It’s late March. Typically, one would think of pulling out their spring clothes by this time. However, a late winter storm has swept across the Plains and is currently sweeping through the Midwest and Ohio Valley. Over a foot of snow has fallen in many places, due to this storm. The 4 graphics below are courtesy of the NWS (local weather forecasting offices). They give you a small idea of how much snow fell on March 23rd and 24th.
As mentioned above, in the time of writing this, the storm is still dropping snow in the Midwest and Ohio Valley regions.
Though winter storms, like this one, in late March are not common, it is not unheard of. One possible reason why it still feels like winter for much of the United States would be because both the Arctic Oscillation Index (AO) and the North Atlantic Oscillation Index (NAO) are negative (below). With both indices being negative, the polar jet stream gets pulled farther south and tends to amplify, or buckle. As a result, stronger cold fronts tend to be associated when both indices are negative. If you look at the AO index, it is strongly negative right now, but is forecast to neutralize and/or become positive by early April.
It is early March 25, 2013 and there have only been 18 total tornado reports this month. According to the Storm Prediction Center, only a preliminary total of 12 tornadoes have occurred this month. Over the past 3 years, the average number of tornadoes for the month of March is around 90. Yes, the US is well below normal for the month of March. So where are all the tornadic storms? It shouldn’t surprise you, with the explanation above, why it has been a relatively quiet start to the tornado season, albeit April and May are the two months that average the most tornadoes during the year.
I strongly believe April and May will be different, especially if the AO index goes positive. The numerical forecast models are already hinting at a much “busier” pattern for the beginning of April. Both GFS and ECMWF models show a line up of short-wave troughs and cut-off lows from the north Pacific that will move eastward, beginning last few days of March and into April. They also keep the polar jet stream in the northern Great Plains and Great Lakes region. This will allow for moisture return from the Gulf of Mexico, a key ingredient for severe storms, to advect into the South and Plains.
I will post another blog as the chances and locations become more clear.