On Wednesday, CWG released its summer forecast. Overall, it’s a bearably hot outlook for D.C. with temperatures close to average. We’ve seen worse (the last three years). What if our forecast is too cool, and the worst is yet to come?
Granted, it would be a tall order to repeat any of the blistering heat Washingtonians had to sweat off in 2010, 2011 and 2012. But in this time of climate extremes and record-toppling weather, another summer scorcher is not too hard to fathom.
As with any weather prediction – and especially considering those of the seasonal variety – CWG’s forecast carries a number of risks. What could go wrong?
1. Cold air stays north
First, the cold air mass that has repeatedly invaded the Plains and Midwest this spring may bottle up over and around the North Pole. The AO and NAO, two atmospheric signals that can control the global temperature pattern at times, have switched signs from negative (early spring) to positive (starting in mid-April). A strong ridge of high pressure and associated warmth in Greenland (per the –NAO) and other Polar locations (per the –AO) have given way to a large dip – or trough – in the jet stream as the “blocking” in the atmosphere dissipated (and the AO/NAO correspondingly turned positive). The cool air did not completely fade from the pattern after mid-April, however, as evident by our Memorial Day weekend (snow in Adirondacks!) and persistent bouts of chilly, and even snowy weather in the Midwest well into May.
Cool air is, now, on the retreat. Much of the East and Midwest – including D.C. where temperatures will reach or exceed 90 through the weekend – will bask in major warmth over the next few days. If the Polar-based cold air mass never breaks free (or does so infrequently) this summer, the forecast risk of prolonged heat waves – subjectively defined as seven or more consecutive days of 90-degree temperatures – could be realized. How about 100-degree heat, which the CWG forecast did not predict? That would be very much in play, too.
2. Pacific pattern (that favors heat in our region) holds firm
The reemergence of colder sea-surface temperatures (SSTs) in the North Pacific Ocean presents another hot risk to CWG’s forecast. We are currently in a cold phase of the Pacific Decadal Oscillation (PDO), a long-term atmospheric signal that defines the composition of SST anomalies (or differences from normal) in the central North Pacific and along the west coast of North America. The PDO returned to its long-term cold phase in 2008 and – most of the time since then – the cold-phase signature of above-normal SSTs in the central North Pacific and below-normal SSTs along North America’s west coast has been evident.
Over the spring, the PDO has tried to take on more of a warm-phase signature, as that central North Pacific warm pool has intensified and expanded east to the west coast of North America. The monthly PDO value in April was -0.16 (negative values mark the cold phase, while positive numbers denote the warm phase). This value flirted with positive territory in January (-0.13) as well. Why is this important? Even just the development of a short-lived warm PDO event would increase the chances of a cool summer.
But recent observations of SSTs in the eastern equatorial Pacific Ocean seem to argue against a full-blown transition to the warm PDO. The latest weekly SST anomaly in the Niño 3.4 Region came in at -0.4°C, which is more reflective of La Niña conditions. La Niña or La Niña-like summers are more likely to be hotter than normal in D.C. and the East, Plains and Midwest in general.
The larger point here is that even if the PDO manages to trend positive this summer, it could do so sluggishly given the existing cold SSTs in the Niño 3.4 Region and the outward expansion of cold anomalies throughout the entire eastern equatorial Pacific since late April. This could work against a cooler summer pattern in the D.C. area.
3. Hot summer pattern analogs
There are some analogs – or close matches to the expected long-range pattern – that incorporate the factors explained in risks one and two.
Take, for example, the summer seasons of 1991 and 2002. Much like in 2013, these two years featured ENSO-neutral conditions in the spring (though El Niño had developed by May in both 1991 and 2002). Also like in 2013, cold PDO conditions had prevailed prior to summer in 1991 and 2002. The PDO eventually flipped to positive by August of both years. In addition, much of the available cold air stayed put over the far northern reaches of Canada and Alaska, which the mostly positive AO values had favored in those summers.
Another summer that could present a good pattern match – if the late-week setup becomes established as the “base” pattern for most of the season – under our “what could go wrong” scenario is 1999. The PDO remained in a cold phase throughout that summer and a La Niña event had been firmly established, but the AO/NAO averaged positive.
The composite of all three analogs shows a core of heat over the Great Lakes underneath the center of an intense high pressure ridge. Note that this pressure pattern is not much different from that shown on the NCEP upper air analog graphic for the period centered on June 4 (compare graphics below). Of course, the pattern that is currently expected to develop by early June may not be the pattern that prevails for all of summer. But it’s a risk that should be kept in mind.
To conclude this post, I’ll leave you with some warm thoughts. The chart below compares notable heat statistics for the risk-driven analog summers mentioned above with those from 2010, 2011 and 2012. There is little difference in the numbers, outside of the stunningly higher numbers of 95 and 100 degree days in 2012.