Sunday, August 18, 2019

Rain: Feast or Famine

Another round of heavy rain fell across the central interior late last week, taking the month's rainfall total to over 5 inches in Fairbanks, and exacerbating flood problems in the area.  According to the NWS, the Salcha River to the southeast of Fairbanks is the highest it's been since 2014 (the wettest summer of record in Fairbanks); here's the hydrograph:

A webcam view of the Tanana River at Nenana from yesterday morning (the latest available) shows a rather swollen river, and according to the NWS there is some flooding in the town.

Only a couple of years in the past have seen so much rain in Fairbanks in the entire month of August, with one of course being the flood year of 1967.  With flood control measures now in place, a repeat of that flood will not occur in Fairbanks, and indeed the Moose Creek Dam is operating tonight to regulate the Chena River flow volume.

Readers in south-central Alaska may well wish that some of the rain would make its way south of the Alaska Range, because while Fairbanks is near-record wet, Anchorage has seen no measurable rainfall at all this month so far.  This is an extraordinary contrast: Fairbanks is second-wettest on record for the month to date (only lagging 1967), while Anchorage is record dry.  The chart below shows August 1-17 rainfall totals for the two cities over their common climate history since 1952.

How can we explain the difference in rainfall fortunes over such a relatively short distance?  The map below shows the 500mb height anomaly (departure from normal) for the first 16 days of the month; the glaring feature is the high pressure anomaly just to the southwest of Anchorage, which has deflected the jet stream and associated weather systems farther north into western and central Alaska.

Below is the actual 500mb height map, instead of the departure from normal.  The ridge near southwest Alaska extends far to the south and southwest, and on the west side of the ridge a long fetch of southerly flow can be inferred, stretching from the central western Pacific up to the Bering Sea and then east into the interior.  This is the moist flow that has created the persistent rain in the interior.

Here's what the flow looks like at 700mb, or about 10,000 feet up in the atmosphere.

Let's compare the recent pattern to what typically occurs when Fairbanks is considerably wetter than Anchorage in the first half of August; the map below shows the average 500mb height anomaly in 6 such years.  There are some distinct similarities to the height anomaly map above, but in the historical (and less extreme) events the unusual ridge was farther west along the Aleutians, rather than sitting in close proximity to Anchorage.

In the opposite scenario where Anchorage is notably wetter than Fairbanks at this time of year, it's not too surprising to see that there is a trough (lower heights) near Bristol Bay, rather than a ridge.  This pattern would allow a vigorous jet stream to affect south-central Alaska, but a downslope component to the flow over the Alaska Range would bring much drier weather to the interior.

Sunday, August 11, 2019

Rainy Season

Parts of interior and western Alaska have seen a lot of rain in recent days, which is of course good news as far as wildfires are concerned: there has been minimal growth in fire acreage this month so far.  But the copious rainfall has brought other problems to some locations, including flooding and (at least in Denali NP) mud and rock slides.

Here are maps of daily estimated precipitation since the beginning of the month:

Rain was especially remarkable across the Seward Peninsula and the northwest interior in the first few days of the month, with Nome reporting its highest 24-hour rain total of record (2.47").  Rain amounts from some other sites were even more remarkable; see below for a graphic published by the NWS on the evening of August 3rd.

This time of year does of course tend to be relatively wet in interior and western Alaska, but there's a very pronounced west-east gradient in when the wettest conditions tend to occur.  Consider the contrast in timing of peak rainfall frequency between Nome, Fairbanks, and Northway - see the charts below.  Far up the Tanana River valley in Northway, measurable rainfall is most frequent at the beginning of July, and August actually sees much less rain than July on average, so the rainy season, such as there is, ends quickly there as summer wanes.

In Fairbanks the peak in precipitation frequency is about 4 weeks later than in Northway, according to 1981-2010 normals from NOAA, and August tends to bring considerably more hours of rain, and more cloud, than July.  (However, the total August rainfall is a bit less than in July, because there are fewer heavy rain events.  For more discussion of July versus August, see posts here and here.)

In contrast, if we go all the way over to Nome, August is much wetter than July and clearly stands out as the wettest month of the year.

The map below gives a sense of how the date of peak precipitation frequency varies across the entire state.  It generally advances from early July to early August from east to west across the interior, reaches late August or early September along the west and southwest coast, and then most of the south-central region sees the height of its rainy season in late September (when interior weather is usually much improved).  Of course the epic rainy season of southeastern Alaska doesn't reach its peak until October.

Here's a zoomed-in map to reveal a bit more of the local variation in the southeast interior and south-central.

Finally, the peak climatological value of precipitation frequency is depicted in the map below.  Unlike the seasonal timing, the maximum frequency of precipitation is surprisingly uniform from west to east across central Alaska, with peak values of 40-50% in many locations.  In the Fairbanks area, the only locations with normal precipitation frequency rising above 50% are the Keystone Ridge and Two Rivers co-op sites; and North Pole apparently never exceeds 35%.

Friday, August 2, 2019

Sea Ice Update

An update on Arctic sea ice conditions has been long overdue on this blog, although Rick Thoman has been providing excellent updates all summer via his Twitter feed.  Nevertheless it's worth reiterating the main headlines, which are that Chukchi Sea ice is at record low levels for the time of year, and Arctic-wide sea ice stands a chance of breaking the record low from 2012.

Unsurprisingly, the extraordinary shortfall of ice in the Bering Sea last winter set the stage for extremely early meltout farther north, and Chukchi Sea ice extent has been almost continuously at record lows ever since meltout began in early May.

The figures below show the NWS sea ice analyses from today (top) and last year on this date (below).  It's quite a difference in both the Chukchi and Beaufort Seas, although the Beaufort Sea ice deficit is not at record levels.

For the Arctic Ocean as a whole, sea ice is running just about neck-and-neck with 2012, which saw the lowest seasonal minimum on record.  However, a new record low this year isn't necessarily likely, because the rate of meltout was so great in August 2012 that it may be tough to keep pace this year.  Much will depend on the weather pattern in the next several weeks, including the degree of storminess and the amount of warm air that is imported into the Arctic from southerly latitudes.  In 2012 an intense low pressure system in early August probably contributed to the rapid ice loss by breaking up ice and mixing it with warm water.  The forecast for the next week or two looks more suggestive of high pressure in the Arctic, but on the other hand there's no lack of warmth in waters south of the ice edge, so I think a challenge to the 2012 record must be a real possibility.  We'll know soon enough.

Wednesday, July 24, 2019

Northwest Warmth

Blog posts will be scarce in the next week or two as I'm traveling, but here's a quick post to highlight the really extraordinary warmth that northwestern Alaska has seen in recent weeks and months.  In large part owing to the lack of nearby sea ice (in winter) and much above-normal nearby ocean temperatures (in summer), air temperatures have been almost constantly above normal for nearly two years now.  According to data from Kotzebue, the last calendar month that was cooler than the 1981-2010 normal was August 2017.  Click to enlarge the figure below.

But as if the events of 2017-2018 were not enough, the warmth has kicked it up another notch since the middle of last winter, as each of the last 6 months has been at least 10°F warmer than normal.  Remarkably, Kotzebue hasn't seen a single day below normal since March 18, and that was only by a fraction of a degree; the last "significant" cold spell was in the first half of January.  Daily temperatures have been frequently more than 1 standard deviation above the daily normal, and it is becoming unusual to see a day that is not in that upper 15% of the climatological distribution (above +1SD).

Here's one more statistic to illustrate how warm it's been this summer so far: Kotzebue has seen 16 days this year with a daily minimum temperature at or above 60°F.  That's easily a record for any year, and it's more than occurred in total in the 3 decades from 1940 to 1970.  They even managed a daily minimum of 69°F on July 9 this year, which is an all-time record high value and more typical of the tropics than the Arctic.

Thursday, July 18, 2019

Lightning Distribution

As a follow-up to last week's post on extreme lightning activity, I thought it would be interesting to take a look at the spatial distribution of lightning in Alaska, as documented by the Alaska Lightning Detection Network.  As many readers know, the ALDN system underwent a transition in 2012 to a new network with different sensors, so the data are not directly comparable pre- and post-2012.  However, we can look at each period separately and make some qualitative comparisons.

The map below (click to enlarge) shows the density of lightning strikes in the 1986-2012 data, and you can zoom in by viewing the map on the ArcGIS platform at the following link:

The ArcGIS link also includes a second layer that shows the 2012-2018 data; you should be able to toggle the layers by navigating to "Content" on the left side of the interface.  Here's a static version of the 2012-2018 map:

Note that the data values are essentially arbitrary; they are obtained by counting the number of lightning strikes in latitude-longitude grid boxes, so there is a small north-south discrepancy in terms of the land area within each grid box.  Also, as mentioned above the new ALDN sensors record lightning strikes at a different rate than the old network, so we can't do a comparison of total strikes between the two periods.

However, it does seem that we can make one very interesting observation, which is that recent years have seen much more lightning in the southern interior than in the rest of the interior, and this is very different from the spatial distribution in the earlier decades.  The 1986-2012 history shows the highest concentration of lightning in the Yukon-Tanana uplands, and especially in the hills just to the east and northeast of Fairbanks.  This seems reasonable and agrees with earlier results here:

In contrast, the data from the past 7 years shows the highest lightning density along the west and north side of the Alaska Range and in the upper Tanana River valley.  It's a striking difference (pardon the pun).

To confirm the result, I re-calculated the number of strikes within two similar latitude/longitude rectangles: one focused on the 2012-2018 hotspot to the south of McGrath, and the other centered on the hills near Fairbanks.  In the 1986-2012 data, the Fairbanks area saw about twice as much lightning as the McGrath region, but in 2012-2018 the ratio was approximately reversed.

Of course the past 7 years is a short period from which to draw any conclusions, especially when the sum total of lightning activity is so heavily influenced by a relatively small number of very active days.  For example, 20% of all 2012-2018 lightning in my box south of McGrath occurred in a single day: June 23, 2015 (see below).  So the apparent southward shift in lightning density since 2012 might be related to just a few very unusual events, and therefore might not be statistically significant.  But it is quite intriguing nonetheless, and I can't help wondering if the exceptional North Pacific warmth of the past several years might have something to do with it.  Some interesting research could be done here.

Friday, July 12, 2019

Lightning Outbreak

Yesterday was an extraordinary day for lightning in Alaska, with the highest number of lightning strikes recorded for any single day in the 2012-present history of the modern Alaska Lightning Detection Network.  The previous daily record from 2016 was broken by about 25%, based on strike counts for 24 hour periods ending at 6 am.

Note that the ALDN database includes plenty of lightning data from northwestern Canada, but I'm excluding anything east of 141°W (the north-south Alaska/Yukon border) so as to focus on Alaska.  (But yesterday also broke the record for the entire domain, and also for days defined as midnight-to-midnight.)

The annual totals and peak daily numbers for each year since 2012 are shown below.  It will be interesting to see if this year sets a record for the annual total; we are already past the climatological peak in lightning activity for Alaska, but a few more big days could push 2019 into first place.

Monday, July 8, 2019

Heat and Smoke

As I'm sure all readers are fully aware, an extraordinarily intense heat wave has been occurring across much of southern Alaska in the past week, and longstanding temperature records have been falling left and right.   Here's a quick summary that Rick Thoman posted yesterday:


There's far too much to comment on, but the remarkable July 4 heat in Anchorage may be the most astonishing statistic: the previous all-time record for the city's official climate site was only 85°F.

Dense smoke from wildfires has held temperatures down in some parts of the interior, including Fairbanks, but many locations in western and southern Alaska are well up into the 80s today.  Particularly striking to me is the 87°F currently being reported from Noatak in the northwest - above the Arctic Circle - and 92°F earlier today at the high-quality CRN station to the southeast of Ruby.

Here's an annotated true-color satellite image, courtesy of Rick Thoman and UAF:

Click on the image below for a higher resolution version:

 As bad as the smoke is at the moment in Fairbanks, this summer has a long way to go to equal the awful summer of 2004 in terms of duration and choking density of smoke - see the chart below, showing data from the Fairbanks airport ASOS instrument.  Here's hoping that 2004 remains an outlier.