Sunday, February 19, 2017

More Threatening Weather at the Oroville Dam

The latest forecasts are worrying regarding the heavily damaged Oroville Dam in California.  And I am surprised there is to so little talk by CA state officials and the media about the danger.

During the past few days, with little rain and active drainage of water, the dam level has been reduced substantially (see image)

But central California and the Sierra Nevada mountains are about to be hit again...quite hard, with a strong atmospheric river (see moisture plot for 1 AM Monday).  The red and white colors shows high column integrated water vapor values.  Aimed right at the dam.
The forecasts show substantial precipitation with this atmospheric river.  Here is the accumulated precipitation for the 24h period ending 4 PM Monday from the excellent NCAR high-resolution ensemble system (the ensemble mean). 4-8 inches over the Sierra.


But that is the ensemble mean or average.  But there is a chance it could be more, so here is the ensemble maximum, the ensemble member with the highest amount of precipitation.  Much more (30-50% more in some locations)..and a serious problem for Oroville Dam.


The 48h precipitation total ending 4 AM Tuesday from the GFS model projects 5-10 inches in the mountains.

And the European Center model has the same idea, with 5-7 inches


Looking at the NCEP SREF ensemble system at Blue Canyon, about 15 miles away from the dam,  shows an ensemble mean of 7 inches, with a range from 5.5 to 13 inches.  Great confidence for a major rain event.


Hydrological forecasts by the National Weather Service, project a rapid rise of the Feather River, the main feed into the reservoir behind the dam, with the flow quadrupling from around 7.5 thousand cubic feet per second to around 32 thousand.

This is a dangerous situation and, as always, there is uncertainty with our forecasts.  Why is there so little discussion of the rapidly increasing precipitation?
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And if you want to read an interesting blog about an unusually strong cold front that hit Hawaii, check this out:

http://hawaiianweather.blogspot.com/2017/02/cold-front-passing-hawaii.html

Saturday, February 18, 2017

Will Seattle Beat the All-Time February Precipitation Record? Almost Certainly Yes

Sunday morning update:  SEA how has 8.01 inches,  1.10 inches below the record Feb total.
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People love records.   And precipitation records are falling over the West Coast.

Here is Seattle, the record February precipitation is 9.11 inches, which fell in 1961.

As of midnight, Seattle Tacoma Airport had 7.85 inches so far this month.
Since then, Sea Tac got another .07 inches, so we are now at 7.92 inches or 1.19 inches behind the record.  A plot of actual (red) and average (blue) cumulative precipitation at Seattle shows the story...we are about 6 inches ahead of normal now.

Looking at the forecasts, we have a very good chance exceed this.   For example, here is the precipitation forecasts for Seattle from the European Center ensemble and high-resolution models.  1.3 inches for the ensemble and 1.7 inches for the high resolution forecast.   If true, the record is toast.

Or the UW WRF forecast for the 72 hr starting at 4 AM this morning. The purple color is more than 1.28 inches.... very close.  And there is more rain after that.

So the chances are very good that a new February rainfall record will be set at Seattle Tacoma Airport and a number of other western WA locations.

All this precipitation has caused our soils to become highly saturated, resulting in mudslides and slope failures, such as the one that closed down I-90 near Issaquah and another that closed I-5 near Woodlawn (see below).



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Northwest Weather Workshop:  

The Northwest Weather Workshop, the big annual weather gathering in the Northwest, will take place on March 3-4, 2017 in Seattle (NOAA Sand Point Facility).   Everyone is welcome.  For more information and to register, go to https://www.atmos.washington.edu/pnww/

Friday, February 17, 2017

Northwest Weather Hits Southern California with the Most Substantial Storm in Years

Southern California is about to be hit hard with heavy rain of 3-9 inches and powerful winds.  Flooding and slides are inevitable.   But it will have a silver lining:  any talk of drought in California should be ended as the last below-normal reservoirs are filled.

Let's start with the 2-day precipitation totals (ending 4 PM Sunday) from the wonderful NCAR high-resolution ensembles (the ensemble mean--the average of the ten ensemble members are shown). Totals exceeding 3 inches covers a wide area from LA north, with some locations in mountains north of Santa Barbara getting 7-9 inches.  That amount of rain would be huge anywhere, but in dry southern CA that is immense.     For a number of locations, this will be the greatest rain event in years.

The reason southern CA is so wet, while the Northwest is relatively dry, is because the jet stream has been deflected way to the south.  This is illustrated by an upper level forecast map for 4 PM Friday, with the sold lines presenting the heights of the 500 hPa pressure surface (around 18,000 ft).  A deep trough of low heights (pressures) is found along the west coast, with the jet stream/strong winds heading into southern CA.


Equally impressive is the latest infrared satellite image (Friday AM).   A extremely moist frontal band is headed right into the LA Basin, which will bring both heavy rain and strong winds.  There is clearly instability (convection/thunderstorms) embedded in the cloud mass.


The latest radar image from Vandenberg AFB shows extensive moderate rain over the region, with the yellow/orange areas being the most intense.


And now we have the problem--southern CA is ringed by high terrain and as the moist flow climbs the barriers, precipitation intensity is greatly increased (by 5-10 times is not unusual).  This air is relatively unstable, so there is also the risk of thunderstorms with locally intense rainfall.   The result is the potential for flash flooding and slides, with debris-laden regions that have recently burned of particularly concern.  The National Weather Service has a flood warning out for the Santa Barbara area and a flash flood watch extending down the coast. It is not going to be pretty down there.


Wednesday, February 15, 2017

For Some in California, the Worst is Yet to Come

After a few day break, California is about to experience a period of very wet weather, with the potential for flooding and slope failures in southern California and threats to the Oroville dam in the north.   For the 24h ending 8 PM Wednesday, the fire hose has been mainly directed to the Pacific Northwest, with some locations in the Olympics getting 5-7 inches of rain.  But substantial rain (greater than 2 inches) extends into the Cascades and down the coast to northern CA.


Now it is California's turn.  The U.S. GFS model forecast for the next 7 days is amazing, with portions of the Sierra Nevada near the Oroville dam getting 10-20


 inches of rain.  Very bad.  And heavy precipitation will hit southern CA as well, where flood and slide warnings are now in effect (see NWS warning page)


The European Center model precipitation for the same period (shown below) has a similar solution, with as much as 7-9 inches over the Sierra Nevada.

But how much uncertainty is there in this forecast? For Chico, CA (near the Oroville dam at lower elevations), the European Center ensembles (many forecasts) are all on pretty much the same page. (each line in the upper panel  \represents one forecast).  One event starting tomorrow, with the heaviest precipitation on Friday.  Then another event later Sunday and Monday.

The ensembles for Santa Barbara are also in agreement for a big event on Friday, with even heavier precipitation in the mountains.


This could well be the wettest storm of the year in southern California and damaging slides and floods are quite possible.

Tuesday, February 14, 2017

The West Coast Will Face An Extraordinarily Wet Period

Northwest Weather Workshop:  Open to AllThe Northwest Weather Workshop, the big annual weather gathering in the Northwest, will take place on March 3-4, 2017 in Seattle (NOAA Sand Point Facility).   Everyone is welcome.  For more information and to register, go to https://www.atmos.washington.edu/pnww/
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Northern California is experiencing its wettest winter on record, with reservoirs full, the ground saturated, and many of the rivers at or near flood stage.

Oroville Dam, northeast of Sacramento, is on the brink of failure and downstream residents have been warned to evacuate.  Water is being released through the emergency spillway in a desperate attempt to save the dam.


During the past two weeks, immense amounts of precipitation has fallen over the West Coast (see image), with some locations in the Sierra and coastal mountains getting more than 20 inches of water.


Considering the last 60 days, large sections of California, Nevada, Utah, and southern Oregon have received 200-400% of normal precipitation, with limited areas exceeding 400% of normal.  Generally, Washington State is slightly drier than normal.


Folks, it is not over yet.  Although a ridge of high pressure temporarily has dried things out the last few days, the fire hose of rain will be returning on Wednesday.

Here is the forecast precipitation total for the next 9-days from the U.S. GFS model.  Nearly all of the Cascades, Sierras, and coastal mountains will get 5-10 inches of additional precipitation.   This includes the last hold-out from serious wetting, the mountains behind Santa Barbara.

The forecast for nearly the identical period from the European Center model shows a similar story.   Very wet conditions along the entire West Coast.



A number of rivers are now at flood stage in California (red dots on map below from the California/Nevada river forecast center) and they will be pushed higher during the next week.

There are a number of individuals and groups that talk about drought in California being a threat from global warming.  But heavier precipitation during major precipitation events during the winter is perhaps a more probable expectation, as atmospheric rivers become more potent as the earth warms. Warm air can "hold" more water vapor than cold air.

The atmospheric river that will approach the Northwest on Wednesday will be a strong one (see map, blue colors indicate very large water contents), and will then head towards California.  And more atmospheric rivers are to follow.


An historic wet winter and one that was not forecast last fall.  My profession has a lot to learn about seasonal forecasting, assuming it is possible.

Sunday, February 12, 2017

How much salt falls naturally out of the sky?

During the past several weeks, as snow has covered the roads of Washington and Oregon, salt is being used to prevent an ice layer from bonding to the concrete and to help melt the snow.  Clearly, salt can make travel far safer, prevent injuries and deaths, and keep our roadways functional. It can prevent a city from being crippled, like Seattle was in 2008 and Portland experienced last month.


But some folks wonder about the environmental impacts of roadway salt and the potential to corrode cars and other vehicles.   But to answer this question, one needs to start with another question.   How much salt is naturally in our environment, particularly salt that reaches the ground in precipitation or by dry deposition?

The answer is going to surprise you.  A lot of salt is falling out of the sky!  In fact, probably more than being spread by State and local departments of transportation.

Where is it coming from?  Breaking ocean waves!    The ocean is salty, of course, and when waves break or when wind produces spray, the air is filled with salt water droplets.  These droplets can evaporate, leaving small particles of salt in the air.   Here in the Northwest, salt particles can be easily blown inland by the


prevailing westerly winds and  brought back to the ground by precipitation or even dry settling (deposition).

To illustrate, here is a map created by the National Atmospheric Deposition Program, based on combining observations and models, that shows the wet deposition (by precipitation) of one component of salt, sodium ions.  (Remember,  from your basic chemistry that salt is sodium chloride, NaCl).   The red areas show large depositions (more an 4 kilograms per hectare).   And remember, a kilogram (kg) is 2.2 pounds and 1 square mile is 259 hectares.


You note there is more salt deposition near coastlines and particularly stormy coastlines.  That is why the Northwest is well salted but California is not.   You can see the effects of the Great Salt Lake as well.  Other years look very similar to 2015.

So based on such research it has been found that the Puget Sound lowlands get around 7 kg per ha per year of salt and the coast (closer to the breaking waves) receives about 30 kg per ha.

So there is considerable salt falling on the surface by natural processes, with little obvious problems for animals and plants.

OK, now let's have some fun, comparing the amount of salt falling on Seattle each year against the amount of salt resulting from roadway protection in the city.

Seattle encompasses 83.78 square miles.  Each square mile is comprised of 260 ha.  Thus, the city has 21,783 ha.    So if 7 kg  of salt falls per year per ha, then the total amount of salt reaching the ground in Seattle each year is 335,455 pounds of salt or 168 tons.

Mama mia!  We have a salty city!

Let's compare this total to how much salt is used to protect the roads for one snow storm, which seems to be the typical number per winter the last few years.  From a little digging, I believe that Seattle uses about 100 tons (200,000 pounds) for a single snow event--less than the 168 tons noted above.   But even I am off a bit, I suspect the bottom line is reliable:

The natural precipitation of salt from the ocean over Seattle is roughly equal to the salt spread for reducing icing during a single storm.  

Thus, I suspect roadway salt is not a significant environmental hazard. Furthermore, since the salt is spread over roads much of the salt goes into drains, some of which go directly to treatment plants and the Sound.  This concern is further reduced by the fact that Puget Sound is a salt-water estuary, with considerable salt content (although a bit less than the ocean, 2.9% versus 3.4%).

Roadway salt, spread during heavy snow periods, does not go into drains immediately, but rather over several days, thus reducing pulses of salt.    Furthermore, it is accompanied by a surge of melt water that dilutes its concentrations.  As an estuary expert I consulted stated:   "dilution is the solution".   You've got to love sayings like that.


It is interesting to note that the natural deposition of salt does not fall in a uniform way, but accompanies the major storm/rain events that also bring large waves and lots of sea spray.  I have confirmed this by looking at the weekly data available from the national deposition site.  Here is an example showing the weekly amounts of sodium from salt at La Grande, in Pierce County from January 2010 to now.  Lots of variability.


Thus, there are natural spikes in salt deposition, with only a few weeks each year providing much of the annual deposition.

I am not saying the salt on roadways is absolutely harmless, but that the risk is small compared to the alternative loss of life, injury, and economic damage. Like is about risk versus reward, costs versus benefits, and in this case the benefits of using road salt one or two times a year far outweigh the risks.

 Finally, modern vehicles have far better primers and paints, and thus are less susceptible to salt-associated corrosion.


Clearly, the environment implications far salt would be far greater for cities with a lot of snow that require many salt applications over the entire winter--such as Chicago or Buffalo.   Thus, Mayor Murray and the folks at SDOT can probably sleep soundly knowing the Seattle's occasional salt spreading is not undermining the environmental quality of the city.  A previous Seattle mayor learned the hard way the dangers of not using salt.

Acknowledgments:  I received guidance from Joel Thornton and Dan Jaffe, experts in atmospheric chemistry at the University of Washington.

Friday, February 10, 2017

Hold on to your hats! Western Washington winds are about to blow!

Update:  here are the maximum gusts yesterday (mph).  In exposed locations, there were gusts to the mid-50s.  Lots of gusts to 30-40 mph around Puget Sound, with elevated and water locations getting to 40-50 mph.  Windy in the mountains.



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A tight, but strong, low center is about to pass north of the Puget Sound region, bringing strong wind gusts that could get as high as 50 mph in exposed locations.

The UW WRF forecast model shows the predicted low at 8 AM (see below), with a large pressure difference on its periphery (solid lines are isobars, lines of constant pressure)

 By 1 PM, the low is north of Puget Sound, with the area of large pressure gradient right over western WA.  That means wind.  

To illustrate, here is the sustained or average wind predicted for 1 PM.  Around Seattle there will be sustained winds of 25-30 knots (29-35 mph).  The gusts will be more.

The swirl of precipitation associated with the low is apparent on the latest (8:15 AM) radar image off the NW WA coast.


And the 8 AM satellite imagery clearly shows the swirl offshore.


So batten down the hatches and get ready...the wind is about to blow.  City Light and the other utilities should get read...some folks are about to lose their power.
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Announcement

The Northwest Weather Workshop, the big annual weather gathering in the Northwest, will take place on March 3-4, 2017 in Seattle (NOAA Sand Point Facility).   Everyone is welcome.  For more information and to register, go to https://www.atmos.washington.edu/pnww/

Global warming will bring milder weather to the Pacific Northwest

A recent article in the journal Climate Change analyzed how the number of mild days will change as the Earth warms due to human-caused global warming.

There are winners and losers, with most the world's population being losers, but for us here in the Northwest the changes will be favorable, with substantially more mild days, particularly in the spring and fall.


Before giving you the results, a few details.   The definition of mild weather in this paper is:
  • Daily maximum air temperature between 18 and 30 °C (64 to 86F).
  • Daily total precipitation not exceeding 1 mm (.04 inches).  This is not much.
  • Daily mean dew point temperature not exceeding 20 °C (68F).  High dew point are associated with sticky, humid conditions.
(If I was going the study, I would have had mild temperature ranging from 60 to 80F, precipitation not exceeding 2 mm, and dew point not exceeding 15°C, reflecting my Northwest U.S. tastes.)

This research made use the NOAA Climate Model (NOAA/GFDL HiFLOR global coupled model.) to project the world's climate for the rest of the century.  Their simulations assumed only modest growth in greenhouse gases (RCP4.5, for those that know about such things).

The following plots show the changes in the probability of mild days from now to 2081-2100) for each month, with blue indicating more mild days, and orange denoting fewer mild days.   In places with a warm climate (e.g., Miami), the changes are not good.  There are no mild days during summer in Miami. Quite frankly, temperatures changes is the least of Miami's global warming problems--rising sea level will probably flood the place.


Los Angeles and Denver, with cooler climates than Miami, will see improvement during winter and the adjacent months, with serious degradations during the warm months. Same thing for Chicago and New York City.

But Seattle is different.  We are real winners.  There will be substantial increases in the number of mild days from June to October, while the loss of mild days in mid-summer are relatively small.  But it is really better than that.  None of our winter days will be in the mild category (64 to 86F), but they will be considerably more pleasant than today (46 feels a lot warmer than 39F) .

And did you note the percentage of mild summer days in Seattle has been and will be around 70%...higher than almost any place else in the country any time of the year?  Only LA in November and May are as good.  And who would want to live in LA, anyway?

The Climate Change paper also took a global view, as shown in the figure below. Folks living in locations that are relative warm in summer (tropics, subtropics, India, SE Asia, SE U.S., China) will be losers.  That includes most of the Earth's population.  In contrast, Canada, Russian, the NW U.S., southeast Australia, lower South America, and northern Europe will be winners--more mild days.  Canada does particularly well.  Africa will be hit hard as will Brazil.



Western Washington and Oregon will relatively favored (at least in temperature) by global warming because we start off on the cool side, with warm extremes reduced by the cool eastern Pacific.  The warmer the land gets in summer, the more onshore (cool) flow develops--our natural air conditioning. And offshore (downslope) flow over the Cascades is predicted to decrease with global warming.

But although our temperatures will get progressively more pleasant during this century, other changes will not be as favorable, such as more extreme rainfall in winter, reduced snowpack, and worsened flooding in the fall.   So lessening the increase in greenhouse gases would certainly be in our own best interests.  You can always put on a sweater.


Wednesday, February 8, 2017

Northwest WA Snow, the Big Warm Up, and Flooding

Today, while much of western Washington warmed and was hit by moderate rain associated with a warm front, Northwest Washington, stretching from Whatcom County to San Juans, experienced snow.

Why localized snow?  Because of cool air jetting out of the Fraser River Valley northeast of Bellingham, as illustrated by this surface map at 10 AM today (Wednesday).  The Fraser River Valley acts as a conduit of colder air from the interior of British Columbia and the approaching low pressure increased the pressure difference across the gap.
The warm front has brought dramatically warmer temperatures over our region.  Check out the temperatures and winds above Seattle during the past 24 h using a time/height cross section (time is along the x axis, height is the y axis, see below).  Huge warm up from roughly -6C to 3 C at 850 hPa pressure level (about 5000 ft).  About 15F increase. The freezing level at Quillayuate along the WA coast rose to over 8000 ft this afternoon.

The warm southwesterly flow was associated with an atmospheric river, a current of large moisture values out of the tropics (see map of moisture values at 10 PM)
As this moisture ascended our terrain, moderate to heavy rain fell, as shown by the 24h values ending 8 PM.  Particularly heavy rain was found over the southern WA and Oregon Cascades and over the coastal mountains (some places received more than 2 inches).   That moisture is being added to rapidly melting snow at lower elevations.

But it doesn't stop there.  Here is the precipitation total forecast for the next 48h from the UW WRF model.  Wow....5-10 inches over the coastal mountains and the higher Cascades.  California shares the fire hose of water as well.

All this precipitation (and a very high freezing level, rising to over 7000 ft), is causing the regional rivers to rapidly rise, with a number of them at or near flood stage (red dots).

And a combination of high astronomical tides and low pressure could cause coastal flooding.

Did I mention the potential for strong winds mid-day Friday as a strong trough of low pressure moves through our region?

Lots of activity to keep meteorologists busy...but remember, the start of  meteorological spring in the NW is only a few weeks away:  February 25th!
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Announcement

The Northwest Weather Workshop, the big annual weather gathering in the Northwest, will take place on March 3-4, 2017 in Seattle (NOAA Sand Point Facility).   Everyone is welcome.  For more information and to register, go to https://www.atmos.washington.edu/pnww/
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