On the heels of a low pressure system which brought light snowfall to the area on Friday, a shortwave will drive southward through the Plains states on Saturday. Coming over the top of a mid and upper level ridge on the West Coast of the United States, the shortwave is forecast to amplify southward from the Plains states through the Mississippi and Ohio Valleys by Sunday. As a result, a surface low pressure will form and move from the Mid Atlantic States to a position off the Northeast Coast by Monday morning and afternoon.
Forecast models have been struggling to pin down details in regards to the mid and upper level features which will characterize the storm system. Earlier this week, models were in good agreement that the storm system would take a more southerly track. With the surface low moving from the Mississippi Valley to the Southern Mid Atlantic Coast, the main concern for our area would be precipitation staying too far south for meaningful snowfall. But in the past 24 hours, a noticeable trend toward a more amplified solution has emerged. Forecast models now track the system off the coast of New Jersey — raising concerns that precipitation type may change to sleet or rain across portions of the area on Monday.
Much of the meteorology behind the setup has to do with the position of the baroclinic zone. A baroclinic zone is an area where a temperature gradient exists on a constant pressure surface. These zones can become favorable areas for low pressure centers to deepen. Additionally, enhanced baroclinicity can lead to increased lift for heavy precipitation. Forecast models have been struggling to pin down exactly where this baroclinic zone will set up during the upcoming weekend and, as a result, have been waffling around with the track of the surface low and mid level centers.
The mid level pattern for this storm system is being controlled mostly by an upper level low over Southeast Canada, which is elongating as a result of a transient mid level ridge to its north over Greenland. You’ve seen our forecasters explain in detail how important high latitude blocking can be for winter storms in our area. This time, although we have a mid level ridge over Greenland and the NAO region, it is transient — and not an established blocking ridge of high pressure. Forecast models have been adjusting the baroclinic zone as a result of shifts with the high latitude ridge — and the resulting position of the upper level low over Southeast Canada.
Essentially, if the blocking ridge over Greenland is more progressive or transient, the upper level low over Canada can shift farther north. This would open up more room for the baroclinic zone over the Mississippi Valley and Mid Atlantic States to shift farther north. With the shortwave tracking farther north as well, mid level warming would be able to work into at least southern portions of our forecast area. Precipitation, however, would become more widespread throughout the area and into New England.
Presently, forecast models are in agreement that the surface low will track from the Mid Atlantic states to the New Jersey Coast. But the baroclinic zone position and mid level center tracks (the low’s at 700, 850, 925mb in the atmosphere) will become essential to pinning down where the transition line from snow to rain or mix progresses during the storm. Confidence is rising in a moderate impact winter storm, but it remains to be seen exactly how far north mixing will get — and that obviously will have a major impact on the forecast and snowfall totals in many areas.
In terms of forecast models, the trend has been noticeably north. The Euro tracks the surface low off the coast of Ocean City Maryland, while the Canadian remains a far south outlier. The GFS and NAM have both trended north over the past day, now taking the surface low off the coast of New Jersey. The SREF, recently, have trended north to agree with those American models. Confidence, given this information, is obviously low. But we are leaning toward the idea that a farther north track is more likely than a farther south one.
This weekend, for meteorologists, will be all about pinning down the tracks of the the mid and upper level features, as well as pinning down the details of any dry slots or warm layers in the atmosphere. With new forecast model data over the next 24 hours, we should have much more detailed information and updates. So, as we love to say: Stay tuned.