Stratospheric Disruption

Meteorologists around the Northern Hemisphere have been excitedly watching events high up in the stratosphere recently, as the circulation aloft has become very unusual for the time of year.

Normally at this point on the calendar there is a robust vortex of cold air circulating high above the Arctic, but in recent days this vortex has been pushed well off its axis by a big ridge over the Canadian side of the Arctic.  The anomaly is ongoing; here's a forecast pressure map for Friday at the 10mb level, or about 100,000 feet above sea level.

The average 10mb westerly wind at 60°N is hovering around zero, which is approximately the weakest it's ever been at this time of year; more typical values are 20-40 m/s (45-90 mph).  The green line in the chart below shows the recent evolution of this closely-watched "U10/60" index, and the red line shows a forecast.

What does this have to do with Alaska, you may ask?  Well, the high pressure anomaly aloft also extends down through the lower stratosphere and the troposphere, so it's connected to the surface-level weather pattern over Alaska.  Here are maps for Friday at 100mb (about 50,000 feet) and 500mb (18,000 feet):

As of this morning, the 500mb ridge is centered over the Chukchi Sea, bringing relatively clear skies and cool northeasterly flow to northern and western Alaska:

Forecasters like to watch the stratospheric winds because they tend to foreshadow circulation patterns at lower levels, sometimes giving valuable information weeks in advance of important pattern changes.  In other words, what happens in the stratosphere doesn't stay in the stratosphere.

For instance, here's an analysis of 500mb heights 10-40 days following previous dates this century when the 10mb 60°N westerly wind dropped to zero in winter.  Above-normal heights near Greenland in the map below signify a negative Arctic Oscillation, a well-known effect of such "sudden stratospheric warming" events.

Closer to home in Alaska, there tends to be a trough over northern Alaska and a ridge over the Gulf of Alaska, and that is usually a mild setup for southern Alaska:

More strikingly, there is a strong wet signal for western Alaska: liquid-equivalent precipitation is commonly above normal in the month or so after these major stratospheric disruptions.

It will be interesting to see if the historical pattern plays out this time around.  CPC does show a strong wet signal in the 6-10 day period, but focused on the eastern interior at this time: