Sunday, October 22, 2017

Heavy Rain, Flooding Rivers, and Profound Rain Shadows

As advertised, a moderate atmospheric river event occurred on Saturday bringing heavy rain and limited flooding to our region. 

The water vapor satellite imagery at 5 PM yesterday said it all....a huge plume of moisture stretching back thousands of miles into the central Pacific. (this type of satellite imagery highlights water vapor content in the middle to upper troposphere).

The 24-h precipitation totals ending 7 AM Sunday were impressive (see image, click on it to enlarge), with over four inches on the SW side of the Olympics and the western slopes of the Cascades and 1-2 inches over the south Sound lowlands.   Look closely and you will see only about 1/4 inch fell around Port Townsend on the NE side of the Olympic Peninsula...the rain shadow in action.

 Even heavier rain fell over the coast mountains and Cascades of western Oregon, with several locations receiving over 6 inches in that 24-h period.  Roughly 2 inches in Portland.

If you really want to appreciate the rainshadow, here is the 24-hour precipitation total from Seattle RainWatch, which calibrates radar totals using rain gauges.  .1-.2 inches over southern Whidbey Island, with 4-5 inches southeast of Puget Sound over the western slopes of the Cascades.

Even though the region has been dry this summer, such heavy amounts quickly fill major rivers.  Thus, several of the rivers draining the western Cascades are now or soon will be in flood stage (red and blue dots)

To illustrate, the Snoqualme River, near the town of Snoqualmie, is now at moderate flood stage.  Fortunately, it should decline rapidly today.

A plot of the winds and temperatures above SEA-TAC Airport for the past day, shows the surge of warm, southwesterly flow late Saturday (red is temperature in C, wind barbs are shown, time is in UTC (Z, GMT) on the bottom and the y coordinate is height in pressure (850 hPa is about 5000 ft).  This morning the temperatures cooled and the winds turned more westerly aloft.

As a result of the westerly winds aloft, Seattle and much of Puget Sound is in the rain shadow of the Olympic Mountains today, with lots of sun.  In contrast, the western slopes of the Cascades and Olympics are getting hit with moderate showers (see radar below).

A nice day around Seattle and Portland today, even with some moist, unstable flow reaching our coast.  You get to appreciate the rain shadows around here.

Friday, October 20, 2017

5000 Mile Atmospheric River Will Hit the Northwest on Saturday: Heavy Rain in the Offering

The atmosphere is kind of funny when it comes to moisture.    Rather than move water vapor out of the tropics in wide currents, the moisture tend to move in relatively narrow atmospheric rivers.

For us in the Pacific Northwest, the atmospheric rivers that reach our shores often come from the southwest, starting somewhere near Hawaii (see graphic of water vapor content summed up in the vertical).  Thus, they are often called a pineapple express.

But sometimes, a very different configuration occurs, with moisture extending northward in the western Pacific, then heading nearly due east into our region.  Traversing an amazing distance of over 5000 miles.  Let's call this the Midway Express.  And one of them will reach us on Saturday.

Here is the satellite view of water vapor content over the Pacific region for Friday afternoon.  Lots in the tropics (warm air holds more water vapor than cold air), with large values surging northward in the western Pacific.   Do you see a narrow east-west feature in the north Pacific, with what looks like a fist at the end.  THAT is the atmospheric river that is heading our way.

A model forecast for tomorrow morning at 11 AM suggests the "fist" of moisture will reach our shore by lunchtime.

A warm-front, will accompany the atmospheric river, something illustrated by the surface pressures (solid line) and temperatures (colors) at 5 AM Saturday morning.   Behind the warm front, temperatures will rise, moisture values will surge and winds will accelerate.  The warm moist air, forced to rise by our regional terrain, will drop copious moisture.

How much?  Here is the 24h total ending 5 AM Sunday morning.  The heaviest rain will be in the Cascades and coastal mountains from roughly Seattle southward.  Some locations could be hit by 5-10 inches.

A close up view for western Washington suggests a strong rainshadow northeast of the Olympics, with only about a tenth of an inch.  In contrast, the southern Cascades get hammered.

And there will be strong wind speeds, particularly near the San Juan Island and coast Saturday morning.

For those of you worried about the water supply, be comforted.  As shown by the water levels at Seattle's Tolt Reservoir, recent rains have not only stopped the summer drawdown, but has caused a rapid rise during the past few days.

Wednesday, October 18, 2017

The First Significant Storm of the Season

The Seattle Times called it the onset of the "Big Dark", but those who know Northwest weather call it the "Big Normal".   Right on time we are enjoying the stimulating strong winds and heavy rain of a potent Pacific front.  And quite honestly, it feels good.

Winds have gusted to 50-60 mph along the coast and over parts of Northwest Washington, and even here in Puget Sound we have had gusts above 40 mph at exposed locations (and 25-35 mph elsewhere).  The map of the max winds in the 24h ending 7 PM wed is shown below.

Strong winds, untested vegetation with summer growth, and trees with leaves have resulted in thousands of customers losing power around the area.   Here is the Seattle City Light outage map at 7 PM.  Puget Sound Energy has 36,000 customers in the dark.

As I write this blog, a very wet front is moving through the region, as shown by the 7 PM radar.

In fact, we are experiencing a modest atmospheric river that is bringing warm, moist subtropical air into the region (it was near 60F last time I looked).  A satellite image of the vertically integrated water vapor in the atmosphere shows the plume of moisture from the southwest, originating from north of Hawaii.  You can almost smell the pineapples.

The latest infrared satellite image clearly shows the juicy frontal zone clouds associated with the "river."  And potent instability clouds (the popcorn-looking clouds out in the Pacific) are ready to move in tomorrow

A strong front is embedded in the plume of moisture, something a trained eye can tell from the coastal Langley Hill radar image late in the afternoon (see below).  You see the corrugated pattern offshore, with orange/red colors?  That is a strong cold front with a narrow cold frontal rainbands.   These always reflect strong fronts. And that front is now moving across Seattle, with pouring rain and strong winds.  I have to admit, I love it.

Plenty more rain will fall during the next few days.  Here is the total for the 72 hr ending 5 AM on Monday.  5-10 inches over the western slopes of the Olympics and Washington Cascades.  Fire season is over. Our reservoirs will start to fill.  Our rivers will be refilled.  And the "Big Normal" is back..

Monday, October 16, 2017

The Real Story Behind the California Wildfires

There has been a huge amount of media coverage regarding the tragic northern California fires, documenting the terrible loss of life and billions of dollars of damage to buildings, infrastructure, and the economy.  As I write this, the death toll has risen to 41, over 5000 buildings have been destroyed or damaged, and the estimates of the financial loss are in the tens of billions of dollars.

Media stories have blamed the catastrophic fires on many things:  a dry environment after the typical summer drought, unusual warmth the past several months, excessive rainfall producing lots of flammable grass, strong winds, global warming, and  the lack of vegetative maintenance (clearing of the power lineright-of-ways) by the local utility (PG&E).

But none of the stories I have read get at what I believe is the real truth behind this unprecedented, severe, and explosively developing wildfire event:

A unique mountain-wave windstorm produced the strongest winds in the historical record at some locations.  An event produced by the unlucky development of just the right flow regime that interacted with regional mountains to produce extreme winds beyond contemporary experience.

In short, this blog will make the case that the extreme nature of the wildfires was the result of a very unusual weather event, one that our weather models had the ability to forecast and warn about, if only their output were applied more effectively.  The blog also suggests that better use of state-of-the-art weather prediction offers the hope of preventing a similar tragedy.

The Unique Wind Event

Although there have been a lot of media reports about windy conditions, few have described the extreme, often unprecedented, nature of the winds on Sunday night and Monday morning (October 8/9th).   Some have even mocked PG&Es claims of hurricane-force winds, suggesting wind speeds of 30-40 mph.

Let's clarify a few things.  There was a wide range of winds that night, with the strongest winds on ridge tops and on the upper lee slopes of terrain.  Some winds was startling.

For example, at 10:30 PM on 9 Oct 2017 the wind gusted to 96 mph on a 3400 foot peak NE of Geyersville, about 20 miles NNW of downtown Santa Rosa. They reported sustained 74 knots (85 mph).  Those are hurricane force winds (sustained of 64 knots or more).

At the Santa Rosa RAWS station (U.S Forest Service and Bureau of Land Management) at 576 ft elevation, the wind accelerated rapidly Sunday night to 68 mph (see below).

A few miles to the NNW and a bit higher (2000 ft), winds at the Hawkeye site accelerated abruptly to 79 mph.


What is really amazing about the winds at these sites, was that they were unprecedented:  the strongest winds on record, with records going back to 1991 (Santa Rosa) or 1993 (Hawkeye).  And we are not talking about winds during the fall, but winds any time during the year.  Even during the stormy winter season when powerful storms can cross the region.

At low-levels, the situation was more mixed.  For example, at Napa Valley Airport (36 ft), the sustained winds at 11:15 PM October 9 (37 knots) were the strongest observed (looking back to 2001) at that location from July 1- November 30, while at the Santa Rosa Airport (KSTS) the sustained winds only reached 28 mph, with 40 mph gusts.

So why were the winds so strong and unprecedented at higher levels in the hills?  These winds were key for causing the wildfires to explode and to quickly move into populated regions.  And the winds undoubtedly damaged power transmission lines and thus helped start electrical fires, which may, in fact, have initiated the big wildfire runs.  And why were the lower-level winds less severe?  What can explain such differences?

Mountain Wave/Hydraulic Jump/Downslope Winds

When strong flow interacts with terrain, the air can be greatly accelerated.  The schematics below show you some situations, with air accelerated over and downstream of mountain crests.

Such acceleration is well know in Washington State, with some locations experiencing huge winds (like Enumclaw where winds reached 120 mph on Dec. 24, 1983 while it was calm in Seattle.)

That night (Sunday evening), strong to moderate easterly/easterly flow was approaching the terrain north of San Francisco, something shown by the 6hr forecast of height (like pressure) and winds at 850 hPa (about 5000 ft)--a forecast valid at 11 PM Sunday night (see below).  The strong winds and their orientation was the result of cooler air and high pressure moving into the Northwest during the previous day.

So we had modestly strong winds (30-50 knots) approaching the terrain.  A very favorable situation for strong mountain-wave winds is a stable layer at or just above crest level.  A stable layer can be noted when temperature is steady with height or increases with height (an inversion).   The nearest vertical sounding (radiosonde launched weather sensors) was at Oakland, CA.  The sounding at 5 PM Sunday DOES shows an inversion at the right levels (see plot), roughly between 850 and 800 hPa (roughly 5-6 thousand feet ASL).

A group at the Desert Research Institute runs a forecast model (WRF) at very high resolution (2-km grid spacing).  Here is their 6-h forecast for sustained surface winds at  11 AM Sunday.

OMG...there it is.  You can see the banded structure of strong winds over and immediately downstream of major terrain features, with lower speed winds near sea level.  I inserted a terrain map can see how the wind maxima were oriented the same way as the ridge lines.  And the model reveals something else:  the enormous  horizontal variability of the winds during such events.

Other major modeling systems also predicted the strong mountain-wave winds.  For example, here are the max wind gusts predicted by the NOAA/NWS High Resolution Rapid Refresh Model for 2 AM Monday.  Same banded structure, with gusts near Santa Rosa of 50-55 knot (58-63 mph)

Professor Rob Fovell of the University of Albany completed another high resolution simulation, one initialized at 5 PM on Thursday.  Here are is a vertical cross section though the Tubbs fire that affected Santa Rosa.  You can see the acceleration of winds (sustained) on the slopes.

The predicted winds at the Tubbs site was scary strong, with max winds around 70 mph.

The creation of such downslope mountain-wave type windstorms is very sensitive to the characteristics of the air moving towards the mountains.  You not only need strong approaching flow, but the proper vertical structure of temperature and winds.  Clearly such conditions don't happen often--otherwise similarly strong winds would have occurred before.  There is no reason to expect that such extreme wind conditions were made more probable by global warming.

So I think we can outline what happened Sunday/Monday of last week.

The vegetation was dry after little rain over the summer (quite normal).  The ground vegetation was perhaps drier than normal because the summer had been usually warm (by 1-4 F as shown by the NOAA Western Region Climate Center map for the last 90 days.)

On Sunday afternoon, winds approaching the mountains of northern CA increased, and the vertical structure of an inversion over cooler air was established.  A strong mountain wave/downslope wind event was initiated, bringing winds of 60-90 mph to the crests and upper lee slopes of the regional terrain.  Such winds helped initiate the fires (possibly due to interaction with power lines) and then caused the resulting fires to explode.  The fires, driven by the strong, gusty winds, pushed very rapidly into populated areas.

The good news in all this?  Our models seemed to be able to simulate this event, providing some warning of the imminent wind acceleration.

What could be done with such information?  Much better warnings of a potential blow up?   Shutting off the power to threatened communities?  There are lots of possibilities.   But one thing is for sure:  we can not let this happen again.  And the first step is to really understand what happened, without assuming we know the answer beforehand.

Too many people are suggesting the wildfire event is all about climate change, when it may prove to reflect a severe weather event unrelated to global warming.  Similarly, some of the same folks claimed that the great rainfall with Hurricane Harvey was all about global warming, when a stalled storm was probably more to blame.  Only by knowing the true cause of disasters and acting on that information can we protect people in the future.

Saturday, October 14, 2017

Cold Weather and Snow Hits the Pacific Northwest

Snow has hit the Cascade passes and below-normal temperatures have spread over the Pacific Northwest, with temperatures dropping into the single digits in portions of eastern Oregon.

And ironically such cold temperatures are a bad sign for those battling the wildfires north of San Francisco.

To "warm up" this blog, lets start with the latest cam shots at Snoqualmie and Stevens Passes.  White stuff.  Enough to make folks think about the upcoming winter season (which may be a good one because of La Nina).

During the last day, many NW folks have observed frost, with temperatures dropping below freezing on both sides of the Cascades.  The map of minimum temperatures for the 24-h ending 8 AM Saturday (below, click to expand), shows 20s in eastern Washington, with teens and even single digits in the valleys of the high plateau of Oregon.  Klamath Marsh RAWS site east of Crater Lake dropped to 5F and Burns, Oregon set a new record for the date (10F).

The below-normal temperatures were clearly evident at Sea Tac and Pasco, WA--here are plots for the past two days at these locations, with average maxima (purple) and minima (light blue).    At Sea-Tac, Thursday was crazy cold (about 12F below normal) and this mornings low temperatures were clearly the coldest so far this fall.
And many days dropping below normal at Pasco.
The air morning into our region is near record cold, something shown by comparing the incoming temperature at 5000 ft (850 hPa) at Quillayute (Wa coast) with climatology (see below).  The gray dot shows the observation at 5 PM Friday and the blue line shows the record low for the date.

The cold air that moved into the Pacific Northwest is associated with higher pressure, something shown by the 12-h forecast for 5 AM today (Saturday) for sea level pressure (solid lines) and lower atmosphere temperature (color shading).  At the leading edge of the cold/high pressure there is a large change in pressure (pressure gradient) that is associated with strong winds.  Unfortunately, some of that pressure-change zone is now over northern CA, which is revving up the winds, particularly over the northern Sierra.   Not as bad as Sunday/Monday, but enough to bring concerns of reinvigorated fires.

The winds yesterday were mostly northerly (from the north) over northern CA, and that blew the smoke southward toward San Francisco (see MODIS image).

I am working on an analysis of winds during this fire event, particular an evaluation of how unusual they were...stay tuned.

Thursday, October 12, 2017

Could the Northern California Wildfires Have Been Prevented Using Preemptive Power Outages and High-Resolution Weather Forecasts?

The catastrophic fires in northern California are still burning, with the death toll rising to 21 and damage estimates ranging into the tens of billions of dollars.

Is is possible that this tragedy could have been prevented or minimized by cutting the power to threatened areas before the fires started, using the best available weather forecast models to guide decision making?

This question is explored in this blog.

The proximate cause of the explosive fires were discussed in my previous blog.  We started with a very dry landscape, following the typical rain-free summer.  Wind picked dramatically on Sunday evening, with gusts to 40-70 mph over northern CA.  With offshore flow, relative humidities were very low.  And temperatures had been above normal.

But something initiated the fires and did so at multiple locations within a period of a few hours during the late evening on Sunday.  Although there is the possibility of arson, the most probable fire starter was arcing power lines damaged or shorted by falling trees and branches.

There is a history of California wildfires started by falling trees/branches during strong wind events, such as the 2015 Butte Fire near Sacramento that killed two and destroyed 550 homes.  And the there were reports of downed and arcing  power lines Sunday evening prior to the wildfire conflagration.

So if downed or arcing power lines was the key initiator of Sunday/Monday's fires what can we do to lessen the chances of a repeat of the tragedy?

Some media reports have suggested that the relevant power company (PG&E) was not effective in trimming vegetation around its power lines.   Certainly, an effective program of vegetation control around powerlines is essential.  And burying power lines in vulnerable areas would be wonderful, but very expensive (I have seen estimates of 1 million a mile)--but perhaps a reasonable investment.

But perhaps there is something else that can be done, which could provide substantial protection:  cutting the power to regions that are directly and immediately threatened by powerline-induced wildfires.

This is how it would work. 

This approach would only take place in regions and periods in which there are threatening amounts of dry fuels on the ground, making wildfires possible.

Thus, portions of California near vegetated area during the dry season (last summer, early fall) would be candidates.

Only areas with appreciable population would be candidates, thus remote areas would not be considered.

Only when dry atmospheric conditions and high winds are imminent and threatening, would pre-emptive blackouts be considered.  I would suggest using periods when gusts are predicted to exceed 40 mph (35 knots) with relative humidities less than 30% as a potential criterion.  If those conditions are forecast to exist within 6 hours or if they are observed, the power would be cut for the affected areas.

A very promising modeling system for such a purpose is the NOAA/NWS HRRR (High-Resolution Rapid Refresh) high-resolution model, which is run every hour, out to 18 h.    Here are the ten and four hour forecast of wind gusts valid at midnight Sunday/Monday (0700 UTC) for central CA.  Both forecasts are threatening, with predicted winds over 40 mph in many of the areas north of San Francisco where they were, in fact, observed.

Both showed the winds revving up above 40 mph around 8 PM and dropping below that value around 9 AM Monday morning. 

So the pre-emptive black out would run for 11 hours (8 PM to 9 AM) and folks would get a series of warnings that it would occur.   With modern numerical prediction, warnings of a potential blackout would be given a few days before, with a penultimate warning 6 hrs before, and a final warning an hour before.

Yes, there would be some inconvenience, but that would be minor compared to the benefits.  In the present case we are talking about saving roughly two-dozen lives and tens of billions of dollars of economic impacts.  And we haven't even touched on the negative impact on air quality for the heavily populated San Francisco metro area.

And there is the issue of false warnings or the "crying wolf" syndrome.  But a casual look at the climatology at a few locations around the area suggest that this was an unusual event, and that picking a realistic criterion (e.g., 40 mph gusts, August through October only, relative humidity below 30%) would produce very few blackout events.   I will explore this more during the next few weeks.

Is this a crazy idea?  If so, why?