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Wednesday August 8, 2018

Assignment: Predict the 24 hr precipitation amount (inches) for Juneau, AK (PAJN)

In Juneau, the state capital of Alaska, the summer is usually fairly dry, and then the precipitation increases sharply in the fall. According to Monday night's National Weather Service forecast discussion for Juneau, it seems that the summer-to-fall transition could be happening suddenly this week: "A fall like weather system will affect the Panhandle over the next 72 hours. A stream of moisture tropical in origin has set the stage for an atmospheric river and heavy rainfall to impact Southeast Alaska beginning with Yakutat and slowly drifting southeast through Thursday. Rainfall during this timeframe has potential to be record breaking." The MOS forecasts are for several inches of rain, so let's forecast Wednesday's 24-hr accumulated precipitation for Juneau and see how much they get.

Question: What do you think is an "atmospheric river"?

What Happened?

Well, the rainfall total at Juneau was an unimpressive 0.5 inches, much less than all of the models expected. Other sites received heavier amounts -- Yakutat and Sitka had 1 or 2 inches or more and there is a flood warning active for areas near Juneau, but there were not the record amounts mentioned in the earlier forecast. Precipitation amount is often the toughest variable to predict.

An atmospheric river is a long, narrow band of of very moist air aloft that can occur in many parts of the world and can sometimes produce extreme precipitation events because of the persistence of rain in one area over an extended period of time. One atmospheric river that sometimes forms from near Hawaii to the west coast has been given the nickname "The Pineapple Express".

Forecasts for Juneau, AK (PAJN)
Forecaster Precip24, inches (error) Comment Day Score