The Summer of 2016 is on the way and that means fun in the sun, going to the beach and boardwalk, and tons of baseball games. This summer though, well, you might need that umbrella and a diverse ability to dress for hot and humid along with cool and chilly.
DRIVERS AND ENHANCERS:
Much like the winter forecast, you have to step back and look at what is influencing the weather pattern for the next 3 months (June to August). In the Summer, the influences on the weather pattern start to decrease for several reasons. For one, the thermal gradient from the land to oceans naturally decreases as do the stratospheric factors. There are some clues to keep an eye on though, mostly in connection to what is happening in the Pacific and Atlantic Oceans.
Sea Surface Temperature Anomalies Drive The Pattern
The key to the Summer Forecast in my opinion is understanding what are the current drivers in the atmosphere and how will those drivers change? This Spring we have seen El Nino collapse to what is now close to a neutral state, which is a major change in our equation. However, we have also seen a sustained +PDO/+AMO pattern reman a major player in the weather pattern.
If we look and rely completely on seasonal models, you see that the support for a hot and humid summer for the Philadelphia and New York City metropolitan areas is driven on the idea of a rapidly developing La Nina. If this evolution took place, in theory, you would have a building ridge over the Southeast which would push into the Mid Atlantic. The Bermuda High returns and thus the three H’s of Summer; Hazy, Hot, and Humid return.
However, that’s not reality. Frankly, the decline to neutral is taking longer than what the models had earlier forecasted and as we head into mid May, NINO 3.4 is still in a very weak El Nino state (0.8C, threshold 0.5C). While there are clear changes in the development of convection and changes in mid and upper level winds, we are clearly not in a La Nina state.
This is important because there is naturally a lag time between a change in ENSO states and a change in the 500 MB pattern in North America. So, if we assume that tomorrow all regions go to La Nina (-0.5C or lower) and all the wind component and convective influences in the Tropical Pacific change overnight, you still have a 4 to 6 week lag time until those changes influence the 500 MB pattern. There will be hints but nothing sustained. Then you would have a slow build towards a La Nina pattern on the East coast, which means you get your first true sustained ridge with multiple days above 90 around August.
Of course we know that the pattern is not going to flip to La Nina tomorrow. We know we will be in this neutral state through at least the rest of May and likely through much of June before any sustained La Nina influences show up. As such, I don’t expect a true La Nina to show up until late July or August and influences to show up in the Fall and beyond. Between now and August, I expect any shift to La Nina will amount to a very weak La Nina state by August.
With ENSO becoming a more neutral factor, what we should be keying on is the North Pacific and north Atlantic Oceans. We’ll discuss the Pacific first and the Atlantic second.
The PDO is going to play a major factor in our weather pattern over the next 3 months. Currently we are in a positive PDO state which leads to what is called a negative Eastern Pacific Oscillation. Basically a trough to the south of the Aleutian Islands. We get this trough to the south of the Aleutian Islands because of the thermal gradient between the warmer waters along the coast from Alaska to California and the colder waters over the central Pacific. This intense thermal gradient leads to lower atmospheric pressure and a focus of 500 MB troughs to the south of Alaska. To counter these lower atmospheric pressures, a ridge typically builds over western Canada and thus a trough sets up over Southeastern Canada.
Now, note I am saying Canada and not North America. The reason being is the Polar jet stream is naturally repositioned primarily to the north of the Northern Mid Atlantic. The heart of these troughs more often than not will be over Ontario and Quebec, Canada. However, the northern Mid Atlantic will be impacted by these base of these troughs and weak cold fronts.
The Atlantic Ocean meanwhile, no longer being overwhelmed by a strong El Nino, continues to grow supportive of a negative North Atlantic Oscillation pattern which basically keeps lower heights over the northwestern Atlantic which in return keeps a trough in the Eastern United States.
The stratosphere becomes an enhancer, not a driver, in this type of weather pattern. If I am looking for a hot and humid Summer, what I want to see are cold temperature anomalies in the mid latitudes. The reason being, if the stratosphere is cold at the mid latitudes, where we are, then 500 MB heights throughout the Mid Latitudes tend to average above normal. As a result, you are more likely to have strong ridges and stronger hot air masses in a cold stratospheric environment than in a warm or neutral.
As you can see based on the trends over the past several months, the stratosphere continues to remain pretty close to neutral and I have seen no signs of a cold stratospheric environment developing. If this should change, then the potential for prolonged heat waves would increase but is not guaranteed.
The above is how I see the jet stream configurations mostly evolve for the next three months. The Polar jet stream is the dominant over the Sub Tropical jet stream due to the collapse of El Nino. Given some minor hints of La Nina developing and the focus of convection remaining around the date line, yet very weak, I suspect a trough will be a constant feature around the desert Southwest and thus a ridge will start to develop over the Gulf Coast and Southeast. We are already seeing signs of this now.
With no changes observed for several months and no signs of any change in the near future, the forecast of the Polar jet stream is based on typical influences observed in a positive PDO regime. In short, a trough will constantly try to dig south from Canada into the Great Lakes, Ohio River Valley, New England, and Mid Atlantic. Basically, when the Polar jet stream is amplified, cold fronts will be dive south through the Northern Mid Atlantic keeping temperatures near to slightly below normal with a constant threat for showers and thunderstorms. When the Polar jet stream does relax, this is when the ridge over the Southeast will be able to build and a period of hot and humid weather will show up, however those periods of hot and humid conditions will be short lived based on the date I am seeing now.
The conditions in the northern Atlantic keeps the negative NAO pattern in play all Summer which means that back door cold fronts will remain a threat along with high pressure systems more likely to anchor themselves around the Gulf of Maine than Bermuda in terms of influence.
So in short:
TEMPERATURES: 0 to -1 PRECIPITATION: Near Normal-Slightly Above
WHAT COULD CHANGE FORECAST:
1. If the PDO cools(warm water along West coast disappears) and a positive EPO pattern shows up (ridge over Aleutian Islands and trough along the West coast), the combination of a developing La Nina and a +EPO pattern would lead to a strong ridge in the East and a sustained heat wave.
2. Any tropical impacts, even from a Tropical Depression or remnant low will lead to the precipitation forecast ending up well above normal.
For weeks now, the Long Range Guidance, take your pick, constantly is trying to forecast a ridge over the central Pacific leading to a major heat ridge over the Eastern United States. Data this morning is showing that once again. I want to point out, this ridge was supposed to show up on the 20th of May, and now guidance in the medium range has backed off on this idea to a grand total of one day of a transient ridge. I personally am not putting a lot of stock into these model solutions until I see a fundamental change in the current SSTA driving this weather pattern in the first place.