Friday, March 23, 2018

Late Winter Snow Showers

My visit to Fairbanks ended on Monday with a rare March instance of strong westerly breezes, adding further to the tally of "strong" westerly wind days in recent weeks (see here and here).  While it's typical for winds to pick up in late winter in Fairbanks, the seasonal breezes are usually from the northeast; in the past 20 years, there have only been 2 other days in March with similar winds (20-30 mph) from the western half of the compass.

In this week's event, the wind was associated with a strong cold front aloft; the 500mb chart below shows the trough axis near Fairbanks at 4pm AKDT on Monday.  Note the temperature of -39°C reported above Fairbanks; by Tuesday morning the 500mb temperature was down to -43.5°C, which was very nearly the coldest air of the entire winter at this level.


However, even as the cold air moved in aloft, temperatures at the surface were up near the freezing point on Monday afternoon.  The chart below shows the full sounding from 4pm on Monday; the (red) temperature trace shows a strikingly steep vertical temperature gradient, indicating an environment with potential for some convective overturning.


In response to the increasingly unstable environment, heavy snow showers emerged on Monday afternoon and moved through the Fairbanks area.  See below for a sequence of radar images at hourly intervals from 2pm to 7pm AKDT.







Interestingly a similar round of showers occurred on Tuesday afternoon as surface temperatures made their way up to the freezing point again, while very cold temperatures persisted aloft.  Tuesday afternoon's temperature difference (42.6°C) between the surface and 500mb was one of the greatest on record for March in Fairbanks and was actually the largest on record for so early in the year.  Here's a Tuesday afternoon radar image:



Reader Bill suggested that it's not uncommon to see heavy snow squalls like this in late winter in Fairbanks, but without going through a long-term archive of radar data, it's difficult to say what the frequency actually is or whether it might be changing over time.

8 comments:

  1. We got snow showers on Tue. around 4:30. It turned into sleet, then hail. Sent in an observer note to Alaska Weather and Dave Snider said it was grauple, soft hail or snow pellets that form when supercooled water droplets are collected and freeze on falling snowflakes. Never seen that before. Photo posted at: smilesfromnowherre.blogspot.com/2018/03/hail-all-hail-to-spring.html

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    1. Thanks for the comment and photos - I was pelted with something similar while unloading the rental car at the airport on Monday night.

      Graupel is thought to be key to thunderstorm electrification, and summer storms are filled with graupel aloft, but the pellets often melt before reaching the ground at that time of year.

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    2. After I read your reply, thought maybe I was wrong on the date since it seemed unlikely that it'd be "graupeling" two days in a row . But the date on the photo was Mar 20, so no matter where you go, there you are.
      In other news, I think I'd mentioned a while back that my wife runs a weather station for her former boss, who's down in Idaho now. Not sure what he does with the data, but I finally plotted some up, kind of interesting... http://smilesfromnowherre.blogspot.com/2018/02/weird-temps.html

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    3. Yes it appears it was two days in a row... although it was nearly midnight Monday night for me at the airport.

      Thanks for the data comparison - some interesting aspects, for sure. I might have expected more differences in the daily max temps. What's your elevation at the house?

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    4. We're at about the 650 contour if the topo maps are right. The USGS lists the gage datum of the Little Chena River site as 464 ft. But that may refer to the point of zero flow (PZF); the elevation at the gage house is probably around 500 ft according to the topo maps.

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  2. To get a rough estimate, you don't need to slug through a million radar images. You have radiosonde profiles that can give an indication of instability which is needed for the squalls. So calculate the threshold needed and see when the instabilities exist. Since it doesn't happen that often, you will have a much smaller dataset to look through.

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    1. Yes - but decoding and plotting the radar scans for individual events is still quite a task (I'm not aware of an archive of imagery). Radar is great for operational monitoring but difficult to query in an automated fashion.

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    2. I did once briefly look into decoding radar data a little while ago. Your right in that it's not as easy as you would hope. I did find some tutorials online to help, though. From what I've seen, it would be easiest to search for "github weather radar" and use someone else's scripts with data from other places.

      https://www.ncdc.noaa.gov/data-access/radar-data/radar-decoding

      https://www.ncdc.noaa.gov/data-access/radar-data/radar-display-tools

      Or spend money and use a service that has the radar data all bundled up.

      Separate from this, I think looking at the instabilities would be interesting in itself. We could see if there is a spike in March/April and also the thunderstorm activity.

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