Saturday, May 4, 2013

Arctic Outbreak May Strike Mid-May

It looks like an Arctic Outbreak may be in the offing for the middle days of May, just as many began to think that the chilly spring would finally retreat.

The European model is shown above, projecting temperature anomalies 5000 feet off the ground for the morning of May 14th. A large swath of well below-normal temperatures is seen swinging into the northern Plains, Midwest and Northeast. It does appear that this outbreak would occur following the passage of a storm system across southern Canada, which is the typical way an arctic cold event transpires. This outbreak is not yet showing up on the American GFS modeling system, and is only shakily supported by the ECMWF model and its ensemble set. That said, the synoptic set-up isn't all that impossible.

A strengthening Madden-Julian Oscillation wave currently making its way across the waters north of Australia will push east into the open waters of the Pacific. As this transition occurs, the MJO will enter Phases 2 and 3, the latter of which we are watching for cold weather potential. The projected timeframe for this arctic outbreak falls on roughly May 14th, which equates to Phase 3 in this European MJO forecast. Ensemble members from the European forecasting system are shown in yellow lines as being in a decent consensus on a moderate-strength Phase 3 MJO status for May 14.

The typical atmospheric set-up for a Phase 3 MJO during the month of May is shown above. We tend to see high pressure building over the Bering Sea into western Asia and Siberia. The result includes low pressure being elongated into two main centers; one east of Greenland and one in north central Canada. With the presence of another high pressure system in the Canadian Maritimes, the second low pressure area can be coaxed to squeeze south into central or even south central Canada. The situation may then turn towards producing low pressure in the northern US in response to the southern shifting of the Canadian low pressure system. Altogether, it comes down to cold weather being more obligated to push towards the US. The cold weather does not necessarily penetrate the United States, but the likelihood of a cold weather event with a synoptic set-up like the one shown above is increased.

Minimum temperatures for the morning of May 14th tell the story, with widespread temperatures in the 20s and 30s extending across Canada and well into the Plains, Great Lakes and Midwest. We even see the freezing line pushing towards northern Missouri and southern Nebraska. As I told above, model guidance is uncertain on the evolution of this scenario. American model guidance is unsupportive, and European guidance that is supportive of such an incident happening is rather inconsistent from run to run. I prefer to leave this event at the 'possible' level rather than elaborating on how this could all go down- I really want to see more model support for this situation before going ahead with potential implications and other effects of this arctic outbreak.