Saturday, February 18, 2017

Cold Front Passing Hawaii

Cold Front Approaching

A cold front moving down the island chain will bring a period of clouds and moderate showers, with cool and breezy trade winds increasing after the front passes. The front will reach Kauai this afternoon.  Note the rope-like cloud band to the NW of Kauai in the MODIS true color image below, valid for 11 AM 18 February 2017.  Near Hawaii cold fronts often take on the appearance of rope clouds.  





The Japanese Himawari satellite gives a broader view of the rope cloud.


The cold front will pass Oahu and Molokai this evening, Maui overnight, and the Big Island Sunday morning, moving south of the Big Island by Sunday night.  See forecast maps below showing precipitation and sea level pressure (labeled contours) for 2 AM Sunday and 2 AM Monday below.  The line of showers is associated with the front. Notice how the pressure gradient tightens with time, increasing the winds.



Heating and moistening of the air over the warm subtropical ocean (surface heat exchange) is greater on the cold side of the front where the air-sea temperature difference is larger.  As a result fronts tend to dissipate as they approach Hawaii, leading to the rope-cloud appearance of the front (see images at the start of this post).  The result of the surface heat exchange the dew point temperature drop is generally greater than the temperature drop after the cold front passes.  At NOAA buoy 51001 located ~195 miles northwest of Kauai, the time-series graph of temperature below shows a drop of 7˚F in ~5 hours, with an overall drop of 8˚F in 24 hours.  Meanwhile the dewpoint temperature drops a full 20˚ F in 24 hours.   



See WRF forecast sea-level temperatures and winds for 8 AM Saturday and 2 AM Monday below.  Increasing winds and cooler temperatures are in store.


The change in dew point temperature is often larger than the change in temperature following a cold front passage in Hawaii.  After the front passes, a mostly dry weather pattern will prevail into Monday, with northeast winds making it feel cool. Note the change in total precipitable water vapor in the images below, valid for 5 AM Saturday 2/18/17 and 2 PM Monday 2/20/17.  The orange colors indicated significantly drier air flowing over the state behind the cold front on Monday.

Storm Force Winds Possible

Following the cold front, the pressure gradient will increase producing 25 to 35 mph winds over the islands on Sunday and Monday.  In the channels between the islands winds will be 20% higher, resulting in storm force gusts in some of these areas.  Breezy trade winds will continue into the middle of next week.  WRF output sea-level winds at 2 PM Sunday 2/19 shows significant enhancement of the winds in the island channels.  In a future post, I will discuss terrain enhancement of the winds in Hawaii and the phenomena of downslope windstorms.




4 comments:

  1. Just found your blog from a link at cliffmass site. From now on you're bookmarked and my #1 source for Hawaiian weather. At last, someone who knows that Oahu is not the only island!

    The Vog on Big Island has been horrible! I checked out your vog post on 2/4. Wish I'd known about you then....

    I'm appalled at the lack of vog warnings from anyone. The Volcanoes National Park puts out that the air quality is good, when I can clearly see that I can't detect a horizon.

    It is unconscionable that tourism is placed above Public Health! Parents should be warned to keep their children inside. Pets too if possible. A simple triangular household dust mask does wonders. In a pinch, you can adapt coffee filters.

    Don't melt your lungs with acid and sandpaper them with glass! It's bad when you notice that your cat's meow has a "frog in the throat" quality!

    Insist that the public be told the truth about air quality instead of pretending that it's "all good", nothing to see here. Well that's true if you look out over the ocean. Can't see a thing!

    Thanks for your public service! I'll be spreading the word about your excellent reports!

    Joy in Hawaii

    ReplyDelete
  2. Dear Joy,

    Thanks for the encouragement. Did you see that the vog model output is now mobile friendly?

    http://weather.hawaii.edu/vmap/hysplit/

    Also check out our weather server if you haven’t already

    http://weather.hawaii.edu

    me ke aloha, Steven

    ReplyDelete
  3. Mahalo for the blog and the Vog monitoring tools. It's a great contribution.

    ReplyDelete
  4. Thanks Michael,

    Spread the word on the vog model.

    Mahalo, Steven

    ReplyDelete