Saturday, October 28, 2017

Thaw and Rain

Exceptional warmth for the time of year has developed across most of Alaska, putting a halt on freeze-up and even melting away the early snow cover in places in the past couple of days.  Most of the state has seen temperatures above freezing, including parts of the North Slope today; 37°F was observed on the Sag River at 69°N this afternoon.

On a statewide basis the "heat wave" is among the most significant observed this year when compared to normal temperatures for the date.  The chart below, courtesy of Rick Thoman, shows a daily temperature index for 25 sites around the state; the values (ranging from -10 to +10) correspond to percentiles within the historical distribution.  Since April there have been far more days in the upper tercile (red dots) than the lower tercile (blue dots), and yesterday's index value was the second highest of the year after June 8.

In addition to the warmth there has been rain: one-tenth of an inch of plain rain in Fairbanks yesterday, with nary a trace of snow falling.  In Tanana it has been raining steadily since mid-morning today (over 8 hours straight now), and temperatures aren't even particularly close to freezing; the dewpoint reached 37°F this afternoon, which is the highest on record for this late in the year - in fact it's the highest dewpoint observed there between October 19 and March 21 (based on data from 1950-present).

Here's the rather miserable scene in Tanana this afternoon: rain falling on a thin snow cover, with moderate ice moving in the Yukon River.

With the temperature in Fairbanks getting up to 42°F on Thursday, there is now only a trace of snow on the ground in the Golden Heart City.  Based on the long-term history, less than 20% of years have no real snow cover in Fairbanks on or after this date, but this year makes it 4 out of the last 5 years (2013, 2015, 2016, 2017).  The scene this afternoon on the UAF webcam - with more green than white - is not normal for October 28.

On another topic, the webcam shot is of interest for another reason too - notice the bright sun dog to the east of the sun.  Sun dogs are a rather common optical phenomenon caused by refraction and scattering of light through flat hexagonal ice crystals, generally (as in this instance) in high cirrus cloud.  The image also shows a hint of a 22° halo above and below the sun dog.  These phenomena are not unique to the northern latitudes, although the prevalence of ice crystals especially near the surface in the cold season does give rise to far more interesting optical phenomena than are commonly observed in warmer climates.

Thursday, October 26, 2017

New ECMWF Seasonal Model

As the snow melts away this evening in Fairbanks-land under the influence of chinook flow (with temperatures generally in the 40s), long-range forecasters are watching the tropical Pacific to see if the incipient La Niña episode will continue to strengthen.  If it does, then we would not expect abnormal warmth to be the theme of the winter in southern and interior Alaska; instead, some notable cold episodes would be rather likely, but variability would also be high - perhaps not unlike the recent swing in temperatures.

A new tool that forecasters will be using this winter is an upgraded version of the seasonal model from the well-known European Centre for Medium-Range Weather Forecasts (ECMWF).  In addition to having better physics and higher resolution in both the atmosphere and ocean simulations, the system now includes a coupled sea-ice model, leading to a much better representation of inter-annual variability and trends in ice conditions.  In my view the lack of a sea-ice model in the previous version was a rather major shortcoming.

In preparation for the release of the model upgrade (version 5), I've been looking at the performance of the model using the historical retrospective forecasts that are provided for bias correction and calibration.  The figures below show summary statistics (correlation coefficient) for forecasts of sea surface temperature in the equatorial Pacific (Niño3.4 region) and in the North Pacific (PDO and NPM indices); I'm showing results for the NMME models as well as versions 4 and 5 of the ECMWF model.  (See this post from last year for some background information.)

It's encouraging to see that ECMWF's version 5 model is better than version 4 for the PDO and NPM forecasts.  Also, the ECMWF's PDO and NPM forecasts are now generally better than any of the individual NMME models, and for the PDO at 1-3 month lead times the ECMWF is even better than the NMME ensemble mean.  Oddly, however, my results show no improvement in the Niño3.4 forecasts.

I also looked at forecasts of several of the most important atmospheric teleconnection patterns that meteorologists tend to monitor, including the PNA (Pacific/North American) pattern and the EPO (Eastern Pacific Oscillation).  Both of these atmospheric circulation patterns are closely connected to Alaska's winter weather; for example, the 4-panel of maps below shows the 500mb height and surface temperature patterns associated with the positive PNA (top) and negative EPO (bottom) phases, both of which tend to bring unusual warmth to most of Alaska.

The opposite PNA and EPO phases are shown below (negative PNA on top, positive EPO on bottom).  To appreciate the significance of these two patterns, consider that the November-March PNA and EPO index values (which are actually independent of each other) jointly explain about two-thirds of the November-March temperature variance in Fairbanks; so we can "predict" the Fairbanks winter temperature with a mean absolute error of only 2.0°F if we know the PNA and EPO index values.

In view of the significance of these atmospheric "modes", it's of interest to see whether the models can predict them on seasonal time scales.  The leftmost sets of columns in the charts below show that the answer is yes, and the PNA and EPO forecasts are actually quite good compared to several other teleconnection indices such as the North Atlantic Oscillation and the Arctic Oscillation.

It's also encouraging to see that the ECMWF's upgraded model is considerably better at predicting the EPO pattern, and this bodes well for seasonal winter forecasts in Alaska.

Friday, October 20, 2017

Freeze-Up Progress

Here's a quick update just to make a note of the cold temperatures and rapid onset of freeze-up over the past day or so.  A quick freeze-up at this date is neither surprising nor unusual, but I find it useful to have these dates documented for future reference.

Here's this afternoon's scene on the Chena River in Fairbanks, where the temperature dropped to +3°F this morning.  This is the coldest it's been this early in the season since 2012; but in 1965 the mercury dropped to -15°F on October 20 (and the high was only +3°F on the 21st!)

Sub-zero temperatures were observed in several spots around Fairbanks, including -3°F at Goldstream Creek and a very chilly -10°F near Smith Lake on UAF's North Campus.  The Poker Flat Research Range was even colder at -14°F: not bad for this time of year.

Farther afield, sub-minus 10°F occurred in a number of other remote places, including -17°F at the Jim River HADS and -15°F at the Beaver RAWS.  Unsurprisingly, the Yukon River appears to be frozen over now at Beaver:

Looking west, the Koyuk River is building ice quickly, and over in the Yukon Territory the George Black Ferry was making one of the last runs of the season at Dawson today.

Wednesday, October 18, 2017

Winter Descends

After writing only last week about the lack of a hard freeze in Fairbanks - and indeed the first hard freeze last Wednesday was the latest on record - conditions have suddenly turned more wintry.  The temperature hasn't risen above freezing since Sunday, and there's no immediate prospect of it doing so as the trend remains firmly down for now.

It's possible, though unlikely, that the thermometer will not rise above freezing again in Fairbanks this year.  The last time this happened was in 2008, when the last above-freezing temperature was on October 11; but most years see at least a minor thaw in November or December, and about half of recent years have made it into the 40s in these months.

In keeping with the wintry change, widespread snow showers have brought at least a dusting of snow to much of the interior in the past couple of days.  Yesterday was the long-term median date for establishment of the permanent winter snow cover in Fairbanks, and in view of the chilly forecast it's certainly possible that the snow that's now on the ground could remain until spring.

Here are some webcam views of a hefty snow shower that has passed over UAF's West Ridge in the course of writing this post.


Here's the radar image from 6pm, showing the high reflectivities associated with substantial snowfall rates in the localized showers; Fairbanks airport recorded 1/4 mile visibility at 6:04pm.

The snow showers have been forming within an increasingly unstable air mass, as cold air moves in aloft over a relatively warm land surface.  The three charts below show the Fairbanks balloon soundings from 4pm AKDT yesterday (top), 4am this morning (middle), and 4pm today (bottom); note how the temperature (red) curve becomes steeper in the lower levels while the surface temperature remains nearly constant.

As far as freeze-up across the interior is concerned, webcam views suggest there is not a great deal happening yet, but that should change quickly in the next few days.  Temperatures have not risen out of the teens today in the Yukon Flats, so the apparently open Yukon River at Beaver will soon ice up:

To the east, the Yukon is still open at Dawson, and far to the west, the Koyuk River is still open, and there's open water at Selawik.  I'd expect a freeze-up in a matter of a very few days in the latter locations, given the cold air now working its way down across the state.

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:

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.