Hail Reports

The Fairbanks area saw some hefty showers and thunderstorms yesterday and last Friday, and hail was reported around town on both days.  The storm on Friday afternoon was isolated but quite intense, with three-quarter inch hail reported.  Here's a radar image from 3pm, when the storm's intensity peaked as it moved from northwest to southeast over the north side of town (click to enlarge):

Another cell dropped pea-sized hail at the NWS office on the UAF campus a little while later.

Yesterday saw a larger and more organized storm track into town from the northeast, bringing both hail (up to half-inch reported) and damaging wind gusts.  The radar reflectivity reached an impressive 65 dbZ over quite a lengthy track.

Courtesy of the NWS on Twitter/X, this was the view to the northeast from the airport at the same time as the radar image above:

There was an impressive display of mammatus clouds after the storm passed.  These clouds form as pockets of cold air, loaded with ice crystals, sink out of the high-level cloud shield that forms in the wake of a mature thunderstorm.

Here's a simple animation of the storm evolution on radar:

As of yesterday, Fairbanks is up to 7 days so far this season with thunder reported at the airport.  That's the same as last year at this date, and it's a little ahead of the long-term normal of 4-5 days.  The normal for the whole season is 9-10 days.

Statewide lightning activity has resumed climbing since Friday, and the year-to-date strike count total has nearly reached the median for the seasonal total.

The Lower Yukon region is now well above the 2012-2025 record.

Meanwhile, there's a heat advisory in effect for the interior North Slope, and the NWS forecast for Umiat is currently calling for 87°F tomorrow.  The RAWS thermometer there will no doubt be a few degrees too high as usual, so it will be interesting to see if we can get a 90°F reading - it would be the first since 2013.

[Update: it reached 83°F, a typical value for the summer's highest temperature in Umiat.]

Fire Suppression

The multi-day spate of widespread lightning activity in the second half of June produced a 9-day total of about 57,000 cloud-to-ground lightning strikes, according to the Alaska Fire Service's detection network.  It was a substantial wave of activity, but not all that unusual in the context of historical data since 2012; four other years had larger 9-day totals (2025, 2015, 2019, 2022), with the record being last year's 69,000 strikes that occurred just a few days earlier on the calendar.

As noted previously, the lightning outbreak was unusual for the level of activity in the southwest and northern interior regions.  The Lower Yukon zone, in particular, saw a very intense onslaught compared to the climatological normal, and the year-to-date activity there is already tied with the 2019 record for the entire season.

Here's the actual strike density in absolute terms for the month of June:

The northwestern and southeastern interior were relatively inactive, with the most notable shortfall being in the Koyukuk and Upper Kobuk zone:

It has been something of a surprise to see wildfire activity remain very subdued despite all the lightning.  There have been fire starts, of course, but they haven't grown; statewide acreage is only 29,000 acres, a mere 10% of the climatological median.  The chart below shows that the disparity between the number of fire starts and fire acreage is very unusual; no other year since 1993 had so little acreage with more than 100 new fires in June.

Similarly, the fire acreage relative to the number of lightning strikes is extremely low:

Although last year had more fire, it was qualitatively similar: somewhat subdued fire growth relative to the extensive lightning activity.  At least one key factor is that both years had a wet spring across most of the fire-prone interior:

Contrast this with 2022, for example, which saw very efficient fire growth (a lot of acres per fire and per lightning strike) after a dry (but not warm) spring:

Of course the weather in June also makes a huge difference for the fate of lightning-sparked fires; 2004 and 2013 were extremely warm in June, and fire growth was rapid.  Last month had some warm days, but the second half of the month was near-normal for Fairbanks temperature overall; and there's been a substantial amount of rain in many areas in the past two weeks.  Obviously when lightning is accompanied by widespread and significant rain - as opposed to "dry" thunderstorms - the fire activity is more often suppressed.

Not surprisingly, there is a correlation between June fire acreage and the rest of the season (fires tend to keep burning), but the weather is the key factor.  For instance, 2009 was extremely warm and dry in July, producing big fire activity after a slow start.  Hopefully we won't see anything similar this year.

More Lightning

Yesterday was the seventh day in a row of very substantial lightning activity in interior Alaska, and it was the second most active day on record (2012-present) for the Lower Yukon region, with nearly 3500 strikes in that part of the western interior.  Only July 11, 2019 had more lightning strikes in a single day in the Lower Yukon zone; but this year's activity is far ahead of 2019 year-to-date:

Here's a preliminary look at the spatial distribution of lightning this month, with data through the 25th.

The historical normal for June tends to be much reduced to the west of Galena and McGrath, and also across the northern interior, so the westward and northward extent of heavy lightning activity is the most unusual aspect of this persistent lightning outbreak.

On the other hand, the lightning "hole" around Fort Yukon this month is pretty typical.

Like Clockwork

[June 24, June 25: see updates below]

The calendar can be a remarkably good guide to seasonal weather changes at times, and especially in very continental climates where the pace of change is typically rapid.  The onset of widespread lightning across interior Alaska over the past several days is a case in point: check out the similarity between this year's accumulated lightning strike count and the climatological median (thick black versus dashed black lines): 

Based on historical data since 2012, the climatological median takes off right at June 15, and this year major lightning activity began on June 19.  Each of the past four days has seen more than 4000 lightning strikes statewide, and while the 4-day total of about 20,000 is not particularly noteworthy, the impeccable timing is interesting.

Last year saw a similar but more dramatic surge beginning on June 16: over 50,000 strikes in 5 days at that time.

In terms of spatial distribution, the most pronounced activity has been in the Kuskokwim and Lower Yukon regions, as well as the central interior (South and West Tanana zones).  Here's a map of the AICC Predictive Service Areas and other geographical divisions that I use to track totals (click to enlarge):

Compared to the historical median, the Lower Yukon region has seen the largest excess of lightning strikes, with the season's total already having reached the normal for a whole season:

In contrast, the northern interior has seen less lightning than normal, and the North Slope has seen only 10% of its typical 1200 lightning strikes to this date:

As for fire, statewide reported acreage remains minimal, despite the rather significant lightning outbreak.  There have been plenty of fire starts, of course, but "so far so good" in terms of containment and limited growth.  Significant rains were widespread today in the upper half of the Tanana Valley, and the southwestern interior has also seen good rains in the last few days - so that helps.  Perhaps lingering moisture from the very wet spring has been enough to hinder quick fire growth after all, despite concerningly dry conditions earlier this month.

The 2026 statewide fire acreage is still nearly indistinguishable from zero on the year-to-date chart below, with total acreage at only 17,000 acres compared to a median of 184,000 for the date.

Looking ahead, distinctly cool conditions appear likely to return next week, and there's also rain in the forecast, so the fire risk appears set to subside for a time.

June 24 update:

Yesterday - the fifth day of widespread lightning - was the most active day so far, with over 7000 strikes statewide.  There were 3000 in the southern Tanana zone by itself, pushing the year-to-date total well above normal in the heart of the interior:

The statewide year-to-date total is slightly above normal now, but the current lightning outbreak is still much less intense than last year's mid-late June onslaught.

June 25 update:

And another big day: yesterday produced the largest statewide strike count so far, over 7400 strikes.  The northern interior had major lightning activity for the first time, with the northern Tanana zone seeing nearly 2800 strikes, suddenly taking the year-to-date total into second place locally.  Last year was far and away the most active year in the 2012-present history for the north-central interior.

The southern Tanana zone is now almost at a record high for year-to-date activity:

Last Wednesday's comment that the "pattern looks conducive to the onset of more widespread lightning in the near future" turned out to be more correct than I expected.

Minimal Fire So Far

Interior Alaska finally saw some genuinely warm weather last weekend, with temperatures pushing up toward the 80°F mark in many areas; and Fairbanks saw its first 80°F of the season - a few days later than normal.

The mini-heatwave didn't last long, but renewed warmth is on the way in the coming days, and the predicted mid-atmosphere pressure pattern looks conducive to the onset of more widespread lightning in the near future.  The map below shows the expected 500mb height anomaly (departure from normal) for the next 7 days, indicating a broad axis of above-normal heights (i.e. a ridge) from eastern Russia to northwestern Canada:

Contrast the pattern in the first two weeks of this month, when a strong trough over the Arctic Ocean brought cool northwesterly air flows to interior and northern Alaska:

Atmospheric moisture has been well below normal for the time of year, and that means a lack of atmospheric fuel for thunderstorms.

No surprise, then, that statewide lightning activity is below normal and is now moving into "significantly below normal" territory; the climatological median ramps up quickly from June 15 on.  Click to enlarge:

Thanks to the lack of lightning, year-to-date fire activity is close to zero: only 5600 acres have burned statewide so far, compared to a historical median of 85,000 acres for this date.  In view of the weather forecast, this seems likely to change soon.

It's also worth noting that the past two weeks have been quite dry for most of the interior, a marked change from the wet spring; fire fuels have no doubt dried out quite a bit.

Summer Review

I've been meaning to post a few comments on the climate patterns and anomalies during summer - better late than never, I trust.  On a statewide basis it was a fairly warm and fairly dry summer, but neither aspect was highly unusual in terms of the state averages: according to NOAA it was the 10th warmest summer in the last 30 years (warmer than 2024 but cooler than 2023), and statewide precipitation was the fifth lowest in 30 years (driest since 2019).

However, regional differences were significant, especially for precipitation.  The summer was much drier than normal along the central and eastern Gulf Coast, but much wetter than normal for most of western Alaska as well as the northern interior and western Brooks Range.  Here are the ERA5 (top) and NOAA (bottom) percentile rank graphics for the three-month period:

Here's a chart of accumulated precipitation at Yakutat, to illustrate the dryness (click to enlarge).  The June through August period was the second driest on record, following 2007, with total rainfall amounting to only 37% of the long-term median (9.6" versus 25.8").

Precipitation deficits were very pronounced in July along the northern Gulf Coast, but August really exacerbated the drought (relative to normal) and also set up the big contrast with excessive rainfall in northwestern Alaska:

We covered the late August stormy pattern for western and northern Alaska here, and the August 500mb height anomaly highlights the pressure gradient across western Alaska: the ridge over southern Alaska deflected the jet stream northward into western and northern Alaska.

In terms of sunshine and wind, it was a very poor summer for the Y-K Delta region, with all three months bearing part of the blame.  However, only June was cooler than normal in the southwest.  Eastern and northern Alaska were generally sunnier and less windy than normal, although not in August.

Looking back at lightning and wildfire activity, both were above the long-term median, but lightning much more so.  For a time in late June and early July, lightning strike counts were running second only to 2015 (with data back to 2012), and the seasonal total ended up in 4th place, similar to 2019 and 2022.  In contrast, wildfire acreage ended very close to 1 million acres, which is about twice the long-term median but well below 2019, 2022, and of course 2015.

It seems likely that Alaska's very wet spring was largely responsible for the relatively subdued fire season.  The statewide average for the April-May total precipitation was easily the highest on record, with both April and May ranking in second place statewide; and importantly, the wet anomaly included large parts of the fire-prone interior.

 

May was also cooler than normal, and June was only a little warmer than normal in the interior.  This is a big contrast with, say, the enormous warmth of May 2015 that dried out the state in advance of that huge fire season.

Here's a map to illustrate the distribution of lightning activity relative to normal.  There was far more lightning than normal in northern Alaska, and the more typically active southern interior also had more lightning than normal.

A plot of actual strike count density shows that the bulk of the activity occurred across the interior, as always, but it was very unusual to see the high strike counts extending so far north into the Brooks Range.

I suspect that the combination of fairly high temperatures and high humidity in the northern interior was part of the reason for the very active lightning season there (warm, humid air is fuel for thunderstorms).  Here's a map of dewpoint percentile relative to the 30-year climate:

From a broader climate perspective, widespread and extremely unusual warmth in the North Pacific this summer is one of the biggest stories in climate science at the moment.  June-August sea surface temperature anomalies were as much as 3°C above normal in the northwestern Pacific, leading Japan to have its third record-hot summer in a row.

Remarkably, the August area-average SST reached 20°C for the Pacific basin north of 30°N, according to NOAA's ERSST data.  The acceleration of warming in the past few years is quite something:

I'm out of time for this post, but I'll append below the rest of my August climate percentile maps for the sake of completeness.