Wednesday, August 20, 2014

Will this be the warmest summer in Northwest history?

Although summer still has one more month to run, the extraordinary warmth of the past two months is putting us on a track whereby we might achieve that record.  Here is the average departure of the maximum temperature from normal from June 21st until now.  The Northwest is well above normal, particularly over and east of the Cascade crest.

Minimum temperature?  Same picture.

We can compare the temperatures at Seattle Tacoma Airport, Yakima, and Spokane against the normal highs and lows (see figures).  For roughly 2/3 of the days, the max temperatures are higher than  normal (the red line), with many days 5-10F warmer than normal.
July was the first or second warmest for many stations around the Northwest and, as shown above, August was even more anomalous.

The latest Climate Prediction Forecast for the next 6-10 days is...  you guessed it.  Warmer than normal over Washington and Oregon.

Temperatures this summer are running roughly 3F above normal.  Following the median global warming scenario, this is like advancing 30-50 years into the future.  So this summer you are getting an idea of the conditions you or your children will experience in approximately 2050.

Monday, August 18, 2014

Why Is Washington State Experiencing More Wildfires than Southern/Central California?

The drought in California has been given extensive coverage in the media, with stories last spring predicting a heavy toll from major wildfires over the summer.   This has not come to pass.

After a few early wildfires near San Diego in April, California has seen few major wildfires, but Washington State has been hit very hard, with many major fires in the Cascades and eastern Washington (see graphic).  Eight major fires are now burning over Washington (red circles), twice the number found in California.

Looking at the U.S. Drought Monitor diagnostics (which show the moisture content of the surface layer), you would think that California would be FIRE CENTRAL, with FAR drier conditions than in Washington (I show the maps for August 12th and July 1st).

But that has not been the case.  So why are so many fires occurring in Washington while California has been spared the worst so far?

As I will explain below, the initiation of wildfires is a complicated issue, with far more involved than drought and a dry surface.   A number of factors need to come together to get major fires, and Washington (and particularly eastern WA) has had them.  These situation also has implications for what might happen under global warming, as the Southwest dries out.

The situation over the Northwest has been ideal for fires this summer.   We have had long periods of warm, dry conditions that has dried out the surface and caused relative humidities to plummet.  And then weak troughs have approached us, covering the area with lots of lightning that have initiated fires.

Consider Wenatchee, on the eastern slopes of the Cascades and close to several major fires.  Below you can see the temperatures, relative humidity, and temperature.  In mid July temperatures rose into the 90s and low 100s F, with relative humidities dropping below 15%.  And then rain and thunderstorms hit around July 23, setting off a series of fires.  Then the warming started again and the humidity plummeted.   And then ran and thunderstorms hit again last week, causing more fires.

Each of the lightning events was caused by the approach of an upper level trough, like the one shown below, that interrupted a warm, high pressure regime.  The passage of the trough was associated with strong winds on the eastern slopes that stoked the fires, sometimes causing them to explode into uncontrollable infernos.

So for us in the Northwest, we have had perfect conditions for fires...long warm, dry periods interspersed with brief periods of thunderstorms.

By why has California lucked out?    First, other than April they have had a general absence of dry, Santa Ana conditions in which strong, offshore flow brings extremely low humidities, heat, and powerful winds rev up fires.

They have not had much lightning because the troughs moved in over the Northwest and not over southern and central CA.

And they have actually had quite a bit of precipitation over the Sierras, with a stronger than normal wet Southwest Monsoon.  To show this, here is the percent of normal precipitation over the western U.S. for the last 60 days.  The blues and purples are well above average.

The summer is not over yet, and the great Santa Ana fires generally hit California in the fall, so California is not out of the woods yet.  But the big message is that the initiation of major wildfires is a complicated affair, and simplistic arguments about drought and fire can be pushed too far if we are not careful.

Saturday, August 16, 2014

When More Rain Falls on the "Wrong" Side of the Mountains

Here in the Pacific Northwest we are used to the heaviest rain falling on the western side of the mountains (such as the Cascades), with rainfall amounts rapidly declining to the east of the crests.  The climatological precipitation maps certainly show this (see graphic).   Eastern Washington is not known as sagebrush country for nothing.

And we understand in general why this distribution exists.  Many of our barriers (such as the Cascades) are oriented north-south and winds approaching the terrain are generally westerly (from the west), since the large scale flow in the midlatitudes is westerly.    So air is forced upwards on the western (windward) side, which leads to clouds and precipitation.  On the other hand, air descends on the eastern (leeward) side, resulting in drying and less clouds/precipitation.  95% of the time that is what is happening around here.

But there are some situations that produce heavier precipitation on the other (eastern) side of the mountains, in this the Cascades, with heavier precipitation over eastern Washington than western Washington and far heavier precipitation on the eastern slopes than the western slopes.

Such a situation happened Wednesday to Friday of the week.  Let me demonstrate this.

Here is the forecast precipitation for the 48-h ending at 5 PM Friday.  Way more rain over eastern Washington than over the western lowlands.

This forecast was confirmed by the observations.  For example, here is a weather radar snapshot for Thursday at 7:14 AM.

So why was precipitation on the eastern rather than the western side of the mountains?

The winds had reversed from the normal westerly flow to easterly (from the east) flow.

Why?  Because a small upper level low had moved in over our region and this low caused the winds to reverse.  Here are two maps (for 2 PM Thursday and 2 AM Friday) showing  the upper (500 hPa, roughly 18,000 ft) flow.  The solid lines are the heights of this pressure surface (you can think of them like pressure).  The low is in the center and the winds are parallel to the lines with higher values to the right of the wind. The winds are also shown by the standard wind barbs.  As the low center slowly passed south of Washington and then along the border with Oregon, there was easterly flow approaching the Washington Cascades.  This air was forced to rise by the Cascades, enhancing precipitation on the eastern side.

Satellite pictures  showed the swirl of clouds associated with the low.  Here is an infrared picture from Thursday AM.

Now there are other contributors to producing more rainfall over eastern Washington, such as when the air there is more unstable and warmer at low levels there, producing thunderstorms to the east of the crest, while the cool marine air on the west side is too stable for such showers.   In fact, this was also happening on Thursday.  Occasionally, some of the summer "Southwest Monsoon" moisture streams northward east of the Cascades, giving them some showers.

Thus, summer is a favored time for more precipitation over eastern Washington from thunderstorms/convection.  You can see this from maps of climatological precipitation.  On the left side, you see average July precipitation.  A bit drier on the eastern side, but not by much (a few tenths of a inch).  The Willamette Valley and eastern Oregon are almost the same.  In contrast, the annual precipitation is another story, with the western lowlands getting about four times more than the Columbia Basin.

                                              July                                                               Annual


Thursday, August 14, 2014

No Drought in Western Washington this Summer

The rain this week has broken the drought in western Washington.

Here is the precipitation at Seattle Tacoma Airport compared to normal (blue line) for the past 12 weeks.  Almost exactly normal!
Or perhaps you would prefer a larger view showing the % of average rainfall for the entire summer--June 15th to yesterday.  Western Washington are greens and light blue:  a bit above normal.  Nevada, parts of Utah, and Arizona were also above a lot.  This was caused by the very active SW monsoon this summer.  Not so good over central California.

What about the last year? Western Washington and Oregon are one of the few areas of the West Coast near normal  California is a disaster area and eastern WA is dry.

 The question you may ask is about the upcoming winter.  The latest Climate Prediction Center forecast is for a wet southern Ca and dry NW:  the typical expectation during an El Nino year (see graphic)

Tuesday, August 12, 2014

Serious Rain

Midnight Update:

Heavy precipitation has moved into Puget Sound country.  Here is the lightning over the past 24 hr (basically all day Tuesday).  The most recent is yellow, followed by reds and purple.   The Cascades have been hit very hard with lightning and right now the folks around Bellingham are getting the flashes.


Last night thunderstorms hit portions of Western Washington and right now (3 PM)  a band of heavy showers is moving northward towards Puget Sound.  Note that particularly heavy precipitation is associated with thunderstorms over the southern Cascades (red colors).

Here are recent lightning strokes.  The Cascades are getting hit by multiple strokes.

The forcing for all this activity is an upper level trough/low that is moving towards us now and which will circle around the area for the next day.  To illustrate, here are the upper level forecast maps (500 hPa) at 5 AM and 11 AM on Wednesday.

Take a look at the 24h precipitation ending about 6 PM today.  The south Cascades have been hammered by the thunderstorms, with some locations getting 2-3 inches,

Large amounts of precipitation for this time of the much as 1-2 inches over the Cascades..are forecast during the next 72 h (see map).  Eastern Washington will get a lot of precipitation, and they need it.