Seasonal Outlook

Veteran Alaska climate expert Rick Thoman delivered the latest in his longstanding series of monthly climate outlook seminars today at UAF, providing all the ins and outs of the seasonal forecast for the next few months in Alaska.  Here's a link to the presentation slides; it contains a wealth of information and is well worth a look.

https://accap.uaf.edu/sites/default/files/2019_May_Alaska_Region_Climate_Outlook_0.pdf

The bottom line from the seasonal outlook is that the probabilities favor a warmer than normal summer across the entire state, and there's a tilt towards above-normal rainfall in all but Southeast Alaska and the Alaska Peninsula and Aleutians.  In this context, "normal" is defined by the 1981-2010 period, so the ongoing exceptional warmth in the oceans surrounding Alaska is a major impetus for the warm forecast.

After enjoying Rick's presentation, I was reminded of a curiosity that I noticed a few weeks ago involving an apparent connection between late summer rainfall in northwestern Alaska and the phase of the QBO.  The QBO (Quasi-Biennial Oscillation) is a slowly-evolving, semi-periodic fluctuation in winds up in the stratosphere above the equator.  These winds shift from westerly to easterly and back again on a timescale of a couple of years.

Now you might well ask what stratospheric winds above the equator could possibly have to do with Alaska weather, and if you did, I wouldn't be able to give a good explanation.  According to my very limited understanding, the QBO phase is thought to affect the way in which large-scale waves propagate around the globe at higher latitudes and in the troposphere; the details have to do with atmospheric wave physics, but I believe there's still considerable debate about the detailed mechanisms.

Fortunately we don't need to understand the QBO to know that it exists and is predictable, and to see the historical relationship with weather at ground level.  For example, here's a map showing the July-August precipitation pattern for years with a strongly positive QBO:

And here's the analysis for the negative phase:

The QBO is currently strongly positive and is likely to remain so for a few more months, so this obviously favors a drier July-August period in northwestern Alaska.

Below are scatter plots of the full joint distribution of QBO and July-August precipitation at Kotzebue and Bettles since 1979.  I won't claim the relationship is "strong", but it is statistically significant.  Note that the two wettest July-August periods on record in Kotzebue (2012 and 1994) also had the most negative July-August QBO on record (the two points are almost on top of each other on the chart).

So it will be interesting to see if the QBO influence shows up later this summer in the northwest; or will CPC's expectation for wetter conditions prove more reliable?

ERA5 Data for Alaska - Including Download Link

Back in November I took an initial look at the new ERA5 reanalysis data set from the world-leading ECMWF weather modeling and forecasting center in Europe.  At the time, the ERA5 data was only available for 2000-2017, but the reanalysis now extends back to 1979, and a further extension to 1950 will soon come online.  The high quality of the data assimilation and modeling framework that's used to produce the reanalysis makes this a real treasure trove of historical climate data.

It's an interesting exercise to compare the ERA5 data for Alaska to NOAA's climate division data, produced by NCEI.  For many years the climate division data was only available for the lower 48, but in 2015 the data set was expanded to include 13 climate zones in Alaska; here's a map.

To facilitate a direct comparison, I calculated area-averages for several ERA5 variables within each of Alaska's climate divisions.  For example, there are 605 ERA5 grid cells that at least partially intersect the Southeast Interior division; so I calculated the area of the intersection for each grid cell and added up the fractional contributions to the total area of the Southeast Interior zone.

Here's a chart showing the mean temperatures for January and for July in the Southeast Interior (which includes Fairbanks).  Aside from a modest cold bias in the ERA5 values in January, the performance is outstanding.

The situation is not quite as good in the North Slope division, which is not surprising as the observing network is more sparse, and moreover weather analysis and forecasting models (like the ECMWF model that underpins ERA5) often have a more difficult time with atmospheric physics in the Arctic.

Interestingly the 1979-2018 linear temperature trends are similar for January, but the ERA5 trend is much smaller than NCEI's trend for July in the North Slope division.

Looking at precipitation, ERA5 does fairly well for the Southeast Interior in both January and July, but again the agreement is not as good for the North Slope.  Precipitation is always a major challenge for reanalysis, and so these results are pretty good.

Finally, I did a quick comparison of ERA5 solar radiation to the CERES gridded data for the Southeast Interior, and again I used an area average for both data sets.  The results show a very close correspondence for the month of March, but there is only modest agreement in July.

We could of course keep going with all sorts of comparisons between ERA5 and other data sets, and between ERA5 and historical climate observations around the state, but there's no doubt that ERA5 is a very high quality reanalysis.  Beyond the pure fidelity of the data, however, the real value of the reanalysis is that it's spatially and temporally complete; and ERA5 even includes uncertainty estimates, although I haven't looked at that aspect yet.

For readers who might like to take a look at the Alaska data themselves, the following link provides the area-averaged data for the 13 climate divisions, including mean temperature, precipitation, solar radiation, and 10m wind speed.

https://drive.google.com/drive/folders/15RUOjHKVmxFJnnW1a9ZaMpQRwR-G6yPD

Follow-Up on Break-Up

As a quick follow-up on the topic of predicting breakup dates, I looked at whether breakup is more closely related to hourly temperatures above freezing rather than daily mean temperatures above freezing.  This is something that reader Eric suggested a while back, and reader BJ re-emphasized that daily average temperatures could be misleading.

I pulled out hourly temperature data from 1998-2018 in Fairbanks and calculated the accumulation of thaw degree hours up until the date of breakup in Nenana, i.e. the sum of the hourly temperature excess (if any) above 32°F.  Historical hourly data from Nenana is not quite complete enough for this task, so data from Fairbanks will have to do.

Here's a scatter plot of thaw degree hours (TDHs) versus thaw degree days (TDDs) accumulated through breakup; click to enlarge.

Not surprisingly, it's a very good relationship, but the best-fit line is flatter than would be expected for a one-to-one relationship.  Superficially, this suggests that hourly data is indeed a better predictor of breakup date than daily data, because there is less variance in TDHs at breakup than in TDDs at breakup.  (Imagine if TDHs were a perfect predictor: the best-fit line would be horizontal.)  The standard deviations of the two variables demonstrate the difference: the standard deviation of TDHs at breakup is 25% of their mean value, whereas TDDs have a standard deviation of 31% of the mean value.

A bit more work confirms that the typical range of thaw degree hours at breakup is associated with a slightly smaller date window than for thaw degree days.  See the chart below, which attempts to illustrate this.  If we're using daily data, we find that a typical (80%) range of TDDs at breakup corresponds to a 7-day climatological window, but if we use hourly data the window is narrowed to about 6 days.  It's obviously a small difference - hourly data is no holy grail for breakup prediction - but it seems that readers were correct to suggest that we can do better than daily average temperatures.  A more thorough investigation would require a more careful modeling effort with daily data... perhaps I'll return to that next spring.

Snow After Green-Up

According to the National Weather Service office in Fairbanks, green-up occurred on West Chena Ridge on Wednesday evening after the temperature reached a summer-like 70°F earlier in the day.  This means that the hillside turned distinctly green for the first time as birch and aspen leaves emerged en masse.

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE FAIRBANKS AK
1101 PM AKDT WED MAY 1 2019

AKZ222-021400-
MIDDLE TANANA VALLEY-
INCLUDING FAIRBANKS, FORT WAINWRIGHT, EIELSON AFB, ESTER, 
NORTH POLE, MOOSE CREEK, TWO RIVERS, FOX, CHATANIKA, 
CHENA HOT SPRINGS, AND SOURDOUGH CAMP
1101 PM AKDT WED MAY 1 2019

...GREEN UP OCCURRED IN FAIRBANKS THIS EVENING...

GREEN UP OCCURRED IN FAIRBANKS THIS EVENING, MAY 1, 2019. CHENA 
RIDGE WAS BEGINNING TO SHOW A FAINT SHADE OF GREEN THIS 
AFTERNOON, BUT THE RIDGE TURNED MORE DISTINCTLY GREEN BY THIS 
EVENING. 

THE MEAN DATE OF GREEN UP IN FAIRBANKS IS MAY 9. THE EARLIEST 
GREEN UP DATE RECORDED IN FAIRBANKS WAS APRIL 26, 2016.

The 70°F on May 1st was one of the earliest occurrences on record for such warmth, but a wild swing back to colder weather has occurred in the short time since.  This morning rain turned to mixed rain and snow at valley level, and then finally to plain snow before tapering off.  Some accumulations were observed in the hills, but apparently not in Fairbanks itself.

Looking back at the history of Fairbanks hourly weather data since 1950, I can't find any other instance of plain snow in the hourly observations after the date of the first 70°F.  There have been a number of instances of light mixed rain and snow, but most were very light with visibility of 10 miles or more, and plain snow (as occurred for a couple of hours this morning) appears to be unprecedented after the first 70°F day.  However, it should be noted that May 27, 1978 brought enough wet rain/snow to accumulate 0.1" , and that was 2 weeks after the first 70°F of the year.

More substantial snow occurred today at locations farther up the Tanana River valley, such as Tok and Northway.  And the history shows this isn't unprecedented; in mid-May 1995, 5 inches of snow fell in Northway less than a week after the temperature reached a remarkable 82°F.  The temperature only reached that level on 3 more days that year in Northway (elevation 1715').