Back to Rain

After a remarkable spell of warm and dry weather to close out summer, rain returned to Fairbanks earlier this week, pushing August's rainfall total to 2.08"; and this means that all three summer months have been wetter than normal.  Perhaps surprisingly, even in the wet regime of recent years, only 2014 achieved that distinction, although 2017 was close (July 2017 just missed the mark).  Other years were 1930, 1932, 1948, 1955, 1962, 1990, and 2008, although we perhaps shouldn't compare earlier decades to the current standard "normal" of 1981-2010.

Before saying a few more words about rainfall trends, I'll just make a note on the recent warm spell: with 11 consecutive days (Aug 13-23) having a high temperature above 70°F in Fairbanks, this is actually the longest such spell since at least 1930 for so late in the season.  However, it's a long way from being the greatest late summer heat wave, as other years reached into the 80s with warm spells of similar duration.  For instance, 1950 saw highs in the 80s on 4 of 5 days from August 19-23, with most days above 70° for two and a half weeks.  August 2004 had 10 days of 80°F or above (the last on the 20th), and August 1977 saw an amazing 87°F and 86°F on the 21st and 22nd respectively.

Returning to rainfall, I got to wondering whether the persistently high rainfall of recent summers has a clear signature in the duration of rain: is it raining more of the time, or is the rain just heavier when it occurs?  Here's a quick analysis to address the question: the chart below (click to enlarge) shows the fraction of Fairbanks hourly (top of the hour) observations reporting rain since 1973.  I'm showing June-September here, because these are the months with by far the greatest increase in precipitation since 2014 (see this recent post); and here we're looking at observations of rain, not snow.  There's a clear signal for more frequent rain in each of the last 7 years, so we can say that yes, it has been raining more of the time in summer and early autumn since 2014.

 

Interestingly, however, the number of days with measurable precipitation has not increased as obviously; it's up this year, but 2018 and 2019 did not see an unusual number of wet days, and nor did 2014.

 

Here's the total June-September precipitation, illustrating again the sustained nature of the new wet regime.

 

So what do we have in summary?  Since 2014, it has been raining more of the time in summer and early autumn, and the frequency of heavy rain events has gone up considerably (as shown in the earlier post), but the number of days with rain hasn't changed much.  So when it rains, it rains for longer and comes down harder, but the frequency of dry versus wet days is not substantially different.

Warmth for Summer's End

An interesting feature of the interior Alaska climate is that early autumn tends to bring drier and more settled, sunny weather after the typical rains of August, but this year the improving trend seems to have arrived several weeks early.  It's warm, too, with today being the 7th straight day that the temperature has exceeded 70°F in Fairbanks; that hasn't happened this late in the year since 2009 (and that was in early September).  Prior to that, we go back to August 2004, which was very hot and led to a highly unusual late-season extension of that record-breaking fire season.

Of course most places at lower latitudes are still in the throes of summer heat; Death Valley roasted at 130°F just the other day.  The more rapid drop-off in temperatures up north in most years is directly related to the quick decline in solar insolation at high latitudes.  Compare the two figures below: the first showing the annual cycle in solar radiation near Fairbanks, and the second showing the same for my neck of the woods far to the south.  The quick decrease in Fairbanks is partly related to increased cloudiness as summer advances, but it's also a result of the solar geometry.

The summer maximum is even more fleeting at Alaska's northernmost city (see below); but notice how high the peak is - almost as high as the summer peak in Georgia, and that's with much more cloudiness at Utqiaġvik.  (Another interesting feature is that even the cloudiest days in Utqiaġvik still receive a large amount of solar radiation in early summer, owing to 24-hour daylight.)

The late-summer decline in solar energy in Fairbanks means that it's increasingly uncommon for afternoon temperatures to achieve their full "potential" based on temperatures aloft.  Increasing cloudiness of course bears responsibility for this, but even if it's sunny with warm air aloft, the weakening sun in August is increasingly unable to produce a strong surplus of heating at the surface.

For example, here's yesterday's 4pm balloon sounding from Fairbanks; there's certainly a nice well-mixed boundary layer, and there's a small excess of temperature (a super-adiabatic layer) at the surface from the solar heating.  The near-surface temperature of 72°F at the balloon release site is 24°F higher than the 850mb temperature of 48°F; this equates to a lapse rate of 10.0°C/km, which is just slightly above the dry adiabatic (well-mixed) rate of 9.8°C/km.

 

Back on a sunny July 4th, the additional solar heating at that time allowed the 4pm surface temperature to be 27°F higher than at 850mb, for a lapse rate of 10.5°C/km; the sunshine eked out another 2-3°F compared to what it might have done with the same air mass six weeks later.

 

The seasonal changes are more noticeable when we compare the distribution of surface-to-850mb temperature differences by month - see below.  For simplicity, I've used the daily maximum temperature at the surface, even though this does not always occur in the afternoon.

 

As summer advances, the highest lapse rates (like that on July 4) become much less common, and surface temperatures tend more and more to underachieve relative to conditions aloft.  Of course this only means one thing: we're on the long road back to the semi-permanent temperature inversion of the winter months.

 

Reduced Temperature Variance

If residents of central Alaska have a sense that this summer has seen unusually steady temperatures, without extremes of either cold or heat, they are correct.  In fact, according to daily Fairbanks data, the standard deviation of daily mean temperatures since June 1st is the lowest on record.  Since at least 1930, there's never been a summer with less variability in daily temperatures through August 12.

The slight downward trend over the 90 years of data is not statistically significant, but the trend since 1990 actually is marginally significant at a 95% confidence level.

This summer's daily temperature variance is also the lowest on record - and by a larger margin - at McGrath, and it's the second lowest on record at Bettles.

Here are daily temperatures since May 1 in comparison to normal for the time of year in Fairbanks.  Lots of small deviations from normal.

What explanation might we propose for this summer's unusually steady conditions?  It would be interesting to look at seasonal climate drivers that tend to impart low variance, but for now the answer is rather simplistic: the general weather pattern has precluded the strong ridge/trough waves in the atmosphere that create larger swings in temperature from day to day and week to week.  In particular, it's the ridges that have been missing, as 500mb heights have been lower than normal over eastern and/or southern Alaska.  This seems to have been a relatively stable configuration, with few significant perturbations to bring much warmer air to Alaska's interior.

Here are 500mb anomaly maps from June, July, and August so far:

 

As for the downward trend in the past 30 years, it may well be the reduced frequency on the cold side of the climate distribution that explains the reduction in summer temperature variability.  In other words, as the Arctic has warmed far faster than the rest of the Northern Hemisphere in recent decades, air from the north will have warmed much more than air from the south.  It would be useful to look at this in more detail when time permits.

Update August 14:

It turns out there's no meaningful correlation between the mean and the variance of summer daily temperatures (after removing the long-term trend from the mean), so it seems we can't say that cool summer weather patterns (for Fairbanks) are inherently more stable.

 

Historical Rainfall Extremes

In the wake of last weekend's epic rains in central Alaska, I've been poking around in historical rainfall data from the Alaska RAWS network (data courtesy of Rick Thoman) and a few other sources.  The question is whether any previous events can be identified in interior or northern Alaska that approach or even exceed the 5.35" peak 24-hour total that occurred at the McKinley River site last weekend.

According to Rick's earlier research, the previous record for rainfall in 24 hours is 4.58" at Clear, about 20 miles south of Nenana; this occurred in the same event as the Fairbanks flood of August 1967.  The observation was a standard once-per-day measurement, so it's almost inevitable that a higher peak 24-hour total occurred.

Does the RAWS data reveal anything more extreme than this in recent decades?  The short answer is no - if we're talking about interior Alaska, i.e. north of the Alaska Range.  Epic rains occur along Alaska's southern and southeastern coastline, of course, and I found an event with over 6" of rain in 24 hours at the Ruth Glacier RAWS on the south side of Denali NP (at 3300' elevation).  But it's obviously a very different climate on the south side of the divide.

In terms of a new finding, the highest I uncovered for the interior is 4.20" in 24 hours at the Salmon Trout RAWS, which is at 2200' elevation above the Porcupine River, not far from the Canadian border (and 66.8°N latitude).  This occurred on August 7, 2010, and the peak 12-hour rainfall was 3.05".

Another high-latitude one that surprised me - although it's not quite in the same league - is 3.01" in 24 hours at the Red Dog CRN in far northwest Alaska (68.0°N, elevation 940').  That was in mid-August 2012.

I would certainly be remiss not to mention also the remarkable 4.16" last summer (August again) at Denali NP's Eielson Visitor Center (3760' elevation on the north side of the park).

For context, the record 24-hour rainfall in Fairbanks is 3.44", which was of course in August 1967.  Nothing close to this has occurred at valley-level in Fairbanks since, but up in the hills there have been some 3" rains in 24 hours; for instance, the Fairbanks 11NE CRN reported 3.07" in 24 hours in July 2014, and interestingly the Munson Ridge SNOTEL has reported over 3" in three recent years (3.76" in August 2014, 3.23" in July 2016, 3.09" in August 2019).  Given the frequency with which Munson Ridge sees big rains, it may not be many years before they see 4-5" in a day; we can only imagine what occurred up in the hills in 1967 (e.g. the Gilmore Creek co-op north of town recorded 6.18" in two days).

On a 6-hour time scale, the 3.32" at McKinley River last weekend easily beats anything else I can find for the interior.  The closest is 2.48" in 6 hours on July 6, 2007, at the Preacher Creek RAWS to the west of Circle.  There are just a handful of other events exceeding 2" in 6 hours.

If anyone can suggest other notable historical rain events that might be contenders, I'd be glad to dig into the data a bit more; the RAWS data obviously only goes back a few decades at best.

Extreme Rain

Here's a look at observed 48-hour rainfall totals from the weekend's storm; click to enlarge the map below.  As usual the data is sparse, but it suggests that over 3 inches fell across a large area on the northwest side of Denali NP, as well as between Fairbanks and Livengood.

The highest totals on the map are as follows:

5.76"  McKinley River RAWS

4.17"  Wien Lake RAWS

3.88"  Wonder Lake RAWS

3.61"  Wickersham Dome HADS

3.16"  Chatanika RAWS

The high-quality Denali 27N CRN sits in close proximity to the Wonder Lake RAWS and reported 4.10", so that provides strong support for the highest amounts.

Rainfall totals in the middle Tanana Valley were somewhat lower than expected, although Nenana received a hefty 1.71" and Fairbanks airport reported a respectable 1.22".

Part of the reason for the lower totals east of Nenana is that the initial disturbance from the east arrived much later than expected on Saturday evening and did not produce the widespread strong afternoon thunderstorms that were advertised by the NWS.  It wasn't until the front moved off to the west in the early hours of Sunday that it started generating prolific rainfall, as illustrated by the rainfall accumulation chart from the McKinley River RAWS:

In this chart the green dots and the blue line show the same thing, but they pertain to the left and right vertical axes respectively.  Incredibly, the instrument recorded 2.01" of rain in 2 hours and 3.0" in 4 hours.

The NOAA precipitation atlas does not have return interval estimates for this particular site, but for Wonder Lake the 1000-year return interval for a 2-hour total happens to be 2.01".  It's almost the same at Minchumina, and the McKinley River RAWS is half-way between the two sites, so it seems this was indeed about a 1-in-1000 year event for the specific location.

And according to Rick Thoman:

The 48-hour rainfall total for central Interior Alaska thru 9am Mon. The McKinley River station 24-hour total 5.35" (135.6"mm) looks like the highest reliable 24-hour precip north of the Alaska Range on record. Previous highest 4.58" at Clear in Aug 1967. #akwx @Climatologist49 pic.twitter.com/A33g7AiPbi

— Rick Thoman (@AlaskaWx) August 3, 2020

All in all, I'd argue the forecasts were quite good, and the strong wording of the NWS messages was justified even though rainfall underperformed a bit in Fairbanks.  In particular, the forecasters' assessment was highly prescient when it spoke of "very heavy rain... in the Alaska Range especially near Denali National Park where 2 to 4 inches of rain is expected with local amounts pushing nearly 6 inches".  It's not easy to predict extreme and historic weather events, because history provides no guide, so this was quite a triumph of modern technology.

Copious Rain and A Permafrost Study

I wasn't expecting to return to the topic of summer rain quite so soon, but for two reasons it makes sense to do so.  First, the current weather forecast from NWS Fairbanks is worthy of comment, with a flood watch in effect over the weekend for the central interior, middle Tanana valley, and Denali region.  The rainfall forecast for the next 3 days is striking to say the least; any time you have 15% or more of your normal annual precipitation predicted for a weekend, it's worth paying attention.

The special statement issued by the NWS includes quite a collection of hazards:

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE FAIRBANKS AK
508 AM AKDT FRI JUL 31 2020

AKZ220>226-011415-
YUKON FLATS AND SURROUNDING UPLANDS-CENTRAL INTERIOR-
MIDDLE TANANA VALLEY-DELTANA AND TANANA FLATS-
UPPER TANANA VALLEY AND THE FORTYMILE COUNTRY-DENALI-
EASTERN ALASKA RANGE-
INCLUDING FORT YUKON, VENETIE, CENTRAL, CIRCLE, STEVENS VILLAGE, 
BEAVER, CHALKYITSIK, BIRCH CREEK, CIRCLE HOT SPRINGS, 
EAGLE SUMMIT, TWELVEMILE SUMMIT, NENANA, ANDERSON, TANANA, MINTO, 
MANLEY HOT SPRINGS, RAMPART, LAKE MINCHUMINA, LIVENGOOD, 
FAIRBANKS, FORT WAINWRIGHT, EIELSON AFB, ESTER, NORTH POLE, 
MOOSE CREEK, TWO RIVERS, FOX, CHATANIKA, CHENA HOT SPRINGS, 
SOURDOUGH CAMP, SALCHA, DELTA JUNCTION, FORT GREELY, 
HARDING/BIRCH LAKE, DRY CREEK, DOT LAKE, HEALY LAKE, TOK, 
TANACROSS, EAGLE, TETLIN, NORTHWAY, ALCAN, CHICKEN, BOUNDARY, 
HEALY, DENALI NATIONAL PARK, CARLO CREEK, KANTISHNA, 
MENTASTA LAKE, BLACK RAPIDS, DONNELLY DOME, TRIMS DOT CAMP, 
EAGLE TRAIL, AND MINERAL LAKE
508 AM AKDT FRI JUL 31 2020

...COPIOUS RAINFALL LIKELY OVER MOST OF THE EASTERN/CENTRAL 
INTERIOR AND ALASKA RANGE SATURDAY EVENING...

...INCREASING LIKELIHOOD FOR A SIGNIFICANT OUTBREAK OF STRONG 
THUNDERSTORMS OVER THE EASTERN INTERIOR...

STRONG THUNDERSTORMS ARE EXPECTED BEGINNING LATE SATURDAY 
AFTERNOON OVER THE FORTYMILE UPLANDS AND WHITE MOUNTAINS. THESE STORMS
WILL LIKELY PRODUCE DANGEROUS LIGHTNING, LOCALLY TORRENTIAL 
RAINFALL RATES, HAIL, AND STRONG WINDS. EXPECT CONDITIONS TO 
DETERIORATE RAPIDLY FROM THE HOT AND MOSTLY SUNNY AFTERNOON 
CONDITIONS. OUTDOOR RECREATORS, IN PARTICULAR, SHOULD EXERCISE 
SIGNIFICANT CAUTION AND BE WEATHER-READY. 

EXPECT TORRENTIAL RAINFALL RATES WITH UP TO AN INCH PER HOUR OVER
LOCALIZED AREAS. TOTAL RAINFALL WILL RANGE FROM 0.5-2" TOTAL OVER
THE EASTERN INTERIOR UP TO 2-4" OVER THE CENTRAL INTERIOR, WITH 
THE HEAVIEST TOTALS CURRENTLY EXPECTED FROM TANANA TO MINCHUMINA 
TO THE MINTO FLATS. HEAVY RAIN WILL TRANSITION TO LIGHTER RAIN, 
BUT CONTINUE THROUGH THE WEEKEND OVER THE CENTRAL INTERIOR. 

RIVER LEVELS REMAIN ELEVATED WITH MUCH OF THE SURROUNDING TERRAIN
STILL SATURATED FROM WEEKS OF CONTINUOUS WET WEATHER. THIS MEANS 
THERE IS POTENTIAL FOR SHARP RISES ON RIVERS INCLUDING THE 
CHATANIKA, CHENA, LITTLE CHENA, SALCHA, AND GOODPASTER, AS WELL AS
SMALLER CREEKS AND TRIBUTARIES ACROSS THE EASTERN INTERIOR. 

LARGER RIVERS INCLUDING THE TANANA RIVER WILL BEGIN RISING EARLY 
NEXT WEEK AS WATER MOVES THROUGH THE SYSTEM. 

MUDSLIDES AND QUICK RISING STREAMS IN THE ALASKA RANGE, ESPECIALLY
NEAR DENALI, WILL ALSO BE A SIGNIFICANT CONCERN. 

FOR THE LATEST RIVER INFORMATION GO TO WWW.WEATHER.GOV/APRFC. 

FOR THE LATEST UPDATES VISIT WEATHER.GOV/FAIRBANKS

The weather pattern responsible for all this involves a strong upper-level trough over the Gulf of Alaska, with warm and moist flow reaching the interior from the southeast.  This bears some resemblance to the rainy weather setup in late June.  The precise origin of the forcing for this weekend's expected deluge is not quite clear at first glance, but there's no doubt about the magnitude of the rainfall signal in the model forecast - see below.

This latest onslaught of moisture will add to the year's precipitation surplus and will favor increased permafrost thawing this summer.  That's according to the conclusions of a new paper published by Tom Douglas of the Army Cold Regions Lab in Fairbanks:

Increased rainfall stimulates permafrost thaw across a variety of Interior Alaskan boreal ecosystems

This work was highlighted by Ned Rozell back in June:

https://www.adn.com/alaska-news/science/2020/06/13/when-thawing-permafrost-kills-a-moose-story/

The paper shows that permafrost active layer (seasonal thaw layer) depth increased significantly across a diverse set of locations near Fairbanks from 2013-2017, and there seems to be a correlation between the annual thaw depth and summer rainfall totals, with wetter summers tending to produce more thaw.  This makes sense, as surface water above 10°C carries a substantial amount of sensible heat as it percolates down to the 0°C top of the permafrost; and the effect may be multiplied in areas where thawing permafrost is subsiding, leading to greater liquid water influx from surrounding areas.

One slight surprise with the results (as discussed in the paper) is that the dramatic thaw in the record-wet summer 2014 was not reversed in 2015 or 2017, despite those summers being less wet.  But curiously, the rainfall data in the paper seems to be slightly off: the stated rainfall totals for 2015 and 2017 are quite a bit lower than what actually occurred in Fairbanks, so the "problem" is not as great as the discussion suggests.  As a recent post here showed, every year since 2014 has been considerably wetter than normal in Fairbanks (with the vast majority of the increase coming as rain), so it's no surprise that permafrost hasn't had a chance to recover.  And from the point of view of rainfall, it certainly isn't doing any better this year.

Update Saturday 5pm:

Here's a satellite loop showing the frontal zone developing over the southeastern interior.  Progress is a bit delayed, but big rain is coming.

Today's GFS forecast:

This afternoon's NWS forecast discussion is worth saving for future reference:

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
222 PM AKDT Sat Aug 1 2020

.SYNOPSIS...
A significant storm system is expected to impact the Southern
Interior and Alaska Range beginning late this afternoon and
continue through late Sunday or Monday morning. A strong front is
expected to stall along a Fairbanks to Denali National Park line
and will produce very heavy rain, perhaps record setting rainfall
in the Alaska Range and Southern Interior. The front has slowed
down and will not arrive during peak heating this afternoon so
the chances for strong thunderstorms in the Central Interior have
diminished some, however chances for strong thunderstorms remains
high in the Eastern Interior this afternoon and evening. Very
heavy rain is expected through monday Morning including the Tanana
Valley where 1 to 3 inches of rain is expected through Monday
morning and in the Alaska Range especially near Denali National
park where 2 to 4 inches of rain is expected with local amounts
pushing nearly 6 inches. The potential for local flash flooding is
a very high across much of the Alaska Range, especially around
the Denali area. Elsewhere, cooler and showery conditions will
persist over the West Coast and Western Interior with continued
fog and stratus over the North Slope.

&&

.DISCUSSION...

Upper Levels and Analysis...
A strong ridge over the Central and Eastern Interior will weaken
and retreat to the north as strong upper level low over the
panhandle pushes northwest into the southeast interior this
evening and continues to push northwest into the Central interior
near Fairbanks tonight. 850 mb temperatures currently around 14
above will drop rapidly to around 6 above by Sunday afternoon.
Rapid cooling aloft combined with diluent flow aloft will produce
a closed low centered near Fairbanks late tonight and Sunday.
This closed low will produce northerly flow at the surface and
aloft over the Central Alaska Range and will enhance rainfall with
moderate upslope flow. The low stretches along an upper level
boundary along a line from Fort Yukon to Fairbanks to Denali
National Park. Very heavy rainfall is expected to fall along this
stalled front tonight through Monday morning. Over the North
Slope, southwest flow and a stationary front remain in place,
keeping showers and clouds and cooler weather there, as well.

Model Discussion...
The models continue to struggle badly on the timing and placement
and amount of heavy rainfall over the Central Interior and
Central Alaska Range even at less than 6 to 12 hours before the
start of the event. We utilized a blend of models again with a
heavier weighting of the ECMWF due to its consistency over the
last several runs on the timing, placement and amount of heavy
rainfall and retained reasonable continuity with previous forecast
packages and latest model updates. The heavier weighted ECMWF
forecast favors widespread heavy rainfall from the Alaska Range to
the Tanana Valley, thus ongoing flood watches will remain in
place. Very small differences in feature location will result in
significant differences in timing, placement and amounts of heavy
rainfall especially with the upslope precip enhancing and
downslope precip limiting terrain of the Alaska Range with very
subtle changes in wind direction.

Central and Eastern Interior... A very nice afternoon with sunny
and warm conditions ongoing over the Central and Eastern Interior
as the upper level ridging continues to build and temps rise into
the 80s. We will likely make a run at the warmest day of the
summer this afternoon before a strong front pushes in from the
east. The sunny, nice weather will deteriorate rapidly beginning
late afternoon over the Eastern Interior and this evening over the
Central Interior as a strong front pushes into the area from the
southeast. Strong thunderstorms are expected to develop over the
Fortymile Uplands, Eastern Alaska Range and White Mountains then
move westward late this afternoon and evening. Instability will be
very high as CAPE values will exceed 1500-1800 j/kg, a high value
in Alaska for instability. Storms will produce may produce
torrential rainfall in excess of an inch per hour in some areas,
along with hail, deadly and frequent lightning, and strong winds.
By late evening and overnight, the storms will merge into a
complex and shift west over the Tanana Valley and Western Alaska
Range northward into the Central Interior. There remains some
uncertainty on the exact timing, location and amounts of the heavy
rainfall but at this point we expect heavy rain rain over the
entirety of the Alaska Range with moderate to heavy rainfall over
the Tanana Valley by late tonight and Sunday morning. Expect
rainfall to continue through Monday morning, at least, with the
potential for 1-3" of rain in the Tanana Valley including
Fairbanks and 2-4" across the Alaska Range, highest values near
Denali possibly reaching 6 inches through Monday morning.
Thunderstorms and moderate rainfall will also impact areas north
including the southeast Brooks Range and Yukon Flats, although
flooding rainfall is not expected.

West Coast and Western Interior...
Quiet weather will persist out west, especially along the coast as
a northerly wind gradient at the surface increases. Some fog and
stratus will remain a problem over the immediate coasts, however,
Interior will gradually warm up with threats for isolated
thunderstorms each afternoon through Monday.

North Slope and Brooks Range...
A stalled front continues to sit over the North Slope, bringing
light rain and continued clouds. This will weaken, and southerly
flow will increase over the eastern half of the Brooks Range and
North Slope tonight and Sunday, bringing rain and possibly an
isolated thunderstorm over the southern slopes of the Brooks
Range. This will last through Monday while the remainder of the
North Slope will continue to see cooler and stratus/fog conditions
given the light onshore northeast flow.

Coastal Hazard Potential Days 3 and 4...None.

&&

.FIRE WEATHER...
One final hot day is in store for the Central/Eastern Interior
this afternoon before a powerful storm rolls through this evening
and overnight. There will likely be a lightning outbreak over the
Eastern Interior with widespread lightning, but storms will also
bring copious rainfall. Extensive thunderstorms will also impact
the drier areas of the Interior including the Yukon Flats. Expect
heavy rainfall from the Alaska Range to the Central Interior and
Tanana Valley through Monday. Cool abut mostly dry with isolated
thunderstorms will be the main story over the Western Interior.
The West Coast will remain cool and dry.

&&

.HYDROLOGY...
The focus continues to be an incoming heavy near record setting rain
event for parts of the Central and Eastern Interior beginning
late this afternoon and into Monday. Models continue to struggle
with the placement of the heaviest precipitation in the Interior
(see model discussion above), but the emerging concensus is a
broad area from Delta Junction to Fairbanks to Lake Minchumina
southward into the Alaska Range with 1 to 3 inches of rainfall;
the highest bullseye of rainfall of 2 to 4 inches with 6 inches
locally remains the Alaska Range near Denali National Park through
Sunday. Flood Watches remain in effect for these areas. Expect
sharp rises on smaller rivers including the Chena, Little Chena,
Salcha, Nenana, Delta, and Goodpaster Rivers with continued rises
along the Tanana from Fairbanks west. Rock and mud slides are
possible in the steep terrain of the Alaska Range along with the
possibility of local flash flooding in torrential rainfall.

Sea Ice Update

We're at the time of year when many eyes start looking north to the Arctic Ocean to see how sea ice is faring as melt season passes through its summer peak; the perennial question is whether we might see a new record low for ice extent by September.  The current record was set in 2012, and before that 2007 was the big record-breaking year for ice loss.

So far this season, melt-out has been very rapid, and currently we're sitting at record low ice extent for the date.  Here's a nice figure, courtesy of Thomas Lavergne on Twitter.

The particularly rapid loss this month has been closely related to extreme warmth over Arctic Russia that has persisted more or less continuously since winter, and as a result, current sea ice deficits are strongly concentrated on the Russian side of the Arctic.  The latest update from NSIDC is worth a read:

http://nsidc.org/arcticseaicenews/2020/07/siberian-downward-slide/

Here are charts showing the record pace of melt-out this year in the Laptev Sea and the East Siberian Sea.  Interestingly, 2012 was not the record low ice year for either basin; in the Laptev Sea the record low occurred in 2014, and for the East Siberian Sea the fastest and most overwhelming melt-out occurred in 2007.

In contrast, the sea ice situation is quite different in the Western Hemisphere, with Beaufort Sea ice in particular holding up well so far compared to many recent years.  Both here and in the Chukchi Sea, 2012 was the record low ice year.

An interesting coincidence in the current weather pattern is that a very strong storm has developed over the Chukchi Sea in the past couple of days, just as in early August 2012.  The similarity is striking, although the 2012 storm was slightly stronger; compare the Canadian surface analysis maps below:

3pm AKST July 27, 2020

9pm AKST August 5, 2012

The 2012 storm was the strongest summer cyclone of the modern era in the Arctic, and it probably contributed to the record low ice extent that year by breaking up already-vulnerable ice and increasing ice-water mixing.  It will be very interesting to see if there are any discernible effects on sea ice from this year's storm.

Here's a reference on the 2012 storm, for further reading:

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2012GL054259

Still More Rain

In keeping with recent posts on increased warm-season precipitation, rainy weather has become persistent in Fairbanks-land this month.  Each of the last 7 days, and 9 of the last 10, have seen measurable rainfall in Fairbanks, and although total amounts have not been excessive, the year-to-date precipitation now stands at 9.47", which is not far off the pace of 2014.  At the CRN site to the northeast of Fairbanks, 2020 precipitation is just about tied with 2014.  Here's a chart of the accumulation relative to the 2002-2019 normal for the CRN site.

After a notable cool spell a few weeks ago, daily average temperatures have been hovering very close to normal, although daily highs have remained on the cool side because of cloud and rain.

More interesting than the overall average temperature has been the lack of significantly unusual warmth this summer; the temperature has not exceeded 80°F in Fairbanks since May 10.

Given the date on the calendar, the 82°F on May 10 is now somewhat likely to go down as the highest temperature of 2020, and this would be the earliest date on record for setting the year's high temperature.  The current record for that climate statistic is the amazing 88°F on May 11, 1995, and interestingly 1995 was also a year with a developing La Niña during summer, a very warm North Atlantic, and a very active Atlantic hurricane season.  1995 also saw extreme warmth in September in Fairbanks, with an incredible 78°F on the 21st, and I've always found it fascinating that the same year holds the records (by some margin) for most extreme warm spells on either side of summer.  I wonder if something similar could happen this year...

Rainfall Trends

As a follow-up to last week's post on increased rainfall in Fairbanks, I did a broader analysis with a couple of gridded data sets to examine the spatial extent of the changes.  This provides a quicker view of broad trends than digging through data from individual observation sites, which often suffer from missing days or other quality issues.

Looking first at a gridded analysis of observed precipitation (courtesy of Germany's Global Precipitation Climatology Centre), I calculated the percentile of the June-September precipitation in each of the six years from 2014-2019, relative to the distribution of June-September precipitation from 1979-2013.  In other words, if (for example) June-September 2014 precipitation was higher than any year from 1979-2013, the percentile would be 100% (the top of the distribution), but if it was equal to the median of the 35-year history, the percentile would be 50%.

I then took the average of the percentiles from the six years; see the map below.  The analysis shows an area centered just to the west of Fairbanks where the 6-year average percentile is around 90%, and this is consistent with the Fairbanks data; the last 6 years have all produced June-September precipitation near or above the 90th percentile in Fairbanks.

The gridded analysis shows unusually wet conditions extending northwestward from the central interior all the way up to Utqiaġvik, and there's another area of sustained high precipitation near the north end of the Alaska Peninsula.  We should regard these results with caution, however; the number of Alaska sites with consistent long-term precipitation data is small, and the gridded analysis is only as good as the data that goes into it.

For an independent view, the map below shows the same calculation from the ERA5 reanalysis, which is simply a long-term sequence of short-range model forecasts; actual precipitation data is not included as an input.  ERA5 confirms that Fairbanks is not the only place with very unusual precipitation during June-September in recent years, and the wet signal is particularly striking along the eastern North Slope.

Out of curiosity, I also looked at the average percentile of June-September shortwave radiation, which of course is mostly tied to cloudiness in June and July.  This shows reduced sunshine in the eastern interior and much more so over the eastern North Slope and the Beaufort Sea.  But over the Bering Sea summer sunshine has been persistently high, according to ERA5.

The percentile of sea-level pressure illustrates the weather pattern that has tended to dominate mid-late summer in recent years: a ridge of high pressure over the Bering Sea.

A global view of average 500mb height anomalies since 2014 at the same time of year reveals that the Bering Sea ridge has been one of the largest flow anomalies worldwide - see below.  (Note that heights are higher almost everywhere because of rising global mean temperatures, but the spatial distribution of the changes is particularly interesting.)

The Bering ridge seems to be part of a pattern of stronger high pressure ridges in the upper mid-latitudes of the Northern Hemisphere (in contrast to smaller height increases in the tropics and the high Arctic).  I believe this is also connected to persistently recurring high ocean temperatures across the North Pacific; I'll write more on this another day.

Fairbanks Thunderstorms and the rest of 2020

Okay, for old times sake…

The highest annual frequency of thunderstorms in Alaska is over the Tanana-Yukon uplands. Many weather-aware Fairbanksians know that there are thunderstorms, say, out Chena Hot Springs Road than at the airport, but the airport is where there are long terms records. Here's a graphic showing the number of days each year (April-September) when thunder was reported. I have no explanation for the jump in 1988 beyond that it's thundering more frequently nowadays. It's not from automation, since thunder reporting in aviation observations has never been automated in Alaska. When I first noted this some years ago, I asked a former long time, now retired, NWS Fairbanks observer, and he could not recall any changes in procedure or instructions on when to report thunder in the late 1980s, or at any time during his career (coding, of course, changed with the advent of METAR code as the standard in 1996).

So far (through July 11) this summer there have been eleven days with thunder at the airport, already above the recent year average. The typical drop-off in thunderstorm frequency is coming soon, so can we come up with a climatologically based estimate of what will be the total number of days for 2020? To do that, I looked at 1988-2019 (32 years) and asked how many thunder days occurred from July 12 through the end of September. That ranged from zero (three years) to 10 (two years), with an average of 3.5 days. But of course, thunderstorms days only come as whole days (technically, this is an example of "count data"), i.e. there can never be a year with 14.5 thunderstorm days. So to create an outlook for the 2020 seasonal total, I fitted a Poisson distribution (good for count data since it is a discrete probability distribution) to the 32 years of data (the era of more thunderstorms). The primary assumption of a Poisson distribution is that the values are independent. In this case, the number of additional days with thunder this year is not conditioned by the days in 2019, or 1988, etc. That seems like a reasonable assumption in this case. So, putting all the pieces together yields this outlook with is based on nothing but history (no meterology): better than 50-50 chance that there will be 2 to 4 four additional thunder days by the time the snow flies.

Such is the value of Deep Cold for those that are interested in getting "under the hood" of Alaska climate. 😋

10 Years of Deep Cold

I managed to miss the day itself, but yesterday was the 10th anniversary of Rick's first post here on the Deep Cold blog.

http://ak-wx.blogspot.com/2010/07/first-post.html

Rick maintained a high frequency of posts for the first 3 years, and then the authorship transitioned to me (Richard James) in summer 2013.  Brian Brettschneider also contributed a number of posts for a time.  The "official" hand-off was nearly 7 years ago:

http://ak-wx.blogspot.com/2013/07/blog-changes.html

My approach has generally been to write more detailed but infrequent posts, and regrettably the frequency has declined as other priorities have intruded; but I've found it a most enjoyable "hobby" to explore a wide range of topics over the years.  I've often told my wife that I would probably still write the blog even if no-one reads it, because it serves as both a repository of my informal research and a record of a few of Alaska's major weather and climate events.  Even now I find it very helpful to refer back to material from earlier years.

I often think it would be nice to have more daily weather commentary on the blog, along the lines of what Rick used to do, but I do find brevity to be a challenge.  I'm also not sufficiently immersed in day-to-day weather in Alaska, as I don't live in the Great Land.  I'd be delighted if one day that might change, and then I'll report firsthand; but for now, I'll continue with comments from afar.

Inactive Fire Season So Far

Interesting reading from the Alaska Fire Service today:

https://akfireinfo.com/2020/07/09/alaskas-fire-season-shaping-up-to-be-a-low-year/

"As of Wednesday, there had been 299 wildfires reported this season that had burn an estimated 173,743 acres, according to AICC statistics. Based on historical data, the chances of reaching the 500,000 mark is less than 20%, Strader said. In a typical fire season, Alaska burns about 650,000 acres."

I'll attribute the relative lack of fire to the trend towards La Niña in the tropical Pacific.  See here for my comments back in May.

Wet Summers

A few days ago Rick Thoman posted a striking chart on Twitter, showing clearly that Fairbanks has seen unusually high precipitation on a sustained basis since 2014.  Click to enlarge:

Remarkably, the 12-month running total precipitation hasn't dipped below about 12" since the record wet summer of 2014, and such a lengthy wet spell is clearly unprecedented in the roughly 100-year period of record.  When we consider that the long-term median annual precipitation is less than 11", it's quite extraordinary to see six straight years well above that level.

If we assume that each year is statistically independent of the last - which is not true but may not be too far off the mark (at least prior to 2014) - then we can do a crude estimate of the probability that 6 straight years will have such high precipitation by random chance.  The driest of the last 6 years was 2018, with 13.75", and this level was only exceeded 14 times from 1930-2013, i.e. 14 of 84 years, or an annual probability of 16.7%.  The chance that this happens 6 times in a row is then only 0.002%, and (naïvely) the recurrence interval is about 50,000 years.  It seems overwhelmingly likely, therefore, that the climate has changed; the rainfall statistics are not stationary over time.

An interesting question to consider in connection with this change is the seasonal distribution of the increase in precipitation.  June of this year was very wet; did June contribute significantly to the overall increase in 2014-2019?  The answer is yes, but larger increases have occurred in July through September, and in percentage terms the change in September is most striking (see below).  September 2015 and 2014 were the wettest and 3rd wettest on record, respectively, and September 2016 and 2018 were also very wet.

It's inevitable that the largest changes in total precipitation have occurred in the warm season, because that's when most of Fairbanks' liquid-equivalent precipitation falls; and if we imagine a percentage increase in water vapor content (related to, say, warming oceans), the impact in absolute terms would be most noticeable in summer.

The increase in September is interesting, and it's tempting to suggest it has to do with reduced Arctic sea ice; we would expect a large increase in lower atmospheric warmth and moisture at high latitudes as the Arctic Ocean opens up in autumn.  However, I suspect the Arctic warming only provides a background influence on September precipitation, and that other, more immediate causes (such as recent storm tracks and circulation anomalies) can be identified to explain the dramatic difference in the past few years.

Finally, it's fascinating to look at how the distribution of daily precipitation amounts has changed during June through September, the months that contribute most of the total precipitation increase.  The chart below shows that the frequency has increased in all daily precipitation categories above 0.1", and the percentage change in frequency has been very substantial for the wettest days, i.e. those with more than 0.5".

Of course a change in the frequency of the wettest days makes a relatively outsized contribution to the total precipitation increase, and this is illustrated below.  The lion's share of the total increase has arisen from a jump in daily precipitation totals above 0.5".  In 2014-2019, there were 32 of these events in June through September, amounting to an average of 4.62" per year; but from 1930 to 2013, these events occurred at less than half the frequency, and amounted to only 1.83" per year on average.  This difference accounts for about two-thirds of the overall increase in total annual precipitation.

If we consider the rate of observing events of 0.5" or more, we can do another crude estimate of the probability that the past six years could have occurred by random chance.  Using a Poisson distribution with an occurrence rate (1930-2013) of 2.35 events/year, the probability of seeing 32 or more events in 6 years is 0.003%, which is surprisingly close to the 0.002% estimated above from the annual totals.  Either way, the level of statistical significance is very high, and so I think we can say with very high confidence that the past six years represent a different climate regime from earlier decades.  As noted in this 2018 post by Rick on a similar topic, this has important implications for city management and planning.

July Frost

A very chilly air mass originating over the Arctic Ocean has produced frost in many of the usual cold spots of the central and eastern interior in the past few days.  Here are a few of the readings I noticed:

Fairbanks Area:
UAF Smith Lake   27°F
Ester 5NE Coop   28°F
Goldstream Creek Coop   29°F

Elsewhere:
Tok 70 SE CRN   26°F
Chicken Coop   27°F

Here's the 7-day temperature trace from UAF's Smith Lake 2m sensor:

According to the balloon sounding from Fairbanks airport, the 850mb temperature fell to -0.5°C on Tuesday afternoon, which is the first time since 2009 that the 850mb temperature has been below freezing between mid-June and the end of July.  The last such cold spell in 2009 (June 25) was also the last time that Chicken fell to 27°F in the same high-summer window.

The 26°F at the Tok CRN is the coldest in the 9-year history of the site for this time of year; it's the first instance of 26°F between June 14 and August 21.  Data goes back to autumn 2011.

Here's the reason for the cold: according to a back-trajectory estimate from NOAA, the lower tropospheric air that was over Fairbanks on Tuesday afternoon had traveled from near the North Pole in the previous 10 days.