Sunday, October 11, 2015

Tropical Air Brings Warmth and Torrential Rains to Western Washington

The last few days have seemed almost tropical in western Washington, with high humidities, warm nighttime temperatures, and finally intense rainfall.

The rainfall totals for the 24-h ending 6 AM this morning (Sunday) were impressive (see regional and close in maps below).  1-1.5 inches over Puget Sound and 2-4 inches over the western slopes of the Olympics and the Cascades.

We can look at the storm-total precipitation graphics from the local weather radars.  At the coastal Langley Hill radar, there is profound orographic (terrain ) enhancement with roughly .5 to 1 inch offshore and 1-3 inches over the southwest slope of the Olympics.  As I have discussed before, the rising motions forced by terrain result in substantially increased precipitation on the windward slopes.

The Camano Is. radar shows similar enhancement over the western slopes of the Cascades where precipitation is greatly increased (as much as 3-4 inches)

Not surprisingly, this intense precipitation has caused western Cascade and Olympic Mt rivers to rise rapidly above normal levels, as illustrated by the Snoqualmie River at Carnation, WA:

Temperatures have been mild the last few days, with minimum temperatures about 5F above normal (see below). Dew points (a good measure of water in the atmosphere) were also unusually high, reaching the mid to upper 50s in many locations.
So why the intense rain and why have been so tropical lately?  

The reason is a very large scale area of low pressure pressure that developed over the North Pacific just south of the Aleutians, with a ridge of high pressure over the Rockies.  As shown by a forecast of the flow at 850 hPa (about 5000 ft ) for 5 PM Thursday (colors are temperatures, winds shown by wind barbs, heights (like pressure) shown by solid lines) this configuration produces strong southerly/southwesterly flow over the eastern Pacific, bringing warm and moist air northward over our region.

There are satellites that can measure the amount of water vapor from space and can clearly delineate the plumes of moisture from the tropics--also known as atmospheric rivers.  Here is such moisture imagery for Friday and Saturday.  You can see the conduits of moist, warm air into our region.  Hawaiian air has come to you!

A weak front will come through tomorrow bring clouds and light rain, followed by a dry period into Saturday.  Then rain returns.   A very typical weather sequence for October.


JeffB said...

One has to get a chuckle remembering the recent talk of drought from our Governor and other doom and gloom prognosticators who think they have this weather and climate thing all figured out.

It is simple, Earth runs the show, not mankind.

Mark said...

From Meteorologist Scott Sistek Blog

'Water year' rainfall nearly normal, so how are we in a drought?

1. Much of last autumn and winter had warm, heavy rains that counted quite a bit in the official rain gauge in Seattle, but with little mountain snow to establish any kind of "water bank" for our region to draw off in the summer.

2. In addition, most of the rest of our rain this year also came in heavy spurts -- especially in late winter and August. It's better to have lighter rains spread out over greater days than heavier rains in a punch that allows some of the water to run off and not be absorbed. While our total rainfall is somewhat near normal, the number of rainy days so far this year is below normal.

3. Late winter through spring and into mid summer were very dry across the state with several extended periods with no rain.

4. It's been much hotter than normal -- several sites have smashed temperature records by leaps and bounds this year -- and that causes more rapid evaporation and decreasing soil moisture content. Because it was so hot and dry for so long, and we didn't have a winter snowpack to draw from, aquifers and reservoirs that supply water to fish, farms and communities across the state took a major hit, according to the Washington Department of Ecology.

From the Weather Underground, Jeff Masters:

Second-warmest September in U.S. weather records
With a 48-state average that came in 3.7°F above the 20th-century average, last month was the second-warmest September for the contiguous U.S. in 121 years of record-keeping, topped only by September 1998 (Note: Following a super El Nino). A set of nine widely dispersed states had their warmest September on record: Connecticut, Colorado, Maine, Michigan, Minnesota, New Mexico, Rhode Island, Utah, and Wisconsin. The month was also on the dry side, coming in as the 21st-driest September of the past 121 years. (Note: Washington state was the coldest in the CONUS coming in at 48th warmest out of 121 years)

Locations where Sunday (Oct 11, 2015) was the warmest day notched so late in the autumn include the following (thanks go to Nick Wiltgen and Linda Lam at for some of the record-heat data used below):

International Falls, MN: 88°F
Duluth, MN: 84°F
Redwood Falls, MN: 90°F
*Fargo, ND: 97°F
Grand Forks, ND: 90°F
Aberdeen, SD: 93°F
Pierre, SD: 95°F
*Broken Bow, NE: 98°F
*Grand Island, NE: 97°F
Lincoln, NE: 94°F
**Colorado Springs, CO: 87°F
(asterisk = October record broken;
double asterisk = October record tied)

Ansel said...

I got 1.7 inches of rain on the Bothell-Mill-creek line, Friday through Saturday.

faronium said...

One wet storm does not break a drought nor does it portend a wet winter. Time will tell. It's possible to look at analogue summer's after an extended warm ENSO period and see both wet summers with record few fires as well as the record breaking (in BC in terms of area burned) summer of '58.

The coupled climate system runs the show but humans are heavily modifyimg the arena as the show goes on.

richard583 said...

"Let's", hope that the broader pattern evolution (even general trending.) works to steer some of that warmer, more tropical moisture .. a bit more south, with the general progression of the main colder season months ahead. ....

Michael Gersh said...

Drought refers to a dearth of rainfall, but our political class thinks it needs continuous crisis, so if they don't have one they make it up. Calling our last year's rainfall a "drought" is a flat out lie, covered with the fig leaf of a dearth of typical levels of snow. If we ever see an actual drought there will be no need for experts to point it out to us.

John Marshall said...

Seems like we need to expand the general public's definition of drought to include potential evaporation, soil moisture and river and aquafer levels along with types and frequency of rainfall and of course snow levels. Total rainfall is the most important piece, but what I expect (and I think we largely got) from the state drought monitors is this more complete and realistic picture.

When the salmon can't go up the rivers and we have to irrigate established, native plants to keep them alive, that's a drought, regardless of any other metrics.

That said, the dictionary definition of drought is simply the lack of rainfall. Perhaps we need to change the dictinionary definition of drought. Or use a new word to describe the lack of available water resources. This is how hydrologists and soil scientists and others in related fields look at it, but the general public not so much.

This is likely to be especially important if climate change is going to create years like this last one more frequently in the future.
I see no reason we can't have greater than normal annual rainfall and still be in a severe 'drought', depending on these other factors.

Daniel said...

Dear Cliff

I really enjoy your blog and all the incredible information you make available. Thank you for this! Many years ago, I took a meteorology class at a community college and have promptly forgotten all but the absolute basics. Do you have a book to recommend for someone to brush up and learn more?

Thanks in advance

Duncan Mowatt said...

Cliff - I was recently reading an article you wrote in September about the forecast for this winter. Your thought at the time was a 20% reduction for our average snow pack (better the 80% reduction from last year).

Do you still believe that this is true, or have you modified your thoughts?

I REALLY would love to have a ski season this year :)

Mark said...

Dear Michael Gersh,
Here are descriptions of the four main categories of drought.

Meteorological drought is specific to different regions. For example, 20 inches (51 centimeters) of rainfall in a year is normal in West Texas, but the same amount would be less than half the yearly average in Virginia.

Agricultural drought accounts for the water needs of crops during different growing stages. For instance, not enough moisture at planting may hinder germination, leading to low plant populations and a reduction in yield.

Hydrological drought refers to persistently low water volumes in streams, rivers and reservoirs. Human activities, such as drawdown of reservoirs, can worsen hydrological droughts. Hydrological drought is often linked with meteorological droughts.

Socioeconomic drought occurs when the demand for water exceeds the supply. Examples of this kind of drought include too much irrigation or when low river flow forces hydroelectric power plant operators to reduce energy production.

John23987 said...

I recommend " The Weather of the Pacific Northwest" by Cliff Mass

Michael Gersh said...

Dear Mark,

There are only two types of drought. There is the one where there is not enough rainfall, which is a meteorological drought. Then there is a political drought, where politicians play with words to confuse their constituency in order to get their way or avoid blame, otherwise known as "pulling the wool over the eyes of the ignorant masses." That's the kind we have today, where the political class and those who are in their thrall pretend that the failure of our incompetent government to provide proper infrastructure is the fault of the weather. You may call it a hydrological or sociological drought if you wish, but pardon me if I disagree.