|Fig 1 -- Hemispheric plot of 500mb heights (shaded) and mean sea level pressure (contoured) valid 00Z, Feb 8, 2011.|
But a pattern change is currently happening. Here in Seattle we're getting our first trough to come on shore in the last two weeks--this means that that ridge that has been just parked over the west coast is finally moving inland...
|Fig 2 -- Hemispheric plot of 500mb heights (shaded) and mean sea level pressure (contoured) for 12Z, Feb 12, 2011.|
But note another difference in these pictures. There is a persistent low-pressure center in the north Atlantic that's usually referred to as the Icelandic low (even though it can meander away from Iceland). We can see it in both the images above as that big, heavily-contoured surface low in the north Atlantic. Notice how in the first image back on February 8th, the Icelandic low is not as deep as it is in the current image. In fact, it seems that as the trough over the eastern US is lifting out, some of the "energy" associated with that trough might be deepening the Icelandic low. The fluctuations in the strength of the Icelandic low are a phenomenon that is called the "North Atlantic Oscillation", and its dynamics are still being researched. However, there is an index that measures the NAO which basically looks at the strength of the Icelandic low. Here's a graph of how the NAO has varied over the past few months (we're only really concerned with the top frame).
|Fig 3 -- NAO index for the previous four months (top panel). From the CPC NAO page.|
Speaking of models, I've started seeing a lot of buzz in various weather discussion areas online about the potential showing up in our models for the first severe weather event of the plains this season. What's the buzz about? Well, most of what I've seen has been about the model forecasts for next Friday, which at this point have a surface low moving across the northern plains with rather warm air (compared to the frigid air we've been seeing in the central US) brought up in the warm sector.
|Fig 4 -- GFS 144 hour forecast of sea-level pressure (contours), temperature (shaded) and winds (barbs) for 12Z, Friday, Feb. 18, 2011.|
|Fig 5 -- GFS 144 hour forecast of dewpoint temp (shaded) and winds (barbs) for 12Z, Friday, Feb. 18, 2011.|
So when does the convective season usually start in the plains? I did a quick survey of the past ten years on the SPC's severe weather event archive and documented the date of the first events in the year that had significant severe weather reports of any type in the plains. I did not include events that were primarily in the southeastern US and also used some discretion as to when the first event was supposed to be--for example, the random early January severe weather outbreaks we often see were not included. Averaging across the ten years, the average date I came up with was...
Though we did see first events occurring as early as February 5th and as late as March 23rd. There also have been month-long gaps between the first and second severe weather events on the plains. So it's a bit rough. But, on average--March 2nd seems to be the time when our severe weather kicks up. But we're already within the envelope of possible dates based on past records--so it's time to start thinking ahead for this year's convective season.