When you have an isolated thunderstorm, how can you tell that it’s not a volcanic eruption?
Brandon Aydlett, the Science and Operations Officer (SOO) at the NWS forecast office in Guam (where the National Weather Service day begins) relays the following message:
Yesterday we received a call from a local radio noting something suspicious in IR imagery over the Philippines. Of course, it looked eerily like a volcanic eruption, of course, in a volcanically-active area
What steps might one take see if something is/is not a volcano? Consider the image below that the radio personality saw, from 1820 UTC on 2 April 2025. What is that isolated feature over the island of Samar in the central Philippines? If you saw that very cold cloud top, how might you decide if it’s a thunderstorm vs. a volcano?
NOAA-20 overflew the region shortly after 1700 UTC on 2 April, and a toggle between the Day Night Band imagery and the I05 infrared (11.45) imagery, below (from the NASA Worldview site), shows a very faint cloud signature (the waxing crescent moon was providing no illumination!), but a faint circulation can (just barely) be discerned in the Day Night band imagery.
MIMIC Total Precipitable Water fields, below, from 0000 UTC 1 April through 0000 UTC 3 April show a concentrated area of moisture moving towards/over Samar, arriving at about 1200 UTC on 2 April.
ASCAT winds from Metop-B, shown below (source), also show a circulation moving towards the island of Samar.
Himawari-9 provides animations (and multispectral observations) that might help differentiate between a thunderstorm and a volcano. The animations (clean window infrared and Ash RGB) below are from 1000 to 2000 UTC; at 2000 UTC the cold cloud tops are starting to warm. Structures in the warmest clouds in the Band 13 imagery show arcs that are consistent with the circulation detected by Metop-B ASCAT. The Ash RGB lacks the pink hues associated with volcanic ash (Ash RGB Quick Guide is here). So far, a lot of satellite indicators — from both geostationary and polar orbiters — argue against a volcanic event.
It’s not unheard of that tropical convection and volcanic eruptions coincide in this part of the Philippines. (Here’s the ash cloud from Pinatubo in 1991 mingling with the clouds of Typhoon Yunya; in Pinatubo’s case, the daytime imagery allowed for an easy visual distinction between the ash cloud and the cumulonimbus. This event on 2 April was at night. More Pinatubo imagery is available at this blog post.) However, neither the Tokyo VAAC nor Seismic monitoring nor imagery at the CIMSS VOLCAT site suggest any kind of volcanism.
Answer: This is a thunderstorm!
I am indebted to Brandon Aydlett, SOO at WFO GUM, for alerting me to this interesting case. He also sent along this pdf outlining what happened when he got the request. You’ll note that the blog post includes much of what he said.