Improved Chances for a Big Northwest Windstorm This Winter

We have gone several years since the last big windstorm from off the Pacific.

This year may be different.  This blog will tell you why.

The Chanukah Eve Storm of December 14, 2016

Major Northwest windstorms are associated with intense low-pressure centers reaching our coast and then moving to the northeast.   As illustrated below, these storms possess intense pressure differences around them, which produces strong winds.

Big Pacific cyclones hitting the Pacific Northwest are rare during strong El Nino and strong La Nina winters.

To illustrate this fact, the figure below shows you the sea surface temperature difference from normal in the central tropical Pacific, the key indicator of El Nino and La Nina, from 1860 through 2000.   Normal (or neutral years) are found from -0.5 to 0.5 C.  Moderate to strong El Ninos occur when temperatures are more than 1°C above normal.  Moderate to strong La Nina's when temperature is more than 1°C cooler than normal.  

The red dots show years with strong storms and high winds hitting our region.    Notice how they never occurred for strong El Ninos and La Nina.  Most windstorms occur in neutral to near neutral conditions (tropical Pacific sea surface temperatures are within 1°C of normal)

And now the key point.  The latest forecasts suggest we will be in neutral conditions or a  weak La Nina this year, a situation in which many big windstorms have occurred in our region.

Below is a graph showing a collection of predictions using different methods, some statistical, some using physical models.  Most are predicting neutral or weak La Nina conditions.   Good windstorm potential.

Why are neutral years better for strong low centers approaching our coast?

Major Pacific low centers/cyclones are closely associated with the jet stream.

During strong El Nino years, the jet tends to go south into California (see figure)

In contrast, for La Nina years, the jet stream tends to swing north of us, leaving our region in a northwesterly flow not favorable for strong windstorm development.  The last few winters have been dominated by El Nino conditions.

No guarantees, but I suspect we have a decent chance at a major blow this winter.

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Announcement:  Free Public Lecture at Kane Hall on October 10:  Global Warming, the Jet Stream, and Cold Waves

All of you are invited to attend what should be an excellent public lecture by Professor Jonathan Martin on how global warming affects the jet stream and cold air outbreaks.

It will be a timely and interesting lecture accessible to non-meteorologists and given in honor of UW Professor Peter Hobbs.

The talk will be at 7 PM in Kane Hall room 210.

If you would like to go, please register online here:

Parking is available (at a modest cost) in the UW Central Garage, which is located directly under Kane Hall.  Or take the light rail (the UW stops are a 5-10-minute walk away).

Why Such Catastrophic Flooding with Hurricane Helene?

 Hurricane Helene has brought terrible destruction to the southeast U.S., with the worst damage and loss of life in the higher terrain of western North Carolina.

The death toll is now well over 100 and the total damage is estimated to exceed 100 billion dollars.

As we will see, localized rainfall totals for the storm were extreme, with some locations receiving as much as 30 inches.

To understand what happened, first consider the terrain of the region (below).  You can clearly see the high terrain of the Appalachians over the northwest portion of the domain.

A map of rainfall totals for the event shows values reaching 15-25 inches on the eastern side of the Appalachians, where the hurricane pushed air against the terrain.

In fact, one location (Busick RAWS) had a 3-day total of 31.33 inches of rain, more than the annual total of many locations in the U.S.

The track of the hurricane was perfect for creating a very strong, moist, southeaserly flow that was forced up the eastern side of the barrier, dropping immense amounts of precipitation.

Northwesterners are very aware of the dramatic effects of geographic enhancement of precipitation:  as air rises on terrain it cools and is forced to give up its water vapor, which is converted into precipitation

Hurricane Helene at 5 AM PDT on Friday still had a very strong circulation (see pressure forecast for that time below).    Winds rotate counterclockwise around lows, so that the winds were from the southeast over western North Carolina

The hurricane entrained a plume of very moist air right into the eastern side of the Appalachians, something shown by a simulation of the winds around 5000 ft (850 hPa pressure) on Friday morning.  The purple color indicates the strongest winds...and the wind pennants show the direction.  The hurricane center was in the perfect location to push air into the North Carolina mountains.

The NOAA/NWS High-Resolution Rapid Refresh  (HRRR) model predicted the heavy precipitation in the right location  (see below).

Now the editorial portion of this blog.

The ability to forecast severe weather like this has gotten immensely better, with weather modeling technology enjoying profound improvement.   

But even with far better weather forecasts, there are still events with large death tolls, like this event and the Lahaina wildfire on Maui.    

Society must learn to better use advancing weather prediction to reduce the large death tolls in such events.