Edit: NWS Honolulu Science and Operations Officer (SOO) Robert Ballard graciously provided some insight into this case, and this post has been updated with his input. Thanks, Bob!
In the morning hours of 8 May 2026, some interesting low level cloud plumes appeared over or near the Hawaiian island of Oahu. The presence and cause of these clouds sparked some discussion around the Honolulu Forecast Office (who were gracious hosts of this blogger this past week) as it wasn’t immediately clear what was happening instance. Fortunately, satellites can provide some clues as to what was going on. This was mostly an overnight event, so we’ll start with the GOES-18 (GOES West) Band 13 (10.3 micron) loop. Here we see cloud blooming over the central Hawaiian island of Oahu as well as to the northeast and southwest. The leading edge of these clouds also exhibits noticeable clearing as the clouds propagate outward.
So what’s going on here? Let’s see how satellites might provide some insight. Our first clue is that this is a fairly warm set of clouds. The contrast in the above loop isn’t great using the default color settings in AWIPS. Switching color maps and changing the limits brings out some more detail. Here’s a static image with a data readout. Our suspicions appear to be correct: the cloud at its coldest was still above freezing. The royal blue parts of the image that represent the expanding cloud shield are around 5-7 C, while the green areas represent the ocean surface and are around 19-20 C.
Next, let’s take a look at the Night Microphysics RGB product. According to the RGB Quick Guide, these pink and purple colors are associated with cloud-free regions. Obviously, that’s not happening here. However, the standard interpretations of these products were crested with the midlatitudes in mind, and in regions further afield the standard interpretations may not apply due to very different environmental temperatures and water vapor concentrations.
Perhaps a look at the 12 Z radiosonde launch from Lihue may help. Lihue is on the very eastern edge of the island of Kaua’i, the island on the left side of the loop above. It’s around 100 miles from Honolulu (on the bay of the island at the enter of the image) and thus reasonably representative of the environment. Shown below, it’s a fairly typical sounding for Hawaii this time of year, with a prominent trade wind inversion between 650 and 800 hPa caused by the large-scale subsidence found at the boundary between the tropics and sub-tropics. Below that is a nearly-saturated and well-mixed boundary layer. Looking at the raw data, we can see that the inversion started around 780 hPa (2250 m) with the freezing level around 570 hPa (4850 m).
Adding in the surface observations from Honolulu brings some additional insight. The observations show the base of the cloud deck was 7500 feet (2300 m) which means that in essence the base of the clouds was at the base of the inversion. Contemporaneous radar-derived rain accumulation indicate that precipitation remained on the northeast side of the island, and no rain was recorded at Honolulu. At the time the clouds initiated (around 10 PM local or 0800 UTC), the surface temperature at Honolulu was 76 F (24 C) with a dew point of 66 F (19 C). That temperature might be the key to all of this. Let’s zoom in on the bottom of that sounding. I’ve added the Honolulu temperature and dew point observations in red and green respectively, and approximated a parcel trace in blue.
If we assume that the vertical profile at Lihue is reasonably representative of the conditions over Honolulu, then it could be possible that surface-based parcels would be positively buyoant. If we look at the start of the start of the cloud formation, it appears to form on the northeastern edge of Oahu, as marked in the figure below.
Winds on the north side of the island were weak and from the north at the time that the convection initiated around 10 PM local time. The last bit of the puzzle is the terrain. Oahu is dominated by two significant mountain ranges, the Wai’anae range in the west and the Ko’olau Range in the east, which are the remnants of shield volcanoes of geological ages past. Courtesy of Open Street Map, here is a topographical map of Oahu.
Note how the initial cloudiness seems to parallel the Ko’olau Range, and the observed rain did the same. NWS SOO Robert Ballard notes the following, lightly edited for context:
At 10 pm, if the background flow is light enough (usually something like 8 knots or less background flow), you’d expect to see offshore/downslope land breezes, rather than onshore/upslope sea breezes dominate. From the Night Microphysics loop, the low level background flow in the low cloud field seems to be fairly light and from the east-southeast, which is a favorable flow pattern to pin showers to the windward side of the Ko’olau mountain range, especially if you have some sort of mid-to-upper support coming near or over the islands. This type of overall pattern, when a little more favorable aloft, can produce impactful rainfall and even flash flooding.
We may finally have enough information to put everything together. It appears that upslope flow in a relatively warm and saturated environment was enough to initiate what would normally be deep, moist convection. However, the strong cap of the trade wind inversion was high enough to allow convection to initiate but low enough to limit it to the 600-700 hPa level. This meant the clouds stayed relatively warm, and since everything stayed below freezing there was no lightning. In essence, these were cumulonimbus anvils at 1/3 the height of normal.
A few other “anvils” can be seen in locations over the ocean. These aren’t as vigorous as the main anvil over Oahu, but then again, they didn’t have the benefit of terrain enhancement. Given how warm the surface was and how saturated the air was, it’s very possible that there was surface based localized convection that didn’t need orographic lift to inititate.
The final question is the clearing that we see around the edges of the anvils as they propagate outward. That is likely subsidence compensating for the vertical lift. As the updraft slams into the trade wind inversion and the anvil spreads out, there has to be sinking motion elsewhere to maintain mass continuity. That subsidence means warming and drying, creating a clear band around the anvils.
Now that we’ve got all of this context, scroll back up and look at the second image again. Because, after all, how often do you see an overshooting top below the freezing level?
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