Mauka Showers

Starting off with some good news…I was able to resolve the issue with the unviewable satellite loops in the 2015 hurricane season post from a couple of weeks ago. The loops appeared fine in the draft, but I guess when I published it, there was a file size limitation that prevented viewers from seeing it on my webpage. Stupid technology. I pushed the “publish” button using the WordPress app while sitting in a hotel in Japan thinking I was being clever. Unfortunately, the whole plan failed miserably and I couldn’t troubleshoot it without my laptop. My irritation was amplified by the thought of how much time it took to create the loops, but it didn’t last long because, well, I was in Japan! While I still can’t display the loop within the post itself, I can provide links to the loops. They’re at the bottom of the post so please check it out if you’re still interested.

Another piece of good news is that late last week we had our first cold front passage of the 2025 – 2026 wet season, or at least to me it’s good news. The first cold front usually reaches the main Hawaiian Islands around mid- to late-October, so it was a little later than usual. There was a weak cold front that almost made it to the state in mid-September, but it stalled and dissipated over the coastal waters north of Kauaʻi. Since 2000, I’ve seen our first cold fronts reach the main Hawaiian Islands as early as the first week of September and as late as the first week of December.

The front reached Kauaʻi and Oʻahu on November 8, and pushed across Maui County and the northern portion of the Big Island on November 9. The surface analysis from the Honolulu Forecast Office (HFO) on the morning of November 8 shows the cold front over Kauaʻi (below).

Surface analysis from 8 AM HST, November 8, 2025, posted by the HFO. Blue lines with triangles are cold front positions.

A loop of satellite images from November 9 shows a weakening cold front after it reached the Big Island. The cloud band associated with the front, shown as medium gray shades from the Big Island to Kauaʻi, consist of low clouds with embedded showers. Most of these clouds have tops below 10,000 ft. The deeper cloud layers, shown as light grays and blues far northeast of the Hawaiian Islands, are a part of the main circulation of the front’s parent extratropical cyclone. Because the cyclone was so far away, and the trailing high pressure system already north-northwest of the state, surface winds near the state to the north of the cold front were northeasterlies, with easterlies to the south of the front. If the cyclone was closer to the state, winds after frontal passage would have been northwesterlies or northerlies.

(Top) NOAA/NESDIS GOES infrared image loop from 1850-2040 UTC, November 9, 2025 shows the leading edge of the cold front over the Big Island. (Bottom) Annotated satellite image from 1850 UTC, November 9 shows the location of the cold front’s leading edge for reference.

Rainfall totals for the three days covering the entirety of the event are shown in the graphic below. The data indicate a widespread 1 to 4 inches along the windward slopes of the state, with isolated totals over 6 inches on the Big Island, and over 9 inches on Maui. These are respectable amounts for a frontal passage, especially one without significant upper level support. The windward emphasis of rainfall is due to the low level winds being from the northeast and the shallow nature of the clouds. A frontal passage with a parent low pressure center closer to the main Hawaiian Islands would allow pre-frontal southeasterly to southwesterly winds to develop rain bands that move over the leeward sides of the islands.

Rainfall totals for November 8 – 10, 2025, based on daily rainfall grids from the Hawaiʻi Climate Data Portal.

Some of you may be wondering if this cold front was enough to kill the drought affecting the state. The short answer is no, not completely anyway. The figure below shows the U.S. Drought Monitor map posted on November 13, 2025. Moderate Drought (D1 category) covers more than half the state, and Severe Drought (D2) covers a little under a third. Areas of Extreme Drought (D3) cover Maui’s central valley and portions of the Big Island. Significant amounts of D2 and D3 areas are in the leeward sides of the state, so the rainfall from the recent frontal passage would not have been enough to ease drought impacts.

Another issue is that with any significant drought (D2 or worse), you need to have multiple rain events to break it completely. Producers over the years have always told me that you need to have follow-up rainfall to get the soils to fully recover. Most of the windward areas should be able to see at least some improvement over the next couple of weeks from not only the rainfall associated with the November 8 – 9 frontal passage, but also due to the decent amounts of rain over the windward slopes since then. However, leeward areas will need an event that focuses on the southwestern slopes of the islands, and some follow-up events to ensure a full recovery.

 U.S. Drought Monitor map posted on November 13. Moderate or greater drought covers more than half the state. The Abnormally Dry (D0) category is not considered to be drought and can be thought of more as a “pre-drought” condition.

The significance of the drought during the past three months can be seen in the Standardized Precipitation Index (SPI) data. In the figure below, areas in the Very Dry and Extremely Dry categories cover most of the Koʻolau Range on Oʻahu, and the windward slopes of Maui. Portions of the Hāmākua region and the Kohala Mountains on the Big Island also were in the Very Dry category, with a speck of Extremely Dry near Waimea. When you’re in these SPI categories, you’re seeing well below average rainfall. Effects of this lack of rainfall included the declaration by Maui County of a Stage 2 Water Shortage for West Maui, and a Stage 3 Water Shortage for Upcountry Maui (downgraded back to Stage 2 on October 28). I don’t recall ever seeing a Stage 3 Water Shortage declared before, and even Stage 2 Water Shortage levels aren’t too common. These declarations impose significant water use restrictions for residents on these systems. In addition to supplying the Upcountry Maui water system, rainfall on the windward slopes of Haleakalā also provide irrigation water for agriculture operations in the central valley. Reductions to irrigation water will ultimately result in reduced production and economic impacts. You can see many more impacts in HFO’s recent Drought Information Statements.

Standardized Precipitation Index 3-month analysis from the Hawaiʻi Climate Data Portal (HCDP). The SPI is often used for drought monitoring, but is applicable to all rainfall conditions. The map shown here covers the 3-month period ending in October 2025.

Last year, the May through September dry season was also drier than normal, but Hurricane Hone brought much needed relief in late August. The August through October (ASO) SPI picture from 2024 shows much different conditions compared to this year, with the wetter than normal conditions mainly due to rainfall from Hone’s passage south of the Big Island. One rancher told me last year that without Hone, they would have been in serious trouble. You know it’s bad when people wish for a tropical cyclone to get the rainfall they need!

Same as above, except for the 3-month period ending in October 2024.

We’ll see if the leeward areas of the state get their turn at some drought relief. Long range forecasts over the past couple of months have been consistent in indicating that November would be a transition month from the dry summer and early fall conditions to a wetter than normal winter.