Saturday, December 17, 2016

2016 Kona Low

This weekend our weather is dominated by a kona low.



What are Kona Lows?

Kona lows are subtropical cyclones that occur during the cool season in the north central Pacific.  They form as a result of forcing by upper-level lows cut off from the polar westerlies in winter.  Kona lows are cold core with the strongest circulation in middle and upper atmosphere.  They are relatively long lived storms - often affecting Hawaii for a week or more.  Erratic tracks and convective-scale organization of precipitation makes forecasting Kona Low impacts a challenge.


Kona Low Impacts Include:

  • heavy rains, flash floods, and landslides
  • high winds
  • large waves and swell
  • blizzards at higher elevations on Mauna Kea and Mauna Loa
  • severe thunderstorms
  • large hail 
  • tornados 

Below is an F1 tornado over central Oahu.


Blizzard on Mauna Kea came with 130 MPH winds in 2001.



The Kona Low Impacting us This Weekend

A well-developed Kona Low is dominating our weather this weekend.  Cool and showery weather has descended on the Hawaiian Islands as a Kona Low approaches the state from the north.  Here is the IR satellite image for 9 AM on Saturday 12/17/16.




The surface low is about 200 mi north of Kauai as seen in the surface pressure analysis that is overlaid on the IR satellite image for 8 AM today.




Kona Lows are generated by cold air and low pressure aloft, aka a trough aloft.  Below is the 250-mb height field overlaid on the water-vapor image for 8 AM today.




MKWC high-resolution weather model output shows showers circling the low center where the coldest air is aloft.  A vigorous convective band is seen just west of the Big Island.  




IR Satellite image for 8 PM today shows deep convection forming over Kauai and the Big Island, with the threat of heavy rain over Maui indicated.


Heavy rainbands are already impacting the Big Island this evening. See the radar reflectivity for 7 PM this evening below.


As time goes on, the model forecasts the low to slip southwest and for the bands of showers to continue to move over the Big Island.   See the forecast for 8 PM Sunday below.  The smaller islands may escape the worst of the heavy rains, if the model is correct.  However, weather models have difficulty correctly simulating the details of the convective showers and rainbands in Kona Lows.  In addition, the models tend to weaken the lows too quickly.  From past experience I would say we are not out of the woods yet.  So be prepared for the possibility of thunderstorms forming over Oahu during the next couple of days.  


3 comments:

  1. What a wonderful blog!! Thanks to C. mass im more than interested! You are a resource and a half for very meteorology! Keep up the good work!!!

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  2. Thanks Harrison. Glad you enjoy my posts.
    me ke aloha, Steven

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