Windless Winter

Reader Gary made a comment late last month about unusually heavy hoar frost in Fairbanks and a lack of wind all winter.  Presumably the two are related, as any significant wind would tend to disturb delicate frost formations and also enhance sublimation.  Naturally I got to wondering about what the data show for the wind speed this winter as compared to prior years - how unusual is the lack of wind?

The chart below shows the number of days in each winter on which sustained wind speeds of at least 5 knots (blue) and 10 knots (red) were observed between November 1 and February 8.  The discontinuity in 1996 is definitely related to the introduction of ASOS instrumentation, so the numbers are not directly comparable between the pre-1996 and post-1996 years; but the longer-term history is interesting nevertheless.

Remarkably, this winter since November 1 has seen only 3 days with a wind speed report of at least 10 knots, and only 17 days of 5-knot winds or greater; both of these are record lows.  Put another way, the wind speed has remained below 5 knots for every hourly observation during 83 of 100 days since November 1.  The peak sustained wind speed since November 1 is only 11 knots (on January 1), which is also a record low.  Therefore based on the data in the hourly observations, it would seem that no winter since 1950 was so lacking in wind as this winter (through February 8).  (We should bear in mind, though, that we can't be sure about the comparison to pre-1996 years.  The late 1960's appear to have been relatively windless.)

The next question is - obviously - why has this happened?  It's surprising, because we normally associate calm conditions with unusual cold in winter - but of course it was extremely warm until late January, and the inversion strength was generally a bit weaker than normal.  The mean pressure has been a little higher than normal (see below) over much of the state but does not suggest a major reduction in the normal north-south pressure gradient.  I quickly looked at the 850 mb wind speed and it has been slightly higher than normal since November 1, so we can't blame the overall flow pattern.  Thus we have a mystery that is worth investigating more carefully in a future post.

*** Chart below added by Brian on 2/10 at 8:45 p.m. ***

Figure. Eielson AFB wind speed between November 1st and February 10th. 

Obviously there are a few outlier years. The winder of 2010-2011 has no days with calm winds and a lot of Missing ("M") observations. Most likely, those are calm wind observations. The other years 1996-1997 to 2003-2004 are a little more uncertain. 1997 was when the ASOS was installed so that explains part of it. If we ignore the anomalously high years, 2014-2015 appears unremarkable.

Yukon Quest Temperatures

The Yukon Quest sled dog race kicked off this morning in Whitehorse with temperatures close to -30 °F at the start and with colder temperatures awaiting the mushers down the trail in the first couple of days.  The race is notorious for cold, wind, and other difficulties, so this is no surprise, but it's interesting to look at how temperatures have varied in the past several years.

The charts below show the range of temperatures observed at four locations along the trail during the period each year that the Yukon Quest was running, based on the start date and the winner's ending date, since 2001.  The range of conditions is very large, of course, and even within one race period (10-12 days) it's typical for temperatures to vary by 50-60 °F.  Temperatures above freezing are quite frequently observed during the race in the southern Yukon Territory (e.g. Carmacks), but -40° or colder is not unusual at the interior locations (Eagle, Dawson).

Based on this limited sample over 14 years, Dawson is by some margin the coldest of the 4 locations in terms of mean temperature; but even this is only slightly below zero F, which is (I'm told) a rather pleasant temperature for running dogs.  The data show that truly extreme cold is actually not the typical condition - and thus there's no basis for a claim like that of Wikipedia, that "temperatures commonly drop as low as -60 °F").  A cold race, yes - but not absurdly so.

Upper Level Pattern Length

Last year I began looking at how to quantify the length of temperature patterns at the surface. It seems like when the temperature goes above normal, it stays that way for a while. Conversely, cold patterns tend to hang around a while too. Unfortunately, what happens at the surface does not alway reflect what is going on synoptically. Just this week, Anchorage has stayed quite cold even though an above normal airmass moved overhead. At the surface, the airmass change went nearly unnoticed. Therefore, I decided to use 850 mb temperature changes as a proxy measure of airmass changes. There are certainly other metrics as well. We could looks at 500 mb or 300 mb winds or height changes. For now, those will be left for another day.

There has been discussion in the scientific literature over the last few years about changes in jet stream "blockiness." A blocky pattern should be reflected in longer duration of upper air patterns. Once again, because the data is handy for me, I will look at the Anchorage International Airport balloon soundings (1948-2014). As we noted last week, the temperature at 850 mb in Anchorage has been steadily rising. Therefore, we need to compute daily normal temperatures and standard deviations based on 30-year climate periods. Figure 1 shows the normal 850 mb temperature for Anchorage.

Figure 1. Normal temperature at 850 mb for Anchorage International Airport. Four 30-year periods are shown.

How do we decide if an above normal or below normal pattern is present? For this study, I arbitrarily decided that if 6 out of 7 days are in the upper tercile of temperatures (>= 0.43 standard deviations above the mean), all 7 of those days are part of a "warm spell." Conversely, if 6 out of 7 days are in the bottom tercile of temperatures (<= -0.43 standard deviations below the mean), all 7 of those days are part of a "cold spell." This is a "gut feeling" definition but will suffice for now. Figure 2 shows a sample of my Excel calculation output. Note the columns titled "Above" and "Below." They denote warm spells and cold spells in December 2013. In that month, there was a 9-day warm spell and an 8-day cold spell based on the aforementioned criteria.

Figure 2. Excel screenshot showing calculation of warm and cold spells.

Now for the fun part. Has there been a change in the length of these patterns since 1948? The answer is yes and no. Let's look at a chart first. Figure 3 shows the length of all warm and cold spells since 1948. Since the minimum length of a warm or cold spell is 7 days, that is the minimum value on the y-axis. It is difficult to discern patterns from all the dots; therefore, I added trend lines to the data.

Figure 3. Length of time for all warm ad cold spells at 850 mb in Anchorage. Each dot represents the end point of a period.

At the beginning of the balloon record, warm spells and cold spells each lasted 12 days on average. Over the next 67 years, the length of cold spells dropped slightly to 11.5 days and the length of warm spells increased by 25% to an average length of 15 days. This is a quite dramatic increase in my opinion. If we look at it on a decade basis, we see that the patterns are quite consistent on that time scale. Figure 4 shows the average length of warm and cold spells by decade.

Figure 4. Length of time for all warm ad cold spells at 850 mb in Anchorage grouped by decade.

Not only are temperatures rising at the 850 mb level in Anchorage, but the average length of a warm period is increasing. What is most interesting is that the increase in the length of warm spells is not coming at the expense of the cold spells.

Anatomy of a Cold Spell

Regular reader Eric asked about the upper-level flow pattern associated with the late January cold spell and how it contrasted to the earlier persistently warm pattern.  We'll start with a few maps (see below) showing the 500 mb height pattern and anomalies in November (top row) and December (middle row), and for January 1-22 (bottom row).  The basic flow pattern was very persistent, with a strong ridge over western Canada and eastern Alaska creating a zone of southerly flow to its west that transported warm air up to Alaska.  The 500 mb heights were particularly anomalous in November and early January.

500mb Height	500 mb Height Anomaly

We can contrast this flow pattern with the events that unfolded in late January.  The charts below show the daily mean 500 mb height and sea-level pressure beginning with January 22, which was two days before the temperature dropped below -20 °F for the first time in Fairbanks with this cold snap (but Huslia reached -51 °F on Jan 23).  Fairbanks saw minimum temperatures of -40 °F or lower January 26-30.

500mb Height	MSLP

Obviously the upper-level trough was primarily responsible for bringing cold air into interior Alaska, but as so often happens the surface temperatures did not really drop off until cloud cover dissipated in response to rising pressure and subsidence aloft.  The chart below shows the evolution of temperatures at 850 mb and at the surface, as well as the hourly cloud cover observations.  Interestingly the 850 mb temperature bottomed out right when the sky cleared and the surface temperature dropped late on the 25th; by the time the surface temperature reached -40 °F, the temperature aloft was already on the rise.  This provides a very nice example of the inverse correlation between cloud cover and inversion strength.

Alaska Winter Weather Advisories: Part 2

** Note: I posted a nearly identical post focusing on the Lower 48 on my other blog. **

Slightly over a year ago, I wrote a post about the number of winter weather forecast products issued in Alaska between 2009 and 2013. This is an update of that post using data through 2014 and adding several more categories. Additionally, there are both Experimental and Operational categories that were not fully captured last year – now they are.

Introduction

In 2005, the National Weather Service (NWS) made a significant change to the way they issue watches and warnings. They introduced the VTEC coding system to standardize the procedure for issuing these products. The VTEC code allows various software packages to quickly decode the forecast product and display it on a map or transmit the information to subscribers and the public. Figure 1 shows a sample forecast product with the VTEC code outlined in red. The VTEC line notes that it is operational, a new issuance, from the Fairbanks NWS Office, a winter weather advisory, and the start/end times.

Figure 1. Winter Weather Advisory issued by the Fairbanks, Alaska, NWS office on January 1, 2015. The VTEC is outlined in red.

The University of Iowa generously logs all NWS forecast products from every office in the country. Their archive site is quite handy for analysis of historical data. Unlike many data archive sites, they are perfectly happy to let you grab data using your own scripts and other tools. They even provide some sample Python scripts to use. Thank you Daryl Herzmann!

Winter Weather Products

In this analysis, I am interested only in winter weather products (not Dense Fog Advisories, High Wind Warnings, etc.). Each NWS Office is responsible for all forecasts within their zones. As most readers of this blog know, there are three Forecast Offices in Alaska – Fairbanks, Juneau, and Anchorage. Figure 2 shows the NWS Office boundaries and forecast zones for Alaska.

Figure 2. Map of all NWS Office boundaries and forecast zones in Alaska.

Local Criteria

With the exception of Blizzard Warnings, the criteria for every winter weather product is defined by the local NWS office. For example, the Wind Chill Advisory criteria for most of Alaska requires wind chill value of -40° or lower with a sustained wind of 15 mph or higher for three or more consecutive hours. In southern Florida, a wind chill of +35°F is sufficient for issuance of a Wind Chill Advisory. Therefore, southern Florida has seen more Wind Chill Advisories than most of Alaska! Another post from last year shows the criteria established by each Alaska Forecast Office for several winter weather products.

Maps

In this section we present nine (9) maps of winter weather advisory products. For all maps, we use the 2009-2014 time period. The reason for this is because significant changes were made to the suite of forecast products in 2008. For example, Snow advisories and Freezing Drizzle Advisories were consolidated into Winter Weather Advisories. Due to this change, and others, I thought it best to start with 2009 instead of parse out the pre-2009 data. Please keep in mind that these are forecast products. No attempt has been made to assess the accuracy of the forecasts. When looking at the Lower 48 inset, remember that each forecast office down there has their own criteria too. Therefore, a comparison of the number of advisory and warning products is an apples to oranges comparison – but it is fascinating nonetheless. Also keep in mind that the legend categories were chosen to encompass all values in the U.S., not just Alaska.

And here are the maps .....

Figure 3. Combined number of Frost Advisories, Freeze Warnings, and Hard Freeze Warnings between 2009 and 2014.

Figure 4. Number of Freezing Rain Advisories between 2009 and 2014.

 Figure 5. Number of Ice Storm Warnings between 2009 and 2014.

Figure 6. Combined number of Wind Chill Advisories, Wind Chill Warnings, and Excessive Cold Warnings between 2009 and 2014.

 Figure 7. Number of Winter Weather Advisories between 2009 and 2014.

Figure 8. Number of Winter Storm Warnings between 2009 and 2014.

Figure 9. Number of Blizzard Warnings between 2009 and 2014.

Figure 10. Total number of winter precipitation-related advisories (Winter Weather, Lake Effect Snow, and Freezing Rain) between 2009 and 2014.

Figure 11. Total number of winter precipitation-related warnings (Winter Storm, Lake Effect Snow, Ice Storm, And Blizzard) between 2009 and 2014.

There are far too many patterns in the maps to mention. The patterns are a result of climatology, forecast criteria, and forecaster judgement. Is there anything that surprises you?

Note: While I attempted to accurately portray the data depicted above, I cannot guarantee it is error-free.

Canadian Wind Chill

Chilly weather is continuing in the Alaskan interior for now, but a consideration of current conditions about 1500 miles to the east provides some interesting perspective.  This morning the town of Baker Lake, Nunavut, reported sustained winds of 40 mph along with a temperature of -38 °F, which equates to a wind chill of -82 °F.

What's remarkable is that Baker Lake is close to sea-level (elevation 18 m), in a region of relatively subdued topography, and is about 35 miles south of the latitude of Fairbanks.  It is doubtful whether any low-elevation sub-Arctic observing site in Alaska has ever observed such a combination of cold and wind - though I imagine it may occasionally happen in regions of terrain-channeled flow in the southeast.

Here's this morning's upper-air sounding from Baker Lake (courtesy of U-Wyoming) and a table of hourly surface observations (courtesy of Weather Underground).  Amazingly, the wind chill is lower at the surface than at 500 mb.

Will the PDO Influence Return?

Yesterday was the fifth day in a row with a minimum temperature of -40 °F or below in Fairbanks.  This is the longest such sequence since December 2012, which saw 9 straight days of -40 °F temperatures.  The Fairbanks record (1930-present) for longest such sequence is 18 days in January 1965 and January 1971 (the latter month also produced the all-time state cold record at Prospect Creek).

It is of interest to note that the PDO phase is still strongly positive as it has been throughout the winter so far (see below).

Is it therefore a surprise that colder than normal conditions have emerged over Alaska this month?  Wouldn't we expect a positive PDO episode of such magnitude to prevent deep cold from developing over Alaska?  Perhaps - but this is so far only a brief cold spell by historical standards, and this month's mean temperature will still end up above normal in Fairbanks.  Moreover, as we discussed back in December, the PDO influence on Fairbanks temperatures is smaller in January than in early or late winter.  I don't know why this is so, but it's clear in the summary statistics I showed before and in the monthly scatterplots shown below.  Note that for these plots I computed the monthly temperature anomalies with respect to the contemporary 30-year normals.

The historical data reveal that the PDO typically reasserts itself in February, with the scatterplot showing a considerably better correlation than in January.  Given the magnitude of the current PDO anomaly, we would therefore NOT expect February to remain cold overall in Fairbanks.  Of course, there are always exceptions - for example, February 1936 was the 10th coldest February in Fairbanks but also had the second most positive PDO index for the month.  Perhaps this year will be a similar outlier - indeed the first 10 days of the month certainly look cold - but if cold persists throughout the month, then it will be counter to the long-term PDO correlation.