Trade winds, windward showers could prevail through spring

The National Weather Service has updated its wet weather forecast.

While it calls for above-average rainfall, it looks like it may be focused more on the windward side.

“If it gets strong, then what we see in recent La Nina events is more frequent trade winds during winter months, so it keeps rain primarily on windward sections, so leeward area remains dry,” explained Kevin Kodama, National Weather Service hydrologist.

Kodama says they’re not sure how strong La Nina will be this year. That will be determined by ocean temperatures.

During stronger La Nina years, there have been more persistent trade winds and windward showers.

It’s the opposite during weaker La Nina years, with more storms that can displace trade winds.

Wet Season Rainfall Outlook for the State of Hawaii

Summary of the dry season (May through September 2017)

Statewide: Most locations had near to below average rainfall.

  • Drought present on the Big Island at the start of the dry season.
  • Spread to the other three counties and intensified during the summer.
  • Mainly affecting ranching operations and localized water service areas on Maui and the Big Island.

13th driest dry season in the last 30 years (based on rankings from 8 key sites).

  • 2015 dry season was the wettest in the last 30 years.
  • 2003 dry season was the driest in the last 30 years.
  • Expected wet conditions for windward slopes did not occur.

Unlike 2015 and 2016, below average tropical cyclone activity in 2017 did not provide a boost in rainfall during the summer months.

Outlook for the wet season (October 2017 through April 2018)

NOAA’s Climate Prediction Center (CPC): The current ENSO-neutral conditions are likely transitioning to a La Nina state (cool phase) with a 55 to 60 percent chance of La Nina developing during the fall.

  • CPC issued a “La Nina Watch” on September 14, 2017.

After development, La Nina is forecast to persist until the spring when conditions may transition back to ENSO-neutral.

There is uncertainty in the eventual strength of this La Nina episode.

Probabilities favor above average rainfall through the wet season.

  • Above average rainfall is reflected in the climate model consensus predictions which captures large scale conditions but not Hawaii’s microclimates.
  • Wet seasons during recent moderate to strong La Nina events have had wet conditions over the windward slopes but dry conditions over leeward areas.
  • Weaker La Nina events had more rainfall make it to leeward areas.

Recovery from existing drought probable for windward Big Island and Maui.

Existing drought may persist or worsen in some of the leeward areas, especially on the Big Island and Maui County.

Wet season preparedness reminders

Do not drive on roads with fast-flowing water.

  • Just 2 feet of fast-flowing water can sweep most vehicles off a road.
  • Road may also be severely undercut.

Do not walk across flooded streams.

  • If you’re hiking and get stranded, wait for the water to recede.
  • Streams in Hawaii generally recede quickly.

Expect more 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.
  • Move indoors when you hear thunder.
  • If you travel through a flood-prone area, identify alternate routes ahead of time.

If you live in a flood-prone area, have an evacuation plan in case flood waters quickly threaten your home.

Stay informed of conditions that could change rapidly.

  • Sunny skies can turn cloudy with intense rainfall in less than an hour.
  • Check out the latest forecasts, watches, warnings, and advisories via the media, NOAA Weather Radio, the Internet, or one of several weather mobile phone apps.
  • Wireless Emergency Alerts (WEA) on mobile phones notify you that you’re in a flash flood warning area.

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