Extreme Weather in McGrath

The heatwave is on the way out, and Fairbanks didn't reach 90°F after all, but McGrath did - and it's a much more unusual event down there.  McGrath tends to have a more maritime influence, being closer to the sea and to the west of the Alaska Range.

With data back to 1941, McGrath has only reached 90°F on 5 days: once in 1969, on 3 consecutive days in 2013 (reaching an amazing 94°F), and once in 2019.  All were in June.  However, July and August have both seen 89°F, most recently on July 8, 2019.

To dispel doubts about the accuracy of yesterday's measurement, the AWOS instruments measured 88°F upstream (on the Kuskokwim River) at Nikolai and 86/87°F downstream at Sleetmute and Aniak.  As usual, RAWS thermometers in the area were higher, but they should be disregarded.

However, the heat is only half the story.  Shortly after the temperature peaked at 6pm, a very intense rain shower developed, dropping over an inch of rain in a very short time.  Brian Brettschneider notes that the rain rates exceeded the estimated threshold for a 1000-year recurrence interval!  Here are the precipitation frequency intervals from NOAA's Atlas 14 (click to enlarge):

Between 7:23 and 7:53pm, the ASOS reported 0.78" of rain (compare to 0.73" for a 30-minute event with a 1000-year recurrence interval), and 1.22" fell in 1 hour 24 minutes.

The notion of return interval is based on historical statistics and the assumption of a stable climate, which is of course never quite true, and we do expect more frequent heavy precipitation events in a warming climate with more atmospheric water content.  Nevertheless, this was clearly an extremely rare event for McGrath.

Putting the two aspects together, the combination of rare heat and rare heavy rainfall was obviously way outside the historical climate record for McGrath.  For instance, the warmest day in the past 84 years with a daily precipitation total of 1" or higher was only 72°F.  There have only been 3 days with a high temperature above 80°F and a daily rainfall above 0.5" (most recently - again - in 2019).

Second Summer

It's all change again in Alaska's weather, with summer returning this week for many places.  The temperature rose into the 80s F yesterday for much of the interior, and the next couple of days will be the hottest of the year for many.  The NWS expects Fairbanks to see 90°F, which has only happened 6 times this century so far (2009, 2010, two days in 2013, 2017, and last year).

Here is yesterday afternoon's 500mb analysis, courtesy of Environment Canada.  Instead of a strong Bering Sea trough dominating the circulation over Alaska, we now have a strong west-east ridge with a connection to western Canada and the western lower 48, where heat and wildfires have been running rampant lately.

This has the look of a classic early/mid summer heat wave; it's not as typical of late July (and would be very unusual in August).  It's certainly interesting to see this unfold after a very "westerly" first half of July.  Here's how this year compares to previous years in terms of the Seward-Deadhorse westerly wind speed index that I described a few weeks ago (when the previous pattern change occurred).

By this metric, July 1-15 this year fell in the top 10 for westerly winds aloft.  Not surprisingly, very strong westerly flow at 700mb (approximately 10,000' above sea level) corresponds to suppressed daytime temperatures in Fairbanks, owing to increased cloud and rainfall, as well as the predominance of cool North Pacific air masses.

Is it unusual for such a strong westerly regime to be followed by a heat wave later in summer?  No: the historical data shows no relationship between early July westerlies and the highest subsequent weekly temperature (looking at daily high temps):

And if we look at the highest daily temperature in the rest of summer, it's interesting to see that a strong westerly pattern in early July tends to have at least one subsequent episode with temperatures into the 80s in Fairbanks - unlike some years.

It's possible that the high-amplitude pattern that produces a strong westerly regime in early July is also conducive to subsequent hot weather, at least for a day or two.

In other words, don't write off summer in early July.

Finally, for completeness, here's the July 16 - August 31 total precipitation in Fairbanks, in relation to early July westerly flow.  Strong westerly patterns tend not to be very dry for the rest of summer, but neither are they typically very wet.

Weather Notes

Large parts of Alaska have been much wetter than normal recently; here's a map of estimated 14-day precipitation totals (NOAA data) and the "percent of normal" (click to enlarge):

Several longstanding climate observing sites saw near-record rainfall over the weekend, including:

Bethel 1.98" on Friday-Sunday: the highest 3-day rainfall total in the month of July since 1955.

Anchorage  2.28" on Saturday-Sunday: the second highest 2-day total in the month of July; only 2001 had more (July 4-5).

Sitka  2.35" on Saturday, the highest calendar day total in July since 1948.

Of course all these numbers would be less notable in August, when heavy rain storms are less rare.

Wet and unsettled weather is common across southern Alaska in summer when the PDO (Pacific Decadal Oscillation) is strongly negative, as it is now.  Here's a map showing the percent of years with above-normal sea-level pressure in summers with a bottom-10 PDO index since 1950:

And the PDO is certainly entrenched in the negative phase: here's the daily index for the last several years.

Sea surface temperatures are much lower than normal in the eastern Bering Sea, which is both cause and effect of the atmospheric trough (low pressure) that is tending to recur in that area.

The weekend rainstorm was produced by a strong trough over that very location; here's the 500mb chart from 4pm AKDT on Saturday afternoon (click to enlarge):

The deep, strong southerly flow into south-central Alaska is amply illustrated on the 4pm balloon sounding from Anchorage: observe all the wind barbs pointed out of the south on the right side of the profile.

Also notice the very high freezing level - over 11,500 feet above sea level - and the high water content (precipitable water) of well over an inch (PW=3.02cm).  No wonder it rained a lot.

The combination of cloud, rain, and a chilly ocean nearby also produced unusually low daytime temperatures: Anchorage and Bethel haven't seen back-to-back days with such low daily maximum temperatures in July since 1970 (54-55°F in Anchorage) and 1971 (48-49°F in Bethel).  Guess what: the PDO was significantly negative back then too.

Regional Lightning Data

For my latest data project, I have been looking at the distribution of lightning within Alaska's 21 fire management regions, i.e. the Predictive Service Areas:

Using the 2012-present lightning data from AICC, I've assigned each lightning strike to one of the zones, or to an adjacent region in Canada or over the ocean - I'm still working on the ocean zones.  This will be useful for looking at historical lightning variations by region, and for assessing realtime activity compared to the (modern) historical average.

The greatest density of lightning occurs in the three zones to the north of the Alaska Range, what I'll call the Tanana valley zones (AK01E, AK01W, AK03S).  (There's also a Tanana Zone-North PSA, but that's considerably less active.)  Here's the annual number of ground lightning strikes in these 3 zones, both for the full year and through July 12:

These zones comprise just over 10% of the area of Alaska, but they receive about a third of the lightning strikes in the state.  This year's activity is below normal so far: the 4th lowest in 13 years of data.

Interestingly the last two years were polar opposites in terms of the timing of lightning: in 2022 it was nearly all prior to this date, but last year most of the lightning activity was after July 12.

The region of Alaska with the largest lightning deficit so far this year is the northeast interior, i.e. the large Upper Yukon Valley zone.  This zone is very nearly as large as the 3 Tanana valley zones combined, and it gets the most lightning of any single zone in terms of total number of strikes.  Here's the chart of annual numbers:

As of July 12, the total activity in this zone is above last year, but that will change imminently.  It's interesting to see how active the first half of summer was in 2012-2017 in this zone, whereas since then only 2022 was very active before mid-July.

Now that I have this data set at my disposal, blog readers should brace for a few more posts analyzing the numbers.

June Climate Data

June climate data are in from NOAA/NCEI, so it's time for a recap.  As we all know, it was a very warm and dry June for the state as a whole: the sixth warmest and fifth driest since 1925, according to the NCEI data.  The only years that were both warmer and drier in June were 1936 and 1957, although June 2022 was very similar in temperature and was the driest on record in the NCEI dataset.

The warmth was most anomalous for the northeastern interior, and the ERA5 climate model (reanalysis) data backs this up:

Only June 2004 was warmer in the northeastern interior, and that was also the warmest June statewide, leading into the worst fire season in modern history.  This year's 300,000 or so acres that burned in June pales in comparison with the nearly 1 million acres that burned in June 2004 (followed by another 5 million in July and August).

ERA5 data confirms that unusual dryness was widespread from the Panhandle to the West Coast and across most, though not all, of the interior.

The circulation pattern that produced this outcome involved high pressure over the East Siberian Sea and low pressure in the Gulf of Alaska:

In winter this would be a cold pattern as cold air sinks into Alaska from the high Arctic, but in the height of summer a warm outcome is favored by ample sunshine heating up the continental interior (see below).  It's also worth noting that the exceptional warmth in the last third of June was driven by a trough in the Bering Sea and a big ridge over the interior, i.e. a more amplified pattern than the monthly mean suggests.

Wind was above normal in the Aleutians and western/southwestern Alaska, but mostly lighter than normal elsewhere.

But June is firmly in the rearview mirror now, as the pattern has been completely different in the last week.  McGrath tied a daily record low temperature yesterday, and CPC suggests the generally cool weather will continue for at least the next couple of weeks.  There have only been 4 years since 1925 when July was cooler than June on a statewide basis - will it happen again this year?  It's seems quite possible.

Big Pattern Change

What a change a few days can make.  A dramatic onset of westerly flow has brought much cooler weather to western, interior, and northern Alaska, and substantial rains have provided major relief for the wildfire situation in many areas.  All burn suspensions have been lifted statewide.

Yesterday was really quite chilly in the interior: with cloud and rain, the high temperature was only 60°F in Fairbanks and 55°F in Bettles.  These are distinctly unusual conditions for this early in July, but would be less so in the second half of July.  Rain totals in the past couple of days exceeded 3" in some spots in the hills around Fairbanks:

Earlier in the week, even higher rain amounts fell in northwestern Alaska: as much as 4-5" was measured in the hills to the north of the Kobuk River delta:

Well over 3" fell in 12 hours at the ASIK RAWS site, elevation 1329': it was an onslaught of heavy, wind-driven rain on Tuesday afternoon.

The immediate explanation for this pattern change is the development of a strong trough on Alaska's side of the Arctic, bringing strong westerly flow across central and northern Alaska.  Furthermore, a powerful ridge has developed over the Aleutians, boosting the north-south pressure gradient that's driving the westerly flow.  Here's the 500mb chart from 3am today, courtesy of Environment Canada:

The estimated 500mb height of 597dm just south of the Aleutians is extraordinarily high and may be an all-time record for the location; I'll need to look into this.

It's interesting that a westerly episode of this magnitude has developed so early in July, when we typically think of late July and August as the time for onset of rainy, cool weather across interior Alaska.  The chart below shows estimates of the normal seasonal cycle of 700mb westerly flow across the 150°W meridian between 60 and 70°N, i.e. a north-south line from near Seward in the south to roughly Deadhorse in the north.  Two 30-year periods are shown: 1961-1990 and 1991-2020.

This confirms that westerly flow tends to see a seasonal minimum around late May and ramps up quickly to its seasonal maximum in August.  There seems to be a hint that recent decades have seen weaker August westerlies, but this difference may well be within the random error from a 30-year sample size.

In a subsequent post I'll take a look at the historical variability of "westerly onset" timing.  

Fire Update

A quick update on fire: the weather is now changing significantly, but the hot conditions of the last several days have pushed statewide fire acreage up to 371,000 acres, per the AICC's daily report.  That total is now above the historical median for the date.

Unfortunately for Fairbanks, almost a third of the acreage has accrued in the nearby McDonald fire, and air quality was atrocious in town yesterday, with the visibility reported as low as 1/4 mile at the airport.  The last time the visibility was that low from smoke (not fog) was way back in 2005.

The following chart illustrates the relationship between the 7-day increase in statewide fire acreage and the weekly-mean high temperature in Fairbanks.  For reference, the latest 7-day mean high temperature in Fairbanks was 81.6°F.

Of course there's much more to fire growth than just high temperature - there have been large acreage increases with quite chilly temperatures in the past - but the warmest weeks do tend to produce at least moderately large acreage gains, as we would expect.

Hot and Smoky

As alluded to in Mike's post yesterday, an upper level ridge has brought very warm conditions to much of Alaska, and yesterday was downright hot in parts of the interior and north, with temperatures reaching well into the 80s.  The Whitestone Farms co-op site near Delta Junction reported 90°F, and that should be quite reliable, unlike the RAWS thermometers that always read high in the strong summer sunshine.

Elsewhere, Eielson AFB reached 87°F, North Pole saw 86°F, and Eagle reported 87°F.  Fairbanks airport, in contrast, only reached 81°F because of dense smoke in the morning: the McDonald and Clear fires are burning to the south of the Tanana River.

The enhanced satellite image below, posted by NWS Fairbanks, shows the location of active fires with the bright red colors.  Note the smaller but still substantial fires to the northwest of Fairbanks too.

Here's a view of yesterday's high temperatures.  Even on the North Slope temperatures jumped up into the 80s, including 85°F (maybe too high) at the Umiat RAWS and 80°F at Nuiqsut.  Up in the Brooks Range, Anaktuvuk Pass reached 78°F, which is pretty remarkable.

Statewide fire acreage has reached 148,000 acres according to the AICC.  This is a significant increase from a week ago, but that's to be expected at the time of year, and the running total remains somewhat below normal.

The relatively modest increase in fire acreage despite very warm conditions recently is probably related to the fact that lightning activity has not been especially problematic.  Sunday and Monday had about 11000 lightning strikes between them statewide, which is notable, but other than that lightning activity has been fairly subdued.  In fact, the total strike count in June is running about 25% below normal, based on a short history since 2012.

But the wet spring probably also has a lot to do with the (so far) modest fire season: ground moisture coming into the season was certainly higher than normal in many places.

https://ak-wx.blogspot.com/2024/06/may-climate-data.html

https://ak-wx.blogspot.com/2024/05/april-climate-data.html

Omega Block and the Upcoming Alaska warmspell.

 As like most avid weather hobbyists, I often spend my time studying the plethora of weather maps now available online.  With the upcoming warm spell forecasted for the majority of the state I found myself interested in the upper air patterns that is going to create it.  

The first and foremost player in this game is an unusually deep low-pressure system churning over the Bering Sea. While this area of the globe frequently experiences stormy weather and is the home of the semi-permanent Aleutian low it is unusual to see a storm this deep in the heart of summer.  Computer models have this storm dropping down to 977mb in depth.  Quite surprising for this time of year.  This deep of a storm would be more common in early fall.   See map below courtesy of NOAA.

Rising, cooling air releases a lot of latent heat when water is condensing, building up the existing ridge to is east.  Temperature difference at 850 mb (about 5000 feet in the atmosphere) is strong.  Temperatures are as low at -4.5C (mid 20's F) near the low center and +15C (upper 50's F) over eastern mainland Alaska!  See below 850 mb map courtesy of Ventusky:

Here's the corresponding surface temperatures, quite toasty over the majority of the state! 70's, 80's and perhaps a few 90's in the 40-mile country.  Even the arctic coast will get in on some of the action depending on prevailing wind direction. Southeast or south surface winds are conducive of warmth.

Durning these next few exciting weather days I noticed the upper atmosphere was getting into a stable pattern called an Omega block.  These can hang around for quite some time before breaking down.  Here is the basic definition courtesy of NOAA: Omega blocks are a combination of two cutoff lows with one blocking high sandwiched between them. Because of their size, Omega blocks are often quite persistent. So basically, warm and dry lingering in location while wet and stormy conditions remain day after day.  See computer model below of the forecasted Omega block at 700 mb or 10,000 feet.  Our second low pressure system (which is much weaker in comparison) is located off coast of the Pacific Northwest.  Seattle should have some cool weather the next few days.  Thanks for reading!  

North Slope marine layer and Brook's Range Thunderstorms

Summer is moving forward up on Alaska's north slope.  The majority of the snow cover has melted, rivers and arctic lakes are opening up.  In response to some of our reader's queries about the recent warm spell affecting the 49th state I thought I'd share a little weather lore for the upper part of the state.  

The majority of Alaska is relatively warm now and will experience the warmest climatological weather in the next month or so (unless you are in the Aleutians when it comes much later).  That said the coastal areas are not as warm or lucky as the mainland.  In this article I am focusing on areas north of the Brooks however any community near large bodies of water will experience similar local microclimate effects.

Today it is bright and sunny across the majority of the North Slope.  As you can see from the infrared satellite, (courtesy of NOAA) northern Alaska has a large area of black north of the Brooks Range.  That dark color indicates warm air all the way to the ground.  Also interesting to note is the thunderstorms blossoming over the mountains.  Plenty of heat from the sun hitting the ground to allow convection along the spine of the mountains. There is additional instability support to the SW from a shortwave. Perfect recipe for storms! The coastal areas however are not near as dark as they are under the influence of the arctic marine layer.  The Arctic Ocean is still mostly frozen over and is reflecting a lot of solar radiation.  

The marine layer over the frozen ocean is cold, hanging around 32 degrees F which is a stark comparison to the landmass just to the south (60'sF).  See below picture courtesy of Ventusky.

One should note the cool greens near the coastal areas.  The winds are onshore, with a much colder marine layer invading coastal areas.  30's and 40's are spread across coastal areas while just a few miles inland its much warmer and in the 50's and 60's. Air inland from the coast being warmer and less dense is being forced to rise.  Colder, denser marine air is rushing in to replace it. Add in a little upper atmosphere instability, some low-level moisture and some elevation rise and it's the perfect recipe for thunderstorms. Here is another example of the marine layer pulled from Utqiagvik's afternoon weather sounding.  The strong June sun can't overcome the inversion caused by the marine layer.  Poor folks on the coast are still wearing winter hats while anyone to the south is in a t-shirt and shorts.

What would it take to get coastal areas to warm up and overcome the marine layer?  That would be the topic of another blog but needless to say it involves a south to north pressure gradient, with down sloping, offshore winds.  On a side note, the mosquitoes are not out yet, but I bet if I went 20 miles inland, they would be.  Thanks for reading!  Kaktovik Mike.

Wildfire Update

Alaska's fire season has been fairly subdued up to this point, although residents of the Tanana Valley near Salcha might be forgiven for thinking otherwise, as a large fire is burning on the west side of the Tanana River.  The McDonald fire, as it's named, has burned about 40,000 acres, or nearly two-thirds of the state's total of 63,000 acres so far this season.  It's by far the largest fire in the state and has produced dense smoke in the local area.  Here's the latest update from the Alaska Fire Service:

https://akfireinfo.com/2024/06/20/mcdonald-fire-remains-west-of-the-tanana-river/

Elsewhere there are a number of fires in southwestern Alaska, sparked by widespread lightning activity on Monday:

https://akfireinfo.com/2024/06/18/recent-lightning-sparks-multiple-new-fires-in-southwest-alaska/

Also, a few large tundra fires are active in the northwest.

The 1995-2023 median statewide acreage for this date is about 100,000 acres, so the season is slightly less active than normal so far, but it's still early.  The median acreage jumps four-fold in the next 30 days, so things can change fast.

Here's a photo of pyrocumulus cloud over the McDonald fire on Monday, courtesy of the NWS via X/Twitter.  The fire was stirred up on Monday afternoon by winds related to nearby thunderstorm activity.

Speaking of thunderstorms, Bettles reported a heavy thunderstorm with quarter-inch hail yesterday.  This is unusual but certainly not rare: I found the following hail reports in the last 30 years at Bettles.

May 15, 1997   1/4" hail

May 11, 1998   1/2" hail

June 19, 2000   hail reported but no size specified

June 11, 2002   1/2" hail

July 1, 2005   1/4" hail

June 28, 2008   1/2" hail

May 26, 2017   1/4" hail

July 12, 2017   hail reported but no size specified

June 5, 2019   less than 1/4"

Fairbanks has had fewer hail reports in the same time, but the official data is from the airport, and other parts of town are more prone to thunderstorms.

June 8, 1997   no size specified

August 9, 2002   1/4" hail

May 14, 2012   3/16" (reported as graupel or small hail)

June 2, 2019   3/8" hail  (blog post here)

June 16, 2021   less than 1/4"

Some Unusual Warmth

Temperatures have been quite variable for much of the state in the past couple of weeks, as the flow pattern has been generally "blocked" with high pressure in the Arctic and (unusually) low pressure in the Gulf of Alaska.  In a circulation with high-latitude blocking, the jet stream tends to meander more unpredictably than normal, and of course the surface weather depends on where you find yourself with respect to those atmospheric eddies and swirls.

The northern interior has been perhaps the main beneficiary (if warm weather is considered a good thing); Bettles has seen temperatures above 70°F on all but 4 days this month so far.  The 12 days with a high temperature of 72°F or higher so far this year is unusual, although the record is 17 back in 2010 (through June 16).

Nome also had a mini-heatwave a week ago, with the temperature rising to 77°F last Monday.  This isn't the earliest on record for such warmth - it reached 78°F at the end of May in 1981 - but again it's unusual.  It also illustrates the positive skewness of the temperature distribution at this time of year in Nome: large warm departures from normal are more common than equally large cold departures.  For example, the normal daily high temperature in Nome on June 10 is 54°F, so last Monday was 23°F warmer than normal.  Imagine a day 23°F colder than normal: the high temperature would be only 31°F, but that has never happened (since 1907) in Nome in the month of June; the latest is May 28 (1934), and in more recent years May 24 (2001).

A decade ago, Brian Brettschneider illustrated the spatial distribution of daily mean temperature skewness across Alaska: check out the maps at this old post.

https://ak-wx.blogspot.com/2014/03/temperature-skewness-maps.html

Note that this is for daily mean temperature, and there would be some differences for daily max and min temperatures.

May Climate Data

The month of May was the coldest since 2013 for Alaska as a whole, according to NOAA/NCEI data, and it was also wetter and cloudier than normal across most of the state - a rather unpleasant end to spring, and problematic for breakup flooding, especially for Kuskokwim River communities.  The only somewhat dry region was the Arctic northwest and Brooks Range, according to ERA5 data.  Also notable is the fact that it was the first wetter-than-normal month of the year for the southern Panhandle.

The far southwestern mainland and upper Alaska Peninsula saw the most anomalous cold for the month, with the "Bristol Bay" climate division having the coldest May since 2001, and before that 1985.  Checking in on a few sites in the region, Iliamna stands out as particularly unusual: the monthly mean temperature of 37.1°F was the coldest since 1971.

The cold and wet pattern was a direct reflection of a cold upper-level trough over southwestern Alaska.  The jet stream was well to the south, riding the strong north-south temperature gradient between extreme warmth in the central North Pacific and chilly conditions in the Bering Sea region:

By my calculations, the monthly PDO index of -1.8 was the most negative for May since 1950.  The 3-year smoothed PDO index is also the most negative since the early 1950s, illustrating that we're in one of the most entrenched and significant negative PDO regimes in the modern climate history.

As for wind, both northwestern Alaska and the eastern Aleutians were much windier than normal in May, and so was the southern Panhandle, according to ERA5.  However, the Arctic coast was relatively calm after a rather windy start to the year.