Friday, August 5, 2022

A World of Difference

The last 3-4 weeks have seen a remarkable turnaround in weather fortunes for much of Alaska, with dramatically wetter - and considerably cooler - conditions bringing a very welcome end to the wildfire season.  Compare the two 30-day precipitation anomaly maps below: the top one showing the situation as of a month ago, and the latest analysis on the bottom.


On July 5, 18% of Alaska's land area was classified as having moderate or severe drought, according to the U.S. Drought Monitor, but now drought remains in only a small area (less than 1%) just to the south of Fort Yukon.

Reader Gary commented that "it seems like August in July for Interior Alaska", and indeed the change to notably wetter and cooler weather is very typical for August.  I've pondered whether it would be possible to identify a sudden regime shift that marks this transition in many years; it would be nice to measure the change in an objective way, and look at how the transition varies from year to year and from decade to decade.  For example, has it become earlier over time?  Any suggestions from readers would be welcomed.

In the absence of objective criteria to define an August-like "regime shift", I thought it would be interesting to look at the timing of the warmest week of the year in Fairbanks: see below.  Early July is the most common time for the warmest week to occur, like this year, but there is a lot of variability, with some years seeing the warmest week in early August or before the middle of June.  There's a hint of a trend towards earlier peak warmth, although it's certainly not statistically significant.

A much more significant trend is evident if we look at the wettest week of the year in Fairbanks, based on rain amounts only (I've excluded weeks with any measurable snowfall).  The timing of peak rains really has changed a lot: prior to about 1970, the wettest week was more often in August or September than in July, but in the past 20 years it has become rather unusual to see the peak in August, let alone in September.

From this perspective, then, it does look like the transition from dry spring weather to wet "late summer" weather has become earlier over time.  But we should also recognize that changes in July rainfall have been dominated by an increase of infrequent but heavy events, as discussed elsewhere on this blog (e.g. here), and this is not necessarily the same thing as the August wet regime arriving earlier (i.e. July rains are more convective, whereas August is typically more stratiform).

Another intriguing angle on the timing of the regime shift is found in the midpoint date for the wildfire season: see below for one of Rick Thoman's excellent Twitter graphics.  Only 30 years of data are available, but there's a hint of a trend towards earlier fire season midpoints.  Interestingly this season was a big one (over 3 million acres burned), and yet its midpoint was on June 30 (assuming acreage does not increase much from here).  No other year in the series had both a large acreage total and a relatively early midpoint.

That's enough speculation for one post.  But for the sake of completion, here's the timing of the coldest week in Fairbanks.  Given the profound warming throughout the Arctic in autumn and early winter, it's no surprise to see that it has become less common for Fairbanks to have the winter's coldest weather before the turn of the year.

Tuesday, July 26, 2022

Out of Season Windstorm

Yesterday brought some wild and highly unseasonable weather to southern and interior Alaska, as an extremely powerful cold front blasted east across the state in connection with the intense vortex that is still spinning near the Bering Strait (see the last post).  Indeed the mid-atmospheric cyclone was re-energized in a big way from a new low pressure system that developed within a frontal zone stretching along the Aleutian chain, and this new surge of energy is what pushed the front so vigorously into the interior.  Here's a sequence of 500mb maps from Saturday afternoon through this morning at 12-hour intervals:

The eastward progress of cold air aloft is visible in the 850mb maps below, at 6-hour intervals from 10am yesterday to 10am today.  The location of the front was at the leading edge of the cooler air, passing through Fairbanks around 7-8pm yesterday evening.

The front will long be remembered in Fairbanks for the damaging winds that occurred with its passage, gusting to 44mph at the airport and 46mph at Fort Wainwright.  Power was knocked out for many thousands of homes and "untold property damage" occurred, to quote the News-Miner.  Remarkably, true chinook winds occurred earlier in the day farther up the Tanana Valley, peaking at 63mph at Delta Junction and around 70mph at a few sites nearby.  This is all very remarkable for the time of year, as there is rarely enough of a pressure gradient to generate widespread strong winds at this season.  (Of course strong winds are quite rare in winter in Fairbanks too, owing to the strong temperature inversion, but high winds occur frequently in the hills and many other valleys.)

Here's the 4pm sounding from Fairbanks yesterday, prior to the arrival of the front.  The wind barbs on the right show very strong flow from a SSW direction aloft, and the temperature profile shows a steep lapse rate through a very deep layer, greatly aiding the downward transfer of momentum to the surface (i.e. the opposite of a temperature inversion).

The sounding's reported wind speed of 56 knots at 700mb (nearly 10,000 feet elevation) is the highest 700mb wind speed on record for Fairbanks in June or July.  Interestingly, both 2018 and 2019 saw similar 700mb wind speeds in early August, but those were big rain events with upper-level winds from a more westerly direction and with much less steep lapse rates and much less wind at the surface.  Here are those soundings:

If we look at only 700mb winds from a southerly quadrant (southwest through southeast), yesterday's event had easily the highest wind speed on record for summer in Fairbanks (data back to 1948).  No other sounding reported 50 knots or higher from a southerly quadrant between April 10 (1956) and September 5 (2012), so from this perspective yesterday was an unprecedented event in at least the last 75 years.  With trees still having full foliage, the extent of the damage is no surprise.

Saturday, July 23, 2022

Arctic Vortex

After an astonishing outbreak of wildfire in southwestern Alaska earlier this summer, the weather has changed dramatically.  The past few days have been chilly, and this morning many locations around the Bristol Bay region were in the 30s, with some close to freezing - notably 33°F at the Aleknagik CRN near Dillingham and also at Koliganek.  Also worth noting is the 31°F yesterday at the CRN site on St. Paul Island; this is the first sub-freezing temperature there in July since 2012.

The mean daily temperature plot from Bethel (below) shows that the cool conditions are not as unusual as the heat was in early June.  Nevertheless, yesterday's high temperature of only 49°F in Bethel is right at the lower end of what's been observed historically for the time of year: the daily record low maximum temperatures (i.e. coldest daytime highs) are mostly around 49-50°F from mid-July through mid-August.

The cool weather can be attributed to a very strong mid-level low pressure system that dropped down from the Arctic.  The mid-atmospheric cyclone can be traced back to the central Arctic, and arguably it is THE summertime tropospheric polar vortex that moved down to the Bering Strait.  Here's a simple animation of 500mb heights at 12-hour intervals since the beginning of the month.

The vortex has been extremely strong for the time of year, and remarkably it set new records for lowest 500mb height in the month of July for quite a number of locations from the central Arctic to the Bering Strait (based on ECMWF and ERA5 data).

As for the larger significance of such an event, random variability plays a large part in the formation of a feature like this, but I'll also note that the dramatically cooler weather is finally more reminiscent of a typical La Niña pattern for summer, as noted in this ill-fated blog post from early June.

[Postscript: here's a photo of snow on Big Diomede island in the Bering Strait on Monday]

Monday, July 18, 2022

Weather Relief

I'm traveling at present and can't comment in detail, but major weather relief has arrived at last when it comes to Alaska's nasty fire and smoke season.  A change of pattern was advertised well in advance by the Climate Prediction Center, and widespread rainfall has been a big help for fire-fighting efforts.  Here's a temperature anomaly plot for Fairbanks and the latest 7-day precipitation analysis from the NWS:

The 30-day departure from normal analysis (below) now shows some green across southern Alaska, but notice how much brown there still is on the 90-day anomaly map (also below).  The drought isn't over yet.

Returning to the topic of lightning, a few days ago I witnessed an impressive night-time display of lightning in Florida, and I got to thinking about the contrast in lightning density between the two states: Florida having the most lightning per square mile, and Alaska the least.  Based on the BLM/ALDN data, Alaska's peak lightning density has been about 5 strikes (cloud or ground) per square km over about 11 years, i.e. approximately 0.5 strikes per km2 per year; this lightning hotspot is just to the north of the Alaska Range, roughly between Minchumina and Healy.

In contrast, the entire state of Florida averages about 7 strikes per km2 per year, according to a 10-year study from Vaisala, and some areas have considerably more.  So even the most lightning-prone parts of Alaska don't come close to what's considered normal in the so-called Sunshine State.  As far as I'm concerned, less lightning is always better, although it can be a visual spectacle: here's a video taken by my daughter last Wednesday evening as we were driving in Florida (apologies for inferior quality).  It would be interesting to dig through the Alaska lightning data to find the single most prolific lightning storm and see how it compares to these flash rates.

Tuesday, July 12, 2022

Flooding and June Climate Data

First, weather news today from the southeastern interior: flooding has damaged the Richardson Highway in a number of locations near the crossing of the Alaska Range to the south of Delta Junction.  Damage looks considerable:


A combination of big snowmelt from the higher elevations and heavy rain are to blame.  The latest 3-day precipitation analysis shows 1-2" of rain in a number of places, but of course amounts may have been much higher in mountainous areas.


A similar washout happened on the Alaska Highway in northern British Columbia less than two weeks ago.  Here's a photo of the remarkable damage there, obtained at the time from Twitter, although regrettably I can't remember the exact source:

On another note, June climate data finally came in from NOAA/NCEI, who confirmed that (based on the limited available data) it was the driest June on record for Alaska as a whole.  Compared to the prior 30 years (see below), June was the driest or second-driest across a wide area, and the Bristol Bay, Cook Inlet, and Northwest Gulf divisions were extremely warm: Kodiak had its second warmest June on record, and King Salmon was third warmest.

Gridded ERA5 data adds more detail to the picture and fills out some additional variables that help explain why the fire season has become so bad:


And here's an interesting result: June average dewpoints were much above normal across southern Alaska, but much lower than normal in the western and northern interior.

Monday, July 11, 2022

Cloud Lightning

I've been spending a bit of time working with Alaska's lightning data recently.  Part of the goal is to have a history of daily lightning strike counts that is consistent with the 6am-6am numbers displayed on the BLM dashboard each day:

The analysis reveals that the current stretch of 10 consecutive days (including today) with over 5,000 lightning strikes is easily the most since the current detection network came into operation in 2012.  The previous record was 6 days in June 2015.  The late June 2015 lightning onslaught was massive, including over 56,000 strikes in 3 days; we haven't seen those kind of numbers yet this year, but in contrast there's been a more sustained period of heavy lightning activity in the past 10 days.

The BLM lightning data distinguishes between cloud-to-ground and cloud-to-cloud lightning strikes, so naturally I thought it would be interesting to examine the relative frequency.  This reveals something quite surprising: nearly all of Alaska's cloud-to-cloud lightning occurs in the western half of the overall lightning zone.  Compare the maps below (note the different scales):

Why would the eastern interior be almost devoid of cloud-to-cloud lightning?  I hypothesize that eastern interior thunderstorms may be typically smaller and more isolated (despite being common), and rarely grow large enough to generate cloud-to-cloud lightning, which occurs between different parts of large thunderstorms or between separate thunderclouds.  In contrast, perhaps higher moisture levels in the western interior more often allows thunderstorms to grow upscale and merge into larger conglomerations that provide more opportunity for discharge within and between clouds.

Those who have spent more time than I have watching satellite imagery might be able to comment on whether this hypothesis is reasonable.

Here's the percent of cloud-to-cloud lightning as a function of longitude.  We only have 10 years of data, but it's a pretty striking result (sorry).

Wednesday, July 6, 2022

Lightning and Fire

As if the fire situation were not threatening enough already, lightning activity ramped up over the weekend across interior Alaska.  There were nearly 30,000 lightning strikes from Saturday through Tuesday, which is more than some years see in all of July.

However, compared to some of the big lightning episodes of recent years, the last few days have not been too unusual.  For instance, the peak 2-day strike count of around 17,000 strikes is about normal for the maximum observed in recent summers, based on data since 2012.  Several years have seen much greater 2-day outbreaks: over 50,000 strikes in mid-July 2016, over 40,000 in late June 2015, and 36,000 in July 2019.

It's interesting to consider whether wildfire acreage tends to increase more rapidly after large bursts of lightning activity.  This is a question that could be assessed in many ways, but for a quick look I examined the fire acreage within 30 days before and after each summer's peak 2-day lightning strike count.

Note that I excluded 2014, 2020, and 2021, which had relatively subdued lightning peaks (on the order of 10,000 strikes), whereas all the other years on the chart had peaks at least as great as last weekend.  The order of the years in the legend at right shows the order of peak 2-day strikes from greatest to least.

It's somewhat sobering to see that all but one of the years saw more acreage burned in the 30 days after the lightning peak than in the 30 days before, although of course the magnitude of acreage differed vastly between years.  A couple of years - 2015 and 2016 - saw a pretty obvious acceleration in fire in the week after the lightning peak, but in other years there isn't a clear connection.  No doubt a more in-depth analysis would provide additional insight.

While the recent lightning is certainly not good news, the medium-range weather outlooks from the Climate Prediction Center have been trending in the right direction lately.  Here's the latest version: this is distinctly encouraging.