March 6-7 2006

Air Temperatures — The following maximum temperatures were recorded across Hawaii Monday:

Lihue, Kauai – 77F
Honolulu, Oahu – 80
Kaneohe, Oahu – 77
Kahului, Maui – 83
Hilo, Hawaii – 78
Kailua-Kona, Hawaii – 82

Temperatures early Tuesday morning ranged between these warmest and coolest spots near sea level at the 4 a.m. hour:

Kaneohe, Oahu – 73
Molokai airport – 62

Precipitation Totals — The following numbers represent the largest precipitation totals during the last 24 hours (as of Tuesday morning) on each of the major islands, including:

0.33 MOUNT WAIALEALE, KAUAI
0.56 MANOA LYON ARBORETUM, OAHU
0.00 MOLOKAI
0.00 LANAI
0.00 KAHOOLAWE
0.39 WEST WAILUAIKI, MAUI
0.15 HONAUNAU, BIG ISLAND

Weather Chart — Here’s the latest (automatically updated) weather map…showing a 1031 millibar high pressure system to the NE of the islands. This high pressure cell will keep the trade winds blowing today…gradually swinging around to the SE into Wednesday. Here’s a Weather Map Symbol page for clarification about what all those funny weather symbols mean.

Hawaii’s Mountains — Here’s a link to the live webcam on the summit of near 14,000 foot Mauna Kea on the Big Island of Hawaii. The tallest peak on the island of Maui is the Haleakala Crater, which is near 10,000 feet in elevation. These two webcams are only available during the daylight hours here in the islands…that is unless there’s a big moon rising just after sunset for an hour or two!

Satellite Images — To view the cloud conditions we have here in Hawaii, please use the following satellite links, starting off with the Infrared Satellite Imageof the islands to see all the clouds around the state. This next satellite image is one that gives close images of the islands only during the daytime hours, and is referred to as a Close-up visible satellite image. The next satellite image shows a larger view of the Pacific…giving perspective to the wider ranging cloud patterns in the Pacific Ocean…out from the islands. To help you keep track of where any showers may be around the islands, here’s the latest animated radar image.

Aloha Paragraphs



Pueo….the Hawaiian Owl

The weather here in the islands will be quite pleasant, with a late winter trade wind pattern in place for the time being. The windward sides of the islands will continue to see off and on showers arriving, which are common when the trades are blowing. The leeward sides will find partly sunny weather during the days, along with warm temperatures for beach excursions. A high pressure system to our north will keep the trade winds blowing through Tuesday.


The computer forecast models continue to show some changes occurring by mid-week. As a matter of fact, there seems to be some similarities between what’s coming up…and what happened last week on Kauai and Oahu. Last week’s weather was wet on those two islands, and we may very well see another round of showers arriving there by Thursday. This locally wet weather could conceivably remain in place into the weekend time frame. It’s still a little too early for exact details, although it looks like the second half of the week could be showery.


As was the case last week, the eastern islands of Maui and the Big Island may remain on the dry side of this deep moisture to the west. The models show a trough of low pressure setting up to the west of kauai. As this trough edges closer after Wednesday, the chances of rain will increase first for Kauai, and then for Oahu by Friday. As I mentioned in the above paragraph, the particulars with this forecast will have to be fine tuned as we move through the next couple of days. It isn’t certain yet whether the shower line will stop before reaching the entire state, so stay tuned for more on this unfolding sitation.


As far as the winds go, the trade winds will prevail through Tuesday. As the trough of low pressure moves closer to Hawaii, our winds will gradually swing around to the SE or even south in places. This will likely bring more volcanic haze to some parts of the state, and in particular around Kona, and then on up to the islands of Maui County. At the same time, SE and south winds bring moisture from the deeper tropics, which help to feed the showery clouds that will be around then. SE and south winds bring warmer air into the state as well, so we could see another record high temperature at the Kahului airport, like last week’s 88F degree readings for two straight days!


I had a good weekend, starting off with the sci-fi film Ultrviolet Friday night. Saturday’s surfing trip to the Lahaina side was fun. Sunday a friend and I went kayaking below the Pali on the way to Lahaina out of Maalaea Bay. Then it was the ping pong tournament with my neighbors last evening, while watching the nice sunset. There was a lot of physical exercise involved, which I enjoy a lot. My health is good, and my body serves me well, which I feel very fortunate about! Speaking of health, my Father is feeling pretty good, and will take the next test this Wednesday, and our hopes are that they will finally determine where the internal bleeding is happening…and then be able to fix the problem.

I asked our friend Hans Rosendal, recently retired NWS lead forecaster at the Honolulu office, to comment upon the recent very heavy, flooding rainfall event on Oahu. By the way, I’m honored that he would grace us with his amazingly deep understanding of the weather not only here in the islands, but worldwide!  Here’s what he has to say about it:


Glenn asked me to comment and help explain the cause of the recent wet weather along the windward slopes of the Koolau mountain range of Oahu here in late February and early March.  The wet weather occurred over several days as more than 2 feet of rainfall fell in spots from Kaneohe northward to Kahuku. We may ask what kind of atmospheric conditions are needed to produce such heavy rainfall? 
First of all we need to have a lot of water vapor present in the local atmosphere.  We can perhaps blame some of it on our newly developed La Nina situation with its cold water on the equator to the south of us that pushes the peak moisture band of the tropics a little north of its usual latitudes and also changes the large scale flow patterns enough to often produce wet weather here in Hawaii. The La Nina pattern in the atmosphere was slow in kicking in here in our part of the world with the flow pattern actually quite dry and more reminding us of an El Nino dry pattern during the early part of the winter.
We measure the moisture content of the air in precipitable water content.  We may talk about a nearly saturated air mass with precipitable water values of 2 inches or 50 mm or more. That is, if you were able to wring out all the water in the air over a large area you would end up with 2 inches of rainfall. Obviously with as much as 12 to 24 inches of rain reported in some areas in the recent storms, the rainfall was very unevenly distributed with most areas getting little or nothing. That is often the case during flooding situations. The thunderstorm clouds and heavy rain producing cells are anchored to some terrain features, such as a coastline or mountain slope, and a moist stream of convergent low level air out of the southeast feeds this moisture continuously into the updraft region where a divergent outflow aloft in the upper southwesterlies removes the air minus the condensed moisture precipitated on the way up.
Helicity is a term used to characterize the vertical wind flow in this sort of nearly stationary supercell thunderstorms where southeast winds at the bottom of the atmosphere becomes more southerly and southwesterly with height with gradually increasing wind speeds. Sort of like a helix or a screw.  The momentum of the air remains nearly constant with height, as the stronger winds are counter balanced by the lower density of the air as you ascend in the atmosphere.  The tornado producing thunderstorm cells on the Great Plains of Kansas and Nebraska are often of this type of high helicity cells and thus may move or develop erratically to the right of the mean flow direction.
What makes the flow latch on to the terrain during these helicity situations and drop the rain in such concentrated fashion over favored areas?  The Koolau mountain range on Oahu is a favored place for this to occur during southeast surface flow and upper level southwesterlies.  The surface flow is basically parallel to the island chain and to the windward shoreline of Oahu in these situations. Some increased convergence of low level moisture is likely taking place already before the flow gets to Oahu  within these downwind island plumes such as the Molokai plume.

On the larger scale we likely have a troughy region of light winds to the lee of the chain with say 20 kt southeast winds some distance offshore along the windward side. We are thus located along the western edge of the High centered between here and California.  Winds may thus be nearly calm along the shoreline itself.  We therefore have cyclonic shear vorticity in the surface layers along this strip of shoreline from Waimanalo to Kahuku in spite of the flow at large being slightly anticyclonically curved. The property of vorticity is very conservative and thus do not quickly form or dissipate.  There are however ready exchanges taking place between cyclonic shear and cyclonic curvature vorticity. Thus small low pressure centers do develop in these terrain favored spots out of the cyclonic shear and cause the convergence and anchoring mechanism.

Cyclonic vorticity in the boundary layer along the slope of the Koolaus means convergence and heavy rain in the otherwise fairly neutral flow vorticitywise.  The upper southwesterly flow east of the trough located to the west of the islands will result in upper divergence due to the positive (cyclonic) vorticity advection (PVA) which in turn supports and induces the low level convergence and rising motion at mid levels, These synoptic effects seemed to favor Oahu during this particular situation. As an example, the island of Kauai has a more rounded shape than Oahu and the flooding at Anahola in December 1991 was a somewhat similar only stronger pattern where the bulk of the rain fell within a 12 hour period rather than over two days or so in this case.
Thus as long as the large scale pattern of low level convergence and upper level divergence stays fairly stationary, this terrain anchored rainfall can continue over several days. A blocking pattern with a strong 1040 to 1050 mb high far northwest of us near 45N 180W with a cutoff low near 30N 180W or just north of Midway Island was a part of this large scale synoptic weather pattern.  This pattern has not entirely gone away yet so there is still a chance of more wet weather for the Islands during the next week or two.

 

 

 

 

It’s early Monday evening here in Kula, Maui. I just came in off the weather deck, watching the gorgeous sunset…all the while listening to the resident Mockingbird singing its heart out atop the Norfolk Pine next door. The sunset lit up all the high cirrus clouds around now, and in the process turning the waters of Maalaea Bay pink in color. As I was driving up through the pasturelands on the way home from work, I happened to see two Hawaiian Owls, also called Pueo here in the islands. I always think of this as a special occasion, and without fail put my palms together in acknowledgement of their presence. Just now as I was sitting outside, I saw a third one in the pastures just down the hill from my weather tower! I hit the beach again today, with the sunshine so deliciously warm on my bare back. The ocean was just the right temperature, and with no waves breaking, my swim was great. I hope you have a great Monday night! I’m pleased to see quite a few local businesses taking advantage of the link advertisement offer (details just below), as I think of it as a win-win-win situation: it helps businesses with a web presence, its helps the folks who are looking for special deals, or good things to buy or do here in the islands, and it helps me too! I’ll be back early Tuesday morning with the next completely updated weather narrative from paradise. Aloha for now…Glenn.

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