Alaska Thunder and Lightning
Alaska Thunder and Lightning
More 2023 Weirdness
The summer of 2023 has been a strange one in many parts of the world, with extreme temperatures, from daily to seasonal time scales, extreme rainfall and of course devastating wildfires. For Alaska we can add “it’s been a weird thunderstorm summer”
Alaska Thunderstorm & Lightning Climatology
It’s long been recognized that while thunderstorms occur everywhere in Alaska they are very strongly favored over Interior Alaska in summer (see technical notes below). The Alaska Fire Service has operated a lightning detection network in Alaska since the mid-1980s. Unsurprisingly, the technology and performance of the network has changed a lot in that time, including changes in sensor type, sensor efficiency and range. In 2012 a new lightning detection network (referred to as the “Time of Arrival” network) came online and remains in use today. Because it detects “strokes” vs. “strikes”, the counts are not directly comparable with the 1986-2012 system The two systems ran in parallel in 2012 and based on this comparison, the current lightning detection network “stroke count” averaged about twice that of the old network “strike count”, though there was considerable day-to-day variability.
Using the older lightning data that has 25 years of archived strike data, Brian Brettschneider (NWS Alaska Region) created the graphic in Fig. 1 some years ago but this remains the best representation of the relative frequency of lightning (hence thunderstorms) in Alaska. The absolute values are less important than the relative values. The highest lightning density shown here (red) is about ten times that in the lowest shaded value (deep green) and highlights that thunderstorms are favored between the Alaska Range to the south and the Brooks Range to north with the maximums over higher elevations.
To only slightly oversimplify, there are three primary ingredients needed to create thunderstorms: 1) sufficient moisture, 2) sufficient decrease in temperature with height into the middle atmosphere (the “lapse rate”) and 3) a mechanism to lift the moist air.
Interior Alaska (extending eastward into the central Yukon Territory) is where these ingredients come together most readily, especially in the early summer: early summer temperatures are regularly high enough that the hills become an elevated heat source and this provides the initial lift. The limiting factor with thunder in Alaska, especially early in the summer is moisture availably. In later summer the middle atmosphere cooling rate is the limiting factor. These small details are important because nearly all Interior thunderstorm activity is of the “airmass” variety. Organized thunderstorm, such as with weather fronts is extremely unusual over mainland Alaska, though this does occur (at low frequency) in the autumn over Southeast Alaska.
The result of the convergence of the two main controls on Alaska thunderstorms results, historically at least, thunderstorm activity is a short but sharp season. Figure 2. represents an analysis of the older lightning detection network data seasonality of strike frequency. This shows the (smoothed) relative frequency of lighting through the course of the summer in the main lightning area (shown in the map insert). This shows the ramp-up of lightning in late May to a “plateau” of sorts in mid-June (likely due to mid-atmosphere warming) and a peak in early July (when low level moisture is high). But to me the most dramatic feature of this graphic is the rapid drop of lightning frequency the second half of July. This is well correlated with the climatological increase in westerly winds aloft and the start of the Interior Alaska rainy season, which greatly reduces the middle atmosphere cooling rate.
Summer 2023 Thunderstorm Season
Thunderstorm activity was very low in and near Alaska through the third week in July. The total stroke count was less than half of the next lowest year (2021). This was the result of the stable weather pattern that produced the wet weather pattern for Southcentral and southwest Alaska for June and much of July (more details in my post here)
A shift in the pattern the fourth week of July allowed for a remarkable late season thunderstorm outbreak. The total stroke count last eight days of July was more than double that seasonal total through July 21. The total strike count on July 24 (Fig 4.) was the highest since July 11, 2019 and the fifth highest since the current Time-of-Arrival network came online in 2012. Dozens of wildfires were ignited in the central Interior during the last week of July (details in my post here) and most continue to burn as of mid-August as the rainy season has not yet materialized.
While not as extreme as in late July, unseasonably high thunderstorm activity has continued through the first two weeks of August. As of August 14, the total strokes in August alone accounts for more than a quarter of the seasonal total. In Fig 5. I use both the older and current lightning detection networks to assess how much of the seasonal (May through September) lightning has occurred in August as a percentage of the total: hopefully this “same year” comparison reduces influence of the network changes. The spike in August lightning since 2013 makes me wonder if there is still a sensor/network change effect, but both 2013 and 2014 definitely saw very limited early summer thunderstorms (hot and dry 2013, cloudy and rainy 2014), with the August totals not especially high compared to other years. This year is different: just through the first two weeks of August more strokes have been logged than the August total for any year 2012-2022. And while the thunderstorm season is winding down over Interior Alaska for another year, the 2023 thunderstorm season has been unique in the modern era, which makes it a good learning opportunity about what’s possible in our changing climate.
