Unusually Low Pressure Caused Sea Level to Rise

Have you felt lightheaded during the past day?  

Perhaps you should have, because the atmospheric pressure has been unusually low.

Low enough to cause a significant rise in sea level.

Consider a plot of sea-level pressure for SeaTac Airport since July (below).  The pressure dropped to 990 hPa, the lowest observed during the period, by a LOT.

Our extremely low pressure was associated with a deep low-pressure center immediately to our west (see the analysis at 4 AM Friday).   At this time, the low center had a central pressure of 968 hPa, which is low for this latitude.

The average pressure for Seattle in November is 1017 hPa (the average around the nation is 1013 hPa), so our pressure on Friday was 27 hPa below normal.

Localized low pressure causes sea level to rise by approximately 1 cm for every decline of 1 hPa.  (See schematic below).

Thus, sea level over Seattle was elevated by 27 cm or 10.6 inches by the low pressure.  Almost a foot!

You want to see the low pressure in action on sea level?   

Below is a plot below of predicted (blue) versus observed (red) sea level in Seattle.   The difference between the two has increased this week as the low-pressure center approached, with sea level 2.2 feet above normal.  

So about 40% of the anomalous sea level rise was due to the low pressure.

But the effects of the low center on sea level do not end there.

The low center is associated with powerful westerly (from the west) and southwesterly winds (see forecast surface winds at 1 PM Friday below).   Winds near the coast were strongly from the south (southerly)

Such southerly winds push on the ocean surface, which flows TOWARDS the coast by the Coriolis force, resulting in a HIGHER water level.  See schematic below of this situation.  Want to impress your friends?  This is called Ekman Transport.

But the high water fun caused by the low center does not end there!

The offshore low with strong winds over a large area (long fetch) produces big waves that can bring the water levels up even more on the coast (see forecast wave heights for Friday morning below).

There are other, more minor contributors to short-term high water levels, such as a Kelvin wave associated with the El Nino moving up the coast (several inches).     

The good news is that water levels should drop as the low center weakens over the weekend.

The Large Effects of El Nino On Our Current Weather

Our current weather is being profoundly influenced by El Nino.   

And that influence is not about to end.

Let me demonstrate this to you.

The following is a map of the anomaly (difference from normal) of upper-level heights at 500 hPa pressure for the past week.  You can think of this as the difference from normal of pressure around 18000 ft.

A huge trough of low pressure is off our coast (purple color).  Higher pressure/heights over the western U.S. (yellow color)

Here is the forecast of the same upper-level heights/pressures for the next 5 days.  Very similar pattern.  Deep low off our coast aloft.

Now, let me show the pattern for very strong El Nino winters. 

Mamma Mia!  A strong offshore trough/low just like the last week!

This upper-level El Nino pattern is associated with a persistent low-pressure area off our coast (see below).

Which is VERY like the pressure pattern we have seen the last few days.

To illustrate, here is the sea level pressure forecast for tomorrow morning at 8 AM. Looks just like the strong El Niñoino pattern!  Impressive low off our coast.

Moisture tends to wrap around such "El Nino" lows, resulting in the heaviest precipitation on the coast and south of us (see plot of moisture transport below)

During El Nino years, southern Oregon and northern California tend to get a lot of precipitation, and the current forecasts show exactly that (see total precipitation forecast through Wednesday morning below).

Vancouver Island gets a lot of precipitation as well because of the strong southerly flow circulating around the low-pressure system, which is forced to rise by the island's terrain.

There is a lot of gloom and doom in some of the media about this pattern, but it is actually quite favorable. 

The northern CA reservoirs are currently below normal, as are the reservoirs of southwest BC.  They will get a welcome refill.

El Niño brings substantial precipitation to the Colorado River drainage, which is badly needed to fill the reservoirs such as Mead and Powell. 

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Announcement

I will hold a special online Zoom session at 10 AM on Saturday for Patreon supporters. I plan to talk about the seasonal forecast in much more depth and answer your questions.

The Atmosphere is Providing Water Where It is Needed

There is a deep connection between meteorology and religion, and there are times when one wonders about divine intervention in our weather.

This is such a time.

Last year brought low snowpack for most of the West.  

Some were worried about water shortages and poor crops.   

But well-timed rain and substantial reservoir storage saved the day, with agriculture doing well and sufficient water available for human consumption.

Divine intervention?

This fall started out relatively dry, and some folks were worried, including a certain local newspaper.

But again, higher powers intervened.   During the past 72 hours, there have been substantial amounts of precipitation over the region (see below)

 A potent atmospheric river was the cause of our wet bounty (see below).  

Local reservoirs are refilling, including some low ones on the Olympic Peninsula (such as Wynoochee Lake below)

Other regions have been in need, and divine intervention has been and will be evident.

For example, California, the source of many of our fruits and vegetables.

Well-timed rainfall last year filled the reservoirs, but snowpack was low.   As a result, some of the big reservoirs are now below normal, such as Shasta and Oroville (see below).

Fortunately,  the current forecasts promise significant intervention, with massive amounts of precipitation over the Golden State during the next ten days (see below).   Remarkable.

This wet bounty includes LOTS of snow in the Rockies and over the drainage of the Colorado River (see below). A miracle.

Not getting religion yet?   What about British Columbia, which has suffered from a lack of precipitation this fall?    

Vancouver's reservoirs are quite low.

Fortunately, impressive amounts of precipitation are forecast for southwestern BC through next Monday morning

The water situation is rapidly improving on the West Coast.  The forecasts are highly promising.

There is only one thing to say about it: 

Forecast Triumph

Yesterday, a strong front, associated with strong winds gusting to 40 mph or more, moved through the region, resulting in numerous power outages.

As I will describe below, numerical weather models predicted this event with great skill, something we could not have done 20 years ago.   

This is a triumph of modern weather prediction technology.

Let me demonstrate this to you.  

Below is the radar image around 4 PM Saturday, based on the radar observations from the Langley Hill radar near Hoquium.  An intense radar return was apparent offshore (the thin yellow and red line, indicated by the red arrow).   This is the front.

Fronts associated with a narrow, cold frontal rainband were first described by  UW researchers, such as Professor Peter Hobbs and associates.

Even more impressive is a video of the cold front approaching the coast (from a wonderful phone app called RadarScope).

Here is a forecast of a simulated radar image, made 24 hours (from 4 PM) before the event is shown below.  It was made by the UW WRF model run by my group.  

Almost perfect.  Very strong simulated front.

A closer-in view is below.

A forecast made 60 hr before the event was virtually the same.  Strong front in the same location

A forecast made on Thursday evening, showing the strongest near-surface wind gusts, indicating strong winds over the region.  Strong enough to worry about.

The winds for Seattle made for Seattle City Light as part of the UW WindWatch system were clearly accurate, showing the threat of strong wind (the black dots are the strongest observed with the city; the gray and colored lines indicate model forecasts)

The models were very, very clear.  

There was a prediction of 30-45 mph winds over the city.  And that means power outages, particularly early in the storm season.

Such an emphatic forecast by the models was the reason I blogged about it on Friday, making it clear that some would lose power.

In fact, over 100,000 customers lost power over western Washington last night (see below).

And over 46,000 customers in Seattle lost their lights.

This kind of skillful forecasting occurs time and time again, based on highly skillful weather model forecasts.

Another reminder that it is no longer appropriate to make fun of forecasters and weather models.   😀

No longer appropriate

A SuperFront Will Cross the Northwest This Weekend

Most weather fronts that cross the region are unremarkable due to their passage over the relatively warm, uniform waters of the northeast Pacific.  

This weekend will be different:  a  SuperFront will make landfall late tomorrow (Saturday).  

Some of you will lose power.

Let me show you simulated radar images for three times:  10 AM, 4 PM, and 9 PM on Saturday from the UW high-resolution forecast system (from the 4-km grid-spacing domain).  Red indicates very strong winds, followed by yellow.  The arrows point to the frontal locations.

Wow. Very intense, sharp front.  Very heavy rainfall.

The high-resolution 1.3 km simulation run by my group at the UW produced an even more intense front (forecast around 8 PM shown).  Very impressive.

What about the predicted winds?

The City of Seattle supports the most high-tech wind prediction system in the country (called SeattleWindWatch, run at the UW), which includes an ensemble of many wind predictions for estimation of the uncertainties.  

The forecast below is for the winds over Seattle.  Several of the simulations predicted winds reaching 40 mph. Over the water, winds could be stronger,

 Winds will rev up after dinnertime and peak overnight.

Such strong winds, this early in the wind season when some leaves are still on the trees, mean power outages.  

As noted above, wind will be strongest over the water (see forecast below for 4 AM).   I suspect most of the power outages will be in such areas.

For the whole region, the strongest winds will be on the coast (see map of strongest wind gusts below).  Importantly, there will be areas of intense gusts on the lee/downstream side of some of our highest terrain, including the eastern slopes of the Cascades.

In addition to wind, the front should bring substantial ( and welcome) precipitation to the region, with the total through 4 AM Sunday shown below

So, if you live west of the Cascades,  get your meteorological popcorn out and be prepared for a dramatic meteorological show later tomorrow and early Sunday.

Wind, Waves, and Wet

This weekend, we will "enjoy" the meteorological trifecta of winds, coastal waves, and substantial precipitation.

First, the winds.

The forecast sea level pressure for Sunday at 10 AM predicts a strong 994 hPa low center to west of the Washington coast.

The associated winds (below) are impressive, with sustained winds of 50-60 knots to the south and west of the low center.    This is known as the "poisonous tail" in the weather business.

Such strong winds over a large area (large fetch) will produce large waves that will reach our coast later this weekend (see below).   These waves will climb to 45 feet offshore and over 20 feet on our coast.

There will be good wave watching on the Washington coast on Sunday.

As the low moves northward up the coast on Saturday and Sunday morning, wind gusts will increase along the Washington coast and   over the western Washington interior  

The forecast gusts at 4 AM Sunday are shown below.  Strong gusts (50 kt and more) on the coast; 30- 45-knot gusts in the interior, particularly near the water.

The UW runs perhaps the most sophisticated wind prediction system in the country for Seattle City Light (called WindWatch), and it is predicting 40 mph gusts in Seattle on Saturday night and early Sunday. (see below)

As the first major blow of the season, with lots of vegetative growth over the summer, there will be power outages.

 

My "pro-tip":  charge up your electronics and lights on Saturday.  You won't regret it.

Finally, there is the wet part of the upcoming weather.

Below is the predicted precipitation accumulation through Monday at 4 AM.  Very wet in the Olympics and the northern Cascades.  Plenty elsewhere.  Reservoirs will begin to fill.

This is just the beginning of the wet/stormy season in the Northwest, with a lot more on the way.   Typical November conditions, and there is no going back.