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.