from today. Note the huge missing energy over the Northeast. This kind of total disappearance is rather incredible - we often see changes in position or timing, which can impact larger things like whether two or more disturbances "phase," but the total liquidation of a disturbance in 12-24 hours is rare. The key to the major change in the forecast is this: if you follow the instigating, southern lobe of energy that was supposed to create Tuesday's storm backwards, you find it has just emerged over the Western Pacific. I've circled it in red. This is a data-sparse area. Vorticity_NH "Initialization" is a huge component of numerical weather prediction (computer guidance) - if you could initialize everything perfectly, you'd have perfect forecasts. Reality is a lot happens between the sparse measurements we can take of our atmosphere at any given time. Initialization incorporates multiple methods of estimating what's currently happening in the atmosphere: weather balloons, airplane information and (hugely) satellite estimation are biggest components. In the current position of the disturbance in question, little of any of those. This sets up an interesting situation where you have a disturbance poorly initialized that was supposed to be a big player. You have to at least consider the possibility that, when sampled more adequately, its impact may suddenly "reappear." For this reason, even though even our NBC10 Boston/NECN Forecast System shows less than a 10% chance of precipitation on Tuesday now, you're still seeing some snowflakes in the forecast for Tuesday. Not ready to let go of it until this thing comes a bit farther east.

Vorticity_Yesterday Vorticity_Yesterday

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