Sunday, January 21, 2018

Very Strong Cold Front Crosses the Coast

Sometimes you see an image that really gets your attention...and the radar image this morning from the Langley Hill radar (near Hoquiam) was one (see below).
This image shows reflectivity, which is related to precipitation intensity--with higher values (pouring rain or hail) shown by red.

A very strong front is seen, with the red lines indicative of a narrow cold frontal rain band, which is found at the actual location of the cold front.

Do you see how the red line is corrugated, with heavier areas and gaps?   This is characteristic of the very strongest fronts, and produced by an instability associated with strong shear across the front.   Just amazing.

Below is a blow up to give you a closer look.  This intense cold front is associated with a temperature change (colder behind), a trough of low pressure, and a very rapid wind shift--from southerly in front to westerly behind. 

The frontal characteristic are not uniform along the front, but are much stronger in the red areas, compared to the break locations (known as gaps).

The National Weather Service has some coastal buoys that can show us what the transition was like at the surface. At Buoy 46041, just off the WA coast, the pressure (green line) fell rapidly until 15 GMT today (7AM), and then rose very rapidly.   Wind gusts (red lines) increased to 47 knots, before plummeting after frontal passage.

At the same buoy, there was a sharp wind shift  from southerly (about 175 degrees) to southwesterly (roughly 240 degrees) with frontal passage (see below).  Temperature dropped several degrees with frontal passage (not shown).

As suggested by the 2 PM visible satellite figure, the front was over the Cascades at that time, with very cold unstable air behind it (strong convective cells are seen over the Pacific as colder air went over warmer water...which makes for big instability).

As a numerical modeler, the first thing I looked at was the simulation that started last night at 4 PM.  

Did it get the strong front?  It did.  

Here is a 16 hour forecasts for the very highest resolution run at the UW (1.33 km grid spacing) for pressure/wind and precipitation/wind, valid at 8 AM Sunday.  Sharp wind/pressure transition and even the hint of the corrugated precipitation pattern in the right hand figure.  Impressive

All that instability and convection should produce significant snow over the Cascades during the later afternoon and evening.

Saturday, January 20, 2018

Newest Cloud Types Seen in the Pacific Northwest

In  March 2017, an updated International Cloud Atlas was released and several new clouds were added. 

One new cloud is Asperitas, characterized by a complex wave-like base.   This week (January 17th), James Dearman sent me this wonderful shot of an Asperitas cloud that he took near Vancouver, Washington (see below). 

Pretty amazing cloud, with undulations looking like a sea surface turned upside down.

Here is another example from Virginia:

These clouds result from wave-like motions in the atmosphere that distort a pre-existing cloud deck.   Such waves can be initiated by fronts, thunderstorms, or some other type of atmospheric disturbance.  Generally no precipitation or severe weather with these clouds--but they do look scary.

Folks normally don't think about it, but the atmosphere is full of waves.    Generally, the atmosphere is stable, meaning if you push an air parcel upward, it will want to return to where it started.  But like a swing set, the air parcel tends to overshoot and goes into an oscillation.  A pendulum is another example.

Another new cloud is one we have talked about in this blogCavum, or the hole punch cloud (see below).   These "holes" are produced when an aircraft goes through a cloud made of supercooled water (liquid water below freezing).  The passage of the aircraft through the cloud causes a transition to ice crystals, which subsequently fall out, leaving a hole.

Thursday, January 18, 2018

Big Waves Hit the Northwest Coast from an Immense Storm Offshore

The Washington Coast is being pummeled by unusually big waves, some as high as 30-40 feet!  For example, buoy 41, right off the central WA coast observed a significant wave height of over 30 ft (see plot and map below)

Buoy 50, right off of Newport, Oregon, got to 35 feet this morning.

And a Westport surf cam shows an angry sea.

Or this scary video from Westport taken by Bryan Benoit:

My colleagues at the National Weather Service currently have coastal flood and high surf warnings up for the Washington and Oregon coasts.

 The National Weather Service runs a wave prediction model, called WaveWatch III, the predicts wave heights (using weather prediction models to drive the wave model).  The WW3 forecast of significant wave height for 1 AM last night predicted a slug of very high waves of 13-14 meter height (43-46 ft) approaching our coast.

No wonder my friends at the National Weather Service had warnings out!

Here is a blow up of the predicted waves at the same time.
Why such big waves?  Because there is a HUGE, intense, and slow moving storm (midlatitude cyclone) over the northeast Pacific.

I mean a stunningly big storm.  Here is an infrared satellite image last can see the immense swirl of clouds circling the system.

The sea level pressure analysis at 4 PM Wednesday shows the extraordinary system, with a central pressure of 964 hPa....very deep for a low that far south.

You notice the extreme pressure gradients around the storm, particular on the south and west sides?  Those are associated with very strong winds.
In fact, the surface wind forecast from the UW WRF model for 7 PM Wednesday, indicates SUSTAINED winds of 50 knots around the south side of the storm.

Wind waves depend on the strength of the winds, the distance the winds are blowing over the water (the fetch), and the length of time the winds work on the water.

With a huge, intense, slow-moving storm like this, all of these elements are maximized to produce big waves offshore, which propagate away from the storm (and towards us) as swell.

Be very careful if you go wave-watching on the coast.  Sneaker waves can come in and inundate previously dry locations. 

And I hope that no cruise ships are in the area....😉.  In fact, the latest real-time ship tracker image shows virtually no ship traffic in the region of strong winds and big waves...

Tuesday, January 16, 2018

West Coast Water Supply Is In Good Shape And About to Get Much Better

There has been some recent talk of drought over the West Coast, but in reality water supplies are in good shape and about to get much better, particularly over Oregon and California.

Let's start by checking out reservoir levels.  In Seattle, the current reservoir levels (red) are well above normal (blue).  And there is plenty more rain predicted for the next few weeks.  Looks good.

The Yakima River reservoir system is critical for water supply in Eastern Washington, and as shown below, the current water storage (blue line) is way above normal (red line).

Examining the current storage in the massive California reservoir system (shown below), one notes the all reservoirs are at or above normal levels, except for Lake Oroville, which is being kept artificially low because of the severe damage of last winter.

The current U.S. drought monitors shows do drought from central CA to Washington, and dry (but not drought) conditions over southern CA and a few locations in Oregon.
Considering the precipitation departure from normal for the past 3 months, one notes wetter than normal conditions from Washington State into Montana, and slightly below normal in eastern Oregon, Nevada, and parts of California.  But modestly drier than normal conditions have occurred over northern CA and coastal Oregon.   This is about to change.

The latest weather forecast model runs indicate a very wet future from central CA to British Columbia.  The 15-day totals from the National Weather Service GFS model has 5-10 inches over much of the Coasts, with 10-15 inches over the Sierra Nevada and the coastal mountains of northern CA and southern Oregon.

And all this precipitation will include immense snowfalls over many of the West Coast mountains, with over four feet over the higher Sierra Nevada, and 3 feet over the Oregon and WA Cascades.

To put it another way, we are starting today with reservoirs in good shape, generally with above-normal levels from northern CA to Washington.  Heavy precipitation will erase much of the modest deficits of the early winter, resulting in near normal precipitation values for the winter season so far.

And considering that we have a moderate La Nina in place, the precipitation will probably be heavier than normal for much of the remaining winter from central CA to the Pacific Northwest.  This is suggested by a composite of winter (Nov.-March) precipitation for a moderate La Nina year (like this year).

The bottom line:  the water resource outlook is favorable for the upcoming winter and summer.

Sunday, January 14, 2018

Record Heat in Western Washington and Oregon

Tuesday Update:  Monday's highs are shown below.  Lots of 60s, including mid to upper 60s on the western slopes of the Cascades and Olympics.  Even a few 70s in favored spots.  Daily records broken at Seattle, Quillayute, Hoquiam and a few other locations.   But places in which the fog held in (southern Sound), remained cooler.

Daily high temperature records were broken yesterday and more will fall today. Sea-Tac got to 58F (old record was 56F) and Hoquiam to 56F (old record of 55).

This morning's sounding at Quillayute, on the WA coast, shows a freezing level over 10,000 ft and an intense inversion (temperature increasing with height) in the lower few thousand feet (see below)

Yesterday's high temperatures got into the mid-50s all over the region yesterday, with some locations even rising into the mid-60s (only the highs above 54F are shown below).

The temperature this morning at 925 hPa (about 2700 ft)--16.4 C--was a record for the date and we have a strong offshore pressure difference this morning (much higher in eastern WA) that will produce offshore (easterly) flow and substantial downslope warming over the western slopes of the Cascades.  Mid-fifties will be commonplace and some locations downstream of terrain will get into the 60s.

Let me show you the latest UW WRF forecasts for surface (2-meter) air temperature--the details are fascinating.  Red indicates the warmest temperatures (check the legend below each figure).

At 8 AM, the State is divided in two:  cold east of the Cascade crest and warmer to the west.  But notice there are ribbons of greater warmth along the western slopes of the Cascades and to the west of the Olympics--this is due to the downslope warming (air warms by compression as it sinks down the slopes).   So at 8 AM, head to North Bend, WA or the Olympic Coast for warmth,

By 10 AM, the warmth along the Cascade slopes and along the coast will be profound, with lots of locations well into 50s and even near 60F.

But the situation at 2 PM will amaze, upper 50s to lower 60s everywhere, with some locations getting above 65F.  It will feel like spring.

The only thing holding things back is the strong inversion (which represent great stability and slow the mixing of warm air down) and the fog that covers some areas this morning (see 9 AM satellite picture).   Fog covers the southern Sound, parts of the Strait, and extreme NW Washington. And, of course the eastern slopes of the Cascades and the passes.

So enjoy the warm days...things will change greatly this week.

Saturday, January 13, 2018

Finally! A Transition to Cool/Moist La Nina Conditions in the Northwest

A warm front is moving through Washington State now, with a freezing level of around 8000 ft.   Rain is falling in the passes, and even the ever-positive Snoqualmie Summit snow report talks about " soft slop."

The last few days have been substantially warmer than normal , as shown by the observed temperatures at Stampede Pass (4000 ft) in the WA Cascades (normal highs and lows shown by purple and cyan).  Really warm today.
But I have some very good news for skiers and snow lovers: a major shift in the hemispheric flow patterns will result in much cooler/snowier conditions over the Northwest next week.  And much wetter over California.

Finally, we will see a more La Nina-like pattern, something that typically occurs in early January when a La Nina is in place, as it is this year.

Let me show you the upper level flow pattern for the next week, and for a change I will present the heights and winds at 300 hPa, roughly 30,000 ft:  the elevation where jets tend to fly and where the jet stream is often strongest.  Heights (like pressure) are shown in black and the wind speeds are color-shaded.  You can see the jet stream core by looking for the high speed areas (yellow and orange).

The jet stream is directly connected with temperature, with warmer temperatures to its south and cooler temperatures to the north.

Today at 4 PM shows a big ridge of high pressure (high heights) over the western U.S.   Warm and relatively dry for us.

Fast forward to Monday at 4 PM shows the shift:  the ridge has moved inland and the core of the jet is moving south towards California.

By Wednesday at 4 PM, a strong jet stream is approaching Oregon, with Washington on the cold side (good for snow here).

And the jet moves farther south on Thursday.

 And even farther south on Saturday...heading into the U.S. southwest.

The flow pattern this week will bring substantial precipitation to the West Coast, importantly including California.  We will cool down and substantial mountain snows are expected.

For example, here is the snowfall total from the UW WRF system for the 72 hr ending at 4 AM next Saturday.  Snow from the Sierra Nevada to southern BC.  Above 4000 ft there were be several feet of snow in the WA Cascades.  Southern BC mountains will be hammered.

The precipitation totals for the same period show 2-5 inches of liquid water equivalent over much of the western coastal terrain.

Good for water resources, good for skiing, and good for putting away the fire threats in the western U.S.  And with days getting longer, most folks won't mind the clouds.

Thursday, January 11, 2018

Scary But Deceiving Climate Statistics

How many times do you see a headline or read an article about climate change, where you are told that threat of some scary extreme event (flood, heat waves, cold wave, etc.) has increased five or ten or 100 times due to global warming?

But as we will see, such scare statistics may not mean as much as the headlines suggest.  In fact, they can be quite deceptive.  Let me explain.

Many of the scary stories talk about the frequency of exceeding some extreme threshold, such as the number of times temperatures exceed 90F in Seattle during the summer, or the frequency of daily precipitation exceeding 20 inches during the winter in the Cascades--that kind of thing.

The nature of extremes is that they are unusual.  So a very small increase of their numbers can result in the number of occurrences above some threshold increasing radically.

So if there are normally one day a year with temperatures over 90F during the summer and there is two one year, the frequency has DOUBLED.   A 100% increase!  Huge.

Let me do this a bit more quantitatively.  The climatological distribution of temperature is often Gaussian (also called the normal and bell-shaped distribution).

Below is an example of such a distribution, whose mean and most probable value is 75F.  The x-axis is temperature and the y-axis is frequency.   A measure of the spread or width of the distribution is the standard deviation (greek letter sigma is often used to denote it).  67% of the observations should be within one standard deviation of the mean (I have assumed a 3F  standard deviation in the figure below)

You notice the frequency (or probability) of observations drops rapidly for values much larger and smaller than the mean.

Now let's think about extremes.  What is the probability of experiencing a temperature more than 85F?   According to the calculation shown above:  .00043 (.043%)  Not much

OK, now lets warm things up by 1F, so the mean becomes 76F (I am keeping the shape the same).  You might not even notice that.  Here is the new distribution.

The probability has increased to .00135 (.135%).  OMG!   The probability of exceeding 85F has gone up by 3.14 times!

But think about it a bit more.   Instead of the scary increase of 3.14 times, think about the actual increase of probability of getting about 85F.

The increase is ONLY .1%.  Yes, a tenth of a percent increased risk of such warm temperatures.  Doesn't seem so scary all of sudden.

I could give a dozen other examples of this...but hopefully you get the idea.

So be very cynical when you read about increases of extreme events due to global warming (or anything else) and particularly when the increases are for exceeding some threshold and given as a factor (5 or 10 or 100 times more). 

Such numbers can be extremely deceptive and imply a big increase in risk in situations with minor changes.  

And there is more.  Even if global warming results in a higher frequency of exceeding some threshold, its contribution might be quite small.  Consider Hurricane Harvey.   Global warming may have increased the precipitation by a few percent, but nearly all of the event was the result of natural processes.  When folks hear that global warming has increased the probability of an event by some factor they tend to assume that most of the event was due to global warming, when that is generally not the case.

Finally, thresholds are generally arbitrary and subjective.  For example, why is 90F more special than 91F?   The use of thresholds for such climate communication is essentially deceptive and should be used far less. 

What really counts is not the frequency of crossing some threshold but the increase of the amount of the threat (e.g., the increase in high temperature, precipitation).   For example, by the end of the century, the Northwest snowpack will probably be down by 30-50%.   That is scary.  Or the heaviest precipitation in atmospheric rivers could be 20-40% larger.  Again, a major issue.

Global warming is too serious of an issue for us to use questionable, if not deceptive, statistics for communications of its impacts.