Somewhat atypical weather patterns have led to rather unusual weather across many parts of the world, with wetter than normal conditions in Galveston despite an ongoing La Niña. These patterns led to near record warmth in Europe this fall and early winter, followed by record cold in January and early February. Meanwhile, Alaska swung from extreme cold through early January to above normal temperatures for most of February.
So, the question is: what is up?
First, lets start with La Niña. This has been an "atypical" La Niña. Normally, during a La Niña winter, the southern (or Pacific) jet stream is displaced to the north, bringing wetter than normal conditions to the Pacific Northwest and dry, mild conditions to Texas. This year, as happens with about one in five La Niñas, high pressure settled over the Pacific Northwest displacing the jet stream to the south.
This has resulted in a wet autumn and winter in the Galveston area. Since November 1, Galveston has received 17.89" of rain, which is 4.00" above normal. Since January 1, Galveston's precipitation total is 10.27", which is 4.52" more than would be expected at this point in the new year. Seattle, by contrast, has had 3.20" less rain since November 1, than they would normally receive.
Second, we have seen impacts from changes in a weather pattern known as the Arctic Oscillation (AO). This pattern, which involves the movement of the jet stream over the Arctic, generally can be found in two distinct phases-a positive phase and a negative phase. In the positive phase, relatively low pressure persists over the Arctic along with a strong jet stream flow. In this phase, cold air tends to stay "trapped" over the Arctic, resulting in frigid weather in places like Alaska and warmer weather in the mid-latitudes of the U.S., Europe and far eastern Asia. This is what prevailed through November and December. In its negative phase, the pattern is reversed...hence the record-breaking cold outbreak in Europe at the same time temperatures warmed up in Alaska.
This brings us to the third pattern, the North Atlantic Oscillation (NAO). The North Atlantic Oscillation reflects the difference between the sea level pressure of the Icelandic Low and the Azores High. When this difference is large (a positive North Atlantic Oscillation), strong westerly winds tend to keep Europe winters mild and wet. When it is high, the tendency is for Europe to experience a major cold wave and severe cold often strikes the Eastern U.s. as well. Normally, the North Atlantic Oscillation changes in tandem with the Arctic Oscillation. This year, they were somewhat out of synch. So, while Europe did get its Arctic outbreak, the U.S. Northeast was largely spared.
So, what are we to make of this? The first, is that most local weather is affected greatly by large-scale patterns and events often taking place thousands of miles away. The second, is that predicting weather is always a challenge due to the need to correlate local factors with more global features. Finally, that our weather for the rest of this year will be determined chiefly by how these large-scale features evolve in the coming months.
Right now, most predictions suggest that we will gradually move from a La Niña pattern to what is called an ENSO Neutral pattern (neither La Niña nor El Niño) with temperatures and rainfall running near to a little below normal in southeast Texas. However, with the atypical southern jet stream pattern still in place and a large upper-level low pressure trough still hanging back to our west, I would assume that we will still see fairly decent rainfall through the next couple of weeks or so. Beyond that...we'll just have to see what develops!
Below are links to information relevant to this blog:
Ocean Temperatures and Current La Niña OutlookENSO Discussion and OutlookGraphic La Niña ForecastsArctic Oscillation ExplainedCurrent Arctic Oscillation OutlookCurrent North Atlantic Oscillation Outlook
