Wednesday, November 4, 2015

Cold Forecast

The medium-range forecast is starting to look pretty chilly for interior Alaska, suggesting that true winter conditions may not be far off.  Here are the latest 6-10 day and 8-14 day temperature forecasts from NOAA's CPC, showing about a 60% chance of lower-tercile (significantly below-normal) temperatures in both periods.

Today's computer model runs show the circulation feature that is expected to be responsible for the cold: a big trough over northwestern North America.  The deterministic GFS forecast shows some impressive cold, with 850mb temperatures dropping below -30°C over the far eastern interior by November 15.  The last time 850mb temperatures dropped below -25°C in Fairbanks before mid-month was in 2006 (-26.5°C on the 13th).  But that is one of the more extreme scenarios that might happen, and probably not the most likely.

Here's a nice webcam photo from Dawson, YT, today.  Ice is still moving on the mighty Yukon River, but freeze-up appears to be imminent.  The temperature was +5°F at Dawson airport this morning, but reached -9°F on Sunday.


  1. If we do have -30℃ at 850mb, what is the temp at ground level? When it gets that cold the probability of inversions increases greatly.

    And for American readers, -30℃ = -22℉. The temperature where most vehicles won't start without external heating.

    Also, Richard, I like your recent use of webcams. Helps keep things real.

    1. Eric, I ran a quick linear regression between 850mb temperature and surface daily minimum temperature at this time of year, and it turns out that -30°C at 850mb equates to -31°F at ground level (on average). But a lot would depend on wind and cloud cover.

    2. Was that -31℉ or -31℃? Please say -31℃. Otherwise that's a big cold inversion. (Though like you said, temps would most likely be quite a bit warmer)

      Don't the computer models do ground temps too? I thought I saw something for ground or sea level or at least 1000mb.

    3. Fahrenheit. But the inversion is actually stronger, on average, when temperatures aloft are higher.

      Yes, the models certainly predict 2m temperature, but those forecasts usually have significant biases one way or the other, so they're not very useful without bias correction. It's more common to look at 850mb temperatures to get an idea of airmass temperature.