A Potent Atmospheric River Will Bring Heavy Rain and Some Flooding to the Northwest

If some folks thought that a strong El Nino would protect the region from heavy rain before the new year, they were mistaken.

A substantial atmospheric river-- a narrow current of high-water vapor content-- will be heading right into our region starting Monday evening, and the precipitation amounts will be substantial.

Let me show you.

Below is a forecast map of total water vapor content through a vertical column of the atmosphere at 10 PM Monday.  The blues and light pinks indicate the largest values.    

Atmospheric rivers like this tap large amounts of water vapor in the tropics and subtropics.   Moisture also converges into the "river" from the sides, not unlike the water of small streams flowing into a major river along its path.

Professional meteorologists generally prefer to look at another measure of the potency of atmospheric rivers called integrated water vapor transport (IWVT),  which is water vapor content times the wind speed.  

It is a measure of how much water vapor is being transported by the wind and is better correlated with rainfall potential.

Here is the IWVT at the same time as the image above (10 PM Monday).  Blues indicate large values.  

You will note that there is a lot of moisture coming into the Northwest coast, but little extending to the northeast. Where did all the moisture go?

It rained out!

Below is the forecast total precipitation through Monday at 4 AM (start at 4 AM Sunday).  Precipitation for the whole region, but heaviest over the Oregon terrain.

Then the atmospheric river comes in (JAWS music should be played now).  The total precipitation through 4 AM Tuesday is impressive, with 7-10 inches over the mountains of Washington and SW British Columbia.  Most of that atmospheric river moisture is being pulled out of the atmosphere by precipitation over regional terrain.

One day later even more, particularly over Oregon.  

Soils are now saturated throughout the region.  Adding this precipitation will result in rapidly rising river levels, some to major flood levels.

Consider the forecast of the NOAA/NWS Northwest River Forecast Center (see below).  A number of rivers will get to major flooding (purple) and others to major flooding (red).

Consider the Snoqualmie River near Carnation.  It will get considerably above major flood stage and will set a record for the period.

Do you want to see something very, very impressive?  Go to Snoqualmie Falls on Wednesday.  You won't believe what you will see.

One thing I can guarantee you.  The City of Seattle will be able to bring the water level of its reservoirs way up and there won't be any talk of low water levels.  Take a long shower if you like....😔

Considering that we are in a strong El Nino period, which will probably reduce snowpack in the spring, reservoir managers should try to save as much of this water as possible.

Strong Winds and Heavy Mountain Snows Coming to the Pacific Northwest

After a period of high pressure, weak winds, fog, low clouds, and little precipitation, the weather in our region is about to get quite intense and interesting.

Strong winds will soon buffet western Washington and heavy snow (multiple feet) will fall across our regional mountains.

A front went through this morning and our region is now in moist, unstable northwesterly flow as shown by the visible satellite image this morning (below).  The white and dark areas offshore are convective showers and you can see the intense cloudiness on the western slopes of the Cascade associated with substantial snow showers.

White-out conditions are now occurring in the mountain passes, illustrated by the WSDOT pass cam below.

Let me show you predicted snow accumulations from the latest University of Washington model forecast.

Through 4 PM the Cascades will get as much as 6-12 inches, and eastern Washington be whitened.

The total through 4 AM Saturday is much more impressive, with 1-2 feet over higher terrain.

And by 4 PM Saturday, many locations will have experienced 2-3 feet.  Substantial snow will fall over northeast Washington and northern Idaho.

The snow season will begin with a bang and many ski areas should be able to open.  Travel across the passes should be dicey during the next day...so keep that in mind.

The regional snowpack had declined to about 50% of normal.  By Sunday, we should be at or above normal levels in most locations.  I should note that other modeling systems (such as the European Center) agree with this forecast.

And then there are the winds.....

 Some of you will experience power outages overnight.  I am charging up my devices as soon as I get home from the UW!

A low-pressure system will move eastward to our north, creating a large north-south pressure difference across western Washington (see forecast surface pressure map for 4 AM Saturday).  That means strong winds.

The latest NOAA HRRR forecast (available on the City of Seattle Windwatch site built by the UW) predicts powerful winds overnight. 

 Below are the forecast gusts at 1 and 3 AM.  First, the coast and NW Washington will get hit and then Puget Sound, some with winds exceeding 50 mph.  Such winds guarantee some power outages.

The only good thing is that many trees have lost their leaves, making them less vulnerable

Considerably more active weather is predicted beyond this weekend, including heavy rain and potential flooding, including a major flood on the Snoqualmie River (see the NW River Forecast Center plot below).  

I will talk about that threat in a future blog......one major weather challenge at a time!

High Pressure and the Northwest's Low-Cloud Affliction

 The visible satellite image this morning was stunning.

Low-elevation clouds covered nearly all of Washington, except for the highest elevations (see image)

Some folks were in fog while others looked at a low-stratus cloud deck.

And this cloudy murk was cold, with much of the state below freezing (see minimum temps this morning).   Freezing fog was widespread.

Cold, cloudy, foggy and icy.  Yuk.

What can you blame for this unpleasant situation?   

Persistent high pressure.  A strong ridge of high pressure aloft.

The upper-level map (500 hPa pressure, about 18,000 ft) on Sunday evening shows the situation (see below).

The difference from normal of 500 hPa heights (or pressure) for the last week was way above normal  (see below, red indicates strong ridging...which is the same as high pressure aloft).

Translation:  the high-pressure aloft was very strong and persistent.

In the summer, such a pattern would give us a heat wave.

But in winter we get a cold wave!  But why?

High pressure is associated with sinking air aloft, which kills middle and upper-level clouds.

Without clouds, the Earth can effectively emit infrared radiation into space.  There are no clouds to get in the way!  Nights are very long and the sun is wimpy, so solar heating is weak.

The ground cools more effectively than the atmosphere aloft and this helps create an inversion, with temperature WARMING with height.   The inversion is also strengthened by the high-pressure aloft, whose sinking motion warms the air aloft (air warms as it sinks due to compression by the higher pressure at lower levels).

The inversion is a very stable feature and prevents the mixing of air at lower elevations.   It protects the low-level cold air from mixing out.

Want to see the inversion?  Here is a plot of temperature (red) and dew point on the Washington coast at Quillayute on Tuesday morning.  Temperature is increasing with height at low levels.  An inversion!

Each day the air got colder near the surface.  Cold air can hold less water vapor than warm air.  Water vapor starts to condense out into low clouds and fog.

And it is even worse than that.  The top of the clouds reflect solar radiation back to space, enhancing cooling.  Clouds are good emitters of infrared radiation.  More cooling.   Low clouds are cooling machines!

So if you want to blame the cold, cloudy weather on something, blame the high pressure!

We escape this run-away cooling in the summer because of the far stronger solar radiation, which is able to heat the surface and destroy the low-level inversion.

Are King Tides a Threat During the Next Two Months?

A number of media stories have been talking about King Tides and flooding during the past few days, and I have gotten several emails from worried readers.

So let's talk about the reason for King Tides and evaluate the threat.

The highest tides of the year...the King Tides... typically occur this time of the year for two reasons.

First, the astronomical setup is ideal, because the Earth is closest to the sun around the New Year.

Second, this is the stormest time of the year, with Pacific cyclones associated with low pressure, and low pressure causes the water level to rise.

Monthly high tides, also known as spring tides, occur twice each month when the sun, moon, and earth are aligned.

And this alignment is enhanced during the winter because the Earth is closer to the sun during our cold period.  Believe it or not, the Earth is nearest to the sun during our winter!  (see below).  Thus, the gravitational attraction of the sun is larger during winter, which produces a greater maximum daily tide:  the KING TIDE.

Finally,  regional low pressure increases the water level.  

How much?  When sea level pressure falls by 1 hPa (or 1 mb), the water level rises by about 1 cm.   So a deep storm that causes pressure to fall by 25 hPa can result in a water rise of around 25 cm or roughly 10 inches.

Now let's look ahead.

Take a look at the predicted astronomical tide levels for November, December, and January below (the green line is today).  For November, tides will max out during the next few days around 12.5 ft above Mean Lower Low Water (MLLW), the average level of the lowest tide for each day computed over a 19-year period.

December will have a number of days (mainly during the middle and end of the month) that will get to the same level.    Mini-King tides.

Mid-January will bring max tides that are a bit higher...around 13 feet....so that will be the time of greatest vulnerability.

There will be little problem of high tides and flooding without a strong low-pressure system moving in...so we need to watch the forecasts for the next few months.   Right now pressure is well above normal...and will remain that way for several days.

Beyond, 10 days the forecasts have little skill...so we are going to have to wait and see.

Want to know about the forecast pressure for the next week?  Check out the ensemble of many forecasts below for Seattle's sea level pressure.  Forecast uncertainty gets very large after December 2 (the forecasts are not in agreement).

The latest model run has a decent storm on December 3 (see sea level pressure forecast below) with a deep low-pressure center heading into Washington State.  Fortunately, the astronomical tides will be low then.

 

The Remarkable Life of Steve Pool

I am very sad to report the passing of Steve Pool, past chief meteorologist at KOMO TV, and a dear friend for many decades.

Steve was one of the most exceptional and generous individuals I have ever known.

A brilliant communicator.  A kind and generous soul.  A highly intuitive and insightful meteorologist. And someone who gave so much to our community.

I first got to know Steve during the mid-80s, when I got a call from KOMO-TV asking whether I would instruct him in meteorology; he had just taken on weather responsibilities at the station.   

For the next year and a half, Steve and I worked together intensively to cover the essentials of meteorology, Northwest weather, and weather forecasting.  An extraordinarily bright guy, Steve quickly mastered a huge volume of weather knowledge and became a very able local weather analyst and forecaster.   He was determined to get his forecasts right, and we would often talk during the late evening before his 11 PM broadcast.

As I got to know him better it became clear to me that he was a preternaturally talented communicator and that I had a lot to learn from him in that domain.  

And I mean much more than a smooth delivery.  He had the ability to connect with people.  A disarming smile, great empathy, and a personality that was upbeat and sunny.  I don't think I ever heard him talk ill of another.

We would regularly talk about difficult forecasting situations, driven by his strong 

commitment to not only get the forecast right but to get the explanation correct.

But Steve had many other dimensions and none were as noteworthy as his dedication to the community. 

One day he called me, suggesting that some UW students could intern at KOMO. This 

internship was a great success, giving roughly 3-4 students a year a chance to 

assist in developing the daily weather offerings at KOMO, including the use of a complex 

graphics system, preparing forecasts, and even practicing in front of the camera. All under the 

eyes and with the assistance of a master weathercaster. 

Many of Steve's student interns went on to successful careers as TV meteorologists, including Shannon O'Donnel, M. J. McDermott,  Kelley Bayern, Nathan Santo Domingo, Jefferson Davison, Scott Sistek, Brandon Wholey and Matt Leach--to name just a few.  Scott  Sistek worked for Steve for 20 years and together they wrote an excellent, conversational book on Northwest weather that received an impressive 5-star rating--Somewhere I Was Right.

Shannon O'Donnell, a past intern, is now chief meteorologist for KOMO.  And in Steve's 

tradition, she is teaching a course in my department on weather communication,  including a Dawgcast available online to the general public. 

Steve's influence will be very, very long-lived.

Steve was extremely interested in outreach to young people, and a good example of his efforts 

was the annual Weather Education Day at Safeco Field. Before the game, THOUSANDS of 

students would enjoy learning about basic weather principles and local weather, presented by 

Steve and his guests (I even did it one time).

Steve became a mega-local celebrity, but this did not change him, and he used that celebrity 

to assist in fun raisers and other gatherings for local charitable organizations and importantly 

for his alma mater, the University of Washington. He helped raise millions of dollars for a 

whole slew of important causes and was the MC for many public events put on by my 

department.  On the side, Steve built a small enterprise, assisting folks in learning how to be accomplished communicators.

Shannon O'Donnell, myself, Steve, and Scott Sistek

Steve loved his family, always talking about his wife Michelle and their talented daughters,
Lindsey and Marissa.  When I went over to his house he talked with great pride of his father, who served in WWII  and was shot by the Germans in northern France after D-Day. Steve was a history buff and had a whole wall of history books in his study, as well as the many Emmys he had won over the years.

But what impressed me more than all his accomplishments was his sheer bravery when his health failed.   I don't want to say any more about it, except that he stayed positive and hopeful to the end, where many would have surrendered to despair.  

An extraordinary human being that has given so much, to so many.  An individual who has profoundly changed our lives in so many positive ways.  He will be missed.

A Very Dry Thanksgiving, But No Record

There will be no precipitation over Washington State during the Thanksgiving holiday.

None.  Zippo.  Nada.

Which is kind of surprising since this is the wettest time of the year for our region. 

To show how unusual it is, here is the climatological probability of measurable precipitation at SeaTac Airport for each day of the year.  The last three weeks of November are wetter than any other period, with roughly a 65% chance of rain on any day.

But this year, there will be no rain--something predicted by virtually every weather modeling system.

For example, the European Center forecast model has nothing through Tuesday morning (see below).

So how unusual is a totally dry Thanksgiving period.  Let's find out!

I found that total precipitation at SeaTac airport for November 23-27 for every year of its record (see below).   2023 will be zero, of course.  There have been several totally dry Thanksgivings, the last being 2015.  About ten Thanksgivings have been dry or very, very close to totally dry.

As shown by the brown line, which shows the trend over the entire period, there is little trend, either up or down, over the period of record (since 1947)

Will rain come back?   The latest model runs suggest the answer is yes, starting later next week.

In fact, the latest NOAA/Climate Prediction Center forecast for 8-14 days is for wetter than normal conditions over the West Coast.  

So enjoy the dry weather....it won't last.

Edgy Clouds

Every week, several blog readers send me cloud photos to identify.  So there is plenty of interest in clouds...and for good reason.  A little knowledge about clouds is not only fun but can be useful as well.  

So let me provide some cloud identification 101 today, starting with what you can tell by the characteristics of cloud edges.

Clouds are made up of either ice crystals, water droplets, or a combination of the two.

Importantly, you can differentiate between ice crystals and water droplets by looking at the cloud edges.

Clouds made of water droplets have sharp edges.

Like cumulus clouds:

Or the top of a fog bank:

On the other hand, ice crystal clouds have fuzzy, indistinct edges.

Such as cirrus clouds.

Or cirrostratus clouds:

Did you know that the bases of low-level clouds change sharpness depending on whether precipitation is falling out of the cloud?

When low-level clouds are not precipitating, the cloud base presents a lot of structure (see below)

But once precipitation starts, the cloud base tends to fuzz out and become less sharp and structured. 

In summary, the edges of clouds provide a lot of information for the knowledgeable observer.

The Olympics Mountains Can Affect the Weather in Ellensburg and Wenatchee

 The folks living on the eastern side of the Cascades might think that the Olympic Mountains have little influence on their weather....but they would be wrong.

Consider what happened today.

A Puget Sound Convergence Zone set up, producing a band of precipitation stretching from the northeast slopes of the Olympic Mountains, across Seattle and then eastward to near Wenatchee!

Just a reminder before I go on..... Puget Sound Convergence Zones are associated with westerly or northwesterly flow that is deflected around the Olympics and which converges together to the east of the Olympics.

When low-level air converges together at low levels, the air is forced to rise, producing a band of clouds and precipitation.

This morning and into the afternoon, a convergence zone formed to the east of the Olympics, creating a cloud band that stretched for many miles,with the cloud band ending near Wenatchee (see the figure below with arrows showing the beginning and end of the convergence zone cloud band).   Interestingly, it was totally clear to the north and south of the band over western Washington... a typical occurrence by the way.

There was rain..some heavy.. within the clouds band, as shown by the radar image around 1 PM.   At higher elevations there was snow.

The high-resolution UW modeling system predicted the convergence zone band and its extension across the Cascades (see below).

The Thanksgiving Forecast

It is very nice to enjoy a walk, run, or any other outdoor activity before the holiday mean.  But unfortunately, Thanksgiving is one of the climatologically wettest and stormiest days of the year in our region.

But I have some good news:  Thanksgiving Day 2023 will be dry.   It will be partly cloudy with highs in the upper 40s over western Washington.

Thursday afternoon, a ridge of high pressure will have built over the eastern Pacific (see the weather map at 500 hPa..about 18000 ft forecast for 1 PM Thursday).  A cool, dry pattern.  I would expect low clouds in the AM, breaking up in the afternoon.  Perhaps you can get enough exercise to deserve a little more turkey or whatever you enjoy...