Tuesday, May 26, 2015

As We Approach the Start Of Hurricane Season: Wunderblog's Summer Weather Watch

My M.D. great-aunts were bemused by meteorologists who introduced themselves as Doctor.
On the other hand, when arriving at some function together, they'd announced they were "a pair-o-docs", so not real sticklers for form.
From Bob Henson at  Dr. Masters' Wunderblog:

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.......MORE