Summer Forecast for 2017


We start off with a look at the pattern for May on through August, the Summer outlook.  The evolution of the pattern is being influenced by three factors.  The first is the stratospheric environment.  The second is the evolution of the sea surface temperature anomalies in the northern Pacific and northern Atlantic.  The third is the evolution of El Nino.


This year, the stratosphere is significantly warmer at the high and mid latitudes than in previous years, which appears to be influenced by the changes developing with the sun.  The stratospheric warming has be focused over the northern Atlantic and North America but can also be seen at times in Eurasia.  The stratospheric thermal environment supports a more amplified Polar jet stream with a greater potential for high latitude blocking.  Of notice is a strong and more persistent negative North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) which supports a block over the northwestern Atlantic and a slow progression of low pressure systems over the eastern two-thirds of the United States.

The evolution of the stratospheric temperature anomalies are coupled with the weakening of the QBO in a favorable state for high latitude blocking.  The QBO has fallen below +15 m/s and the latest observations suggest a fall below 10 m/s is increasingly likely this Summer.  A QBO value between +8 m/s and -8 m/s strong supports the development of high latitude blocking going forward.  In short, the current environment this Spring featuring continuous high latitude blocking, especially in the Atlantic Ocean looks to continue to be supported via stratospheric anomalies into and through the Summer.


There are clear changes evolving in both the North Pacific and Atlantic Oceans, as both the PDO and AMO indices are trending towards a negative state.  Both anomalies still in a weak positive state, but overall trends both suggest both the PDO and AMO will be turning to a negative state this Summer.

The cold water pooling in the Gulf of Alaska is a clear sign the PDO is about to flip, however a true negative PDO features above normal water values to the east of Japan, which has been slow to develop.  However, this evolution has been observed before back in the 1940’s and 1950’s (albeit with less reliable observations), and what was seen was a rapid warming of SSTA east of Japan after the cold pool in the Gulf of Alaska down to the southern coast of Mexico is established, which is still evolving.

Meanwhile, the AMO continues to slowly weaken towards a negative state and has been in this process the last several months.  The last truly negative AMO was last seen in 1997 and is due to flip back soon, so an evolution to a negative AMO state in the Atlantic Ocean would be reasonable.

A negative AMO/PDO state has specific impacts on the 500 MB pattern.  A negative PDO state support a ridge to be in place between the Bahamas and Southeastern United States.  When the PDO is negative, the potential for a ridge to build north from the Gulf Coast into the Mid Atlantic increases in potential.  However, the negative AMO state increases the potential for more rainfall from the Plains to the East coast (a positive AMO increases drought conditions in the Plains and East coast, flips with negative state).  However, a negative AMO also leads to less rainfall over Florida, which would line up nicely with the influences of the negative PDO states and a ridge between the Southeast and Bahamas.


Technically, ENSO is still in a neutral state, however there are clear signs that a weak El Nino is being established.  The SOI index continues to fall.  SSTA to the west of Australia are below normal, a clear sign of colder water enhancing the pressure gradient that drives El Nino states.  Plus, now convection is starting to return to the date line for the first time in close to a year.

What this tells me is that the state of El Nino is not far behind.  The latest guidance is lining up nicely with the latest observations now with a true weak west based El Nino forming by late July or August.  An El Nino in the Summer months supports a trough in the East and a strong Sub Tropical jet stream which enhances moisture transport from the Gulf of Mexico for the development of some extratropical low pressure systems.  Typically, El Nino Summers feature few heat waves and more near to below normal temperature regimes.



May will continue to be dominated by the evolution of high latitude blocking in the Northern Atlantic and an active Pacific jet stream over the eastern Pacific that will lead to a persistent trough from the Plains to the East coast.  The weather pattern will feature above normal rainfall due to the high latitude blocking making for the potential for cut off low pressure systems and an increase in air mass influence from the Canadian Maritimes due to high pressure system slow to exit from northern New England.  Temperatures will be below normal and precipitation above normal, however a brief warm up to possibly above normal temperatures and Summer like conditions for around Memorial Day is in the forecast.


The influences of ENSO are not established yet, so the primary influences will be the PDO, AMO, and the stratosphere.  The PDO influences will keep the ridge in place over Florida and the Bahamas.  The NAO/AO states though will lead to a ridge over the West and a trough over the Plains to the East coast.  Look for a storm track from the southern Plains to the Mid Atlantic or Great Lakes with significant severe weather outbreaks to the south and east of these low pressure system tracks.  Temperatures will be volatile with the best potential for a heat wave (high temperatures 90+ for 3 days straight) when the storm track shifts a bit more west in a neutral NAO period which allows the Southeast ridge to build northward.  However, these heat wave threats will be limited in frequency and duration, if they happen at all.  The theme of a slow moving warm front will be discussed frequently.  The theme of severe thunderstorms focused over eastern Pennsylvania and then collapsing further east is a high possibility as well due to the increased Canadian Maritime influence on the weather pattern.  Above normal rainfall and thus flash flooding will be a high threat.  While temperatures will be near to below average for much of the forecast period, levels of humidity will be high.


By August I expect El Nino to be established or at the very least, the convection around the date line to be more robust with ENSO values in each region between +0.3 to +1.0 Celsius with an expectation of the warmest anomalies in ENSO 3.4/4.  The evolution of El Nino combined with the other factors discussed in June and July would make a heat wave in this month very difficult.  There will be hot days, it is Summer after all, but with a constant stream of cold fronts from the Pacific along with a continuation of high latitude blocking in the Atlantic and stratospheric anomalies making cut off low pressure systems possible over the United States; the pattern in August is expected to support below normal temperatures and above normal precipitation once again.  The influence of El Nino will limit tropical storm/hurricane influences but naturally as we saw in 1993, a quiet season can still lead to impacts so this needs to be watched.

The overall theme for this Summer will be active and cool.  There will be some threats for heat waves, but this Summer is likely to be remembered more for rainfall and cool days rather than heat waves.