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.