Wednesday, 18 August
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With large, violent tornadoes in North Dakota/Minnesota last week and the rope tornadoes that occurred on Monday in Colorado, we continue to see the results of an active late summer severe weather pattern. In fact, there are two more setups expected this week, one today across South Dakota and Minnesota, and another on Thursday over the same region. The tornado setups just keep coming here in 2010, and every time you think it’s over, another photogenic tornado occurs.
There are plenty of reasons why things have been so active as of late, even for the last couple months. But the two main reasons, IMO, are the strengthening moderate La Nina combined with the warm Atlantic Ocean waters (positive Atlantic Multidecadal Oscillation). The La Nina (among other things) has kept the jet stream quit active across the northern Plains, and the warm Atlantic has helped keep robust low level moisture present across the Plains. Both conditions are needed to sustain an active severe weather pattern.
The other developing weather story is in the tropics, where computer models are indicating that a tropical storm (or possible hurricane) will develop by the weekend (August 21/22) over the far eastern Atlantic. I’ve talked about the tropics a little over the last couple weeks, and have mentioned that a combination of factors has kept them quiet relative to what has been expected for this season. Those factors, such as dry air, African dust, and the wrong phase of the MJO, haven’t really changed all that much. But, computer models insist a tropical wave coming off Africa will have enough energy to become a tropical storm this weekend.
This doesn’t really change what I said recently, that the tropics look quiet and less active than expected by many seasonal hurricane forecasts. Having a Cave Verde tropical storm or hurricane in late August is normal. Actually, we’ve only had one hurricane and two named storms so far. By now, we should have had three named storms with one hurricane. So, this upcoming storm in the Atlantic (assuming it develops) will basically keep this season at a normal pace. I do think dry air and African dust are going to be a problem, but given time and distance, things can change and maybe we’ll see our next hurricane by this time next week. However, for it to have a good chance of becoming a major hurricane, those inhibiting factors need to improve.