Tuesday, March 14, 2023 Winter Storm Update

 As typical with phasing systems, computer forecast model guidance continues to struggle with the finer details and exact evolution of this system. The finer details and exact evolution will be fairly critical in local impacts, especially for those on the edge. The big question we're dealing with right now is the potential for a dual low pressure structure. Dual low's always throw a giant wrench into the forecast. The challenge with these is it can be impossible to determine whether that scenario is correct or not. While that can be challenging, this is where assessing the mid-levels of forecast guidance can perhaps provide a clue.

Below we will assess the 500mb vorticity and Sea-Level Pressure charts  from the 18z/12 NAM, 18z/12 GFS, and the 12z/12 European forecast model. We will focus on the northern stream shortwave, shortwave energy within the southern stream, and surface low pressure.

All three models continue to be in strong agreement that the northern stream energy will dig south of Long Island and close off. As stated in my Saturday forecast, this is a classic signature for significant southern New England snowstorms. It doesn't guarantee it, but this is a strong signal. While the graphic below does not show a closed off look at 500mb, in subsequent hours the vorticity tracks southeast into Long Island and closes off. The result at the surface is developing and strengthening low pressure right off the New Jersey coast.

Regarding the southern stream energy, we see shortwave energy pretty far off to the south and east. This shortwave energy is associated with convection (showers and thunderstorms developing off the southeast and mid-Atlantic coast and tracking northeast. One of the questions has always been, how the northern stream and southern stream interact, or phase. Forecast model guidance today has trended in a direction in which there is stronger emphasis on this southern stream shortwave energy. Forecast models largely want to make this the main low pressure which results in a significantly less impactful event for a large portion of the region. This is what we call, "chasing convection". Now, we've seen storm potential get destroyed because this scenario verified where the convective-induced low pressure became the main storm and much of the precipitation was tugged southeast with it. So the question is, how likely is that scenario to occur here? 

I believe the models are placing too much emphasis on the convective-induced low pressure. I come to this conclusion as the degree and strength of convection is not expected to be that intense or particularly deep. Deeper and stronger convection results in more intense vorticity. Also, given the structure and evolution of the mid-levels, this will favorable the low pressure development off the New Jersey coast to be the main low which will strengthen as it moves northeast. 

Even now, there is still some uncertainty with the forecast models, but most of this uncertainty involves surface features and evolution. A strong assessment of the mid-levels may help the forecaster eliminate some of this uncertainty. Based on the evolution of the mid-levels, especially at 700mb with the development of a closed 700mb low and track of this 700mb low, combined with the thinking the convective-induced low is overblown, I am thinking we'll see an intense band of heavy snow traverse a large portion of southern New England. 

I'm also becoming a bit more confident the system will occlude rather quickly as it undergoes rapid development off the coast. As occlusion occurs, this will shut off the inflow of warmer, moist air into the storms core which will result in the CCB (band of intense snow) to weaken rapidly and result in a weakening precipitation shield. Due to the increasing confidence in this aspect, I am reducing the totals of each range a bit. I am also becoming a bit concerned with subsidence within the Connecticut River Valley. This could result in significantly lower totals within the Valley, but should intense banding traverse the valley, this concern would be lessened. 

Outside of the Berkshires and far northwestern Connecticut, most should see precipitation start as rain Monday evening, however, as the storm evolves and dynamic cooling begins to take place, temperatures at the surface should drop close to freezing. As heavier lift begins to evolve, rain will rapidly change to snow. The changeover may be slower within the valley and along the coastal Plain. Snow will be wet and heavy with ratios well less than 10:1. Under the CCB, ratios may be as high as 13:1 to even 15:1 if the profile can become cold enough. Snowfall rates of 1-2'' per hour are likely under the most intense banding. Some forecast soundings also indicate a MAUL (Moist Absolutely Unstable Layer) indicating the potential for thundersnow which could yield locally higher rates. Winds are also expected to be quite strong with the potential for gusts 60-70 mph towards eastern Massachusetts and gusts 40-60 farther inland. Combination of strong winds and heavy, wet snow may result in widespread tree damage and power outages. 

Forecast Changes which reflect narrowing the ranges a bit as confidence in the finer details increases and to account for mesoscale features such as orographic support in Berkshire County, MA and subsidence into the Connecticut Valley (Note: I am a little concerned subsidence could be an issue farther north past Springfield):

• Added a 20-30'' contour within the Berkshires where orographic enhancement combined with the greatest ratios will occur. 
• 18-24'' contour was reduced to 16-20''.
• 12-18'' contour was adjusted to 12-16'' and also pushed farther east through Litchfield County, CT, far northwestern New Haven County, CT, and far western Hartford County, CT to account for the higher elevation and higher totals.
• 6-12'' contour was adjusted to 8-12''.
• 3-6'' contour was adjusted to 4-8'' and this contour brought higher into the Connecticut Valley to reflect subsidence concerns.
• 2-4'' contour for the Connecticut shoreline with a 1-3'' contour added in eastern Massachusetts.

 

Tuesday, March 14, 2023 Significant Winter Storm

 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.