Friday, May 18, 2018

Flooding in Eastern Washington, Driest May on Record for Western Washington

If you like dramatic contrasts, Washington State offers a world-class example--and I am not talking about politics.

Over eastern Washington a number of rivers are at flood stage, some reaching record levels.

Thunderstorms and heavy rain have been pummeling the Columbia Basin and northeast WA. The National Weather Service has issued a flood warning for several counties including Okanogan, Bonner, Chelan, Ferry and Pend Oreille, and the Kettle River, which flows through Ferry and Stevens counties, had a record high on May 10 of 22.54 feet on May 10, busting the previous record of 21.1 feet in 1948.

In contrast, over the west, many locations have had little precipitation in May, and there is chance Seattle will have its driest May on record.

Not black and white.  But dry and very wet.   But why?  And is spring flooding in eastern WA unusual?

Let's start by checking out the flooding situation, with information from the NOAA/NWS River Forecast Center in Portland (below).  Red dots are floods, blue dots indicated moderate floods, and purple are major floods.  And several other locations are at bank-full.

What is going on?   The combination of a healthy mountain snowpack and warming temperatures, resulting in rapid melting of snow, and the filling of regional rivers.

Here is the snowpack numbers for April 15th, from the very nice SNOTEL web site. Washington, northern Idaho, and Montana had snow-water amounts well about some locations above 150% of what is typical.

And snowpack in British Columbia, from which the Columbia River drains was also very high (see table)

So going into spring with a bountiful spring snowpack was step 1. 

Then a combination of warming temperatures and increasing spring solar radiation caused rapid snowmelt.  The departure of average temperatures from normal (below) shows our temperature anomaly (difference from normal) for the past two weeks, with much of eastern WA and northern Idaho being 6-8F above normal for that period.

So the normal rapid snowmelt of spring was supercharged by warmer than normal temperatures. 

But like a late-night commercial, I should note that there's more.    The weather pattern has produced wetter than normal conditions east of the Cascade crest (and drier than normal to the west)--see map below of the precipitation departure from normal for  5/3-5/16.

During the past few days, showers and thundershowers have moistened eastern Washington (see radar image for 3 PM yesterday), with the last 24-h totals being impressive.  A number of locations in northeast WA have gotten over 1 inch during the last day.

So why wet east of the Cascades and dry west?  Because there has been a persistent ridge of high pressure along the coast, with troughs (low pressure) moving into California and northward into eastern WA and Idaho (see upper level--500 hPa--map for 5 PM Thursday to illustrate.)  The ridge has kept western WA dry, while the transient troughs have initiated thunderstorms east of the Cascade crest.

It is important to note that spring-time flooding in eastern Washington and along the Columbia is the normal state of affairs in our region.  Before the Columbia and Snake River dams were in place, major and often catastrophic flooding was commonplace. 

One of the most famous events was the great snowmelt flood of May 1948.  Just as in this year, regional mountain snowpack was above normal, unusually warm temperatures hit during mid-month, and thunderstorms dampened eastern Washington. Richland flooded and the surge of water moved down the Columbia, inundating and destroying the city of Vanport (near Portland), which was never rebuilt.

As global warming reduces the regional snowpack later during this century, the threats of such springtime flooding should gradually decline over time.

Wednesday, May 16, 2018

Moisture Hole Reaches California

Today's water vapor imagery shows a dramatic "moisture hole" crossing central California.

Here are two amazing shots for 1 AM and  6 AM PDT this morning.
It almost looks scary.

These images show the temperatures of water vapor in the atmosphere.  White areas indicate lots of water vapor in the upper troposphere (roughly 15,000 to 30,000 ft), while dark areas indicate little.  Thus, dark colors show dry conditions in the middle to upper troposphere--the moisture hole.

This "moisture hole" is associated with a pronounced upper level low, as shown by the WRF model run last night (the 500 hPa, about 18,000 ft, heights are shown, with winds and temperature).  Moisture swings around the low, but values are less in its core.  Air that swings around the low rises more than air inside the low center.

This year we have gotten an unusual number of spring upper level lows heading into California...and there is another---even stronger--predicted for next week.

This pattern has brought clouds, precipitation and thunderstorms to northern CA, southern Oregon, and northern Nevada.  The NWS radars show lots of showers, some heavy moving westward over Oregon

And the lightning strikes for the 24-h period ending 1 AM this morning were impressive. Quite a number.

Precipitation totals over the West Coast for the 24-h period ending 8 AM had some significant totals over northern CA, which is obviously good for their water situation.  CA reservoirs are in very good shape and the late spring moisture helps keep the ground moist.

With all the action going south, Puget Sound was dry again, with only .08 inches in the Sea-Tac rain gauge so far this month.  Will we beat the all-time record (.12 inches)?   The next model run will probably provide the answer.

Monday, May 14, 2018

Some Record Minimum Temperatures This Morning and Dry May

Walking outside early this morning it was evident that something was up.   It felt really warm.

In fact, the minimum temperatures at locations around the region were warm with upper 50s at several locations, with some stations only dropping to the mid-60s.    Sea-Tac Airport had their record high low temperature for the date (58F).  In fact, it was the highest low for any date during the first half of May.

What was the reason for this morning warmth?    Well, we started out with near record warm air aloft for the date because of a ridge of high pressure overhead.  To show this, here are the climatological temperatures in the lower atmosphere (925 hPa pressure, about 3000 ft) at Quillayute, on the WA coast, with today's observation at 5 AM shown by the silver circle.  The red line shows the record for each date.  The temps aloft were near record levels.

But warm temperatures aloft are not enough since infrared cooling to space from the surface will cause low-level temperatures to fall, working against any record.

But there was another factor that worked against this nighttime cooling:  lots of water vapor in the lower atmosphere.   A good measure of water vapor content is the dew point temperature, with higher dew points indicating more water vapor.  So  here is the climatology of surface dew point at Quillayute, with the circle indicating today's values--quite high...nearly at record levels.

Water vapor is a very active absorber and emitter in the infrared and helps to keep the temperatures up at night.  That is why very dry deserts often get chilly at night even when the temperatures are extremely warm during the day.  So with lots of water vapor, the normal nighttime cooling was lessened.

One factor that helped with keeping the moisture high was the lack of offshore-directed, easterly winds.  Nothing dries out the air like strong downslope, easterly flow.  Only when downslope, easterly flow is very, very strong can it produce record high temperatures.

One more thing....this has been an extraordinarily dry May so far.   Seattle has only received .08 inches as of today...and we are halfway through the month.    The rest of the month looks fairly dry, with the forecast through May 28th never showing more than a 20% probability of rain for any day.  The driest May on record at Sea-Tac only received .12 inches.   So perhaps we have a chance to beat the record.

Ironically, the morning high resolution WRF model simulation showed the greatest rain action over northern CA and southern Oregon, which is unusual this time of the year.  Good for topping off the reservoirs in northern CA.

Saturday, May 12, 2018

Northwest Sudden Drying: The Rapid Transition to Dry Soils in May

People think of the Pacific Northwest as being a wet place, with wet, sodden soils.

Perhaps that is true during the winter and early spring. 

But something startling happens in May in much of western Washington: a rapid drying of the soils.   

A transition so rapid that some folks begin irrigating their gardens by late May.

Can this really be true?   Let's check it out.

Here is the soil moisture (8 inches down) at Langley, on southern Whidbey island, for the past month.  A major drop in soil moisture percentage from roughly 26% to around 15%.

Graph courtesy of the WSU Agweather website

Perhaps a better perspective can be gain by looking at the soil moisture over an entire year at this location.  For example, 2017.

Moisture values are high (20-26%) during the winter, but in May and early June the bottom drops out, with values dropping below 8%.  Slow drying follows in July and August, with a huge increase in October.

How about 2015, a year that was particularly warm and dry?  Again, a big drying in May

Looking at other years and other western WA stations shows that the May superdrying is a normal occurrence, something that accords with my experience as a gardener.

But why such rapid drying in May?  To understand, one must know about an important meteorological term:  evapotranspiration.  

Evapotranspiration or (ET for short) is the sum of evaporation and plant transpiration from the Earth's land and ocean surface to the atmosphere.  Over land, it is mainly water evaporating from the earth's surface and water loss by plants.  

So ET is the loss, while precipitation is the gain.  If ET exceeds the precipitation coming in, surface soil moisture declines.

Evapotranspiration is enhanced by warming of the surface (e.g., by solar radiation or warm air passing over it), drying of the air above the ground, and stronger winds (which help mix moisture away from the surface).

Think about what is happening in May.  

Precipitation is declining rapidly.  Here are average monthly precipitation totals at Seattle-Tacoma Airport.  May (roughly 1.75 inches) is a third of the November and December totals and less than half of the March rainfall.   So much less coming in.

Solar radiation, which warms the surface--promoting evaporation, increases rapidly in May, as shown by the solar radiation for the year at Langley, WA:

Surface air temperature increases in May, relative humidity decreases, and soil temperature (which is very important for surface evaporation ) increases substantially during that month (see below)

In short, a major decline of incoming moisture (from precipitation) and a big increase in evaporation from strengthening solar radiation at the surface and warming temperatures, results in a rapid decline of solar moisture in May.  

And we should not forget the large increase of evapotranspiration from plants, which are leafed out and growing rapidly in may, sucking out moisture from the soil.

Today, I was preparing the soil in my garden and could not believe how dry it was.  The May drying has gotten a particularly good start, with drier than normal and warmer than normal conditions (see graphs below)
Blue lines is normal precipitation, purple is this year. 

Red is this year, purple is normal high, cyan is normal low.

So be prepared to water your garden.  And this year, there is another threat to our plants.  A very cute, furry threat that has been increasing in numbers rapidly:

A sign of global warming?  I will leave that for a future blog.

Thursday, May 10, 2018

Will Hawaii's Vog Head Our Way?

Bellamy Pailthorp, my colleague at KNKX, and I have gotten several emails from folks worried about the effluent from Hawaii's Kilauea volcano and whether it might be heading our way.

So let's talk about it
. A major eruption is currently occurring on the Big Island of Hawaii associated with the Kilauea volcano. This eruption, really an enhancement of an ongoing eruptive event starting 1983, is not explosive and putting only minor amounts of ash into the atmosphere. But it is emitting large amounts of sulfur dioxide (SO2), hydrogen sulfide (H2S), water vapor, and other gases.

The amount of SO2 emitted by Kilauea is immense, with some days the emissions reaching 2000 tons. SO2 can combine with water to produce sulfuric acid and sulfate particles. SO2, sulfuric acid particles, and other volcano effluents can combine to for a grayish haze called VOG, which can not only reduce visibility, but can have detrimental effects on lung function.

And with this eruption that have been plenty of small to moderate earthquakes (see below).

Up until this this point, the northeasterly trade winds have been blowing most of the VOG out to sea, leaving air quality pretty decent on the big island (see below).

But changes in the trade winds may bring the volcanic effluent soon into more populated areas. A friend of mine, Professor Steve Businger, of the University of Hawaii, runs a VOG prediction effort, in which they use numerical models to predict the movement of volcano effluent around the Hawaiian islands.

Let me show you their model-based probabilities of SO2 concentrations exceeding .01 parts per million (ppm) for the next day.  The map from this morning (around 10 AM PDT) shows most heading out to sea.

But as we view the forecasts for the upcoming days, more of the SO2 is swinging around the big island and heading into the heavily populated Kona area. Some even swings up to Oahu.

One thing is clear...none of the SO2 and VOG are headed our way, so we can relax.

Hawaii is a beautiful place, but it has its environmental challenges--like very heavy rain (e.g., on Kauai), VOG, earthquakes, tsunamis, and more.

Tuesday, May 8, 2018

Roller Coaster Weather Season in the Northwest

The temperature variations this time of the year are often like a roller coaster, with a steady temperature increase over a few days, followed by an abrupt cooling over a few hours.

Only half-jokingly I call the period from middle spring into early summer, the roller coaster temperature season here in the Pacific Northwest.

During the last month or so we have experienced a number of roller-coaster temperature changes (see the temperatures at Seattle-Tacoma Airport below).  An earlier major event around March 13 had a rise to about 73F, followed by a drop to 45F.  Or the recent one around April 25th, with a rise to about 80F, followed by a drop to around 50F.   Plus, several more minor declines.

The biggest one-day temperature changes in our area are NOT in the middle of winter when fronts and storms are strong, but during spring.

Don't believe me?  Here is the proof from a paper I did a number of years ago.  The figure shows the average number of days per month with one-day temperature drops of certain magnitudes.

For the biggest one-day declines (10°C or more), May is the biggest month by far.  For somewhat lesser drops (7.2-9.4°C), June takes the lead.   For moderate temperatures drops, the summer is tops, with August taking first place.

 Temperature drops in winter are small in comparison.

How does one explain this bizarre state of affairs?  Why do temperatures drop more when it gets warm around here?

As we will see, the big issue is the vast Pacific Ocean and the seasonally changing temperatures differences between land and water.

During winter, we are dominated by onshore flow off of a cool (roughly 50F) and vast Pacific Ocean.  Air temperatures over land (west of the Cascades) are just a minor tweak of the ocean temperatures.

Weather fronts coming across the vast Pacific Ocean are heavily modified by their long traverse across water, with temperatures changes at low levels greatly weakened.    A strong front coming off of Asia, with large temperatures differences at low levels, barely produces a few degree change when it hits are shores.

Yes, the Pacific Northwest suffers from wimpy fronts at low levels.  Something I try not to admit to outsiders.

But in spring as the sun warms up and clouds abate, something changes.  The land starts warming up, particularly east of the Cascade crest--and  yes, even on our side to a lesser degree.  But the eastern Pacific Ocean sea surface temperatures hardly change.     Thus, a large contrast between ocean and land temperatures can develop over the Pacific Northwest.  And there lies the answer.

When we have days with onshore (westerly) flow off the ocean, our temperatures are controlled by the ocean temperatures, and thus remain cool.  By when we get periods of offshore (easterly) flow, our temperatures zoom up.

Cool, onshore flow is the default.   But in spring, there are still upper level weather systems moving through, which can result in high pressure building east of us, resulting in offshore flow...and thus we can get periods of warming.

Here is the plot of wind direction at Hoquiam on the Washington coast for the past four weeks.  Some major swings of direction as weather systems move by.  But look closely around April 24th when we got warm...the winds were easterly (or offshore), pushing the cool, ocean influence out to sea.

The warming tends to take time as offshore flow brings warm air down into western Washington, but the influx of cool air (often associated with an approaching trough) comes in fast...and we have a name for it:  the onshore or marine push.

Why are the largest temperature changes in May, rather than in early August when the temperature contrasts between ocean and land are greatest?  Because weather systems that cause the offshore flow and incite rapid onshore flow tend to weaken during the summer over the midlatitudes.

Why?  Because north-south temperature differences--the drivers of midlatitude disturbances--are less during midsummer.

Anyway, being on a roller coaster can be fun, particularly if you understand how it works.

Saturday, May 5, 2018

Time to Fix the Yakima Airport Temperature Sensor

With all the concerns about global warming and the need for agricultural interests to have reliable temperatures to guide operations, accurate temperature readings at key airport sites are crucial.

That is why continuing problems with the Yakima Airport temperature sensor is so concerning.  So the message to my colleagues in the National Weather Service  is clear:  it is time to swap out the problematic sensor.

The Yakima Airport sensor provides temperatures that are several degrees too warm.  Let me prove this to you.

Here is departure from average of the maximum temperature across Washington State, averaged over the past year.    There is a major hot spot, with temperatures 4-5F above normal.  Nothing like it in the neighborhood.

The hot location?  Yakima.

There is someone who has been documenting the serious issues with the Yakima Airport temperature sensor:  Mark Albright, who was previous State Climatologist and Deputy State Climatologist.  This guy knows the local meteorology as well as anyone.  His hobby is to drive around the State with a temperature sensor checking out the accuracy of the temperature readings at various locations.  A very noble pursuit and appreciated by all of us worried about the integrity of our key observing systems.

Mark Albright
So let me show you some recent documentation he has come up with regarding the Yakima situation.

Consider last August.    Here are the temperature anomalies for normal for stations around the State (look at the TDPTR column).  The greatest departure from normal (6.5F) was at Yakima...with no other station even close.

Want a winter month?  No problem.  Here is the same information for January.  Biggest warm anomaly in the State?  Yakima.

Now, I could show you a dozen more of these monthly summaries--all prepared by Mark, but the story would be the same.  Something is very wrong at Yakima.

I have heard complaints from some agricultural interests that have used Yakima temperatures to make decisions for planting, spraying, and other issues.  Too warm and not representative of the area.

One thing is clear--this warming is not due to local development, since the temperature sensor is found near a secondary runway, with no development going on nearby (see map).

Anyway, the Pendleton office of the NWS is responsible for calibrating and replacing this sensor, which I hope they will do very soon.  This problem has been going on for years and needs to be dealt with.

And if you think this is the only place in the country with a bad temperature is not.  Here is the mean departure from average for minimum temperatures over Utah for the last year.  Oh....oh.-- a big warm anomaly southeast of the Great Salt Lake.  And then there is the infamous warm bias at Seattle-Tacoma Airport.

The problem with these failures is that they tend be one sided--generally producing temperatures that are too warm.  Often this warmth  is associated with a failing fan, which brings in cooler environmental air into the sensor enclosure.   And most siting problems (too near roads, buildings, or non-vegetated surface) tend to cause a warm bias as well.   

How much such problems influence trends in the surface temperature record is one of some debate and controversy..and I won't get into it right now.  My vegetable garden needs attention.