Tuesday, October 28, 2014

Thin Snowpack Survives

The snow cover that arrived in Fairbanks on October 4 failed to melt out sufficiently to bring the official snow depth below 1 inch, and so we can now say the likelihood is very high that the permanent winter snowpack was established on October 4.  This is tied with 1933 for 3rd earliest in the Weather Bureau/NWS era; the only years with earlier onset of permanent snow cover were 1992 (September 13) and 1956 (October 2).

It is an interesting curiosity that the snow depth was reported at 1 inch for 11 consecutive days ending October 23, and this is tied for fifth longest such period (consecutive days at 1" snow depth).  However, it's nowhere close to the record: in 1953, the snow depth remained at 1" for a remarkable 48 days ending December 9.  The chart below shows the years with at least 11 consecutive days at 1" along with the number of days in the period with a high temperature above freezing.  Most often a lengthy spell of 1" snowpack is accompanied by few or no days above freezing, but this year 8 of the 11 days saw the temperature rise above freezing.  However, we should note that the Fairbanks snow depth measurements are now taken (I believe) at the #2 airport location, which is sheltered by vegetation and often runs several degrees cooler than the official temperature site; only 5 of 11 days rose above freezing at the #2 location.

On another note entirely, here's a webcam shot of shallow steam fog over the Tanana River at Nenana early on Sunday morning, with an air temperature of 4 °F at the airport nearby.  The tendrils of mist form when air that has been warmed and moistened in contact with the water surface mixes with colder air a few feet above the surface.  A lot of interesting microscale physics was occurring within that shallow layer of intense temperature gradient!

Monday, October 27, 2014

Sub Zero Temps

Fairbanks nearly recorded their first sub-zero temperature of the season this morning. If they had, the date would have been only 1 day behind schedule. Figure 1 shows the low temperatures this morning. The first 0°F of the season at the official climate site for Fairbanks has occurred as early as October 3rd and as late as November 22. Figure 2 shows the annual date of the first 0°F day since 1904. Finally, Figure 3 shows the earliest 0°F observation for each climate station in the greater Fairbanks area for stations with at least 15 years of observations that extend into the 2000s. Every station has experienced 0°F temperatures before the end of October. North Pole is the winner with a 0°F reading on September 26, 1983.

Figure 1. October 27, 2014 low temperatures from the University of Utah's Mesowest site.

Figure 2. Date of first 0°F temperature for Fairbanks.

Figure 2. Earliest date of 0°F temperature for all GHCN stations around Fairbanks with at least 15 years of data ending no earlier than 2000.

Sunday, October 26, 2014

Point Barrow Freeze-Up

This is just a brief post to note the arrival of widespread sea ice around and north of Point Barrow in the past few days.  The NWS-Anchorage sea ice analysis from Friday showed a large gain in ice cover that connected the shore ice to the Arctic pack for the first time:

It's interesting to observe that cooler temperatures developed quickly at Barrow in the wake of the sea ice formation; the low was 4 °F yesterday, and a significant low-level temperature inversion was observed for the first time since September 16 (see yesterday's 3am and 3pm soundings below).  Prior to sea ice formation, strong surface warming from the adjacent ocean waters tends to produce a steep low-level lapse rate (warm below, cold above) in the lowest levels of the atmosphere, but the heat source is reduced after most of the nearby ocean surface is frozen.  These changes are evident in the climatological vertical temperature profile as discussed in an earlier post here.

Saturday, October 25, 2014

Fairbanks Forecast Performance - Part 2

In an earlier post I began looking at the performance of NWS temperature forecasts for Fairbanks, with a particular focus on whether the forecasts show enough of a "signal" at the end of the short-term forecast period.  On average through the year, the forecast errors at Day 7 are about 20 percent smaller than they would be if the forecast just called for "normal" every day, so the forecasts are clearly useful even out to Day 7.  But do the forecasts show "enough" departure from normal or are they too heavily weighted towards climatology?  The first post showed that the scaling is about right; the NWS forecasts are close to optimal in this regard.

There is more analysis that we can do, however, if we bring in the computer model forecasts and compare them to the NWS forecasts.  For this purpose, I've extracted the GFS and ECMWF computer model forecasts of 850 mb temperature for every day since mid-August 2013 (when I started collecting the data).  The NWS forecasts tend to track with the 850 temperature forecasts, as we would expect, but the following chart shows a hint of something interesting (detailed explanation is below):

The chart shows the average of the Day 7 temperature anomalies (departure from normal) predicted by the two models on the x-axis, and the error of the Day 7 NWS forecast on the y-axis; and the chart only shows days when the model anomalies have the same sign and agree to within 4 °C.  So I've excluded many cases when the models disagreed, because I'm attempting to isolate what happens when the models agree reasonably well.

There is a lot of scatter, of course, and the overall correlation is very weak, but notice the frequency of points above the horizontal zero line when both models expect very cold conditions; the NWS forecast tends to be too warm (not cold enough) in these cases.  On the right-hand side of the chart, there are far fewer cases with comparable warm anomalies in the model forecasts, but in the top five events it seems the NWS was too cold (not warm enough).

My interpretation of the results is that the NWS forecast has a tendency to be too conservative when both of the leading computer models agree in predicting a very large temperature anomaly.  If both models are very cold, then the NWS forecast ought to be lower; and if both models are very warm, then the NWS forecast ought to be warmer.  The conclusion is tentative because of the scatter in the data, but it does make sense: when the two independent models both show a large signal, then this considerably raises the chance that something very unusual will occur; and it seems the NWS forecast anomaly should be amplified accordingly.

For comparison, it's interesting to look at the same charts using the two models individually, see below.  When either model by itself shows a large cold anomaly, there is no obvious bias of the NWS forecasts, although the data on the warm side still suggests an error pattern in the most extreme warm events.

What do I conclude from this analysis?  A general conclusion - and one that is well known - is that having access to independent model forecasts is very useful for assessing the likelihood of extreme events.  This is obviously one justification for running model ensemble systems such as the GFS ensemble forecast, but using a completely independent system like ECMWF provides even more valuable information.

The more specific conclusion is that there is some potential to improve the Day 7 temperature forecasts in Fairbanks when the GFS and ECMWF forecasts are closely aligned in showing a large temperature anomaly.  In other words, the degree of agreement between the models is itself a useful predictor and should be part of the forecast process.  Each model by itself has limited skill at day 7, but when the models line up, then this sends a signal that predictability is higher, and the forecaster would do well to pay attention.

Thursday, October 23, 2014

Disappearing Sun; Barrow Update

Today marks the day when the sun's angle above the horizon at solar noon has declined half way from the equinox to the winter solstice; or equivalently we have traveled three-quarters of the way from the summer to winter solstice in terms of the sun's elevation at noon.

What does this mean?  It means we hereby enter the dark third of the year in the northern hemisphere; and of course this fact is more inescapable the farther north you go.  We can illustrate the lack of solar energy across Alaska in winter by calculating the theoretical solar insolation under clear skies.  From this theoretical standpoint, the total solar energy received in Fairbanks over the next 4 months is less than is received in 6 days in the height of summer.  In Bettles the dark third of the year receives less radiation than in 4 days in summer.  However, in Anchorage the winter sun provides the equivalent of about 10 days in summer.

Here's the view at close to solar noon yesterday from the Alaska Climate Research Center webcam on UAF West Ridge.  The weakness of the sun is illustrated by the fact that some snow remains on the ground despite most days getting above freezing in the past two weeks; the official snow depth in Fairbanks has been at 1 inch for 10 days now.

On another note, Barrow has had a chilly and very windy time of it in the past several days, with a very strong pressure gradient importing cold Arctic air from the northeast.  Here's the surface analysis from Monday afternoon when winds were sustained at about 40 mph for a time.

The high temperature on Tuesday was only 17 °F in Barrow, which is the coldest day so early in the season since 2002.  As we've mentioned many times before, Octobers since 2002 have been extremely warm in Barrow compared to previous decades, and so this kind of chill would have been completely normal in the last century.  For example, the 1930-2000 normal for coldest high temperature to have occurred by October 21 in Barrow was 10 °F; and a high temperature of 17 °F would normally have been observed by October 12.  In 1996, when sea ice was firmly established from Barrow eastward by late September, the high temperature was -7 °F on October 11!

So, it's been a little cooler in Barrow in the past few days - but even this is only approaching normal from earlier decades.  October 2014 is still running well above both the 1981-2010 and 1971-2000 normals.

Wednesday, October 22, 2014

Anchorage Forecast Performance

This is a follow-up to Richard's excellent post on Fairbanks' forecast performance. What I want to focus on is the comparison of the official forecast to both climatology and persistence. Unlike Richard, I have not been proactively saving forecast products. Instead, I utilize the Iowa State text product finder. One of the forecast products is called the State Forecast and represents point forecasts for first-order stations. The product is issued twice daily and includes a minimum and maximum for the next 7 days (morning issuance) and 6.5 days (afternoon issuance). Figure 1 shows a sample State Forecast product.

Figure 1. State Forecast product issued by the Anchorage NWS Office on October 22, 2010.

In Figure 1, you will notice a series of minimum, maximum, and precipitation probability forecasts. Unfortunately, only the Anchorage and Juneau offices issue State Forecasts; hence, our analysis will focus on Anchorage (I know, not very Deep Cold). A choice must be made as to whether to use the morning or afternoon issuance. In this case, we used the afternoon forecast product, which effectively gives us a 6 day forecast. This option provides the shortest time window for assessing the Day 1 forecast (best case scenario) and makes the day-to-day comparison more meaningful. It also allows for a baseline to conduct a persistence forecast comparison. On the negative side, it eliminates Day 7 as a forecast period.

Forecast temperatures vs. Actual Temperatures

When looking at days 1 through 6, we see decreasing skill in the temperature forecast. This is not surprising, as we would expect this result for every forecast issued anywhere in the world. The question is how much value do we get from the forecast compared to another method. In Figures 2 and 3, we see the 2009-2013 difference between the forecasted temperature and the actual temperature for Day 1 through Day 6. Figure 2 is a summary by year and Figure 3 is a summary by month.

On both charts, we see decreasing skill the farther out in time we get; however, the actual forecast always exceeds the no skill forecast (climatology). The most dramatic forecast skills are in the winter months when the skill for Day 1 through Day 3 is especially high. A noticeable drop off is observed by Day 4. We see in Figure 2 that on average, the Day 1 forecast provides 3°F of improvement versus climatology and the Day 6 forecast provides 1.3°F of improvement.

A caveat regarding 2009 and 2013 in Figure 2. Those years had very large temperature anomalies and so the forecast skill for those years suffered accordingly.

Figure 2. Difference (absolute value) between forecasted temperature and measured temperature for Day 1 through Day 6 in Anchorage, Alaska, between 2009 and 2013. All months are aggregated for each year.

Figure 3. Difference (absolute value) between forecasted temperature and measured temperature for Day 1 through Day 6 in Anchorage, Alaska, between 2009 and 2013. All years are aggregated for each month.

Forecast temperatures vs. Climatology

If you had no access to television, radio, or the Internet, one option for generating a 6-day forecast is to predict that each day will be exactly normal; i.e., just use the numbers from the NCDC normals table. As it turns out, climatology appears to be a factor in the NWS forecast. As a forecaster, you would feel comfortable predicting that Day 1 is 20°F above or below normal based on the numerical models is the situation was warranted; however, the comfort level with a forecast of 20°F above or below normal for Day 6 is much reduced. Therefore, the forecast is tempered somewhat by trending it toward climatology. Figures 5 and 6 show the difference between the forecasted daily temperatures and the NCDC published temperature.

Looking at Figure 4, the Day 1 forecast is slightly more than 5°F different than the published normal temperature. However, by Day 6, the forecasted temperature is 3.7°F difference than the published normal temperature. In Figure 5, we see the breakdown by month. In every month and in each year, the forecast trends toward the climatological daily normal.

Figure 4. Difference (absolute value) between forecasted temperature and the NCDC published normal temperature for Day 1 through Day 6 in Anchorage, Alaska, between 2009 and 2013. All months are aggregated for each year.

Figure 5. Difference (absolute value) between forecasted temperature and the NCDC published normal temperature for Day 1 through Day 6 in Anchorage, Alaska, between 2009 and 2013. All years are aggregated for each month

Which Forecast is the Best?

So how do the point forecasts for Days 1 through 6 compare to a no skill forecast? For this analysis, we add a second type of no skill forecast called persistence. This is where you forecast that the temperature tomorrow will be the same as the temperature today. This can be extended all the way out through Day 6. When we do this, the results are shown in Figure 6.

We see that over the course of out 5-year period, the NWS forecast for the Anchorage International Airport is off by slightly more that 2°F (see Figure 3 for an NWS forecast breakdown by month). This is 1.1°F less (better) than a strict persistence forecast and 3.3°F less (better) than a climatology forecast (see orange line in Figure 3 for a climatology breakdown by month).

By Day 3, the climatology forecast catches up with the persistence forecast. Looking out to Day 6, the NWS forecast sill exceeds climatology by 1.3°F. The maximum differential between the NWS forecast and the no skill forecast is at Day 3.

Due to high variability of temperatures from year-to-year, it is impossible to assess the relative forecast improvement over this short time period. However, there is a large skill improvement when using the NWS forecasts for temperatures as compared to the alternatives.

Figure 6. Difference (absolute value) between actual temperature and three forecast methods for Day 1 through Day 6 in Anchorage, Alaska, between 2009 and 2013.

Monday, October 20, 2014

Fairbanks Forecast Performance

For some time I've been meaning to take a look at the long-term performance of the National Weather Service temperature forecasts for Fairbanks, and particularly with one question in mind: do the forecasts show enough variance at the end of the short-term forecast period, i.e. 5-7 days in the future?

The question is motivated by the idea that sometimes the computer models indicate a pronounced temperature anomaly from about a week in advance, but the early NWS forecasts for the same time show only a small departure from normal.  A recent example was seen in the early October cold spell, when the ECMWF and GFS deterministic forecasts of September 29 both showed a notable cold anomaly in place by October 5, but the NWS forecast for the high temperature on October 5 was 38 °F, only 3.6 °F below normal.  In this case, as time went on and the forecast became more certain, the forecast dropped and the observed high temperature was 31 °F.  However, there are many cases when the computer forecasts are badly wrong from 7 days out, and so it is entirely justifiable for the official forecast to show only a small anomaly at longer lead times.  Indeed, it would be most undesirable for the raw model forecast to be reflected in the official outlook, because the numbers would often swing wildly from day to day.  The question is, does the NWS have the right balance?

It's possible to answer this question using a history of NWS forecasts that I have collected for Fairbanks airport since November 2011.  First, here is the basic "skill" of the forecasts for lead times of 1-6 days, i.e. the forecasts for "tomorrow" through "6 days from now".  Averaged over all seasons, the average error of the high and low temperature forecasts is similar and rises from just over 4 °F to nearly 8 °F over the six days.  Not surprisingly, the errors are much larger in winter, but it is interesting to see that the winter low temperature forecasts improve more significantly at shorter lead times, whereas the winter high temperature forecast error remains over 7 °F even for "tomorrow".

Here's a similarly-formatted chart showing the bias of the forecasts, i.e. the mean difference between the forecast and the observed temperatures.  Negative values indicate that the forecasts were too cold on average.  We see that the winter high temperature forecasts have been several degrees too cold on average in the past 3 years, even at shorter lead times, but the bias is much smaller for the low temperatures.  It would be interesting to investigate this further in search of a possible explanation.

Let's now consider the scaling of the temperature forecasts.  I've examined this by calculating the mean absolute error (MAE) that would result if the NWS forecast anomaly (departure from normal) were multiplied by values ranging from 0 to 2.  On the low end of this range, the forecasts would deviate very little from climatology and the forecast would just show normal values each day; but on the high end, the forecasts would show greater deviations from normal than they currently do.  The chart below shows the results of this experiment for day 7 temperature forecasts from all seasons of the year.

The data from the last 3 years show that (on average through the year) the high temperature forecasts are perfectly scaled at day 7, i.e. there is no way to improve the MAE by arbitrarily reducing or increasing the forecast anomaly.  We conclude that the NWS shows just the right amount of variance on average in the day 7 high temperature forecasts; this is not to say that we can't improve on any given forecast using additional information, but we can't reduce the error by simply adjusting the departure from normal across the board.

The day 7 low temperature forecasts are not quite optimally scaled, according to these results, as the NWS shows marginally too much variance.  In other words, the forecasts would be marginally (but only very slightly) better if they showed smaller departures from normal.

There is one other aspect of the problem that interests me, and that is whether we can show that the forecast variance is too small when the computer models show a large anomaly (as opposed to any size anomaly) and/or when the computer models agree with each other.  I'll return to this idea in a subsequent post.

Thursday, October 16, 2014

Barrow Temperature Update

The midpoint of October has been reached, and the mean temperature so far this month in Barrow is 25.4 °F, which is right in the middle of the pack in terms of the "new normal" since 2002.  Of course it is well above normal relative to the climate in former decades.

The chart below shows the daily mean temperature anomalies in Barrow between August 1 and November 30, for 2014 (black line) compared to the past 12 years, which were all very warm in September and October.  As we noted here, the anomalous warmth in recent years has tended to peak at the end of October; we can see that daily temperatures were above the 1981-2010 normal almost all the time in the second half of October in the past 12 years.  This year we've been following the same script since mid-September, and with sea ice still a good distance offshore, there seems to be no reason to expect a sudden change (see the ice analysis below, courtesy of the NWS in Anchorage).

Tuesday, October 14, 2014

Snow Pack Onset

Fairbanks has reported at least 1" of snow on the ground every day since October 4th. Today makes 11 consecutive days with measurable snow depth. However, the total for today was down to 1" and the forecast for the next few days suggest that it might dip below 1". If that occurs, the snow pack onset date will the date when the snow depth exceeds 1" again and stays that way for the rest of the season. Figure 1 shows the snow pack onset date for Fairbanks.

Figure 1. Date where snow depth stayed at or above 1" for the remainder of the season.

In is not unheard of for snow to be measured for at least 10 days and then drop below 1" before being reestablished some number of days later. Here are the instance when it occurred:

In 1919, at least 1" of snow was on the ground from October 1st through October 11th (11 days), but then the snow went below 1" for all but one day until October 29th.

In 1940, at least 1" of snow was on the ground from October 20th through October 30th (11 days); the snow pack was established for good on November 8th.

In 1949, at least 1" of snow was on the ground from October 9th through October 24th (16 days); the snow pack was established for good on October 31st.

In 1951, at least 1" of snow was on the ground from October 6th through October 25th (20 days); the snow pack was established for good on October 27th.

In 1955, at least 1" of snow was on the ground from October 7th through October 22nd (16 days); the snow pack was established for good on October 24th.

In 1972, at least 1" of snow was on the ground from September 29th through October 16th (18 days); the snow pack was established for good on October 19th.

In 1981, at least 1" of snow was on the ground from October 4th through October 14th (11 days); the snow pack was established for good on October 27th.

Monday, October 13, 2014

Arctic Ice Minimum - Part 2

This is a follow-up to last month's post on the seasonal Arctic sea ice minimum, which occurred on about September 17 based on spatial extent.  It's worth looking also at estimates of sea ice volume, thickness, and age, to see how this melt season compared to recent years.

First, volume: the University of Washington's Polar Science Center uses a numerical model to estimate Arctic sea ice volume based on various observations and calculation of known physical processes.  There is obviously a good deal of uncertainty in the volume estimates, but the results are a lot better than nothing.  The chart below shows the monthly mean volume estimates for April and September since 1979 (blue and red lines) along with the difference, i.e. the estimated volume of melt from April to September.

We see that the estimated volume in September increased this year for the second consecutive year and reached a level similar to 2009.  However, the volume remains very much below the volume of earlier decades, and the volume in April 2014 was comparable to the lowest values of recent years.  The relatively small difference between April and September this year indicates that the amount of melting this summer was significantly smaller than in recent years; in fact the melt volume was less than the 1981-2010 average.  It will be interesting to see if this translates into any recovery in April ice volume next spring.

The PIOMAS estimated ice thickness is shown in the following chart from U-Washington.  It's interesting to note that the estimated thickness this summer was considerably higher than in the past four years, which are closely clustered together at the bottom of the chart.  If the model is correct, this suggests that ice thickness remained very low during the 2013 recovery in ice extent; and but this year has seen a recovery in ice thickness, although total extent remained similar to 2013.

Finally, the estimated sea ice age compared to last year, courtesy of NSIDC:

The extent of 3+ year-old ice appears to be similar to last year, but there was more second-year ice this year in September.  This indicates that a significant fraction of last year's first-year ice (formed in the winter of 2012-2013) survived through its second summer, and this appears to be perfectly consistent with the volume and thickness changes shown above.

Saturday, October 11, 2014

Freeze-Up Progress

After a much colder start to October (first 10 days) than in the past five years, freeze-up is getting under way on area rivers a little early this year.  Through October 10, Fairbanks airport has seen 37 freezing degree days, compared to a 1981-2010 median of 9.5 by this date.  The total of over 3 times the normal for the date sounds like a significant anomaly, but owing to the typically rapid drop-off in temperatures at this time of year, it is only 4 days ahead of normal.

Here are a few webcam images of freeze-up progress at various locations, starting with today and going back about a week:

Tanana River at Nenana today:

Koyuk River from Koyuk today:

Koyuk River on Thursday Oct 9:

First widespread ice on Teshekpuk Lake, close to the Arctic coast, on Tuesday Oct 7:

 Teshekpuk Lake the next day, Wednesday Oct 8:

First ice cover on the lake at Inigok, between Umiat and Tesh Lake, last Saturday Oct 4:

And finally, a grainy shot of Toolik Lake (near the haul road just north of the Brooks Range) freezing over last Friday Oct 3:

Here's a chart showing the lake and air temperatures during the freeze-up of Toolik Lake.  I'm not sure of the depth of the temperature sensor in the lake, but it shows nicely how the temperature stabilized at about 2 °C in tandem with the freeze-up.  No ice was evident on the lake until this temperature threshold was reached, but since freezing began there has been only a tiny amount of additional cooling.  This nicely illustrates the heat exchange processes that are so important in the freezing of fresh water lakes around the world.

Friday, October 10, 2014

Fairbanks Temperature Trends - Part 3

In a couple of recent posts (here and here) I looked at changes in mean seasonal temperature in Fairbanks since 1950 and compared the changes to several nearby stations and to conditions at 850 mb.  Reader Gary asked if changes in surface winds might have contributed to the surface temperature changes; this is a plausible idea because wind over the Alaskan interior in winter disturbs the semi-permanent surface-based inversion and raises temperatures near the surface.

Unfortunately for this study, I'm not at all confident that historical surface wind speed measurements were made accurately and consistently enough at Fairbanks to allow an investigation of long-term trends; part of the problem is that the averaging time and measurement height of surface wind observations can vary widely.  However, balloon soundings from Fairbanks have measured wind speed aloft since 1948 and - in my view - are more suitable for this kind of analysis.  In earlier decades the balloon's position was tracked with radio direction finding or radio navigation aids, and now of course it's done with GPS.

To begin with, I interpolated the balloon-measured wind vectors to various heights above ground, so that a consistent average could be obtained for each height.  This is necessary because the sounding data is not reported at standard heights except for various "mandatory" pressure levels such as 850 mb.  After interpolating, and excluding any soundings from non-standard reporting hours, I calculated the December-February mean wind speed at each height for each winter since 1950-51.  See the chart below.

The black line shows the mean wind speed at 850 mb and suggests that the average wind speed at that level may actually have decreased since about 1975.  However, at each of the lower levels there was a significant jump in mean wind speed in the winter of 1990-1991, if the data are to be believed.  Interestingly the wind speed has trended down since then at the lowest levels (100-300 m AGL), but at 500 m the winter wind speed has remained elevated.

It's not quite clear what to make of these results.  It certainly looks as if low-level wind speed and mixing have increased over time in Fairbanks, although the 100 m wind speed has dropped back in recent years to levels similar to pre-1990.  The puzzling aspect is that 1990-91 does not show up as being particularly significant in the temperature time series (see below), although the inversion was certainly weaker than normal that winter.  With apparently higher mean wind speed throughout the 1990s, we would expect to see a jump in temperatures, but actually it was colder than in the decade following the 1976 PDO shift.

Here's another view of the change in wind speed at 500 m: the chart below shows the changing frequency of wind speed categories.  The long-term trend and especially the change after 1990 is quite striking.  It is tempting to blame all of this on changes in instrumentation or perhaps the vertical density of observations (which could affect the interpolation calculation), but neither of these changed significantly in the vicinity of 1990.

More investigation will be required to examine the physical causes of the apparent change in 1990 and to establish whether it is reflected in any other meteorological data.  As a first, simple step, I plotted up the differences in sea-level pressure, 500 mb height, 850 mb temperature, and sea surface temperature between the pre- and post-1990 winters (see below).  Lower pressure over western Alaska and more southerly flow over the interior is certainly consistent with warmer temperatures, greater mixing, and increased low-level wind speed, but it's not yet clear if the weather pattern changes were in any sense tied to 1990.

Wednesday, October 8, 2014

Rapid Autumn Transition

The hardy residents of the Fairbanks area are used to rapid seasonal changes and dramatic temperature swings, but the change in the past 3-4 weeks has been remarkable even for the usually volatile interior climate.

As we noted here and here, a strong chinook event raised the temperature to 76 °F at Fairbanks airport on September 14, which was close to the warmest on record so late in the season, but only 3 weeks later there was 4 inches of snow on the ground with a high temperature of 31 °F.  Given that the peak normal high temperature in early July is 74 °F, we might call this a transition from summer-like to wintry conditions in only 3 weeks.

Is this a record for rate of transition?  Based on these specific criteria (last 75+ °F day, first sub-freezing day with 2+" of snow on the ground), yes.  See the chart below; click for a larger image.  It's interesting to note the break from the relatively uniform results of the past 18 years.

We can show a similar result for Keystone Ridge.  The highest September temperature of 66 °F did not quite reach the peak normal for summer, but the daily mean temperature on the 13th was above peak normal for summer.  Looking at the time from last 65+ °F day to first wintry day (same definition), we again see a transition that was shorter than in recent years (only 16 days this year).

We don't yet know for sure if the snow cover in Fairbanks is the permanent winter snowpack (although it's looking increasingly likely), but if it is, then we would tie the record for shortest time from last 70+ °F to arrival of the snowpack.  The record is 20 days and was set in 1974 and 1992.  The record for shortest time from last 75+ °F to snowpack is 25 days, set in 1965.

Tuesday, October 7, 2014

First Sub-Zero Day

It appears that the first sub-zero observation of the season occurred today at several locations. The RAWS station at Norutak Lake (west of Bettles and south of the Brooks Range) reported a morning low of -2°F. While RAWS data is not always reliable, the Ruby 44 ESE USCRN station was also below zero (-1°F). This value is certainly bullet-proof. Figure 1 shows a screen capture of the low temperatures in the western interior this morning. The -2°F at Norutak Lake is circled. (The Stoney, Alaska, RAWS station may have been even colder but they are not reporting at the moment)

How does this date compare to other seasons? It is actually later than the long term average. Last year was the latest on record but the 1st of October is more typical. Figure 2 shows the date of the first occurrence of sub-zero temperatures in Alaska since 1950. Data extends prior to 1950 but due to the sparsity of stations, statewide assessments are difficult to make.

Finally, the area around the Brooks Range is a common location for the first sub-zero temperature to be measured. Figure 3 shows the location of the season's first sub-zero observation since 1950, If a tie occurred, the station with the lowest temperature was mapped.

Figure 1. Low temperatures in the western interior on the morning of October 7, 2014. Data courtesy of the University of Utah's Mesowest site.

Figure 2. Date of first sub-zero reading in Alaska from 1950-2014. Values obtained from GHCN v.3 and GSOD.

Figure 3. Location of first sub-zero reading of season between 1950 and 2014. If more than one station was below zero on a date, the lowest reading was selected.

Monday, October 6, 2014

Early Snowpack

Two episodes of persistent light snow in Fairbanks over the weekend brought the first significant valley-level accumulation of the season, and the observed snow depth stood at 4" last night at the airport.  This is the earliest occurrence of a 4" or greater snow depth since the great September snowfall of 1992; and it has only been observed this early in 4 other years in the Fairbanks history since 1930.

What are the chances that this snow will remain on the ground at the official Fairbanks climate site throughout the winter?  The chart below shows the frequency with which snow has melted out to a trace or less after a given date, for snow depths of 1" or greater (green line), 2" or greater (light blue), and 3" or greater (purple).  I required at least 10 instances in the 1930-2013 history to calculate a frequency, so the frequency is not available until e.g. October 12 for a snow depth of 3" or more.

According to the long-term climate history, a snow depth of 1" or more on October 5 is more than 60% likely to melt out completely later in the month.  But what about a snow depth of 4" or more - presumably this is less likely to disappear?  Perhaps; but we don't have many past cases to judge from.  The chart below shows the evolution of the snow depth in the weeks after the 5 previous years in which the snow depth had reached 4" or more by October 5.  Interestingly, the snow melted out to a trace or less in 3 of the 5 years - in 1972 a snow depth of 8 inches on September 30 melted out by October 17, and in 1957 a snow depth of 4 inches melted out twice before October 20.  The early snow in 1981 was followed by a snow-free period of 12 days.  However, the early snow of 1956 and 1992 survived.

In summary, the history of previous early "heavy" snows in Fairbanks suggests that it is not uncommon for warmth later in October to melt out the early snow.  However, if 2 inches or more is still in place a week from now, then the odds are against a melt-out after that point.

Update October 7: reader Mike asks if unusually warm temperatures are responsible for the melt-out of an early snowpack, or if a return to normal conditions is sufficient to melt early snow.  The chart below shows the daily temperature anomalies in the 5 years referenced above.

We see that below-normal temperatures remained in place for most of October in 1956 and 1992, which explains the survival of the snowpack, but in the other 3 years the anomalies turned warm in the second half of October (and remained warm for much of November).  In 1972 and 1981 the disappearance of the early snow cover occurred just after the arrival of unusual warmth on about the 13th, so we can definitely blame the flip to warm weather in these cases.  However, in 1957 the early snow melted by October 5, when temperatures were near normal, so this was just the calendar at work.

The apparent preference for relatively warm conditions in November is interesting and mirrors an analog signal I discussed last autumn for early snow years; it seems to be fairly common for early wintry weather in Fairbanks to be replaced by relatively mild early winter conditions.

Mike also asked about the relationship between snow onset dates in Fairbanks and Juneau.  Time doesn't permit a more detailed examination now, but a quick scatterplot of the dates of first 2" snow depth indicates that there is probably no correlation.  In 1953, Juneau received its first significant snowfall (8", no less) 37 days before Fairbanks!