EPAWA Summer 2015
Summer 2015 Forecast/Outlook
It’s that time of year when EPAWA releases the Summer Outlook. This year it seemed to have started early, with a very very warm month of May and so far in the month of June, an above normal start.
Below we take a look at the technical discussion that made up the EPAWA Summer Outlook for 2015, and below it, a month by month breakdown of temperature and precipitation departures by location.
As we approach the midway point of June we can see a lot of similarities to the pattern from May and June 1957. Both years can be seen below in May and in June with the cold dominating in the middle of the CONUS and the warmth in the Eastern US.
June: The projection from the GEFS (GFS ensembles) is on top with strong support from the European ensembles and the MJO (Madden-Julian Oscillation) heading into phase 2-3.
The reason we are seeing a close match with 1957 is because of the intensifying El Nino. The image below shows the warming intensifying during the spring into the summer. Very similar to our last surge in temps for this year:
We have seen a recent leveling off of the warmer SST anomalies across the eastern regions of the ENSO. This appears to be temporary as another downwelling phase of a Kelvin phase moves eastward. We can see the increase in westerly trade winds allowing the warmer water to shift west once again. But with the MJO in a phase 2-3 and the Kelvin wave headed east from 170°W-160°W it appears that we will lack any big warming events in the western basins. At the very least it looks like we will continue to see the focus of convection be across the Eastern basins. This will help keep the Western Pacific tropical season relatively quiet.
One reason we are going to abandon the 1957 analog come July is because of a recurving typhoon at the end of June 1957. This likely played a role in the changing of the pattern favoring troughs in the Eastern US. The Arctic Oscillation was also negative that summer, helping to dig troughs deep into the Eastern US. With the Arctic Oscillation staying positive, and the continuation of the main focus of tropical forcing staying in the Eastern ENSO basins, we think we continue the heat into July (outside of current phase 2-3) . Another reason for the warmth into July is the warm sea surface temperature (SST) anomalies in the Gulf of Mexico and along the Eastern US coast.
Here we can see the kelvin wave heading eastward:
Image below is the track of a super-typhoon that likely altered the pattern in 1957. With the greater forcing expected to be in the eastern basins we are abandoning this potential. However, with El Nino we certainly will be keeping this in the back of our minds.
EPAWA month my month breakdown of temperature and precipitation departures from normal:
July and August Precipitation: Slightly below normal
September Precipitation: Near normal
A few reasons why we are continuing to side with the warmer temperatures through July into August:
1) The soil moisture anomaly shows we are still below average. The recent heavy rains are helping with drought concerns but these storms have been scattered and some portions of our region have not benefitted from the last few episodes of storms. The lack of a widespread rain is allowing the soil moisture anomalies to be below average which in turn would favor a warmer outlook through June into July.
2) Warm Sea Surface Temperature (SST) anomalies in the Gulf of Mexico and the Atlantic. This is supporting the stronger ridging we are seeing and going to see through the remainder of June. We had this signal last year (a little bit) and we had a warm June. But it faded away quickly resulting in a cooler remainder of the summer of 2014. This time the SST anomalies are much higher and look to have some staying power. The second image shows the departures over the last month with SST rising in the Atlantic/Gulf of Mexico. This will reduce the cold risks with any pattern amplification.
What could go wrong:
1) Recurving typhoons could once again pose a threat to bringing in a cooler pattern. But this year we have much stronger ridging to our south and east that will resist cold air amplifications and the tropical forcing is supposed to stay away from the Western Pacific. These two factors will put us in a much better situation to stay warmer than compared to 1957/2014.
2) 2014/1957 we saw a switch to a negative AO in July/August. Looking over the latest global atmospheric angular momentum data there is no sig of a sudden shift through mid July. If the AO begins to shift negative we open the door to some colder risks to the forecast. It will be interesting to see if we can hold it off. With the QBO continuing to drop and history leaning towards at least a weak negative AO we decided to keep August near normal.
Forecaster: Meteorologist Mike DeFino
Video and other contributions made to the outlook by Meteorologist Bobby Martrich
Effective Date: June 14th, 2015, 11am