Islands' 'regular winter storm' result of several factors
| • | Lightning, more rain hit Islands |
| • | Graphic: What's causing this weather? |
By Jan TenBruggencate
Advertiser Science Writer
After a couple of dry winters, Hawai'i has become unaccustomed to the drenchings and the occasional thunder and lighting of the winter storms like the one that put a damper on the end of the Thanksgiving weekend across the state.
"It was just a regular winter storm, and not a particularly powerful one," said meteorologist Andy Nash of the National Weather Service forecast office in Honolulu.
Tell that to folks across the state whose yards are six inches deep in sticky muck, whose landscaping has been flattened by rushing water, whose carpets are soiled and soggy.
In a normal winter, one of these systems passes every week to 10 days, Nash said, though not always with heavy rain and the thunder and lightning that Maui County residents experienced Saturday night and O'ahu residents heard yesterday.
The Thanksgiving weekend storm arrived on a 500-mile-wide low-pressure system that passed north of the Islands from west to east. Its center was about 400 miles north of Hawai'i.
Such a system sucks in air from its surroundings and spins in a counterclockwise direction. That means, for example, winds that come up from the south in front of the storm curve to the west to approach its center.
Two different processes associated with the low have produced the thunderstorms in recent days.
First, warm air being pulled from over warm southern waters came into contact with cold air over the Islands. The result was thundershowers that pounded the central islands during the weekend.
Second, came the cold front. The rain and thunderstorms that hit Kaua'i Sunday and O'ahu yesterday were part of a cold front that is part of the southern side of the low- pressure system.
On weather maps, this feature looks like a long comma hanging under the low. During the past two days, the low has dragged the cold front with it, right across the islands.
In a cold front, the cold air to the southwest of the low pushes under warm moist air to the southeast. As the moist air is driven upward by the wedge of cold air, it cools and moisture precipitates into heavy rain and, at high elevations, into ice and snow.
That's right. It's snowing at the top of those thundershowers pounding on your roof.
At the same time, electrical charges develop in the high cloud. Ice crystals at the top of the cloud take a positive charge, while droplets in the middle of the cloud take a negative charge. There's another positive charge near ground level.
When the charge builds up sufficiently, energy flashes as lightning. The incredible heat of lightning — hotter than the sun's surface — expands the air, sending out a shock wave that we hear as thunder.
The cold front was expected to stall for a while today between Maui and the Big Island, and then the low pressure system was expected to move on, turning north toward the Gulf of Alaska, where it should be incorporated into an even larger low-pressure system, said meteorologist Tim Craig.
But even though far away, the system has one more surprise. It should generate a swell that's expected to hit the northern shore of all of the Islands tonight and tomorrow, Craig said.
The good news for soggy residents is that for the next week or so, most of the state should be back to normal tradewind weather, Nash said.
Reach Jan TenBruggencate at jant@honoluluadvertiser.com or (808) 245-3074.
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