March 2012 has shown just how variable weather can be in Northern Michigan during the spring. March 2012 began in typical fashion, with high temperatures generally in the 30s with low temperatures in the teens and single digits. Snowfall through the first half of the month was nearly seasonal across much of Northern Michigan, with most areas receiving between 7 and 15 inches of snow. Higher snowfall amounts, nearing 24 inches, were noted over portions of Northwest Lower Michigan March 3rd and 4th, in a late winter snow storm.
However over the past week, the Northern Great Lakes have been influenced by a warming trend and witness to summer like temperatures. Temperatures across Northern Michigan since March 16th have trended more like late May and June, rather than the middle of March. Several daily March temperature records have been broken since the 16th, and even some all-time record high temperatures for the month have been set.
Normal high temperatures across Northern Michigan for the second week of March range between 35 and 42 degrees. Therefore, afternoon temperatures have been averaging 30 to 40 degrees above normal since the weekend.
The chart below shows the average temperature for the period March 1st – 23rd 2012 compared to normal.
Average Temp:March 1st-23rd 2012 Average March 1st-23rd Temp (1981-2010)
Average Temp: March1st-23rd 2012 (Ranking)
Sault Ste. Marie
Average Temp:March 1st-23rd 2012
Average March 1st-23rd Temp (1981-2010)
So why is this happening? The answer lies with the larger scale weather pattern, which has been more or less stuck in place from late fall until now. When we talk about larger scale weather patterns, we often refer to the jet stream, a stream of fast moving winds at the upper levels of the atmosphere that encircles the globe. This stream of fast moving air meanders both equator-ward and poleward (much like a winding river), creating troughs and ridges over parts of the hemisphere. These buckles in the jet stream are caused by a variety of factors, such as ocean-atmosphere interactions (such as La Nina or El Nino), topography and natural oscillations. The jet stream is driven by temperature contrasts between the poles and the equator, acting as a means to balance out these natural existing temperature differences due to more solar heating at the equator than at the poles. Therefore, the jet stream serves as a "divider" between warm tropical air to the south, and cold polar air to the north.
Below are two images, the left image is the mean position of the jet stream over the northern hemisphere from March 11th to March 16th. Note the ribbon of faster moving winds (brighter colors) over the mid-latitudes. This image is at the 250mb pressure level, the level of the atmosphere where you would get a barometer reading of 250mb (roughly around 30,000 feet; sea level barometer readings are around 1000mb, remember pressure decreases as you go up). The right image is what is known as 250mb geopotential height, an approximated distance between sea level (in meters) and the 250mb level; averaged over the same time period. Geopotential heights correlate fairly well with temperatures at the surface, meaning lower geopotential heights (cooler colors) relate to cooler temperatures, and vise versa for higher geopotential heights (warm colors).
Note how far north the jet stream is displaced over the eastern two-thirds of the U.S. This is indicative of an upper air ridge, corresponding to warmer temperatures and generally fair weather. This can be seen by the warmer colors of geopotential height map (think warmer temperatures). Also note that winter did not go away for everyone, with dips in the jet stream (troughs) and corresponding cooler temperatures (lower geopotential heights) over Northern Europe, Siberia, and even Alaska. Some places on that side of the globe had a brutal winter, with Valdez, Alaska still reporting an 86 inch snow depth as of March 21st!
So when does this pattern change with a return to more normal early spring temperatures? Currently, there are no indications of a major pattern change over the short term (next 7-10 days), as the global pattern remains in a happy balance of relatively stationary troughs and ridges. The entity of NOAA that issues the longer range outlooks, the Climate Prediction Center, continues to indicate the potential for above normal temperatures continuing into the month of April (see bottom image). Now keep in mind, there very well may be bouts of colder, and even snowy weather into April (it is northern Michigan after all), but these types of specifics are difficult to forecast this far out...we will just have to wait and see.