Big Waves in Hawaii. Why?

 There is a lot of excitement in Hawaii today, as large waves from the northwest reach the north coast of Oahu, making possible the Eddie Aikau Big Wave Invitational, a surfing competition only open to the most elite surfers.    This competition REQUIRES waves reach  at least 40 ft in height at the venue:  Waimea Bay.

NOAA has a buoy just offshore and so far it has only reported a significant wave height of around 6 meters (20 ft).   But the highest waves can substantially exceed the significant wave height and waves can grow as they feel bottom close to the shore. (The significant wave height is the average of the highest third of the waves).

NOAA runs a computer forecast model that predicts significant wave height and direction, called Wave Watch 3.   Here is its forecast for 10 AM.   The arrows show the wave direction (from the northwest), with significant wave heights of 15-20 ft.

What was the source of these waves?

Well, to get big waves you need three things (in addition to water!).  Strong winds, a long fetch for the winds to act on the water, and substantial duration of the wind interacting with the water surface.

To get perfect big waves on the NW Oahu coast generally requires a strong low pressure center to move west to east north of the islands.     

And you need the low to follow the right path.   Too far away and the waves have a chance to decline before reaching Seattle.  To near the islands and conditions are too stormy and unpleasant at the venue.   

To put it another way, you need the storm in on goldilocks track.

In this case, a low pressure center at 10 PM last night was in the perfect position northwest of the islands and was usually strong (indicated by the shading, which shows the difference of the pressure from normal).  Goldilocks indeed.

Now, it case some of my Northwest readers are disappointed regarding the big wave action reaching the Hawaiian Islands, I have some good news.   Some very big waves should reach our coast on tomorrow (Monday, see below).  Surfing anyone?

An Intense Christmas Atmospheric River. No California Drought This Year

 One of the most overused terms used by the media is "atmospheric river".   Yes, even more hyped than "bomb cyclone."     But this week there will be an atmospheric river worth noting...and I will discuss below.

Atmospheric River Overuse

Although the term, atmospheric river, was first used in the late 1990s, the phenomenon was well-known for decades before that: a plume of warm, moist air leading cold fronts associated with midlatitude cyclones.

Virtually every midlatitude cyclone/low center has such a plume of moist air.   To see how omni-present atmospheric rivers are, below is map from today of the plumes of water vapor (red and orange colors) and heights at 700 hPa pressure (think of this as pressure at 10,000 ft).   LOTS of atmospheric rivers, with most associated with low centers and their associated fronts.

But not all atmospheric rivers are equal in magnitude.  

This week some particularly strong ones will aim at northern California and southern Oregon, releasing intense precipitation as the moisture-laden area is forced to rise by the regional terrain.

One of the best measures of atmospheric river strength is integrated water vapor transport (IVT), which is calculated by determine how much water vapor is being transported by the atmospheric river over a period of time (essentially wind speed times water vapor content of the air).

Here is a plot of IVT on Wednesday evening.    WOW...that is quite an atmopsheric river!

The result or this  atmospheric river and some weaken cousins this week, will be massive rainfall on the West Coast.

For the 72 h ending 4 PM Thursday, there will be massive precipitation totals predicted (by the UW model) from northern CA through BC, with northern CA/SW Oregon getting hit hardest (over 5 inches).

Here are the totals through the end of the year from the European Center model.  Many locations will enjoy over a foot of rain.

Remember all the talk of permanent drought in CA during the global warming.

Well, such predictions have not worked out well.

Most California reservoirs are above normal right now....and  that is before the big rains this week (see the proof below).  Reservoir levels  will surge with all the upcoming rain.

Snowpack is in good shape for the entire Wast Coast. (see below).

In short, water supply looks excellent for the upcoming year.