 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.
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.
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.
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.
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.
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.
Figure 5. A thunderstorm over central Pennsylvania on Sunday, May 17. Image credit: wunderphotographer baxtheweatherman.
What Can Lightning Tell Us about Storm Severity ?
As soon as an incipient thunderstorm spits out its first cloud-to-ground lightning bolt, it’s a potentially deadly threat. A complex of severe storms can generate many thousands of lightning flashes per hour. Along with being a hazard in its own right, lightning can serve as an useful clue to how quickly a thunderstorm is strengthening. New tools along these lines have been developed to help forecasters, and they’re being tested this spring at NOAA's Hazardous Weather Testbed at the National Weather Center in Norman, OK, where I visited earlier this month.
Figure 1. Lightning and the setting sun paint a colorful scene over Washington, D.C., on April 20, 2015. Image credit: wunderphotographer wolfpackwx.
Those of you in warm climates may have seen a weak shower suddenly deposit a burst of rain just minutes after you heard the first clap of thunder. For decades, this anecdotal evidence couldn’t be reliably examined. The advent of lightning detection systems in the 1980s meant that lightning flashes could be quantified for the first time. Most of the country is now covered by a network of sensors that detect the location and timing of cloud-to-ground lightning (CGs) by measuring the electromagnetic signals released at ground level. There are also regional lightning mapping arrays (LMAs) that detect intracloud lightning (ICs) by sensing the very high frequency radiation released at each “elbow” of a zigzagging flash. Together, these systems allow what's called total lightning (CGs + ICs) to be quantified. Some of the earliest work using these data found intriguing connections to severe weather. A 1989 study led by Don MacGorman (National Severe Storms Laboratory) found that the total lightning flash rate in one Oklahoma thunderstorm peaked about 5 minutes before a tornado developed. In another early study, Steven Goodman (NOAA/NESDIS) and colleagues discovered a spike in the total flash rate of an Alabama thunderstorm that preceded small hail and a microburst by several minutes. Goodman and Patrick Gatlin (University of Alabama in Huntsville) also developed the first algorithm designed to automatically identify lightning jumps.
More recently, Christopher Schultz (now at NASA) led a series of studies looking more broadly at whether lightning-jump algorithms could serve as an early-warning tool to help forecasters identify which storms might soon become severe. The results were highly encouraging, especially when the algorithm flagged jumps in total lightning that were two standard deviations greater than the rate observed a few minutes earlier. In a 2011 study of more than 700 thunderstorms (mostly in northern Alabama), this “two-sigma” index detected 79% of all severe thunderstorms, with a relatively low false-alarm rate of 36%. Schultz’s recent dissertation work demonstrated that lightning jumps, as opposed to more general increases in flash rate, were closely related to increases in the storm’s peak updraft speed, and in the storm volume that features updrafts classified as intense (e.g., at least 10 meters per second or 22 mph). These increases in updraft size and speed preceded lightning jumps by roughly 4 to 12 minutes.
Lightning jumps can’t tell us exactly what kind of severe weather a storm will produce, and not every lightning jump will lead to a severe outcome. Moreover, some thunderstorms reach severe levels without producing huge amounts of lightning. However, in general, lightning jumps can provide an almost-instantaneous measure of how quickly a thunderstorm updraft is strengthening. As the vertical motion in a storm intensifies, ice crystals, supercooled water droplets, and graupel (snow pellets)--together referred to as mixed-phase precipitation--grow more rapidly and bump into each other more readily, transferring charge among each other. Different speeds of descent allow for the charged particles to assemble in zones of positive and negative charge, which increases the storm’s ability to generate lightning. In a newly forming thunderstorm, the strengthening updraft typically produce the charge separation needed for a lightning jump a few minutes before it has time to generate one or more of the markers that the National Weather Service uses to classify a thunderstorm as severe: hail larger than 1” in diameter, winds reaching 58 mph, and/or a tornado. (Interestingly, lightning itself is not one of the elements that