As reader Gary noted in comments, the initial round of rainfall was convective in nature with storms forming to the southeast and drifting west-northwest. A steady rain then followed for many hours. Here are simple radar animations for two consecutive 90-minute periods.
We've noted before that easterly winds aloft are relatively more favorable for precipitation in summer than in other seasons, although a westerly wind regime is still the most favorable. In the recent event the 700mb wind direction was close to easterly but not particularly strong; there was a weak pressure gradient between a low to the southwest and a weak ridge to the northeast. Here's the 500mb analysis from 3am AKST on Sunday morning.
Update June 15: here's Saturday's 4pm AKDT sounding from Fairbanks. It appears the balloon was actually released at 3:04pm, which was exactly one hour before the ASOS first reported lightning. The sounding shows very little instability, but forced lifting during the subsequent hour could well have transformed the temperature profile into something more unstable. Another possibility is that higher low-level moisture was available nearby but was not sampled by the balloon. Given sufficient moistening at low levels, the sounding could become very unstable.
Where can I access those types of radar feeds?
ReplyDeleteHere:
Deletehttp://vortex.plymouth.edu/nids.html
Note that the year menu does not have 2016, but you can get 2016 images by editing the URL manually after hitting "click here to make the map".
The identifier for Fairbanks is APD, see others here:
http://weather.rap.ucar.edu/radar/
What would likely have caused the snap-crackle-pop of the initial thunder and lightning. Three of us were watching it approach and one noted cloud to cloud and then later we saw some ground strikes south of Fairbanks.
ReplyDeleteMy previously cleaning the plane windows was the cause suggested by the others...doing the insides as well made it all more probable.
Gary
The reason I ask is that the approaching clouds didn't appear to be the traditional Cb's with the extensive vertical development we typically experience in Interior Alaska.
DeleteGaty
Gary, I added the pre-storm sounding and a new paragraph at the end of the post. The short answer is, it's not clear what allowed instability to develop - but develop it clearly did. I suspect there was a shortwave moving in aloft that created enough deep ascent to cool the air aloft and create instability. It doesn't take much lifting to bring about dramatic changes in vertical stability profiles.
DeleteInteresting observation, Gary - perhaps the cloud base was high? Perhaps the instability was indeed created aloft rather than being caused by excessive surface heating.
DeleteSee my reply #16 in the previous thread for reported cloud bases and surface temperature. As noted it was almost overcast mid-day and then went to Few>Scattered>Broken at 55-6000'. There was a brief interval of solar heating/few clouds prior to the thunder rolling in from the SE.
DeleteTypically we see relatively stable buildups first (N>E) followed by slow movement over Fairbanks. When I looked at the Sat IR when I got home the Copper River Basin showed lots of cloud cover and an airmass that appeared to be moving towards the NW over Delta Jct and on to Fairbanks. Your radar shows the NW tails of that stuff.
It was an interesting event and the soaking rain that soon followed squelched the lightning...so I assume it was leading edge weather that involved instability and lifting and cooling of humid air.
Gary