Saturday, November 30, 2013

Mid-November Wind

According to a News-Miner article on Wednesday, an anemometer atop the GVEA administration building in Fairbanks recorded a peak wind gust of 83 mph during the November 13-14 storm.  While the measurement is not comparable to the official history of wind measurements from the airport, the report is impressive and led me to wonder how the airport observations in the recent storm compared to past events.   The chart below shows the annual maximum wind speed in Fairbanks according to the GHCN daily data; note that various time averaging periods were reported at different times in the history.

The highest 5-second or greater wind speed in the GHCN data is 59 mph on April 18, 2003, but this report appears suspect to me because the 2-minute maximum for the same day was only 12 mph, and there does not appear to have been anything unusual about the weather situation that day; I excluded the report for the chart above.  Ignoring the 2003 event, the official measurement of 55 mph on November 14 is the highest on record for a 5-second period or greater; higher "peak gust" values were reported in July 1990 and June 1997.  The recent 2-minute report of 41 mph also narrowly exceeds the peak 1-minute values in 1970 and 1974.

It's also interesting to examine the annual values of the peak wind speeds in winter - see the chart below.  Note that I have excluded March here, because average wind speeds pick up considerably in March (see second chart below) and I thought it would be most valuable to look only at the deep winter months with similar wind behavior.  Interestingly, in 3 of the last 4 winters (including this one to date), the peak 2-minute wind speed has been at the upper end of the historical distribution - and so it appears we can say that Fairbanks has had a run of unusually strong wind events in recent winters.  The behavior of peak wind speeds in April through September (the "less calm" season) does not show any similar trend (third chart below).

Thursday, November 28, 2013

Thanksgiving Records

Here are the daily temperature and precipitation records for Alaska on Thanksgiving day.

Wednesday, November 27, 2013

Thanksgiving Climatology

Since tomorrow is Thanksgiving, it is a good time to see how Thanksgivings of years past have fared for Alaska's three largest cities. The following charts show the high and low temperatures for Thanksgiving day (gold bars) as well as any daily snowfall (cyan). The normal maximum and minimum lines are based on the 30-year climate normal period that corresponds to the date in question. The lines appear staggered because Thanksgiving occurs on a different calendar day each year. The normal values represent the value on the day that Thanksgiving fell on that year. (Note: snowfall measurements, particularly for Anchorage, are frequently missing in the early years shown on the charts)

Tuesday, November 26, 2013

Persistent Warmth

Six months have now passed since the dramatic late spring reversal in Fairbanks temperature anomalies, from persistently below-normal to mostly above-normal (see chart below).  Since late May, unusual warmth has been quite persistent, with only brief interruptions to the prevailing trend.  Remarkably, over 40 percent of the days since May 24 have been at least 1 standard deviation warmer than normal, while only 14 of 185 days have been an equivalent amount below normal.  The distribution of anomalies is shown in the second chart below.

The immediate cause of the persistent warmth is the upper-level ridge that has repeatedly redeveloped over southern Alaska and northwestern Canada, as shown below in the six-month mean 500 mb height anomaly.

[Update: per reader Eric's request, below are the corresponding 500 mb height maps for the prior 10 years: 2003 through 2012, in order.]

Monday, November 25, 2013

Winter Rain Climatology

In view of recent freezing rain in Fairbanks, and also in many other Alaska locations, it's interesting to look at the history of winter rain events in Fairbanks.  Of course, the phenomenon is very unusual because of the difficulty of getting warm enough air into the Alaskan interior in winter, but when it does occur it's very problematic.  As Rick wrote elsewhere, "Freezing Rain in Fairbanks is a serious pain. Because the ground and road surfaces are well below freezing from October through March, any rain, even if it falls when air temperatures are above freezing, can cause severe travel problems. Significant amounts of rain will usually produce significant ice accretion on roads that persists, at least on some lower traffic roads, until late March or early April."

The chart below shows the annual number of significant winter rain or mixed rain/snow events (November through March) since the winter of 1917-18. The cluster of events in recent years is evident (Nov 2010, Jan 2013, Nov 2013) and has resulted in an up-tick in the 15-year trailing average frequency. However, there have also been periods in the past when winter rain occurred with higher than normal frequency, notably in the 1960's, in the 1920's, and in the remarkable winters of 1935-36 and 1936-37, when rain occurred five times. If anything, it appears that winter rain has become somewhat less common over the past century, although I am not sure if the trend is statistically significant.

Winter rain in Fairbanks is most common in November and least common in December and March, as the chart below shows. It is curious that significant winter rain has never been observed between November 25 and December 23 inclusive; it's not clear if this might possibly be a statistical quirk, but the sudden cessation of rain events in late November does look as if it represents a true feature of the Fairbanks climate. From Christmas on, the distribution of rain events seems fairly random.

Sunday, November 24, 2013

Freezing Rain in Anchorage

Unfortunately, Anchorage was greeted with a freezing rain event just like Fairbanks was a week earlier. According to the Anchorage NWS office, there was at least 0.16" of freezing rain. In one hour, 0.08" of freezing rain occurred. That was the most since 0.11" fell in one hour on 11/3/2004. The driving was extremely difficult to say the least (see photo of overturned pickup truck).

For the winter of 2012-2013, it was the 4th calendar day where the Anchorage International Airport reported freezing rain. The seasonal average is 6 days. The chart immediately below shows the count by season for the last 18 winters.

In looking at the events over the previous 18 winters (that's how far back the easily obtainable hourly observation go back via the NCDC website), the coldest that freezing rain (actually freezing drizzle) was observed was 15°F on 3/5/2012 (at 3 p.m.). Conveniently, this is the exact time of an upper air sounding. The chart below shows the sounding at the time of that observation as well as last Friday's event sounding. Interestingly, the 2012 sounding it is very similar to the Fairbanks sounding from their freezing rain event last week. The chart below shows the sounding from Friday's freezing rain event in Anchorage and the one on 3/5/12. The 2012 profile looks eerily similar to the Fairbanks sounding from 11/13/13. Fortunately the 2012 event in Anchorage produced very little in the way of accumulations and the associated travel difficulties.

Thursday, November 21, 2013

The Climate Below (Ground)

The ground beneath our feet is an important part of the biosphere. What happens in the soil directly affects the weather and climate in the atmosphere above us. From water storage to heat storage, the soil is an often overlooked component of the earth's biosphere.

There are several soil monitoring stations around Fairbanks but the Bonanza Creek Experimental Forest site ( 64.699030N 148.258742W) is the one I check on the most frequently. The FP1A station is approximately 15 miles southeast of Fairbanks along the Tanana River. The site description is as follows: "FP1A is located on a low early successional terrace (1.8 - 2 m above winter low river level) adjacent to the Tanana River. Vegetation establishment occurred in 1982 to 1983. Significant bank erosion has occurred since the establishment of the study area, resulting in loss of some study plots."

The chart below shows the daily normal temperature (°C) for sensors at 0 cm, 5 cm, 10 cm, 20 cm, 50 cm, 100 cm, and 200 cm between 1989 and 2012 (Note: these are depths below the organic layer at the surface). The small black squares are the November 21st 2013, readings. The color of the square corresponds to the color of the line (the 0 cm and 5 cm boxes are nearly on top of one another). As you can see, the soil temperatures are far above normal for this time of year. In fact, if you look at the table below the chart, the 11/21/13 readings were the highest on record (for the date) for 5 cm and 50 cm sensors during the 25 years of observations.

I am interested is seeing if there is a correlation between ice measurements along the Tanana and the subsurface temperatures.

Dark in Barrow, -50 F in Southeast Interior

Continued cooling over interior Alaska has produced the season's first reports of -50 °F today in the southeast; here are some of the colder observations:

-51 F  Bolio Lake RAWS
-50 F  Chicken COOP
-50 F  CW4591 automated station near Tok
-49 F  Marguerite Creek near Healy 13NE
-48 F  Tok #2 COOP
-47 F  Northway airport
-46 F  Tok COOP
-41 F  Eielson AFB

According to the GHCN data, it is somewhat unusual for -50 F to be reported this early in the winter in Alaska; it happens once or twice a decade on average.  The earliest observation of -50 F was on November 4, 1945, at Allakaket (-53 F).

In the far north of Alaska, the polar night has now reached Barrow, where the sun set three days ago for the last time this year.  The FAA webcam image below shows the southern horizon today at close to solar noon.

Wednesday, November 20, 2013

Low-Level Refrigeration

The break-up of low-level cloudiness over Fairbanks yesterday allowed lower-atmosphere temperatures to drop smartly from mid-morning on, with the result that the surface-based inversion was pronounced by early this morning (see below).

This morning's low temperature of -31 °F, which may yet be broken before midnight tonight, is slightly unusual for this early in the season; the -30° mark is reached by this date in only about one quarter of years - and less often since the 1970s.  More unusual is today's high temperature of only -22 °F; cold of this degree has occurred this early in the season in only six years since 1930.  Interestingly, some years (six of them since 1930) never produce a day this cold throughout the entire winter.

Across the border in Dawson, upriver from Eagle, today's high temperature of -41 °F was within one degree of the coldest on record for so early in the season, with historical records back to 1900 or so.

Minus 10°F Climatology

With the onset of classically cold Alaska temperatures, I though it would be a good time to put up a map of the number of seasonal occurrences of days with temperatures of -10°F or lower. This map only uses winters since 1950-1951. Stations with 10+ winters of good data during that time period were included. A table with selected values is included a the end. Note: a winter with more than 15 missing observations was excluded.

Tuesday, November 19, 2013

Deepening Cold

This morning's dawn view from Arctic Village features mostly clear skies, some light ice fog, and -42 °F; see below, courtesy of the FAA webcam.  This is the first -40 observation I've seen in Alaska this winter, though there might have been others I missed.  Across the border in the Yukon Territory, the town of Faro reached -44 °F, and Watson Lake reached -41 °F.  Historical data for Watson Lake go back to 1939 and reveal that this morning's chill is close to the coldest on record for so early in the season, although the cold snap of early November 1989 was colder (-46 °F on Nov 12).

Monday, November 18, 2013

Cloud Warming

Valley-level temperatures in and around Fairbanks are being held above the precipice of radiative cooling (so to speak) by low clouds, as seen in the infrared satellite image below: relatively dark shading in parts of the southern and western interior indicates relatively warm temperatures at the top of low clouds.  In northern areas, a mix of high and low clouds is also keeping temperatures from dropping as low as they otherwise would; the cold temperatures (white shading) in the satellite image show areas of cold high cloud in the northeast, not surface-level temperatures.  However, skies are generally clear in the southern Yukon Territory, and there temperatures have dropped to the -20s F and colder.

The low-level clouds have destroyed yesterday morning's surface-based inversion in Fairbanks, as shown in the two soundings below, and so Fairbanks airport has warmed by 15 °F despite continued cooling aloft.  When the clouds break up in the next day or two, the inversion will return and valley-level temperatures will drop to their lowest levels of the season.

Friday, November 15, 2013

Running Snow Total

If the snow forecast for Fairbanks verifies over the next 48 hours, the 30-day running total will exceed the 30-day normal total. That is actually a somewhat unusual occurrence in the last several years. Since the winter of 2007-2008, the 30-day running snow total has been below normal for 70.6% of the time in the months of November through March. By comparison, the 30-day running snow total for Anchorage has been above normal 58.4% of the time in the months of November through March. Note 1: the 30-day snow deficit of -10.9" on 11/9 was the largest during the time period in question. Since this is the snowiest time of year, it is not surprising that the largest deficit was recorded during this time frame. Note 2: the daily snow normals are from the 1981-2010 climate normal period.

Real Cold Coming

A major change in the weather pattern over Alaska is underway, as flow from the north is about to bring serious cold over most of the state.  The charts below show two of the leading computer model forecasts for Tuesday afternoon; the color shading indicates temperature departure from normal at 850 mb, and the dotted lines show the 500 mb height.  Both models show a cold upper-level trough extending from northwest Canada into interior Alaska, and below-normal temperatures.  The agreement is fairly good, although the ECMWF solution is more extreme, showing 850 temperatures more than 15 °C (27 °F) below normal.  At this time of year, that's cold.

Thursday, November 14, 2013

Freezing Rain Physics

Wednesday's freezing rain in Fairbanks provided a fascinating example of how the atmosphere can sometimes produce weather phenomena that defy expectations and even bend the "rules" of atmospheric physics; so I thought it would be worth trying to explain why the event was so unusual.  After all, freezing rain was predicted to occur during the winter storm, so why is this now a surprise?

The key to understanding Wednesday's events is to recognize that precipitation can form either with or without the involvement of ice crystals.  If ice is not present in a cloud, then precipitation can form by a "warm rain" process in which cloud droplets grow by condensation from water vapor, and then the growing cloud droplets coalesce with each other through collisions and become drizzle drops (~0.1 mm in diameter) or eventually rain drops (~1 mm or greater).  This is obviously the only way rain can occur in shallow tropical clouds that are entirely above freezing.  It also explains the formation of drizzle in shallow cloud or fog layers, often observed over the ocean.  This process can also occur at temperatures below freezing, and in fact ice may be completely absent in a cloud even at temperatures well below 32 °F because the freezing process is not automatic (there is a substantial "activation energy" required); so ironically the "warm rain" process can occur at temperatures well below freezing.

The other mechanism to create precipitation involves the ice phase and occurs in mixed phase clouds. When both ice crystals and cloud water droplets are present, then the thermodynamics of the situation dictate that the ice crystals grow at the expense of the water drops, leading eventually to snow crystals that either reach the ground as snow or melt if they fall through above-freezing air either near or above the ground.  This mechanism of precipitation is by far the most common in middle and high latitudes, where ice is generally present in precipitating clouds.  Note that rain formed by this process is simply melted snow; the rain drops have passed through the solid phase.

How do we know if ice will be present in a sub-freezing cloud? This is a difficult question, but obviously makes a big difference to the outcome.  There is a general rule of thumb that I was taught: for cloud-top temperatures around -10 °C, the chance is about 50 % that the cloud is ice-free.  Warmer than that, the chance of ice is lower; colder than that, it's higher.  Note that some liquid water is generally present in clouds down to -40° or occasionally even lower.

Now we can consider yesterday's winter storm in Fairbanks.  Snow was expected in the early stages of the event, because temperatures would be entirely below freezing above the ground, and cold cloud tops would ensure ice phase precipitation.  Forecasters also saw the potential for above-freezing air to reach Fairbanks later in the event in a layer above the ground, which would melt the snow falling from above and create rain; however, with the temperature still below freezing in a shallow layer at the surface, the rain would re-freeze at the ground, i.e. "freezing rain".  Finally, colder temperatures at the end of the event would turn the precipitation back to snow.

Unexpectedly, however, freezing rain occurred along with light snow for several hours near the beginning of the event, followed by straight snow - in other words, the progression was the opposite of what was expected.  The NWS was kind enough to put up a balloon sounding at 9 a.m., during the freezing rain, and therefore we know the following (see the second chart below):
  • The atmosphere was entirely below freezing above Fairbanks, so there was no melting of snow to create rain.  The rain was not formed by the mixed phase process.
  • The cloud top was at about -12 °C, which would not necessarily dictate the presence of ice, but we know that the cloud was mixed phase, because snow was observed along with the freezing rain.

Given these two facts, it is truly remarkable that rain occurred, because standard cloud physics tells us that the snow crystals ought to have grown rapidly at the expense of liquid water droplets in the cloud, thus removing water drops and producing straight snow.  This is normally an efficient process - except for yesterday!  A possible explanation appears to be that there were simply too few ice crystals to prevent the water drops from growing by the "warm rain" process.  The few ice crystals that did form grew efficiently, forming snow, but there was plenty of water vapor left over to allow liquid drops to become large as well.  As for exactly why the ice crystal population was so sparse, that would be a worthy question for academic investigation.

Below are the balloon sounding observations from Fairbanks yesterday.  In retrospect, what should we learn from this event?  First, it's important to watch the cloud top temperatures, because cloud top temperatures of around -10 °C or warmer (e.g. 9 am and 3 pm soundings) raise the possibility that the cloud may not contain ice; in this case freezing drizzle is a distinct possibility, and as we now know, freezing rain is not impossible.  Second, it is now clear that "warm rain" and mixed phase precipitation processes can occur in tandem; just because ice is present doesn't mean that water droplets will be prevented from growing.  I don't think I'm mistaken in saying that virtually all meteorologists think of these processes as "either/or", but in Fairbanks yesterday it was undoubtedly "both/and".  For me, at least, this is a valuable lesson.

Wednesday, November 13, 2013

Rapid Warming over Fairbanks

There will be much more to come, but here's a quick post with the temperature plots from the upper air soundings from the Fairbanks Airport on Wednesday. Happily, there was an extra release at 9am:

As you can plainly see, there was no above freezing layer at all: even at 3pm the entire sounding was still below freezing. Notice the lowering "lid" of the warm air during the day. The freezing rain early Wednesday morning was perhaps the result of the deep layer with temperatures -4 to -7C, a prime set-up for super-cooled water droplet formation. The cloud tops were cold enough (-15C or so) to support ice crystal growth, but my speculation is that there was not enough upward vertical motion to support much crystal growth. Once that process started, then the freezing drizzle turned to snow and produced 2-4 inches around the area by 3pm.

Record Moisture Over Bethel

This morning's 3 am balloon sounding from Bethel measured an atmospheric profile with higher precipitable water (total column moisture) than ever observed before in November (1.10" vs 0.98" in 1991 and 1970).  Remarkably, today's precipitable water measurement also matches the record for October (1.10" in 1970).  The chart below shows the lower atmosphere profile: note the extremely warm saturated layer up to 6000 feet and freezing level above 10000 feet.

The eastward-moving surge of warm moist air is bad news for the interior, as it translates into a freezing rain event for some.  Freezing rain is already being reported as mixing with snow at Fairbanks airport, though it's not clear if this is correct as the 3 am sounding showed temperatures still far below freezing throughout the column.

Tuesday, November 12, 2013

Seasonal Temperatures Return

After many weeks in a row of well above normal temperatures, a brief respite of normal to slightly below normal temperatures have returned to much of Alaska. For Anchorage, 39 of the previous 41 days have been above normal. For Fairbanks, 44 of the previous 48 days have been above normal. As the charts clearly indicate, there is a much greater correlation between the temperature deviations of Anchorage and Fairbanks (R-squared = 0.72) than with Juneau (R-squared < 0.04) .

Sunday, November 10, 2013

High Tides

A large tidal surge is currently affecting western Alaska. Just after midnight a surge of nearly 7 feet was measured in Nome. According to the NWS Fairbanks AFD, waves have topped the sea wall but no damage was reported.

Saturday, November 9, 2013

Freezing Rain in Anchorage

* Updated on 11/10

Today was a terrible day to drive anywhere in Anchorage. There was a brief period of snow, the first measurable snow of the season, followed by several hours of freezing drizzle/rain. Since this was the first event, many people have not put their winter (studded) tires on their cars.

What was interesting is that while freezing drizzle was being reported, the upper air sounding for Anchorage reported below freezing temperatures from the surface all the way to the stratosphere (orange line on plot below). Apparently the sounding missed a nose of warm air at about 700 meters as the Glen Alps cooperative station was reporting +2.2°C at the time of the sounding.

* Update. Note the dramatic increase in atmospheric temperatures on today's 12Z sounding.

Friday, November 8, 2013

Strong Inversion

You know it's winter in Fairbanks when…the RAOB shows a better than 10C surface based inversion and the warmest temperature in the sounding is more than a kilometer above the ground: here's Friday morning's temperature trace from the 3am AKST sounding:

 This is reflected in the surface observations, with 5am temperatures ranging from 14 below in Goldstream Valley to 10 above at some of the personal weather stations in the hills around town. The very shallow inversion also being reflected in the hourly observations at the Airport, with temperatures changes of as much as five degrees per hour in an otherwise very quite night, with clear skies, faint aurora and no large scale winds.

Wild Weather Ahead

A volatile weather pattern is in place across the northwest Pacific and Bering Sea, with a series of storm events affecting particularly the west coast of Alaska but also inland regions.  Computer model forecasts indicate that Fairbanks should see a significant amount of snow in the next week, and temperatures will once again rise far above normal at times.

The chart below shows a time-height cross-section of temperature in the lower atmosphere over Fairbanks, as predicted by a recent computer model forecast; the second chart shows the expected precipitation during the same period, and the third chart shows expected surface temperature (note that the exact values for precipitation and surface temperature should be taken with a large grain of salt).  For several days now, the forecasts have been showing above-freezing temperatures not far above the surface during the upcoming precipitation event for 12 hours or more beginning around midday tomorrow (Saturday).  A likely outcome appears to be snow in the morning hours, changing to freezing rain in the afternoon, then back to snow after midnight.  Temperatures may rise above freezing in the hills, leading to straight rain, but I would think temperatures should remain below freezing at valley locations.  Regardless, with ground temperatures now far below freezing, it will be a messy situation for a time, especially if rain occurs steadily for some hours.

Very chilly conditions return again by Tuesday, but there are indications of another major warming and precipitation event later in the week.  Fairbanks could soon be looking at a healthy snow pack, as an impressive amount of precipitation is being shown by the computer forecast.  Note however that snow is very unlikely to be as heavy as the model is indicating; the November record for total precipitation in Fairbanks is only 3.32" (1970).  The climatological median total precipitation for November is 0.79".