Tuesday, January 21, 2014

January 27-29 Northeast Coastal Clipper Storm

It looks like there will be a clipper system that will hit the Northeast around January 27-29, and it could bring another foot of snow to regions that will receive a foot of snow today (January 21).

Before we get into the system itself, we'll begin with how the pattern will set up prior to the storm.

This image shows the observed jet stream, as of 12z (6 AM Central Time) this morning. We can see the powerful Pacific jet stream roaring along at roughly 200 knots, which is considered a strong jet stream any way you slice it. The jet stream will be playing a crucial role when this clipper system comes along out of Canada, and it may work in favor of strengthening the clipper system as it traverses the Central US and when it jumps offshore. With strong Pacific jet streams like this, you have to be wary of any jet streaks that might carry a storm system onshore and dramatically strengthen it later on in the storm's life.

GFS Ensemble 500mb anomaly (left) and spaghetti (right) forecast for 6 days out.
When the clipper begins to move down towards the United States from Canada, the pattern will be a predominantly northwest flow regime, meaning the movement of the atmosphere is from the northwest to the southeast. This is portrayed well by the 500 millibar height anomalies on the left panel of the GFS Ensemble image above, as we can see the flow dropping southeast after hiking up the intense ridge of high pressure stationed over the western coast of North America. These northwest flow regimes are conducive to clippers as a whole, but this set-up really looks to favor the Northeast for snowfall, whereas we had been seeing the snow stay over the Midwest, Plains and Great Lakes earlier in December and January.

0z GFS 500mb vorticity (left) and 250mb wind (right) forecast for January 27
For the storm itself, the GFS model indicates we will see the clipper ride down from Canada via the northwest flow, and model guidance does have a jet streak riding southeastward with the clipper system. The panel on the right, showing the projected jet stream winds, indicates the jet streak will contain winds between 110 and 130 knots, in contrast to the 80-110 knot winds in the jet stream around the storm. The clipper is expected to drop some accumulating snow across the northern Plains and the Midwest, mostly on the order of 2-5 inches. In a similar scenario as the storm the Northeast is currently experiencing, the clipper will then be ushered offshore.

0z GFS 250mb wind forecast for January 28.
The jet stream will then shoot northward again as the clipper moves east over the Atlantic, which will make the storm ACT like a Nor'easter solely in terms of track. I want to emphasize that this storm will not be a Nor'easter- it will be a clipper system that will take a Nor'easter-like track, running along the East Coast. As the storm pushes offshore in time for January 30, the snow should stop falling across the Northeast, leaving 6-12" in its wake, as the forecast image below shows.

0z GFS total January 27-29 Clipper snowfall.
I believe we will eventually see forecasts bumped up just a bit more as we move closer to this event, mainly because the GFS wants to bring in some of the same convective elements that will be present in today's Northeast snow event to enhance snow totals. While high-resolution models should catch those mesoscale features the best, the GFS model forecast above shows a pretty reasonable forecast in my opinion, with widespread 6-12" along the coastal regions. The weather pattern is ripe for these coastal clipper snow events, and the Northeast should keep an eye out for another foot or two of snow as a result of these systems.