Severe Thunderstorms

The east-central interior saw a remarkable and rare outbreak of strong to severe thunderstorms yesterday evening, and the NWS meteorologists in Fairbanks had their hands full issuing warnings for the numerous storms.  The first severe thunderstorm warning was issued at 5:47pm AKDT for a storm about 30 miles west of Fairbanks, and 12 more warnings were issued over the next two and a half hours.

Here's the Fairbanks radar image at the time the first warning was issued (click to enlarge).  The storm cell in question is the one just to the north of the Parks Highway (the red line) to the west of Fairbanks.  The storm just to the southwest of that, closer to Nenana, dropped half an inch of rain at the airport there in 22 minutes.

Two particularly severe storms developed a bit later well to the northeast of Fairbanks, near Central, and eventually crossing the Yukon River.  Here are the radar snapshots from 7:08 and 7:24pm: note the very high radar reflectivity values, which are strongly indicative of large hail.  The Weather Service issued warnings for "half dollar size" hail, i.e. 1.2 inches in diameter.

The ingredients for the storms involved very high humidity at low levels and a powerful impulse (widespread lifting/ascent) in association with a strong cold front approaching from the west.  The dewpoint reached 61°F in Fairbanks in the early evening, which is about as high as humidity typically gets in summer there, and Fort Yukon's dewpoint made it up to 63°F.  Low-level humidity greatly adds to convective instability, which is the fuel for deep, vigorous overturning in thunderstorms.

Here's the 500mb analysis at 4pm yesterday.  Note the very sharp trough over southwestern Alaska and the strong pressure gradient to its east, with strong southerly flow between the trough and a ridge downstream over northwestern Canada.

 

That southerly flow and the strong Canadian ridge produced excessive heat over Alaska's far eastern interior, western Canada, and southeastern Alaska: here are maximum temperatures yesterday (click to enlarge).

I imagine Rick Thoman will provide more expansive historical context for the heat wave on his Substack channel, but the 91°F reported by the co-op observer at Eagle is pretty unusual: before 2012 it was about a once or twice a decade occurrence.  Father east, the 95°F at Carmacks is within a degree of the all-time high in 2004.  Canada has been breaking all sorts of heat records this summer.

Finally, here are simple radar animations from yesterday evening: split into two halves for the sake of file size (i.e. the second animation picks up where the first one ends).  What a remarkable event!  I'd be very glad to hear of any reports from those on the ground - feel free to share in the comments.

Strong Storms

After commenting on Friday that lightning activity has been relatively subdued from Fairbanks east so far this season, strong thunderstorms have been popping up in the hills around Fairbanks since the weekend.  The NWS in Fairbanks has issued special statements each day since Saturday to advertise the risk of heavy rain, small hail, and frequent lightning, and as Rick Thoman noted, Fairbanks airport has already reported thunder on 7 days so far this season.  This is tied with 2018 for most on record for so early in the season; the record for an entire season appears to be 1992, with 17 days of thunder.

The most notable storm for some local residents occurred on Saturday afternoon, when intense convective cells moved westward across Eielson AFB just before 5pm.  Here's the radar appearance of the storms at 15-minute intervals on approach to Eielson.

The Eielson ASOS reported 0.78" of rain, including 0.58" in 14 minutes, which is a healthy 2.5"/hour rainfall rate.  The return interval for experiencing such a high 15-minute rainfall amount at Eielson is greater than 50 years, so this was a storm of rare intensity in terms of rain rate.  However, other aspects were not too remarkable: the peak wind gust was 43 mph from the northeast, and the temperature dropped from 75°F to 54°F.  It appears no hail was reported at the airport, which is a bit surprising in view of the intense radar
signature of the storm as it passed over the base.

Fairbanks NWS did issue a severe thunderstorm warning for the storm, and this marks only the third severe thunderstorm warning for the Eielson ASOS location since 2005.  The first was on June 18, 2007, and the second was on June 19, 2017; it's interesting to see these dates falling so close together on the calendar.

Here's an animation showing the evolution of the storm system over a longer period.

Another example of a strong storm occurred the previous day (Friday); this one formed just to the northeast of Fairbanks and dropped south across Chena Hot Springs Road at about 3pm.

It's fascinating (to me) to reflect on the extreme opposites of atmospheric stability that occur in Fairbanks-land at opposite ends of the year.  Six months ago, with effectively zero solar heating, the valley-level atmosphere was profoundly stable, with a near-permanent surface-based temperature inversion.  Now, with daily solar radiation often rivaling locations much, much farther to the south, episodes of intense instability allow these strong thunderstorms to develop.  The atmospheric physics and the sensible weather could not be more different.

Micrometeorology in Fairbanks

Yesterday's hot weather was interrupted in the middle of the afternoon at Fairbanks airport by a quick thunderstorm that dropped almost half an inch of rain.  It was a small and short-lived pulse-type storm, but it happened to occur right around the time of the balloon sounding release, and there were some fortuitous and interesting observations as a result.

The zoomed-in radar image below shows the storm when it was most intense, at about 2:45pm AKDT (click to enlarge).  The red dot and marker indicate the approximate midpoint of the airport; note that the ASOS is located farther southwest, near the southwest end of the airport, and right under the path of the storm as it moved from west to east.  However, the balloon launching site is located near the western perimeter road, to the left of the marker, and on the north side of the storm.  The ASOS reported 0.42" of rain from the storm, while the upper air site measured only 0.10".  I've included below a map that Brian made a couple of years ago showing the locations of the two observing sites.

The simple animation below shows the brief life span of the storm; it moved right over the southwest end of the airport and Fairbanks' official climate site, but just grazed the upper air site.

Now take a look at the sounding from the balloon that was released at 3:03pm, which was just when the storm was moving out and decaying.  Rain-cooled air is evident at the surface, with a temperature of only 69°F, but a very high dewpoint of 66°F.  This is actually the highest dewpoint ever measured at the surface in a Fairbanks sounding (a marginally higher dewpoint was measured in June 2002 just above the surface).

Above the surface we see that the air was much warmer and very much drier, with the temperature rising to 83°F only 35m above the ground.  Clearly the air just above the surface was undisturbed by rainfall; it represents the ambient environment in which the storm formed.  I conclude that the rain had already ceased at the upper air site, and warm dry air had moved back in aloft from the immediate surroundings, but rain-cooled air was still in place at the sheltered sounding release site (which is surrounded by trees).

The 14°F inversion between the surface and 35m aloft is a remarkable occurrence, because it's rare to see any inversion in the Fairbanks sounding at the afternoon observation time in summer.  The sun-warmed surface is nearly always the warmest level in the troposphere, and less than 5% of afternoon soundings have an inversion in June and July.  (This post from 2 years ago showed the typical morning and evening temperature profiles through the year.)  Amazingly, yesterday's inversion was the strongest ever observed at any afternoon sounding from April through September.  Obviously it was a very unusual set of circumstances that produced such a shallow layer of cool air at the surface, and it happened at exactly the right time to be measured by the sounding.  It also didn't last long, because the ASOS temperature was back up to 84°F by 4pm and 88°F by 5pm.  Remarkably, the Fairbanks ASOS reached 88°F twice in one day, with a temporary dip down to 73°F during the brief heavy rain.

Strong Thunderstorms

This is just a quick post to document the strong storms occurring in the vicinity of Fairbanks today, as noted by reader Gary.  The sequence of radar images below, at one-hour intervals, shows the widespread development of thunderstorm cells that showed a tendency to merge into larger thunderstorm clusters.

The radar-estimated total precipitation is above 2" in a number of places and even exceeds 4" west of Fairbanks, which would be enough to cause serious local flooding problems.

The 4pm Fairbanks sounding shows a very high 61°F dewpoint at the surface (see below), although the airport ASOS didn't report anything higher than a 57°F dewpoint - but even so, that's quite a lot of moisture.