Sleet in Summertime?! North Slope Unusual Summer Weather.

 North Slope Mike here.  Summertime is busy time in Alaska.  While we are actively fishing, hunting, camping and enjoying the great outdoors the weather is equally active and can vary drastically over just a few days.  As Rick talked about, yesterday the North Slope was bathed in record heat courtesy of strong southerly flow packed between a strong Bearing Sea trough and a large ridge parked over Eastern Canada, 

However, the topic of this post is about unusual cold weather occurring rather recently this summer.  In particular the night of July 27-July 28th of 2024 was quite exciting. I am a few days behind the weather in the writing about this and I apologize.  If we rewind the weather about a week to 8/1/24 areas on the eastern north slope received accumulating snowfall.  Snowfall in summer is fairly common, in fact Utqiagvik (formally Barrow) has seen as much as 4 inches of snow in the August of 1969.  

So, if it is cold enough at times in summer for snow to fall, what about sleet?  The difference between snow and sleet has to do with temperature dynamics in the lower atmosphere.   While a wet, slushy snow intuitively seems quite probable in summer, sleet however seems rather impossible.  See the definitions for both (courtesy of NOAA):

Snow. Most precipitation that forms in wintertime clouds starts out as snow because the top layer of the storm is usually cold enough to create snowflakes. Snowflakes are just collections of ice crystals that cling to each other as they fall toward the ground. Precipitation continues to fall as snow when the temperature remains at or below 32 degrees F from the cloud base to the ground.

Sleet occurs when snowflakes only partially melt when they fall through a shallow layer of warm air. These slushy drops refreeze as they next fall through a deep layer of freezing air above the surface, and eventually reach the ground as frozen rain drops that bounce on impact. Depending on the intensity and duration, sleet can accumulate on the ground much like snow.

Here is a profile picture to look at that explains this further, again courtesy of NOAA.

As you can see snowfall requires the entire air column in the lower atmosphere to be "cold" or below freezing.  The lower 1000' of the air column can be a few degrees above freezing and thus the observer in this case will see wet, slushy snow instead.  Sleet requires a narrow wedge of warmer above freezing air aloft to melt the snowflakes and then a relatively thick layer of subfreezing air to freeze the droplets into ice pellets.  I've discussed this with the folks at the National Weather service in Fairbanks and they tell me sleet in general is very uncommon for northern Alaska and is almost unheard of in summertime!  This intuitively make sense as the majority of "wintery" precipitation occurs in airmasses that are well below freezing throughout the air column.  Freezing rain is more common in spring and summer as the marine inversion layer over the slope is relatively shallow.  

On July 27th, 2024, rain turned to sleet in coastal areas of the north slope.  This observer is in Kaktovik and reported sleet to the NWS at 1800 hours that night.  Jason Ahsenmacher of the Fairbanks NWS verified my observations and said that the forecast models suggested the possibility of sleet occurring. He was quite excited by my observation and provided me with the following meteogram.  Note, it also hinted at the snowfall Kaktovik ended up seeing on the 1st.

Looking at the nearest temperature soundings in Utqiagvik at that time shows a narrow above freezing layer on top of a "colder" subfreezing layer above the ground.  However, keep in mind that this observation was taken 500 miles away from Kaktovik and it serves to illustrate the warm wedge.  Had this same temperature profile been over Kaktovik at that time the sleet would have been snow due to evaporative cooling.  

You can see the small above freezing wedge of warm air above the subfreezing air layer below it. There was about 1300' of sub-freezing air below the warm wedge.  I even took a photograph for records and submitted it to the NWS.  This sleet was bouncing off my 4-wheeler when it was falling.  

I'd like to thank both Jason Ahsenmacher and Bobby from the Fairbanks NWS for their correspondence and assistance on this post.   Indeed, I'd be curious to see how often sleet occurs in summertime in Alaska.  Mike.