Mauka Showers

Note: This blog post will NOT be updated with the latest forecast information regarding the upcoming potential heavy rain event. For the latest community updates, please stay tuned to your local media outlets, and check on the latest official forecasts, watches, and warnings on the Honolulu Forecast Office’s (HFO) website.

Here we are, two and a half months into the wet season and really not a whole lot of rain to show for it. Since summer, the long range models have been consistently saying it would be a slow start to the wet season, with October dry and November a transition month into a rainier than normal December. Looking at the medium range models over Thanksgiving weekend had me wondering if the “rainier than normal December” would ever materialize. Well, it looks like a solid rain event is finally on its way and it has the potential to be a pretty significant one. The downside for those wanting drought relief is that at the moment it doesn’t look like it will be a statewide event. More on that to follow.

The change in weather is being produced by a shift in the North Pacific atmospheric pattern that will send the southern branch of a split jet stream close to the main Hawaiian Islands. The chart below shows the analysis from the GFS model at 2 AM HST, December 12, 2025 at around 35,000 ft, or roughly jet stream level. The low pressure system about halfway between Hawaiʻi and Alaska is in the process of weakening and moving northeastward. This low is associated with the weak front  that moved over Kauaʻi and Oʻahu December 10th and 11th before stalling and dissipating over Maui County. The high pressure system over the Aleutians is the block that is creating the split in the jet.

Geopotential heights and winds at 200 millibars from the GFS model’s 00-hr panel valid at 1200 UTC, December 12, 2025. The main Hawaiian Islands are within the red oval. Positions of the blocking high over the Aleutians and the weakening and exiting low are also indicated in red.

Over the next couple of days, another low pressure system will move eastward south of the blocking high. This will produce a stronger cold front with much better upper level support that will threaten the main Hawaiian Islands. According to HFO’s Area Forecast Discussion, the “heaviest of the rainfall is expected to develop Sunday night through Monday”. The following loop of forecast 6-hr rainfall accumulations covers the 24-hr period ending at 6 PM HST, Monday, December 15 from the ECMWF (European) model. The main rain band in the model forecast affects Kauaʻi and Oʻahu, and appears to have peak intensity early Monday morning.

ECMWF 6-hour precipitation forecast panels from 6 PM HST, December 14 to 6 PM HST, December 15, 2025. Graphics provided by WeatherBELL Analytics.

A simulated enhanced infrared (IR) satellite image loop from the ECMWF model shows a different perspective and gives you an idea of what actual IR satellite images may look like in the next few days. Like actual enhanced GOES IR images, medium and light gray shades are low level clouds. In this enhancement curve, the blue, green, yellow, and orange colors are the higher and colder simulated cloud tops. The loop shows the coldest forecast cloud tops in greens and yellows as the strongest upper tropospheric support sweeps over the west end of the state in conjunction with the peak in forecast rainfall.

ECMWF simulated color enhanced IR image loop centered on the main Hawaiian Islands covering the period from 8 AM HST December 13 through 8 PM HST, December 16, 2025. In this enhancement scale, gray shades are either the surface or low cloud tops. Blue, green, yellow, and orange shades are high cloud tops.

I’ve been showing output from just the ECMWF model, but other global models are showing a similar general pattern. You can see this in the accumulated precipitation for the 5-day period ending at 2 AM HST on December 17 from the ECMWF, GFS (National Weather Service model), and ECMWF artificial intelligence models. While the details in the amounts vary, all three show the main band of precipitation affecting Kauaʻi, Oʻahu, and portions of Maui County. Unfortunately for Maui and the Big Island, which have the worst ongoing drought (see U.S. Drought Monitor map below), it appears that this upcoming weather system is not expected to bring any relief.

Accumulated precipitation for the 5-day period ending at 2 AM HST, December 17, 2025 from the ECMWF model. The graphic is produced by Tropicaltidbits.com.

Same as above, but from the GFS model.

Same as above, but from the ECMWF artificial intelligence model.

U.S. Drought Monitor map for the main Hawaiian Islands as of December 9, 2025. The D1-D4 categories are the different levels of drought. The D0 category, for abnormally dry conditions, can be considered as “pre-drought”.

There are definitely still chances for Maui and the Big Island to get their turn at drought relief. Extended range forecasts from both the ECMWF and GFS show positive rainfall anomalies for the first half of January (ECMWF 7-day anomalies shown below), so water supply and agriculture interests can hope these forecasts will verify correctly.

ECMWF 7-day precipitation anomalies valid at 2 PM HST, January 2, 2026. Greens and blues are positive anomalies, browns and reds are negative anomalies. Graphics provided by WeatherBELL Analytics.

Same as above, but valid at 2 PM HST, January 9, 2026.

Same as above, but valid at 2 PM HST, January 16, 2026.

As with all natural disasters, having a plan can really help keep you safe. A plan for a possible flood event may not need to be as comprehensive as one for a direct hurricane impact, but it still is very useful. Here are some wet season preparedness reminders from HFO’s 2025-2026 Hawaii Wet Season Outlook/Dry Season Summary:

• Do not drive on roads with fast-flowing water.
  • Just 1 to 2 feet of fast-flowing water can sweep most vehicles off a road. 
• Do not walk across flooded streams.
  • If you’re hiking and get stranded, wait for the water to recede. 
• Expect other rainy weather impacts.
  • Increased road travel times.  
  • Possible detours or road closures due to flooding or landslides. 
  • Outdoor activities may be postponed, canceled, or adjusted. 
• The wet season brings increased potential for lightning strikes.
  • Be prepared for power outages, especially when thunderstorms are forecast. 
  • Move indoors when you hear thunder or see a flash of lightning. 
  • Lightning strikes can occur even in areas where it is not raining. 
  • Although rare, thunderstorms occasionally produce damaging winds, large hail, and tornadoes in Hawaiʻi. 
• If you live in a flood-prone area, have an evacuation plan in case flood waters quickly threaten your home. 

Additional resources:

• Hawaiʻi Emergency Management Agency (HIEMA): As mentioned in my Hurricane Kiko post, HIEMA’s website contains a wealth of preparedness information for many natural hazards.
• County Emergency Management Agencies: Your respective county emergency management offices will have specific information affecting your county, including availability and status of evacuation shelters.
  • Kauaʻi Emergency Management Agency
  • City and County of Honolulu Department of Emergency Management
  • Maui Emergency Management Agency
  • Hawaiʻi County Civil Defense Agency

Be prepared and stay safe!

Finally, in the bin labeled “Oh By The Way”, Hilo Airport is still definitely on pace to have its lowest annual rainfall on record. Back on September 12, I showed a Haywood plot indicating that through the end of August, Hilo Airport was in the “Yikes!” category for low annual rainfall. The Haywood plot below shows that through the end of November, Hilo Airport is still in the “Yikes!” category and had accumulated just 55.20 inches of rainfall. Hilo Airport’s driest year, 2010, had 56.19 inches at the same point in time.

Edited Haywood graph of accumulated rainfall from January 1 through November 30 at Hilo Airport. The blue line shows the rainfall for 2025. Green-ish lines are the 5 wettest years and the yellow and orange-ish lines are the 5 driest years. The gray lines are the remaining years in the period of record going back to 1950. Hilo Airport data actually go back to 1949, but the first full year of data was 1950. The bold black line is the average accumulated rainfall. The graph is from the NOAA National Centers for Environmental Information. The edits in red are my own.