Alaska Winter of 2010-2011 Review

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Everybody has an impression of how the winter turned out, but what do the numbers say? Was the long-range prediction for the winter I made in November at all accurate?

Well, the impression expressed by most folks here in Haines, in the northern panhandle, is that is was a very cold winter. Looking at the two maps below you can see that, yes, the northern panhandle was quite a bit colder than usual this past winter—one of our coldest winters on record, actually. Furthermore the northern and eastern panhandle was the relatively coldest part of Alaska this winter…relative to each station’s long-term average.

The top map shows departures from the long-term average (or “normal”) for November through March, what I and many like to consider the winter months (for interior and arctic areas I might say “core winter months”). Haines was in the winter bull’s eye, with a winter 3.5F (1.9C) colder than average, with Juneau and Petersburg almost as cold. (I’ve not checked all the stations in the area, so I’m only referring to the sample plotted.) The cold anomaly drops away for the far south and outer coast of the panhandle. Yakutat was above average and you can see a similar positive departure of 0.6 to 1.8F (0.3-1.0C) along the rest of the gulf coast and right up into the Bering Sea to Nome. The northwest coast was dramatically opposite the southeast coast in that it was way warmer than usual. Inland areas of Southcentral and the Interior were average to about 2F (1C) below average.

Nov-Mar_tmp

The next map shows the same thing as above but for December, January and February only. This is so a direct comparison with the D-J-F seasonal forecast can be made. The pattern follows the Nov-March map above but the below normal areas are even colder, especially the interior, while the warm pocket in the NW is also a little cooler. How does is compare with the 3-month outlook put out in November by the Climate Prediction Center? (click here for that post) The CPC map had virtually all of Alaska south of the Brooks range being below normal. I’d call it a good forecast for that area. The missed forecast was of the way-above-normal area on the Northwest slope, with Barrow being the bull’s eye of warmth. A closer look at Barrow’s anomalies is in order for another blog post.

Dec-Feb_tmp

How about precipitation? The CPC predicted neither wet nor dry but “equal chances” of it going either way or staying in the middle. I predicted snowier than usual in SE Alaska and possibly snowier in Southcentral. The map below shows that my snow forecast was a bust. Snowfall was light by a fair amount in most of the panhandle (A little heavy in Petersburg), and a little on the light side in Southcentral. The miss in this forecast was the heavier then usual snowfall in the NW third of the state, with over twice the average in Kotzebue and Barrow. For this blog post I’ve tabulated snow and not precipitation (rain plus water content of the snow). It would be good to check actual precipitation, but it is likely that higher snow meant higher precipitation for the the interior and arctic since virtually all the precipitation is snow in winter in those areas. Coast and southern areas on the other hand often have above average precipitation simultaneously with below average snowfall, since in warmer weather rain falls even in winter. Most of the time is was cold and dry, especially for the northern panhandle, so I think you would find that precipitation was low too. My higher snow forecast in these areas was based on cooler temperatures shifting rain to snow. What appears to have happened is that so much of the winter was spent under the influence of the cold offshore (dry) winds that the precipitation was not frequent enough to boost snow amounts (or rain amounts). Meanwhile in the NW third of the state, winters are so cold that increased warmth almost always means increased moisture.

 

snow

Now the Why

Here’s a look at the wind and pressure patterns that are responsible for the lopsided maps above. First is the 500 mb height anomalies for November-March courtesy of NOAA’s Earth Systems Research Laboratory, Physical Sciences Division. Click on the map to see a larger version. What it is telling us is that at the mid-atmosphere level, averaged over the winter, there was a strong ridge (higher pressure) aligned generally N-S over the Bering Sea, while there was slightly lower than normal pressure (a trough or low) to the southeast as seen in the blue. The flow over this average ridge brings much very cold air to Alaska from Siberia and the high arctic, while also protecting most of the state from low pressure systems. There certainly were Bering Sea storms (remember this is a 5-month average), but the average storm track was shifted west, affecting the west coast the most (referring back to the first two maps reminds us that the west coast was warmer than average). For a good example of a strong storm kept well to the west by this pattern see A Far West Alaska Blizzard from February.

compday.69.162.221.33.137.13.58.35-NOV-MAR-500mb- hgt anomaly

Here is the corresponding sea level pressure anomaly map. The red area is higher than average surface pressure.

compday.69.162.221.33.137.13.59.41-NOV-MAR-SLP anomaly

Winter highlights

November 22-24 freezing rain in the Interior: A rare heavy mid-winter rain fell on subfreezing ground causing severely iced roads and runways, broken tree limbs and downed power lines. Rain fell for 39 hours in Fairbanks for a total of 0.95 inches (24mm) while McGrath had 2.10 inches (53 mm).

December cold snap: Most areas of Alaska except the far west and northwest were 5-10F (3-6C) colder than average with Bettles being likely the coldest with a monthly average temperature of –21.4 F (-29.7C), an amazing 14F (7.8C) colder than the long-term average. There were many –40F to –50F (-40C to –45C) days in the interior. Even in relatively mild (for the interior) Glennallen, there were only two days in the month not below zero F (-18C).

February Super High: The sea level pressure was so high that many aircraft could not set their altimeters to the current setting, because most can not be set to over 31.00 inches Hg (1050mb). The high pressure was not confined to the interior, but was also over 1050 mb over the Bering Sea…a rare occurrence….but it also happened in November. See Pressure Extremes and the Migrating High Winds for more.

April super-storm: The storm that hit Alaska on April 6-8 was a ex-tropical storm, and these are known to pack a punch of wind and especially precipitation. This one went out of its way to keep up the reputation. Most of Southwest AK had blizzard or near blizzard conditions and many had damaging winds. In False pass two homes lost their roofs and several other buildings were damaged. Many stations had record wind gusts and/or snowfall. As the storm moves north and east high winds blew through mountain passes and gaps in Southcentral and coastal areas received heavy snow. Valdez received 31 inches (79 cm) of snow on the 7th and 8th.

Summer

Can a 3-month forecast for the summer have as much chance of usefulness as the winter forecast seems to? Generally summer is less predictable than winter, but also less volatile. I will post on this soon. Check back, and in the meantime I’d love to hear what your winter was like.

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