Pattern shift -- when will the convective season start?

The first half of February has been one major snowfall after another.  This includes several snowfalls in places that aren't used to seeing a lot of snow--like central Oklahoma.  The bitter cold weather and several snow events in the central US have been due to a persistent pattern aloft since the beginning of February.

Fig 1 -- Hemispheric plot of 500mb heights (shaded) and mean sea level pressure (contoured) valid 00Z, Feb 8, 2011.

The above pattern is similar to what we've had in place for the past two weeks.  Note the very high amplitude ridge off the west coast of the US. This is contrasted by the general troughing (the cooler colors extending further south) across much of the central US.  Short waves (which are the source of many of our storms) will tend to follow the jet stream, and the jet stream (from our thermal wind arguments) will tend to follow the area with the strongest height gradient (assuming height is a proxy for temperature, which it isn't the best, but still...).  This means that all of our short waves will tend to follow the sharp gradient in colors on the map above, which is right around where the blues transition through greens and into yellows.  This implies a storm track where the shortwaves dive south along the eastern slopes of the Rockies, dig across the southern Plains and then move north up the east coast.  The result? Snow-producing storms a lot further south than they normally are and no northward advection of warm air into the central US--so it stays cold.

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.

Note how there's no longer a ridge off the west coast--instead the ridge has been shunted further south and inland, with its axis (we're looking at the color shadings here, by the way) now over Nevada.  That large-scale troughing over the eastern US no longer extends as far south as it used to.  Warm air looks to be returning to the central US...hooray!

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.

We note that ever since the end of January, we've been in a "positive" phase of the NAO--the NAO index measured positive values which correspond to higher than usual pressures in the Icelandic low--just like we saw, the low has not been as deep as it is getting now.  However, we can also see that the recent trend in the NAO over the past week or so has been downward--the low pressure center in the north Atlantic is getting deeper again.  The red lines at the end of the graph indicate various model projections of how the NAO may evolve. It's interesting to note that several of them bring the NAO back up again, which could be evidence of more troughing over the eastern US as a result.  But, these are just models...

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.

Those are 50 degree temperature projected to get all the way up to northern Illinois by the end of the week--pretty impressive considering how bitterly cold it has been recently.  Of course, in the image above, there's also a relatively strong cold front associated with this low.  At the forecast time shown here, the front stretches from near Sioux Falls, SD through Nebraska, western Kansas and down into western Oklahoma.  Considering the GFS forecast for dewpoint temperatures at the same time:

Fig 5 -- GFS 144 hour forecast of dewpoint temp (shaded) and winds (barbs) for 12Z, Friday, Feb. 18, 2011.

We see marginal dewpoints, maybe in the low 50s at best, across eastern Oklahoma and down into Texas.  So there is some northward moisture advection with this low and with convergence along the cold front, maybe we'll see some storms.  But this is still at 144 hours out--no one should trust any model on details this far out.  We'll see how this evolves in the forecasts for this week...

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...

March 2nd.

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.