PNA- Ahead?

[donsutherland1]

Over the past 10 days, the atmosphere has strongly resembled what one would expect from an El Niño event, complete with classic strong MJO forcing. El Niño events coincide with higher MJO amplitudes in June.

 

Yesterday, the MJO moved into Phase 2. Given the strength of the amplitude, which has rivaled some of the figures reached in June 1997 when June records were established, June 1997 is a possible useful case.

 

June 1997 saw the MJO reach Phase 2 on June 13. Three days later, what turned out to be a 23-day PNA+ regime came to an end. The result was a 19-day stretch with a negative PNA. The PNA fell to as low as -2.079. Out of the 19 days, 2 days had PNA figures at or below -2 and the PNA was -1 or below on a total of 11 days.

 

At present, the PNA is +0.899. 22 of the last 23 days have had a positive PNA with June 2 having the single negative value (-0.078). However, the PNA is forecast to go negative by the GFS ensembles in coming days and its timing to the onset of MJO Phase 2 is remarkably similar to that of 1997.

 

 

Currently, the GFS ensembles are forecasting a continuation of warmth in the Pacific Northwest. However, in 1997, warmth gave way to cool anomalies as the PNA- regime strengthened. Given how closely the ongoing ENSO event has resembled 1997, I suspect that at there will be at least some stretch of cooler than normal readings once the PNA- takes hold. In contrast, the eastern U.S. should turn warmer than normal after having gone through a much cooler than normal start to June.

[andyhb]

Good stuff Don, this would also have a decent shot at setting up an active period for severe weather in the middle and late parts of the month. That said, 6/1997 wasn't exactly notable in this category, but generally a -PNA pattern this time of year leads to better chances.

[donsutherland1]

Good stuff Don, this would also have a decent shot at setting up an active period for severe weather in the middle and late parts of the month. That said, 6/1997 wasn't exactly notable in this category, but generally a -PNA pattern this time of year leads to better chances.

It will be interesting to see if there is a lot of severe weather once the PNA goes negative, as well as how long the PNA- will last. With the strong PDO+, a PNA+ may continue to predominate following the end of the forecast PNA-.

[andyhb]

It will be interesting to see if there is a lot of severe weather once the PNA goes negative, as well as how long the PNA- will last. With the strong PDO+, a PNA+ may continue to predominate following the end of the forecast PNA-.

 

Even with the predominant +PNA, the +PDO has been declining significantly since the beginning of the year. NCDC data had it at 0.37 for May, despite the lingering warm pool in the Gulf of Alaska and adjacent areas. This represents a 1.36˚ decline since January in this dataset.

[donsutherland1]

Overnight, the GFS ensembles have made a shift. For the first time, they are now indicating that the present warm regime in the Pacific Northwest could breakdown over the next 4-7 days, leading to at least a period of sustained cool anomalies. This outcome would be consistent with the 1997 scenario and current progression of the high-amplitude MJO.

[donsutherland1]

The PNA has now gone negative (-0.529). Moreover, the GFS ensembles now suggest that the PNA could fall below -2 for a period, much as was the case in 1997.

[donsutherland1]

Despite the PNA- regime that has gotten underway, much of the Pacific Northwest remains warmer than normal. The ECMWF ensembles and operational model show cooler readings and maybe a cool day or two within the next 10 days. However, the PNA- regime may be moving toward an end at the end of that timeframe.

 

Finally, the University of Washington just released the May 2015 PDO figure: +1.20. This figure was somewhat below the April figure of +1.44. Whether or not the PDO continues to decline slowly or begins to rise again will likely depend, in part, on the continuing evolution of the ongoing El Niño event.