Monday, June 3, 2019

Intense Thunderstorm in Fairbanks

Yesterday evening the Fairbanks area experienced a very strong thunderstorm system that prompted a rare severe thunderstorm warning from the National Weather Service.  Here's the NWS public information statement from shortly after the event:

"Public Information Statement National Weather Service Fairbanks AK 802 PM AKDT Sun Jun 2 2019 ...Strong Thunderstorm Hits Fairbanks... Fairbanks had a thunderstorm move west across the city between 6pm and 7pm this evening. An NWS Weather Spotter in Hamilton Acres reported half inch size hail, and radar indicated hail of up to one inch diameter. There were numerous reports of pea size hail and frequent lightning in the Fairbanks area. The Fairbanks International Airport had 65 hundredths (0.65) of an inch of rain in a period of 21 minutes, which is one of the highest rainfall rates in recent memory in Fairbanks. There have been about 200 lightning strikes in the Fairbanks North Star Borough since the thunderstorms began around 3pm. As of 745 pm this evening, thunderstorms had moved south and west of Fairbanks and North Pole."

As luck would have it, a very intense part of the storm complex occurred over the international airport, and the ASOS instruments reported extremely heavy rain as well as hail of 3/8" diameter.  I have only been able to find a few instances of hail being reported from the Fairbanks ASOS (May 15, 2012, August 9, 2002, and June 9, 1997), and only the 2002 report included a hailstone size observation (1/4").

The ASOS precipitation data also show that 0.65" of rain fell in 21 minutes, and of that 0.55" fell in 8 minutes, which equates to a rate of 4.1" per hour.  This is a truly remarkable rainfall rate for such a high latitude, and I suspect it may contend for an all-time record for Fairbanks; more investigation would be justified.

According to the NOAA precipitation atlas, the estimated recurrence interval for a 0.55" rain amount is near 200 years for a 10-minute interval and over 500 years for a 5-minute interval, so this event lay somewhere in between.  In the graphics below, the black lines represent the recurrence interval estimates, and the green and red lines show the limits of the 90% confidence interval.

Here's a sequence of radar images showing the evolution of the storm system; its development was really explosive.  It's remarkable to see radar reflectivities of well over 60 dBZ, which is strongly indicative of hail.  This would be more typical of, say, the western Plains in Kansas or Nebraska at this time of year.

Looking at the balloon sounding from Fairbanks just a couple of hours earlier, the standard metrics don't show a particularly notable amount of instability (see below).   However, the northerly winds aloft produced a modest amount of wind shear (favorable for organized thunderstorms), and it was a moist environment.  Moreover, yesterday afternoon the NWS forecasters noted an upper-level wave that was expected to kick off some storm activity:

"Central and Eastern Interior: A shortwave moving to the south over the area will bring some scattered showers this afternoon and evening. The shortwave will be moving over an area with decent instability thanks to relatively moist antecedent conditions. The current GFS is showing surface based CAPE values of 300-500 J/kg across much of the Interior south of Livengood with weakly negative LI values. This swill be sufficient for some isolated thunderstorms across the area. Some thunderstorms may produce small hail or gusty winds."

Finally, here's a simple animation of the radar imagery.  An impressive event, indeed!


  1. The cell that blew up over the airport looks like it came out of nowhere. Really incredible radar loop!