January 19, 2021

Colder Air and Limited Lowland Snow Ahead

Our region will soon experience colder temperatures and substantial snowfall in the mountains.  The snow level will fall and virtually everyone will see some snowflakes.

But the truth is that we will NOT be experiencing the primo Arctic cold air associated with most major lowland snow events, with minimal snow falling around Puget Sound near sea level.

But there is a substantial lowland snow threat and for that one will have to look southward towards Portland and western Oregon.

Portland May Get Much More That A Dusting

The transition starts on Thursday as a low pressure center moves south of us, pulling cooler air southward from British Columbia (see sea level map below for 1 PM Thursday, with blue colors being colder air).


The wind gust forecast for Thursday morning shows northeasterly winds pushing through the Fraser River Valley, across the San Juans, and out the Strait into the Pacific.  Cool, northerly winds will also push southward into the Pacific.

But there will be little moisture and no lowland snow:  the low center is too far south and west to do us much good.

But life is all about second chances, and lowland snow lovers will gaze hopefully at a frontal system approaching early Sunday morning (1 AM sea level pressures and low level temperatures shown below).  Relatively cold air will be in place (blue colors) and this is night when temperatures are cooler.


The front will have precipitation with it, but temperatures near sea level will be marginal for snow.

The UW WRF model's accumulated snowfall (NOT SNOWDEPTH) through 7 PM Sunday, shows half a foot or more in the mountains with the snowfall rapidly decreasing near sea level (very little at the lowest elevations).   Snowflakes will be seen everywhere, but only  minimal amounts near sea level.  The European Center forecast is very similar.


But like a late-night commercial, I must interject:  wait, there's more!

The third act.  

On Tuesday, a MUCH stronger and potent low center will be moving southward off our coast.  One with much more precipitation and forcing of upward motion (see sea level pressure map for1 PM Tuesday). 

Notice the large east-west pressure difference down the Columbia Gorge, which could bring strong (cool) easterlies into Portland and environs, while lots of precipitation is falling into the region.

This pattern can produce substantial snow around the northern Willamette Valley and Portland.

Take a look at the 48-h snowfall ending 4PM next Tuesday.  Portland gets hit with nearly a foot of snow.  And lots in the Oregon Cascades.


If the forecasts are wrong and the low is farther north, then Seattle could get a big event.

Finally, let's look at the excellent European Center ensemble snow forecasts for Seattle, based on running their model 51 times!.  A good way to judge uncertainly.  Each horizontal stripe is one forecast and the cumulative amounts are color coded.  The average of the forecasts are shown by the green bars.  

Nothing to get excited about.  Most ensemble members are predicted some snowflakes at SeaTac airport (which is relatively high at 452 ft).  Total snowfall (not snow depth) of around an inch. There will be less at lower elevations.  Bummer.

But what about Troutdate, at the western exit of the Columbia Gorge and just NE of Portland?  Wow...  a lot more!


We are still a ways out and the forecast can change.  The mountains will get plenty--you can bank on that.  Skiers will be happy.  Water managers will smile.   But those wanting snow near sea level may remain looking longingly at the sky.....











January 17, 2021

The Stratosphere Has Warmed Profoundly This Month. What are the Implications?

 Earlier this month, stratospheric temperatures warmed by roughly 100F over a period of a few days, in what is known as a Sudden Stratospheric Warming (SSW).  Should you be concerned?  The stratosphere, by the way, is the layer of the atmosphere from roughly 10 to 50 km above sea level.

As we will discuss, such stratospheric warmings are sometimes associated with distortions and alterations of the winds and temperatures in the lower atmosphere, resulting in anomalous weather from heat waves to snow storms.  And major changes in the infamous polar vortex.

There are actually two polar vortices:  one high in the stratosphere 
and another in the troposphere

Based on this incipient warming, some media has been warning about severe weather for several weeks, including major snowstorms over the eastern U.S. (see below).  




The Warming

    Early in January, the temperatures high in the polar stratosphere started to warm suddenly and profoundly.    Take are look at this NASA plot of temperatures at a pressure at 10 hPa (roughly 85,000 ft above the surface) at the North Pole for this year (red/pink) and last year (blue) by date.  The solid black line shows the average temperatures and the gray shading illustrates typical variability of the temperature at that location.

There was quite a warming in early January to about 250 K, roughly 50 K (or Celsius) above normal.  That is 90F above normal!  Wow.  

This is a sudden stratosphere warming, something we typically see once a year in winter.  You can view another such warming last winter in March of last year (blue colors).  


Importantly, this month's warming did not last long and is predicted to be gone completely during the next week.   

Why do such warmings occur?  A lot of research has shown they are associated with high amplitude waves in the troposphere that propagate vertically into the stratosphere and disturb the flow there.  Waves caused by mountain ranges and land-water contrasts, among other reasons.   I have personally done research on such issues, but I won't get into the details here.

To understand the potential impacts of the stratosphere warming, let's go back to the figure at the top of the blog, showing the two polar vortices (shown again below).  During a normal winter period, there is an area of very cold air in the upper stratosphere near the North Pole, which is not surprising consider since there is virtually no solar heating!   A jet westerly (from the west) jet stream (the polar night jet) surrounds the cold air and in fact creates a protective barrier around it.


In the lower atmosphere (troposphere), there is also cold air near the pole and a westerly jet at its southern boundary:  commonly called the jet stream and strongest from roughly 25,000 to 35,000 ft above the surface.   

The undulations of the jet stream and the associated cold air to the north have a HUGE impact on weather near the surface.   For example, a southward undulation can bring a cold wave and snow;  a northward undulation, warm temperatures.

How stratospheric warming affect surface weather

When a polar stratosphere warming occurs, it is associated with strong sinking (which causes warming by compression); the weakening of the temperature difference between a cold pole and warmer air to the south causes the polar night jet to weaken and buckle, and the stratospheric polar vortex circulation weakens and can get distorted or even move off the pole.

The effects of the stratosphere polar warming and undulations/distortions of the cold air tends to propagate downward into the troposphere, where the tropospheric polar vortex can weaken and the tropospheric jet stream become wavier.

For the event of this month, it was clear that the upper jet  stream weakened during the warming. To illustrate, here is the eastward-directed (zonal) winds at 60N for a level near the boundary between the stratosphere and troposphere (150 hPa pressure, about 45,000 ft).  The winds (purple) dropped below normal after the warming began early in this month.


But has this stratospheric warming had a major impact on the undulations of the jet stream well down in the troposphere where we are?

To examine this, I plotted the difference from normal of the heights of the 500 hPa pressure surface (roughly 18,000 ft).  You can think of this as pressure. And the plots show averages over one week.

For the week prior to the warming (Dec. 26- January 1), there are a lot of strong anomalies (difference from normal)...including some anomalous ridging (high pressure over the pole)

The week following the warming (January 10-16th), the polar warming has weakened but perhaps has spread around a bit.  Nothing to write home about.  One big changing was the ridging (high pressure) over the West Coast and troughing over the eastern US., but hard to point at the warming as the cause.  And, in fact, the eastern half of the U.S. has been relatively mild this winter...no severe cold outbreaks.  And persistent blocking (locking up of the atmospheric flow) has been relatively absent this winter.

Looking ahead this week, the models are going for cold weather later in the week...but not in the eastern U.S, BUT OVER the Northwest.

The latest European Center forecast for 500 hPa height anomalies (think of difference from normal of pressure at 18,000 ft), shows higher than normal heights over near the pole and a major trough (low pressure) over the western US.  The kind of pattern that occurs more frequently in La Nina years (as this year is).

Let me make it clear, this pattern will not produce colder than normal temperatures over the eastern U.S. as some of the media were calling for.

And yes...this IS a snow threat for the Northwest lowlands. But I will wait until Tuesday to talk about that in depth.


January 15, 2021

New Podcast: Upcoming Cold and Potential Snow, and Why Did Wednesday's Storm Forecast Go Wrong?

We have an interesting week ahead (more below).  We start with generally dry conditions for most days through Thursday as high pressure build over the region.   But as the high pressure moves offshore, our region moves into much colder northerly flow.  And embedded in that northerly flow there are some wet disturbances that could bring lowland snow.   Too early to have any confidence in the forecast.  

And then there was the windstorm on Wednesday morning.  Why did the US model underplay it and the vaunted European Center model get it right?   I talk about all of this on the podcast.

Here is my podcast:
Click the play button to listen or use your favorite streaming service (see below)

But since I know many of you are interest in cold and snow, let me give you a view of the latest forecast for the end of the week.

The latest UW WRF pressure and temperature forecast for Friday morning is quite cold, with blue colors showing low-level temperatures cold enough for snow.  But in this forecast, not enough moisture, with strong northeasterly winds.


The U.S. ensemble system (GEFS) that runs the global forecast model many times to get at uncertainty is emphatic and confident that our temperatures will plummet.   Get your wool sweaters ready!


The highly skillful European Center model is forecasting some light lowland snow over portions of the region (see forecast snow totals over the next ten days)
Image courtesy of WeatherBell, Inc.

And the European Center forecast for Seattle projects cooling temperatures and .7 inches of snowfall.


The cold you can bank on.  The snow has a lot of uncertainty.  Stay tuned.

You can stream my podcast from your favorite services:

January 13, 2021

Intense, But Small Low Pressure System and Strong Front Brings Extensive Power Outages around Western WA

It's just after midnight winds are really blowing out there now.....I can hear branches snapping.  Lights are flashing on the horizon and it's not lightning....it is the green colors of transformer fuses blowing.

By 1 AM about 67,000 Seattle City Light Customers have lost power and roughly 308000 Puget Sound Energy customers were in the dark.'  A major power outage for the region.




Winds are gusting to 50-60 mph last night.  The max winds through 5 AM are shown below.  Lots in the 40-50 mph, some as high as the mid-60s.


But if you want to be impressed, the max wind gust hit 121 mph at Alpental Summit (see below).   That is serious wind.


The peak winds at Seattle's West Point reached 51 knot (59 mph)...and peak wind on the Evergreen Floating bridge got to 56 mph.



Where did the strong winds come from?   A small, but intense, low pressure system passed just to our north, producing a large north-south pressure difference (gradient) over western Washington (see forecast map of sea level pressure at 10 PM))


Let me be honest.....meteorological guidance underplayed this seriously.  We knew it would be windy, but the forecast models (such as the UW ensemble) were only suggesting sustained winds of 15-20 knots with gusts to 25-35 kt (see UW ensemble forecast from yesterday morning).  Time is on the x-axis and 06/13 was 10 PM last night.  This shows the forecast of sustained winds (averaged over minutes) at Seattle Tacoma Airport, where the max sustained wind got to 28 knots.


Yesterday evening, National Weather Service forecasters in Seattle could see from the satellite imagery that the low was more intense and tracking upon an optimal path and put out a wind advisory.  

Below is the water vapor image at 10:30 PM last night.  It shows the amount of water vapor in the middle to upper troposphere (roughly 15,000 to 30,000 ft).  There was a pronounced and well defined "dry slot", which shows strong descending air behind the storm....a sign of strength.  The low center is generally at the apex of the dry slot.  This was threatening for western Washington and a short-term warning of the blow that would follow.


Importantly, the European model (ECMWF) produced a far better forecast than the American GFS.  Here are are the maximum winds during the event from the European model initialized (started) at 4 AM Tuesday. Gusts reaching to the mid 50s (mph) over Puget Sound and much higher over the mountains.   Good forecast.

Far less in the U.S. model, with winds only gusting to around 40 mph over Puget Sound.  The European Model did far better with the intensity and track of the low.   I often talk about how the U.S. needs to improve its forecasting capability in this blog...this is a good example to keep in mind.




January 12, 2021

Heavy Precipitation and the Mid-Winter Water Update for Our Region

Note: My next blog will be on the sudden stratospheric warming over the Arctic and the implications for the long-term weather forecast. Major changes in the large scale weather circulation in the offering!
__________________________________________

As promised, a potent atmospheric river has hit our region, with very heavy precipitation on the upwind (windward) sides of regional terrain.

Let be begin by showing you a satellite-based view of total atmospheric moisture from late yesterday.  Purple and red are the highest amounts.   Satellites can sense water vapor content based on the emission of radiation by water vapor.  

You can see the plume of moisture heading into us from the southwest.  Amazingly,  our moisture plume can be traced back thousands of miles to the western Pacific near the Philippines and Indonesia: the region of the warmest water on the planet, known as the warm pool. 

The map below shows the precipitation for the past 24 h (ending 8 AM).  Some locations on the south and southwest side of the Olympics have gotten 5-6 inches so far, as have some rain gauges in the mountains of SW Washington.  Around Seattle about 1.5 inches have fallen, while only a few tenths of an inch have wet the ground over northern Whidbey Island in the rain shadow.  Seattle already has had 5.12 inches this month and by the end of the day we will reach the typical monthly total (5.57 inches).  A whole month's rain in less than half a month!


Weather radars can be used as remotely sensing rain gauges by making some assumptions.  Here is the "storm total" for the past two days from the Camano Island radar.  You can see the profound rainshadow to the northeast of the Olympics, with about a tenth of an inch at its core.   But 3-5 inches not far away west of the Hood Canal.



Several of you have emailed me, asking about the current level of our reservoirs.  As of yesterday, Seattle's water supply was in good shape (see below), with above-normal cumulative precipitation, above normal snowpack, and much above normal water storage.   Today will provide a substantial water addition.



But what about the drought-sensitive Yakima River Reservoir System, so critical for agriculture in the east? (see below).  Currently above normal and WAY above last year.


And Washington State's snowpack is in good shape, as shown below and noted in previous blogs, and Oregon is just slightly below normal.


The bottom line:  water resources and snowpack are in good shape for our region at this point, something not surprising for a La Nina Year.

Many of you are asking about cold and snow in the lowlands.   I will talk about this in future blogs, but the latest forecasts are projecting a turn to radically cooler conditions by the end of the month.  As a teaser, here is the predicted 7-day snowfall ending January 29th.  Yes, lowland snow.  But don't get too excited yet.... forecast skill is marginal that far out.   Seattle and other municipalities should make sure the plows and salt supplies are in good order.

January 10, 2021

A Strong Atmosphere River Approaches

The most powerful atmospheric river of the season so far is now imminent and will reach our coast later tomorrow (Monday).

Atmospheric rivers are fascinating creatures.   Why does the atmosphere concentrate water vapor into fast moving narrow currents?  And why do such currents provide such bountiful precipitation and are so optimal for promoting floods and landslides?    These questions are among those that I and others have researched over the past decades and pretty much understand.

Below is a graphic showing the key parameter that characterizes atmosphere rivers, something called Integrated Water Vapor Transport (IVT),that is forecast for Monday night at 10 PM.  IVT is essentially the magnitude of the water vapor being moved by the atmospheric.  Mathematically, water vapor constant times wind speed.

In tis figure, the blue areas are locations with very high values of IVT (over 800 in the units shown)

Bringing a lot of water vapor in the area provides a source of water for clouds and precipitation, particularly as air is forced to rise on our terrain.

A group at Scripps Institute of Oceanography in San Diego even have an atmospheric rating system, similar to Saffir-Simpson Scale for hurricanes (Category 1-5)--see below-- and it based on the IVT and duration.  The event we will experience this week will reach at least a Category 3 level!


What makes these event particularly threatening is that there will be two major pulses of large amounts of water vapor intrusion.... one late Monday/Tuesday and the other Wednesday morning, something shown by the forecasts of water vapor transports at those times (see below).  Each is associated with separate low pressure centers and strong winds.



You notice how the values decreases as the moisture plumes move inland?  Why do you think that is?

The answer is that water vapor is being precipitated out as rain and thus lost from the atmosphere.  The mountains are squeezing out the water vapor like one would do by squeezing a sponge.

But you want to see the precipitation, I'm sure.

For the 72 hours ending at 4 PM Wednesday, there will be 5-10 inches on the windward side of regional terrain, with some places getting OVER 10 inches near the CA/Oregon border.  Oregon is in the bullseye of this event.    And such heavy rain will be falling over regions that had the big fires last September, which is an issue of concern.


Atmospheric rivers generally are associated with warm air, and this one is no different, with freezing levels rising to 5000-6000 ft on Tuesday.  Not good for skiing at Snoqualmie those days.

Finally, why are there atmospheric rivers in the first place?   For us here in the Pacific Northwest, they are associated with situations with higher than normal pressure over California and lower than normal pressure over Alaska.  The result is a large pressure gradient between the two the produces strong southwesterly flow that pulls warm, moist air into our region.   

To illustrate, check out the forecast map of heights (like pressure) at around 5000 ft for Tuesday morning.  Temperatures is shown by shading and winds by wind barbs.  A low near Alaska and high pressure/heights off of CA.  A big change in pressure and thus strong winds (from the southwest), bringing the warm, juicy air towards us.


This is not going to be an extreme, damaging event, but expect some localized flooding, especially in western Oregon.

Colder Air and Limited Lowland Snow Ahead

Our region will soon experience colder temperatures and substantial snowfall in the mountains.  The snow level will fall and virtually every...