Puna Weather
Thursday 20th August 2026
First I'll talk about 90E, the forecast section is farther down this post.
All models show 90E passing by well to our south on Sunday with deep tropical moisture reaching our island on Sunday as well and lasting into Tuesday. As the storm passes by trade winds will increase to breezy most locations. Expect rain to be heavy at times with the risk of flooding.
This morning's scatterometer data shows a closed circulation and there is heavy convection associated with the low, so the NHC will likely upgrade to a tropical depression soon. Now that we have a center fix, the models well be more valuable soon. Prior to this center fix the high resolution models all missed by 100 miles or more. Some to the north, south, and east, but all showing the same idea, mid-level shear keeping the storm from strengthening much and with a weak storm the steering is more towards the west, so they all have the storm passing well to our south.
The main convection is over 100 miles to the west of the center of circulation due to shear over the storm leaving the center completely exposed and allowing mid-level dry air to flow into the core keeping the storm weak. Shear over 90E can be deceptive because one of the main ways of calculating shear is to look at the difference in the wind between the surface and the upper atmosphere. That calculation shows light shear over 90E, but there's another calculating shear, max-shear, and this one is more relavant when the shear is in the mid-level. While the shear values over 90E for the next couple of days stay mostly under 10kts, which would typically allow the storm to consolidate and strengthen, the max-shear is higher making it difficult for the storm to build convection all the way around its core and strengthen quickly.
In order to significantly strengthen today 90E would have to overcome daytime heating, mid-level shear, and mid-level dry air. Nonetheless, we will continue to watch development, changes in NHC wording, and how the models respond to the establishment of a low level center.
Today very humid with light north winds and scattered showers.
Tonight warm and very humid with light south winds and scattered showers.
Friday and Satuday humid with light to moderate easterly trade winds and scattered showers.
Sunday and Monday very humid with breezy easterly trade winds and periods of heavy rain. Higher elevations are likely to see stronger gusts.
This morning we see the remnant circulation of ex-Hernan pass by to our north dragging a tail of tropical moisture over us. Ex-Hernan is also weakening our trades as it passes between us and the northeast Pacific high. Once the trough is to our west, our trades return and this will push moist air up our slopes for windward rain.
It's not long after ex-Hernan's moisture plume clears the area and moisture from Invest 90E arrives. We see a big spike in moisture from the surface to above our mountain tops early Sunday morning. The moisture field associated with 90E is large and will take a few days to pass by, so warm and humid with periods of heavy rain beginning Sunday.
As 90E passes by it tightens the pressure gradient between us and the northeast Pacific high for stronger trade winds. It should be breezy Sunday and Monday at the lower elevations and pretty gusty at times along the coast and over our higher elevations.
The combination of onshore winds and high moisture brings the threat of flooding rains for our entire forecast area from Hamakua to Hilo and down to Kau.
Models show a return to more normal humidity beginning Wednesday or Thursday.
There lurks a long range threat in the far eastern Pacific, but that's nearly two weeks away, so it's a low confidence threat.
