Background Check

Sorry I've been away from the site for a while. I've been busy with school. However I've not been away from monitoring the weather, and have been tracking some note worthy developments in the Pacific. So, before anymore 10-day forecasts, it's come time for a background check of the atmosphere. In the past 40 days we have seen the progression of an intense MJO wave that maintained an amplitude of between 2 and 3 sigma. It is currently entering octant 8 of phase space, with enhanced convection apparent across the Eastern Pacific. At 200mb, anomalously easterly winds are present across the entire Pacific ocean between 120E and 120W with strong westerly anomalies over the Atlantic and Indian Oceans. In the past few weeks, the resulting strong upper level convergence over the Indian Ocean has suppressed convection allowing sea surface temperatures to rise over 30C (about 1 to 2C above norms). The progression of upper level convergence into the west and central Pacific will continue to subdue convection, with a trail of warmer waters tracking eastward. Again, all due to the recent strong MJO event.

At the moment, anomalously warm waters of +1 to +2C extend down to 200m in the west Pacific and down to 100m in the east Pacific. Surface temperature anomalies are slightly positive (0 to +.5C) in all Nino zones, and westerly wind anomalies have taken over, marking a fairly definite turn to overall neutral conditions, as well as steady movement toward an El Nino later this summer and autumn. Based on the expansion of warm waters near Indonesia and the distribution of subsurface anomalies, the distribution of El Nino looks to be west-based (toward Nino4) at this time.

In the north Pacific, an area of positive temperature anomalies continues, characteristic of a moderately negative PDO. However, the maximum of these anomalies has shifted westward away from the PDO domain, and warming has taken place in the Gulf of Alaska. I think this points to at least some weakening of the negative PDO into the summer, but it should remain squarely in negative territory.

Up in the stratosphere, changes are taking place as well. The long lasting +QBO westerlies have started to subside and propagate downward more. At 30mb, the analysis level for the QBO, wind anomalies are rapidly approaching neutral. The downward shifting westerlies into the upper levels of the troposphere are contrary to El Nino wind anomalies and may act as a countering force at this level through the late summer, before a -QBO officially takes over. Strong easterly anomalies are centered near the 10mb level.

So we are in essence looking at a fairly significant regime shift over the next few months. Looking ahead to the hurricane season, a number of factors are pointing toward subdued activity in the Atlantic. First, as sometimes occurs after prolonged La Ninas in the Pacific, the ocean temperatures in the Atlantic have taken a beating. Notable negative anomalies have been present this spring over the tropical Atlantic, despite still being in the long term positive phase of the AMO. Second, the developing El Nino will lead to unfavorable winds over the Atlantic. As mentioned the downwelling QBO anomalies may help to counter this tendency but only for the very start of the season. In addition, there continues to be a signal for the mean trough axis to align in the western Atlantic, creating a route by which hurricanes would be able to recurve clear of the US coastline. At the same time, these factors (a west-based El Nino, and -QBO) point to the potential for a colder and snowier winter in the eastern US.

Closer to home, a positive NAO has been in control for the past two weeks (now up above +2SD). Again, this has some relation to the strong MJO, by the strengthening of the South America and Atlantic Hadley circulation. In the beginning, this lent to the development of a large heat ridge over the east (the 80's to 90 degree temperatures that we saw the 25th through the 28th. That ridge was pushed southeast and currently the strong north to south height gradient has led to a persistent baroclinic zone acting as a highway for disturbances, keeping clouds and rounds of rain over the Mid Atlantic region and SNE. This boundary will gradually lift northward ahead of the next wave on Thursday, bringing back warmer temperatures. South of I90, there will be the potential for some thunder. The final wave will cross on Saturday followed by high pressure moving in from the north and a shot of cooler air for the start of next week. Beyond that, the ECM and GFS are both advertising a strong piece of energy dropping into the west, developing a trough-ridge pattern over the nation. While there are discrepancies with the timing, the potential is increasing for another wave of heat to overspread the east by the end of next week. Using roll-forward analogs concentrating on the anomalies over the last few weeks, a signal continues for eastern heat in June, as indicated in my summer outlook.