In our previous article, we discussed some of the features in the upcoming hemispheric pattern that could support a winter weather event. These features were particularly evident during the period from January 16th to 20th. As we have gotten closer to this time period, forecast models have come into much better agreement on exactly how these features will evolve.
First, there have been multiple shortwaves involved, running along the subtropical jetstream. The models and ensemble members have had major difficulty determining which shortwave to key in on for significant development over the last several days. This has made it more difficult nail down any specific storm threat. Over the past few days, agreement has developed on a disturbance amplifying this weekend, resulting in a large and complex storm system moving through the Ohio Valley and East Coast.
The evolution of the -NAO blocking and polar vortex has changed on forecast model guidance over the past week. This is largely influenced by this past weekend’s storm and the clipper which impacted our area during the early part of this week. Forecast models now show the -NAO blocking ridge cutting off into small block and retrograde into Hudson Bay. This causes the polar vortex to split into two upper-levels lows, one near Newfoundland (50/50 low), the other one over Central Canada. Heights also build into Greenland, enhanced by the clipper becoming very intense over Canadian Maritimes.
The lead subtropical jet shortwave is interacting or phasing with the upper-level low over Central Canada. This causes a full-latitude trough to amplify and take a negative tilt over Central Plains with heights building into Northeast US. In turn, this forces the 50/50 low and confluence zone to lift out of Southeast Canada. The cold airmass over the region begins to moderate. This shortwave, along with a strengthening upper-level jet streak, causes low pressure develop over the Ohio Valley and another a secondary low to form near the Southeast coast. These lows do not merge into one major storm until they track into the Canadian Maritimes.
With that in mind, winter weather potential for this threat is quite low. A modified airmass leaves warmer air prior to the storms arrival. And despite the transfer to a coastal storm, atmospheric energy isn’t fully transferred until the storm is well north of our area. This means the storm system will pass our area without the necessary atmospheric dynamics to produce dynamic cooling, and without a cold airmass in place prior to its arrival, winter weather is not likely.
After this system, leaves the East Coast, another storm begins to try to take shape. The upper-level low over Central Canada begins to drop south and interact with subtropical shortwave energy on the backside of the mean trough. But the lead subtropical shortwave, amplifying earlier in the weekend, limits the room for this secondary wave to phase and amplify. The offshore baroclinic zone, important for the development of low pressure areas, is pushed farther east — and the second wave develops that way, out to sea.
So here’s the bottom line: It appears that winter weather is highly unlikely this weekend. The antecedent airmass prior to the first threat, on Saturday, does not support snow — and neither does the dynamics of the storm system itself. Some wintry weather is possible to the far north and west, over the interior or higher elevations. But even there, atmospheric profiles are poor for snowflake production. The second storm threat looks likely to pass seaward, a product of the wave spacing between that disturbance and the wave prior to it.
For now, snow lover on the East Coast will have to continue to be patient.