Potential is increasing for a substantial mid-March winter storm to impact southern New England Monday night and Tuesday. This will be an extremely challenging forecast due to some large-scale and mesoscale factors which will play major roles in the storm's outcome. A scenario of rain transitioning to heavy, wet snow with strong winds is likely. Combination of heavy, wet snow and strong winds will also cause potential for tree damage and power outages.
During the day Monday, shortwave energy within the northern branch of the jet stream digs southeast across portions of the Ohio Valley and Northeast with shortwave energy within the southern branch of the jet stream out ahead of it. The greatest questions we've faced the last few days is will the northern stream and southern energy phase, and if so, how does this process impact storm evolution?
Over the past few days, forecast model guidance and their ensembles have been hinting a scenario in which the northern stream energy digs south of Long Island and closes off as it phases with this southern stream energy. If you're a snow lover in southern New England, this is a scenario that you certainly want to see. But with this still comes challenges.
1. If the northern stream is too strong (or too amped) a scenario which pulls the developing surface low pressure too far northwest, hooking into perhaps Connecticut would result in a significantly warmer scenario for many, yielding mostly rain with the heaviest snow perhaps combined to the Berkshires of western Massachusetts or farther west into New York (such as the Catskills).
2. If the northern stream energy is too fast, the southern stream won't have as much time to get out ahead of the northern stream energy. This would have a significant impact on phasing potential and could mean a weaker and much farther southeast low pressure. This could be good for snow, however, a more significant storm would become far less likely.
Of course, perhaps the greatest challenge overall, is the antecedent airmass. We're lacking one major ingredient...cold temperatures. Some of our largest snowstorms and snowstorms which feature rain transitioning to snow, involve a large cold high pressure north of Maine, that is also lacking here. So, if there is no cold air, how are we going to get snow? Dynamics baby...we are going to be HEAVILY relying on dynamical process to help cool the tropospheric column which will help support rain transitioning to snow, especially under the heaviest banding.
Ensembles from the European forecast model (shown below) and GFS forecast model (not shown) are in relatively strong agreement in a bit later of a phase. What this does is allow the surface low to be well southeast off the New England coast, however, as the phase occurs the surface low (not only rapidly deepens) but gets tugged northwest, towards the elbow of Cape Cod and tracks northeast. This is a nearly perfect track for heavy snowfall across a large portion of New England:
The details still need to be ironed out, however, at this stage, confidence has increased enough in the potential for a significant storm to create a first call forecast. Over the next 24-36 hours, the focus will shift towards pinning down and sniffing out the mesoscale details. Below are some factors which need to be closely assessed and taken into account:
1. Subsidence - In all likelihood, one or multiple bands of very heavy snow (CCB - cold conveyor belt) will likely develop. In between these bands will be subsidence (sinking air) which could significantly impact snow totals in this zone (as in much lower totals). Meanwhile. under the heaviest banding, totals could be significantly higher.
2. Dry slot - Depending on storm track, and especially track of the 700mb low, a dry slot could traverse portions of the region. This would cut off precipitation, perhaps entirely.
3. Snowfall ratios - Ratios will be key, especially with the larger end of accumulation potential. Given the marginal airmass, snowfall ratios may be as low as 6:1 to 7:1. Under the heaviest banding where it is more likely to generate powerful upward vertical motion into the dendritic snow growth zone, ratios will likely exceed 10:1.
4. Wind - Strong winds are likely, especially towards eastern Massachusetts. Wind gusts of 60-80 mph are a possibility and this would result in numerous power outages. Farther inland, gusts of 40-60 mph would be possible. Combined with heavy, wet snow this could yield numerous power outages and widespread tree damage.
Below is my current thinking. Again, as mesoscale details become a bit more clear or confidence increases in this aspect, this map will likely be fine tuned a bit more. My next update is scheduled for late Sunday afternoon or early Sunday evening.
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