Saturday, October 14, 2017

Heavy Rain Frequency

In the wake of Tuesday's extraordinary rainfall in Fairbanks, it's worth taking another look at the modern climate record (Weather Bureau/NWS era, 1930-present) to see if heavy rain events have become more common.  Back in July, I commented on the long-term increase in July rainfall in Fairbanks and showed that the change has been dramatic for the heaviest events such as those with 1" or more in a single day.  However, these events have become less common in August, and from a year-round perspective there seems to be little evidence that recent events are unprecedented - see the chart below.


Perhaps a slightly more robust method is to look at 2-day precipitation totals rather than individual calendar day totals, because heavy precipitation events often span the arbitrary midnight boundary of a calendar day.  The chart below shows the annual counts of 1-inch precipitation totals in either 1 or 2 days; I'm calling these "unique" 2-day totals because I've subtracted the number of single-day 1-inch events that would otherwise be counted twice.  See Rick Thoman's explanation here for more detail on this kind of approach.


This chart shows a noticeable cluster of heavy precipitation events in the last few years, but again the running 10-year mean is not higher than at other times in Fairbanks' history, so it's still not clear that the frequency of these heavy precipitation events has increased.

However, this is not quite the end of the story.  Owing to the fact that most of the heaviest precipitation events occur in summer in Fairbanks, the charts above are heavily weighted towards summer; for example, 80% of 1-inch daily precipitation events have occurred in June through August (33 of 41 days since 1930).  But what about relatively heavy events in the rest of the year - perhaps not exceeding an inch in 1 or even 2 days, but still extremely heavy for the time of year?

The chart below shows that 2-day precipitation totals above 1 inch have occurred in every month of the year in Fairbanks, even in the normally very dry months of February through April; and so there have been plenty of events in the dry season that I would label as "heavy precipitation" even though they would not be terribly unusual by summer standards.  So what happens if we ask how the frequency of relatively heavy precipitation events has changed over time?


I've attempted to answer this question by pulling out the top 5 precipitation events for each calendar month, so 12x5=60 events in total.  The chart below shows the number of these events per year; for example 1935 had 4 separate events with 2-day precipitation in the top 5 for the month.  Note that I used unique 2-day totals throughout, and I did allow for more than one event in a single month if they were separated by a few days (this has happened a few times).


The picture is now a little different, as we see that the current 10-year running mean is the highest in Fairbanks history.  Remarkably, the past three and a half years have seen 9 separate heavy precipitation events that were respectively in the top 5 for the calendar month.  Here's a list of them, along with links to comments on this blog:

1.61"   June 2014   (see here and here)
3.36"   July 2014   (see here and subsequent posts)
2.24"   September 2014   (see here)
1.30"   September 2015   (see here and here - note the relevant factoid at the end of this post)
1.43"   June 2016   (see here)
1.13"   September 2016
0.98"   December 2016   (see here)
0.76"   February 2017   (see here)
1.54"   October 2017   (this week's event)

The September events of 2014-2016 are particularly notable, and without them the chart above would be less striking.

In summary, when we classify heavy precipitation events with a threshold suited to the wet season, then we see little evidence of a notable increase in Fairbanks in recent years.  However, if we classify major precipitation events for each month separately - and these events are still quite substantial even in the dry season - then the past few years do stand out as unusual and indeed unprecedented in the modern climate record.

Tuesday, October 10, 2017

Heavy Rain in October

Record-breaking rainfall has occurred in Fairbanks today, amounting to more than an inch after changing over from snow in the early morning hours.  The storm total precipitation is up to 1.54" so far at the airport, breaking the all-time records for 24-hour and 2-day precipitation in Fairbanks in the month of October.  This is also the heaviest single precipitation event in Fairbanks since the excessively wet start to September of 2014 (see here).

Remarkably, the low-lying Fairbanks airport rain gauge appears to have seen the highest precipitation amount of any of the automated stations in the area - see the map below - but obviously manual observations from coop stations are not included here.


Heavy precipitation (e.g. an inch or more) is very rare in October in Fairbanks, and in fact it is surprisingly so when we consider that the long-term normal for October total precipitation is 0.79" - lower than June through September, but higher than the other seven months of the year.  The chart below shows an analysis of the number of precipitation events (unique 2-day totals) in each month that exceeded the normal total for the month; so in April we're counting the number of events with more than 0.16", and in August we're counting the number with more than 1.94".


Obviously the wetter months tend to have far fewer single events that exceed the monthly normal, as precipitation is much more frequent, and the distribution of amounts is less skewed, in the wetter months.  Interestingly the chart reveals a fairly good logarithmic relationship if we exclude October (note the log scale on the vertical axis).  However, October stands out like a sore thumb for its rather small frequency of heavy precip events in comparison to the monthly normal; and this highlights just how remarkable it is that today Fairbanks has seen about twice the normal total for October.

Lest it be overlooked, the first accumulating snow of the season was observed last night in Fairbanks, and amounts up to about 5" were measured in the higher parts of town before the changeover to rain.  Here's a sequence of webcam images from UAF showing the elimination of the winter's first snow cover within just a matter of hours.







Monday, October 9, 2017

Still No Hard Freeze

Temperatures in Fairbanks have defied the seasonal trend in a major way in recent days, with the month of October so far running 18°F above normal at the airport (through the 8th).  Perhaps most notable is the absence of a hard freeze so far this autumn either at the airport or on campus at UAF; the lowest temperature so far this season is 30°F at both sites.  Today, October 9, is the record for latest arrival of temperatures in the 20s in the official climate data for Fairbanks, so this record is about to be broken.

If we define the growing season as the period with continuous temperatures above either 27°F or 28°F (under the assumption that anything warmer may not be a killing freeze), then Fairbanks has already broken the record for longest such period on record: for a threshold of 27°F, it's now 170 days and counting.  Here's some context on historical values in the Fairbanks climate record:

http://ak-wx.blogspot.com/2015/04/growing-season-length.html

The map below, courtesy of xmACIS, shows the season-to-date lowest temperature; several other locations around Fairbanks and in the hills have also failed to see a significant freeze thus far.  (The 19°F in the bottom right is from the Salcha RAWS.)


Looking farther afield in the interior, cold has been very scarce elsewhere too.  Note the amazingly warm lowest temperature of 25°F at the Chalkyitsik RAWS; this site normally dips close to 0°F by this date.  A temperature below 25°F is normally observed by the end of August at this spot.


The abnormal warmth in Fairbanks is a continuation of the anomaly that persisted throughout most of the summer and was particularly noticeable in high daily minimum temperatures owing to high humidity.  The same trend is still very much in evidence: it's still much more humid than normal.  Fairbanks has only had a single day so far this autumn with a sub-freezing average dewpoint, compared with a normal dewpoint of about 25°F for this date.  Even at this time of year, high humidity reduces the rate of cooling by radiational energy loss; although of course the recent warmth is more easily explained simply by the lack of cold air masses over interior Alaska.



Thursday, October 5, 2017

Why Do Some Octobers have Low Snow?

Hi, Rick T. here. This came up on Twitter recently, and it seemed interesting enough to warrant more than a graphic and 144 characters: recent Octobers in Fairbanks have not brought very much snow.

Here's a plot of October snowfall since 1912 as well as the 10-year running median. The usual cavaets apply to the pre-1930 totals, though there are no obvious problems with the daily data underlying these totals. However, overall some of the cooperative era totals are very likely a little lower than would be measured in the post 1930 Weather Bureau/NWS era. From this graphic you can see that the last time Fairbanks had a foot or more of snow in October was 2008, and the ten-year median is now at the lowest level since the late 1960s, suggesting the current October snow drought is not dissimilar to what happened in the 1960s. So a question is: why low-snow Octobers?
There are of course two distinct ways that Octobers can have low snowfall in Fairbanks. One way is that precipitation falls as rain instead of snow and the other is that there is little precipitation of any type. Over the course of the month both these factors can come into play, e.g. a rain event during the first week of October (nothing unusual about that) and then it's dry for three weeks (also not terribly unusal).

Here's an attempt to assign one or the other of these factors to low-snow Octobers using just the monthly data.  Since we're interested in low-snow Octobers, I've ignored years with near to above normal snowfall: that is, snowfall above the 33rd percentile (which turns out to be 5.9 inches). So, for Octobers with 5.9 inches of snow or less, I ask: was the total precipitation for the month below the 25th percentile (0.45 inches)? If so, then I ascribe the reason for the low snow that month to the fact that it just did not precipitate much. Otherwise, I chalk up the low-snow October to the fact that there was significant rainfall during the month. The results of applying this method to the historic record are plotted below. By this metric, most low-snow Octobers (24 of the 35 years in the significantly below average category) are due to low precipitation total. No surprise, since histroically the bulk of precipitation in October in Fairbanks falls as snow. However, in 11 years in the past century (and two in the past four), low October snowfall can be attributed to significant precipitaiton that happened to fall as rain. It's fairly obvious that there is no long term trend, but it is interesting between 1970 and 2011, a 40 year stretch with frequently high snowfall totals, only one October was a "low snow 'cause it rained" month.

A more detailed analysis of the daily data might reveal additional interesting details on this question.

Warm Season Recap: Fire Acreage

This year's fire season produced about 650,000 acres of wildfire in Alaska, which is very close to the median annual acreage since 1990.  Two-thirds of the total acreage burned in July, and this is similar to the past two years, even though 2015 total acreage was far higher.  Over the long term, July is the most active month for fires, but if it's not a very active year then it's not uncommon for June to produce more acreage.  August is only a significant month for fires during very active fire years.


A quick view of the geographical distribution of fire acreage in the northern half of the state is available from the acreage breakdown between the federal fire management zones:


This year 63% of the state's total fire acreage occurred in the Upper Yukon zone, which is the highest percentage on record (1990-present), although of course far higher acreage totals have occurred in previous years.


It's interesting to note that the Upper Yukon fire acreage is easily the most variable of the zones from year to year, while the Galena acreage is the least variable.  It would be interesting to explore how this is related to systematic differences in inter-annual variability of warm season weather conditions from west to east across northern Alaska; presumably the summer weather patterns vary from "very unfavorable" to "very favorable" for fire in the northeast, whereas the northwest is more often somewhere in the middle.

Saturday, September 30, 2017

Fairbanks First Autumn Snowfall: the Long View

Hi,

Rick T. here with a quick follow-up on Richard's post on Friday with the first snowfall of the season at Fairbanks Airport. Below is a plot of the date of first snow (any amount) each autumn since 1904, along with the regression plot for the 10ᵗʰ (very early), median and 90ᵗʰ (very late) percentiles. Note that this list aims to give the first snow date exclusive of convective hail. Unfortunately, snow and hail have been lumped together in the US climate record since early 1950s, so a "snow only" time series for Fairbanks requires a little bit of QC.

Prior to 1930, when the Fairbanks observations were done by cooperative observers, it is quite likely that snowfalls like the Friday morning occurrence (mixed rain and snow for a few hours in the middle of the night, no accumulation) would have not been reported, so some of the pre-1930 dates are quite likely later than would now be recorded. Even in the post-1929 Weather Bureau era, around-the-clock observations did not start until World War 2, though there were observations made every six hours, so there was a "middle of the night" presence.  

The regression analysis shows practically no change in the median and very late dates over the past 114 years. I've plotted the regression slopes, but neither the 90th (top red line) or median (green line) pass statistical tests that would allow us to say with high confidence that the slope (i.e. trend) is really different than zero. In light of the probable "late bias" in the pre-1930 data (at least in aggregate) the 90th percentile seem likely to me to have no trend, while the same bias would serve to "push down" the left side of the median date, i.e. increase the century scale slope a little (though this is quantile regression, so it's less sensitive to outliers than ordinary linear regression).

The big change is obviously in very early "first snows" Prior to 1980, a first snow before Labor Day, while not common, occurred 11 times in 75 years. However, in the past 30 years that has occurred just once. The 10ᵗʰ percentile (bottom red line) can be thought of as the date for which there is a ten percent chance in a given year that the first snowfall would occur on or before that date. So prior to WW2, there was about a ten percent chance of first snow September 1ˢᵗ or earlier. Nowadays, we have to get to September 10ᵗʰ or even a bit later to get to the same threshold.

Friday, September 29, 2017

First Flakes

The first snow of the season was reported at Fairbanks airport early this morning, but so far it's only a trace amount and mixed with rain.  According to long-term records, more than 80% of years bring at least a trace of snow to Fairbanks in September, and in recent years it's been more like 90%; the only years in the past two decades without any snow in September were 2007 and 2010.  Of course two years ago 11 inches fell on this date:

http://ak-wx.blogspot.com/2015/09/historic-snow.html

Up in the hills, rain turned to snow yesterday afternoon as colder air moved in aloft.  Here are a couple of webcam shots from 3pm and 7pm yesterday at 2200' on the Steese Highway.




Temperatures in Fairbanks have now entered the 6-week period of most rapid decline, with peak cooling of around 6°F per week occurring near October 20.  It's interesting to note, however, that daily maximum temperatures cool more quickly at first, and daily minimum temperatures then catch up; the difference in cooling rates is most pronounced from mid-September to early October.


This nuance in the seasonal transition is related to snow cover, because having snow on the ground makes a bigger difference for minimum temperatures than maximum temperatures; with snow on the ground, the temperature can drop very rapidly at night with clear skies and light winds.  Most years do not have snow on the ground in Fairbanks at the beginning of October, but snow cover becomes 30% likely by October 9 and 50% likely by October 14.  So the establishment of snow cover plays an important role in reinforcing the overall downward temperature trend after about the first week of October.

Saturday, September 23, 2017

Mild North Winds in Barrow

The equinox is now behind us, but only in the past few days have temperatures in Barrow (or Utqiaġvik) begun to make regular excursions below freezing.  Of course this isn't too surprising with a vast expanse of open water to the north; the Arctic sea ice edge to the north of Barrow is about 500 miles away.

We've looked at this type of thing before, but curiosity led me to re-examine the average temperature around the year in Barrow when the wind is blowing from a northerly component; see the chart below.  The purple line at the top shows the warming that occurred between the 1951-1975 and 2001-2016 periods.


It's interesting to observe that for 1951-1975, winds with a northerly component tended to bring sub-freezing temperatures beginning in early September, but that date has been pushed back to the equinox in recent years.  The warming has been more pronounced in October, of course, as the heating effect of open rather than ice-covered ocean becomes greater the colder it gets.  The difference between 2001-2016 and 1951-1975 peaks at about 12°F in late October for winds with a northerly component.

Another way of looking at the change is to consider the latest date in the year when a temperature above freezing was observed in combination with a northerly-component wind.  Prior to 1995, early October was about the latest that this ever happened, but since then it has been rare to see the last such date before the end of September; and there have been several very late outliers, including January 10, 1997.


Below is a pair of contrasting sea ice extent maps to illustrate the change: yesterday versus the same date in 1983, when the Beaufort Sea coast was already locked in with sea ice.  It's no surprise at all that NOAA's Climate Prediction Center is predicting a 75% chance that Barrow's October mean temperature will be significantly higher than the 1981-2010 normal (i.e. in the upper tercile); see the third map below.  This may even be conservative, as only 2 of the last 11 Octobers have failed to be at least this warm.