Saturday, January 1, 2022

Cold

It's a cold start to the new year in Fairbanks-land, with an air mass moving through that's worthy of this blog's name, i.e. deep cold.  Temperatures are lower in the hills than at valley level, with nasty wind chills. 

Here are noon temperatures around the area (click to enlarge):


The 3am sounding from Fairbanks reported an 850mb temperature of -28°C or -18°F, but it's closer to -25°F now.


After noting on Thursday the rather extreme cold in the forecast, I did a comparison between the latest GEFS 850mb temperature forecast and the minimum that has been observed in recent decades, based on ERA5 data (1950-present).  It turns out that the predicted 850mb temperatures over the southeastern interior have not occurred since before 2000 (light blue shading in the figure below), and in a few spots not since before 1979 (medium blue shading).


Of course, the comparison may be hampered by systematic bias between the GEFS and ERA5 temperatures, especially over or near high terrain, so we'll have to wait a few days to get the self-consistent ERA5 analysis after the event.

Here's an animation through Tuesday morning.



5 comments:

  1. Cold north of the Alaska Range, but to the south high winds towards Anchorage have caused damage. Here's a link to aircraft flipped near Palmer AK.

    https://www.supercub.org/forum/showthread.php?60307-Hangars-Help-You-Sleep-Better&p=817417#post817417

    Gary

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    Replies
    1. Wow, that's awful. The N-S pressure gradient was enormous.

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  2. Can you help me understand more about the significance of the 850 mb temperature forecast and why these pressure measurements are so important in understanding the weather? Thanks, and I enjoy your blog!

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    Replies
    1. Hi, thanks for the question. Yes, meteorologists think of the atmosphere in terms of pressure levels, ranging from near-1000 mb at sea level to around 100-300mb at the top of the troposphere (where most "weather" happens). Pressure always drops as you go up, of course, because there's less weight of air above you.

      So we often refer to standard pressure levels like 500mb (about the middle level in terms of atmospheric mass), rather than height above ground. If we used height as the vertical coordinate, then all of our maps for the free atmosphere ("upper air") would show major terrain effects, and interpretation would be more difficult. Also, the mathematics of the atmosphere tends to be simpler with pressure as the vertical coordinate.

      For most of the globe, the 850mb pressure level is in the lower atmosphere and correlates well with surface weather conditions. So it's a useful standard level for measuring and forecasting free atmospheric conditions, and unlike weather at ground-level there are no complicating factors from local elevation or microclimate variations. However, in areas of higher terrain the surface pressure is below 850mb (i.e. above roughly 3000-6000 feet depending on latitude and weather), so in those areas the 850mb level doesn't exist.

      Note that the millibar unit is equivalent to hectopascals, so 850mb = 850hPa.

      Let me know if you have follow-up questions.

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    2. As a homeowner in Alaska I use Richards explanation plus the weather service forecast of 850 temps. Mainly if there's cold air overhead it'll soon get cold on the ground. It seems to happen that way often enough to pay attention. In Fairbanks we frequently get colder air close the ground than above in winter. This week's forecast below:

      https://forecast.weather.gov/MapClick.php?lon=-147.761&lat=64.837#.YdNN9S-cYt9

      Gary

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