Wednesday, November 22, 2017

La Niña Influence

I mentioned the other day that there has been a persistent ridge of high pressure over the Bering Sea in recent weeks, and I suggested that this is probably related to the current La Niña episode.  Here's the circumstantial evidence for this idea: first, note the sea-level pressure anomaly for November 1-20, showing a very strong departure from normal over the southeast Bering Sea and the eastern Aleutians.

Compare this to the mean November pressure anomaly in the 10 strongest previous La Niña episodes since 1950, based on November-March mean values of the Multivariate ENSO Index (see below).  The resemblance is, to say the least, very good; even the low pressure anomalies to the southeast and to the northwest line up.  It could just be coincidence, of course, but I think it's probably safe to conclude that La Niña is already having a significant effect on the circulation pattern near Alaska.

Here's a simple animation of recent 500mb heights.  It might seem counter-intuitive that the west coast would be so stormy with a strong ridge nearby, but in fact there has been a very active jet stream over the top of the ridge, and frequent strong disturbances have moved around the northern periphery of the ridge.  Another way of looking at it is that the storm track has been displaced north of its usual position.  Moreover, the actual height pattern has been highly variable, as seen in the animation; the anomaly in the maps above says nothing about the stability or variance of the flow pattern.

In view of the excellent agreement with the typical "strong La Niña pattern", it's interesting to note that the current La Niña's intensity is still rather modest; it certainly doesn't yet count as a strong episode by standard metrics.  A map of the last month's SST anomaly (see below) shows a band of cool water along the equator east of the dateline, but it's a narrow band and the anomalies are, again, modest.  Nevertheless, the spatial presentation of the cool anomalies resembles a classical La Niña (cool near South America), and perhaps we could argue that this episode is behaving more like a strong episode because of the strong temperature contrast between the cool equator and the widespread warmth outside the tropics - in other words, it's a strong La Niña when you consider how warm the globe is now.

Back in September we looked at the implications of La Niña for Alaska's climate over the entire winter season - November through March.  But given what we've seen so far, is there anything new we can say about how the rest of the season might evolve?

I decided to look at this by examining the winter climate patterns in La Niña years when a Bering Sea ridge prevailed in early-mid November - and also in other La Niña years when it didn't.  Despite the very good pattern match in the two maps above, there is certainly no guarantee of seeing this pattern during La Niña, and indeed the connection is much less robust when less intense La Niña episodes (like the present one) are included.

So to match the recent anomaly, I calculated the area-average 500mb height for November 1-20 over the region 40-60°N and 170°E-160°W, and after removing the long-term trend I obtained the Nov 1-20 height anomaly for the 16 strongest La Niña winters.  Of these, it just so happens that 8 years had above-normal Nov 1-20 heights (like this year), and the other 8 had below-normal Nov 1-20 heights.

Looking at the subsequent December-February climate patterns, it's no surprise to see that the November-ridging years tend to see the ridge persist, although its focus shifts south of the Aleutians (top map below).  In La Niña winters without a Bering Sea ridge in November, the MSLP pattern is similar but less pronounced (second map below).

The Dec-Feb temperature patterns are quite similar to each other; November-ridging years tend to see more widespread cold from the west coast to southeast Alaska, but the eastern interior seems to have a better chance at cold in the other La Niña winters.

A more interesting difference, perhaps, shows up in the precipitation maps (see below).  Years like this one, with November ridging over the Bering Sea, have a high frequency of above-normal Dec-Feb precipitation over western, northern, and central Alaska, and this certainly seems consistent with the recent stormy pattern along the west coast.  In contrast there is not much of a precipitation signal for western or interior Alaska in La Niña winters that fail to produce a Bering Sea ridge in November.

Finally, what can we say about sub-seasonal variations during winter?  Interestingly the strongest signal for cold (i.e. colder than normal) shows up in January for the November-ridging years.  The first chart below shows the daily temperature anomalies in Fairbanks for the same 8 years with La Niña and a November Bering Sea ridge.  Mid-late January really stands out as having an enhanced frequency of cold, with the lone exception being January 2000 (when it was in fact cold until mid-month).  A second round of cold occurred relatively frequently in early March in these "analog" years.  Of course these signals could just reflect random variability, but I don't think it's unreasonable to expect the sub-seasonal evolution to be at least somewhat predictable in view of the La Niña forcing combined with a distinct and anomalous pattern in November.

In the 8 years without a November ridge, the Fairbanks temperature patterns were - interestingly - nearly inverse, with more frequent warmth in mid-January and relatively cold conditions around the turn of the year and in early February.  If my expectation proves correct, then this winter is more likely to follow the first trajectory (cold in January) than the second; let's see how it plays out.

Sunday, November 19, 2017

West Coast Storms

Just a quick update this morning to note the stormy conditions in western Alaska; blizzard warnings are in place from Point Hope south across the Seward Peninsula to the Stebbins/St Michael area.  As the weather service discussion put it yesterday, a "very active weather pattern continues to hammer western Alaska".  The effects on the coastline are being exacerbated by the remarkable absence of sea ice even far up in the northwest; and the slow growth of sea ice is also related to the storminess, as noted by Rick Thoman yesterday:

This morning's surface observations show an interesting wind contrast between St Michael and Unalakleet, a distance of only 47 miles, as shown on the map below; the wind direction is completely opposite at the two locations.

The wind contrast highlights the location of a strong front that is moving east across Norton Sound and southwestern Alaska.  It's analyzed as a cold front on the Environment Canada map (9pm AKST), which seems ironic as the surface conditions behind it are warmer than to the east of the front, but that's what you get in the wild world of western Alaska weather.

The sequence of maps below (note the timestamps on the top right) show how the wind at St Michael went round from southeast to southwest as temperatures rose, but meanwhile the temperature fell a couple of degrees in Unalakleet as cold interior air moved over the Nulato Hills.  Note too the rising temperature in Fairbanks as high clouds moved in aloft; the coldest conditions have shifted east to the Yukon Territory.

[Update Sunday 6pm]

I find it interesting to note that the wind continued to veer around to the northwest and eventually north and northeast in St Michael today, and as this happened the temperature dropped sharply.  So in the space of less than 18 hours, the wind (a stiff breeze throughout) at St Michael rotated through three-quarters of the compass, while less than 50 miles away at Unalakleet the wind remained virtually unchanged throughout.  Fascinating!

[Update Wednesday 6pm]

Here's a map showing peak wind gusts for the 24 hours ending 6pm yesterday (Tuesday November 21).  This is an entirely different storm from the one discussed above.  Stormy times in western Alaska!

Saturday, November 18, 2017

Winter's Chill

[Update Sunday 6pm]

Last night was colder, especially in the Fortymile Country, with -35 to -38°F in several locations and a rather remarkable -43°F at Chicken.  Far to the north, satellite estimates suggest it may have been below -45°F in Arctic Village and its environs, but unfortunately the Arctic Village AWOS has not been reporting lately.

[End of Update]

Distinctly chilly conditions have descended upon Fairbanks and indeed most of the eastern interior, with widespread -20s and some -30s Fahrenheit this morning.  Here are some of the coldest spots, with higher-elevation valley sites and the Fortymile Country being well represented:

-38°F  Atigun River HADS site at 2600', near Galbraith Lake (north of Atigun Pass)
-35°F  Chicken co-op site (1800')
-31°F  Fortymile River at the Taylor Highway (HADS site at 2100')
-30°F  Salcha RAWS (900')
-30°F  Wade Creek at the Taylor Highway (HADS site at 1965')
-29°F  Tok 70SE CRN site (2000')

In Fairbanks the international airport reached -23°F, the Goldstream Creek co-op saw -22°F, and Eielson AFB reached -20°F, but North Pole only hit -18°F.  It's quite unusual for the airport to be the coldest spot in the area.  Tonight looks to be colder as the 5-minute observations from Fairbanks airport showed a 10°F temperature drop right after sunset just a short while ago - see below (sunset was at 15:34 AKST today):

This morning's -35°F at Chicken is right on schedule, as the 1997-2016 median date for first -35°F reading is tomorrow, November 19.  In Fairbanks it is "normal" to see the first -20°F or colder on November 17, so again this cold spell is right on time.

Here's the 500mb map (courtesy of Environment Canada) from 3am this morning.  Cold air is being imported from the north in the strong pressure gradient between the Pacific/Bering Sea ridge and the cold low over Canada's Melville Island.  The cold low is really cold - notice the -40s Celsius temperatures around it.  As for the Bering Sea ridge, this has been a persistent feature of the pattern in recent weeks and is a clear expression of a La Niña influence.  I'll have more to say about this in a subsequent post.

Wednesday, November 15, 2017

Frequency of Snow

The scene in Fairbanks is wintry at last, with persistent light snow events in the past week bringing the snow depth up to 9 inches.  This is a little above normal for the time of year.

Beginning last Wednesday, 6 consecutive days produced measurable snowfall in Fairbanks; this seems a little unusual, but it's typical for a 6-day snowy period like this to occur at least once in a winter.  The record for consecutive days with measurable snow is 16 days in November 1994, and the next longest periods are:

14 days ending Oct 24, 1970
13 days ending Nov 24, 1988
12 days ending Jan 10, 1987
12 days ending Nov 8, 1996

It's a bit curious that prior to 1965 (i.e. 1930-1964), the longest streak of snowy days was only 10 days; but perhaps there was a tendency to overlook very small (e.g. 0.1") snow accumulations in the early years.

The fact that 3 of the longest 5 periods ended in November is consistent with the peak in daily snowfall frequency at this time of year.  The figure below shows a smoothed daily frequency of measurable snow (blue line) and also shows the frequency of snowfall when there was snow on the previous day (purple line).  As we would expect, the chance of snow is higher if the previous day was snowy.

Taking a quick look at webcams around the area, freeze-up is still not complete in Fairbanks, although the Tanana River at Nenana is ice-covered now.

Friday, November 10, 2017

Utqiaġvik Wind Events

Back in late September, Alaska's northernmost community of Utqiaġvik (formerly Barrow) experienced a damaging coastal flood event that washed away some lengthy sections of road and caused significant erosion to the coast.  The following article provides detail on the damage:

As noted in the article, a similar event happened in 2015; here's a blog post from the time:

Some of the coastal flooding that occurred in the latest event is evident in this shot from UAF's sea ice webcam:

Later that day the berms were repaired along the beach, but obviously the city's defences are rather fragile.

In view of these events, it's worth taking a look at some historical data to see if high wind events have become more common, and if so, by how much.  To do this I used gridded reanalysis data to estimate the sea-level pressure gradient at 6 hour intervals since 1950 for a grid point close to Utqiaġvik.   Of course the pressure gradient is closely related to the wind speed, so the idea is to use the pressure data as a proxy for estimating changes in wind conditions.

But why not just use the historical wind speed data from Barrow?  The reason is that there are too many uncertainties related to changing measurement practices over time - i.e. it's likely that changes in instrumentation, measuring height, and measuring procedure (such as averaging time), as well as a changing urban environment, have created artificial changes in the long-term wind speed data.  In contrast, the sea-level pressure field in a reanalysis should (by design) provide a consistent estimate over time, although not all reanalysis techniques are equally good, and of course a reanalysis is just a model.  Nevertheless I think the reanalysis data has more potential to provide a useful answer.

The chart below shows annual series derived from the 6-hourly pressure gradient from the NCEP/NCAR reanalysis at a grid point near Barrow.  The two horizontal dashed lines show 1957-2016 mean values and help to highlight subtle changes.  Note that data prior to 1957 is a bit suspect because the number of balloon soundings around the Northern Hemisphere was far smaller in earlier years.

It's interesting to see that there seems to have been a subtle upward shift in pressure gradient since about 2000, and for the middle series (the annual mean of the 12 monthly maxima) most years since 2000 have been above the long-term mean.  The 1957-2016 linear trend in the middle series is statistically significant (p=0.03).

Here's a similar analysis from the 20th Century Reanalysis, which actually goes all the way back to 1851.  Note the suspiciously high pressure gradient values prior to 1957 - this supports the idea that the early estimates are less good.

The annual mean pressure gradient values in the two data sets are correlated at R=0.77, which isn't too bad, although obviously there can be large differences in how the reanalyses handle individual events; the "annual maximum" series are correlated at only R=0.58, and the 20th Century Reanalysis seems to have a very different assessment of the event in 2013 (a blizzard in mid-January).

The 20th Century Reanalysis shows less of an indication of higher pressure gradient in recent years; there is still an upward trend, but it's not really statistically significant (p=0.08 for the middle series).

How about individual wind events that might tend to produce storm surge flooding in Barrow?  To look at this I pulled out all 6-hourly instances when the pressure gradient was above a certain threshold AND Barrow reported a wind direction between 250° and 360°, i.e. winds from west-southwesterly through northerly; we'll call these "flooding winds".  Strong winds from this direction would tend to pile up water on the coast.  Of course there is much more to creating storm surge than just instantaneous wind speed and direction, but this is a quick look.

The chart below shows the number of "flooding wind" hours each year in which the pressure gradient exceeded 3mb per 100km, which is about the 95th percentile year-round.  There was a notable lull in these strong wind events for about 15 years starting in 1976, and this year has been windy, but overall there seems to be little trend for the annual number of hours.

Looking just at the autumn season when shore protection from sea ice is reduced, the picture is a little different - see below.  Interestingly for this time of year it does appear that there has been a long-term increase in strong pressure gradient events that could cause coastal flooding, although the last decade or so has not been notably worse than say the 1990s.

The 20th Century Reanalysis again shows a slightly different result.  Here I've used a slightly lower pressure gradient threshold, because the values are systematically lower, but other than a couple of high totals in 1993 and 2002, the past few decades have not seen an unusual number of "flooding wind" hours.

In summary, the well-known NCEP/NCAR reanalysis provides some evidence of an increasing trend in pressure gradient and therefore presumably increasing winds year-round near Barrow, and the data also suggest that somewhat more strong westerly/northwesterly wind events have occurred in autumn since around 1990.  However, the 20th Century Reanalysis provides only marginal support for the idea, so in the absence of more information it's difficult to draw a firm conclusion.  Perhaps the most useful thing we can say is that there's certainly no evidence of a slackening in pressure gradients, and given the profound reduction in sea ice extent, the rate of shore erosion is likely to be higher than in the past.

As an aside, there's one other interesting aspect of the data that I noted: a pronounced increase in pressure gradient in February, see below.  The other months with the most notable increases were June and July, but November has seen a downward trend.

Saturday, November 4, 2017

Extreme Snowfall In Fairbanks

Hi, Rick T. here with a post on the frequency of heavy snowfall in Fairbanks, specifically the return period (or, as I prefer, the annual probability) of snowfall amounts. You're perhaps familiar with this concept as applied to rainfall or river flows, and is an important factor to be considering in many engineering applications. We don't see so much of this kind of analysis with regard to snow, though we should, e.g. the maximum expected snow load during the lifetime of a building is (or ought to be) an important design criteria in snow county.

To illustrate, I'll go through the procedure with the two-day snowfall.  For each snow season since 1919-20, we find the max snowfall total on consecutive calendar days.  This gives us an "extreme values" time series. This seasonal maximum has ranged from 2.6" in 1952-53 to 26.9" in 1965-66. Now note that this maximum two-day total can occur at any point in the season, though realistically, late September to May. While the highest value typically occurs in the November to January time frame, it can be much earlier or later. For instance, in 2015-16, the highest two-day total was 13.5" September 29-30, while in 1991-92, the highest two-day total of 9.6" occurred in mid-May. So putting this in graphical form, on the left is the annual time series. It is clear just from inspection that there is no temporal trend, which  simplifies the analysis. On the right is a histogram of those annual values. For many cold seasons, the highest two-day snow total is in the range of 5-9", but with a moderately long tail toward higher values.

Now we'll fit the data to a generalized extreme value distribution, and I plot the results as a function of return period, as in the graphic below. The observed seasonal maximum in this plot are rank ordered, the green line gives the best fit and the dashed lines are confidence intervals of the fit.

So from this you can pick off the return period (better: annual probability) for two-day total snowfall. So the 5-year return period (20% chance in any individual season) is about 12", while the 20-year return period (5% chance in any given season) is just over 18". If we do this analysis for various accumulation intervals, we get this (leaving off the confidence internals for simplicity):
For reference I've plotted the highest observed value in the past 97 winters. These are used in the fit and so are not independent, but this does give an idea of the (relative) rarity. So both the 7 and 14-day totals are a bit lower than the fitted 100-year return period, while 72.4" that fell in the 31 days ending January 25, 1937 is more unusual. A word of caution: the 24 hour return periods are only for the Weather Bureau/NWS era (since winter 1929-30) and, since c. 2001 there are no six hour snowfall observations. The only 24 hour snowfall totals currently available are the midnight to midnight, calendar day total and the 3pm-3pm AKST totals (which can be back-calculated from the end-of-day and 3pm AKST climate summary products). The 3pm-3pm totals are not regularly used by NCEI, so for the high end snowfalls since 2009 I've checked them myself. The only difference from the published values (which are maximum calendar day values) is for 2016-17, when the highest calendar day total is 10.4" (Dec 28) but the highest 3pm-3pm total is 12.4" (Dec 28-29).

Thursday, November 2, 2017

Missing Soundings

An article today in the Washington Post brought attention to a somewhat unsettling trend in NOAA's upper-air observing program at sites in Alaska.  In short, there has been a reduction in the number of balloon soundings being launched from several of Alaska's upper-air observing sites, and it is likely that this will have some (unquantified) negative effects on the quality of weather forecasts, especially for downstream locations.

The chart below verifies the issue, as the percentage of missing soundings from Alaska's 13 sounding sites has increased rather substantially since late summer.

Looking back at 2016 (see below), over 90% of days had all, or all but one, of the usual 26 soundings per day across the state, although there seems to have been something of an increase in missing data in the second half of the year.  Presumably the problem has been developing slowly over time, but for reasons cited in the article it has worsened considerably of late.

My perspective on the issue is that it's fairly inexcusable, even if the argument can be made that the coverage of 13 upper-air sites over Alaska is quite good compared to other high-latitude parts of the world.  Given the economic importance of forecasts for the lower 48 and Canada, high-density sampling of the atmosphere upstream over Alaska is really critical, and balloon soundings have tremendous value for numerical weather prediction.  Surprisingly, two of the sites that have been cut back are Utqiaġvik/Barrow and Kotzebue, and these are the only U.S. upper-air sites in the Arctic.

I would argue that just purely on a financial basis, it makes sense to at least maintain the existing sounding network.  Compared to the cost of satellite observing platforms, the value of the forecasts, and the cost of the computers to crunch all the numbers, the radiosonde network provides enormous value for money.