Summer Weather Watch: Keep an Eye on These Five Possibilities

It’s Memorial Day weekend, the traditional start of the U.S. summer season, and millions are wondering what kind of weather the next three months will bring. Seasonal predictions have their limits any time of year, and that’s especially true in summer, when upper-level winds are weaker and local influences play a larger role. Moreover, the largest single influence on year-to-year climate variability--the El Niño/Southern Oscillation (ENSO)--is often at low ebb in the northern summer. Not so this year. An unseasonable El Niño event is now approaching moderate strength and is projected to continue intensifying through the summer, perhaps reaching record or near-record strength for the time of year by August. Instead of the typical lack of a summertime push from El Niño or La Niña, we’re thus left with a much different kind of prediction challenge: a summer setup so unusual that we have few analogs to go by. With that caveat, I’ll stick my neck out and offer a Top Five List (with apologies to David Letterman) of things I’ll be watching for as the lazy, crazy, and occasionally hazy days of summer unfold.



Figure 1. Departures from average temperature across the U.S. for the summers of 1982 and 1997, both of which led into strong El Niño events. Image credit: NOAA/ESRL/PSD.


Figure 2. The weather prediction firm WSI is calling for relatively mild conditions this summer across the bulk of the United States, with unusual heat confined mainly to the western U.S. and Florida. Image credit: WSI.


Cool, man, cool
The summers of 1982 and 1997, which preceded the two strongest El Niño events on record, were cooler than average across most of the United States (see Figure 1 above). No analog is perfect, but based in part on the patterns observed in those two years, “we expect the weakest nationwide cooling demand since at least 2009,” says WSI in its summer energy outlook for 2015 (Figure 2). Other years with at least a moderate Oceanic Niño Index value (at least +1.0) in Jun-Jul-Aug include 1972, 1965, and 1957; all but 1957 had widespread below-average summer temperatures. Precipitation signals for the summer are less straightforward, although during winter El Niño tends to bring wetter-than-average conditions across the southern half of the United States. The strong subtropical jet stream that’s fed much of the low-latitude U.S. rainfall over the last month may weaken as we get into summer, then restrengthen in the fall, but signals remain positive for widespread summer moisture. The average of a variety of climate models assembled through the North American Multi-Model Ensemble (NMME) suggests relatively wet conditions across much of the nation, with a cooler-than-average pocket in the nation’s heartland and relatively warm temperatures close to the coasts. These tendencies are reflected in the National Weather Service summer outlook (see Figure 3 below). The central U.S. already has a head start toward a fairly mild summer due to the extremely wet conditions across most of the Plains over the last month. Even when the rains abate and the summer sun kicks in, some of that energy will go toward evaporating surface-based moisture, rather than heating up the ground and the surface air.




Figure 3. Seasonal predictions from the National Weather Service (June-August) showing where the odds are leaning for temperature (left) and precipitation (right). “EC” denotes equal chances of above- or below-average conditions. Image credit: NWS/Weather Prediction Center.


For fire and heat, head northwest
Landscapes are parched from most of California up to the interior of Alaska, as well as adjacent northwest Canada. A major high-latitude heat wave sent temperatures on Thursday in Barrow, AK, up to 47°F, the warmest ever observed so early in the season and only the second time that temperature has been reached before June (more here from the Weather Channel’s Jon Erdman). The warm temperatures have triggered unprecedented flooding that’s closed more than 50 miles of the Dalton Highway, a key route through northern Alaska. It may be a particular rough season for wildfires across those higher-elevation forests where snow was extremely scant this past winter, from the Sierras north through the Cascades and into British Columbia.

Hurricanes aplenty in the northeast Pacific
All signs point toward a blockbuster year for tropical cyclone activity in the northeast Pacific basin, a region favored by the proximity to unusually warm sea-surface temperatures (SSTs) associated with El Niño. SSTs are already more than 1°C (1.8°F) above average over a large swath of the region where northeast Pacific hurricanes typically form. This morning the National Hurricane Center is investigating two potential tropical cyclones for possible development in the northeast Pacific, and models suggest that one of these could become the the farthest-west tropical cyclone on record for this basin so early in the year. It’s not out of the question that at least one northeast Pacific tropical cyclone will move toward Baja California later this summer, perhaps bringing heavy rain and wind to the southwest U.S. In addition, the northwest Pacific is off to a rip-roaring season, with three Category 5 storms to date, the most ever recorded so early in the year. Conditions should lean toward more tranquility across the North Atlantic, where El Niño effects will likely lead to a below-average year for tropical cyclone activity. NOAA will issue its Atlantic hurricane outlook on Wednesday, May 27.


Figure 4. Unusually early warmth now across Alaska and western Canada may push into the Arctic next week, as shown by temperature anomalies predicted by the 0000 GMT Thursday run of the GFS model for 0000 GMT Wednesday, May 27. Image credit: ClimateReanalyzer.com/University of Maine.


Another crucial year for Arctic sea ice
The extent of sea ice in and near the Arctic is already close to record-low values for this time of year. Now that we’re approaching the summer solstice, a key variable that will shape ice melt is the extent of cloudiness over the next few weeks; clear skies allow the round-the-clock sunlight of the midsummer Arctic to have maximum impact on the ice. Long-range models project that the heat wave across northwest Canada and Alaska will push still further north next week, sending temperatures close to or above the freezing mark across a large swath of the western Arctic weeks ahead of schedule. This could result in widespread formation of melt ponds atop the ice, which absorb additional heat from the sun and hasten further melting. It’s too soon to say for sure--the weather in June and July will play a huge role--but I’d consider a new record-low extent for Arctic ice a real possibility for 2015.

Taking aim at a new global heat record
This year is already the warmest in global records for the period January through April. El Niño tends to warm Earth’s atmosphere (by suppressing the upwelling of cold water and spreading oceanic heat over a large area, where it can warm the air above). So barring any major volcanic eruptions, we can expect the next three months to keep this year rolling toward a likely, if dubious, victory over 2014 in the troublesome global-heat race.

Across the soaked Plains, a memorable Memorial Day weekend
Just when it seemed more water couldn’t possibly fall over Texas and Oklahoma, yet another Pacific storm system will wring out more heavy rains this weekend. Flash flood watches are already posted for much of the two states, plus western Arkansas, with five-day rainfall amounts likely to top 3” over large areas and exceed 6” in spots. With the rains juxtaposed over some locations that have received 10” to 20” over the last several weeks, there’s a high potential for dangerous flash flooding this weekend, as well as major river flooding in the days that follow. Severe weather should be relatively subdued this weekend, with a few tornadic storms possible in scattered pockets as the upper low now over the Southwest slowly progresses east. Outside of the plains, travel impacts should be relatively minimal, although showery conditions in many areas will dampen more than a few outdoor activities. Temperatures will be a bit cooler than average over much of the West, while an early-season “warm wave” begins building toward the East Coast.

Have a great holiday weekend, everyone!

Bob Henson



Figure 5. A thunderstorm over central Pennsylvania on Sunday, May 17. Image credit: wunderphotographer baxtheweatherman.