December 2007

Overview

December was a very wet month in western Oregon and northeastern Oregon, but relatively dry in some sections in central and eastern Oregon. The first week was highlighted by one of the strongest storms in recent years (see summary beginning on page 3) – an unusual combination of a big wind storm and a big rain storm, combined. Temperatures for the month were generally near normal. The snow pack, which was rather puny at the start of the month, rose dramatically in December.

Table 1 is a summary of monthly averages and totals at selected stations throughout the state. Table 2 lists daily temperatures and precipitation for most of the locations listed in Table 1. In Table 3, monthly precipitation totals throughout the state are listed. Figure 1 is a map showing the percentage of normal precipitation statewide for the Water Year.

Basin Summary

Here is a summary of water indicators at the end of the month, by river basin:

n.a. Not available
(1) Percent of normal November precipitation, from NOAA Cooperative sites
(2) Percent of normal seasonal precipitation (since Oct. 1), from NOAA Cooperative sites
(3) Percent of normal seasonal precipitation, from Natural Resources Conservation Service (NRCS) SNOTEL sites
(4) Percent of normal snow water equivalent, from NRCS SNOTEL sites
(5) Percent of normal November stream flow, from U.S. Geological Survey (USGS)
(6) Percent of normal seasonal stream flow (since Oct. 1), from USGS
(7) Surface Water Supply Index, from NRCS (-4 = very dry, 0 = normal, +4 = very wet)

Forecasts

The Climate Prediction Center’s (CPC) forecasts for January-March appear below. Temperature probabilities show equal chances of above-, near-, and below-normal for Oregon, while precipitation probabilities show above-normal probabilities.

Oregon Climate Service predicts below-normal temperatures and above-normal precipitation for the next three months.

ENSO Update

Australian Bureau of Meteorology, January 2, 2008

A La Niña event is firmly established in the Pacific, strengthening over the past month and contributing to the enhanced eastern Australian rainfall since November.

Cooler than average sea surface temperatures (SSTs) now extend further west along the equator than at any time since 2000, while warmer than average SSTs surround northern Australia. The December 2007 Southern Oscillation Index (SOI) of 14.4 is the highest monthly SOI value since April 2006. Trade Winds remain enhanced and cloudiness continues to be suppressed along much of the central equatorial Pacific; both indicators of the now mature event. While sub-surface temperatures remain cooler than normal in the east, some warming has occurred in the western regions.

Although some computer models suggest that the event is nearing its peak, most continue to indicate the persistence of cool Pacific Ocean temperatures, consistent with a La Niña, until at least autumn 2008.

CPC ENSO Forecast (January 10, 2008)

La Niña remained at moderate strength during December 2007, with below-average sea surface temperatures (SSTs) extending from 160šE to the South American coast. All of the Niño region indices remained cooler than -1.0°C , with the Niño-3.4 and Niño-3 indices persisting near -1.5°C. The upper-ocean heat content (average temperatures in the upper 300 m of the ocean) in the central and east-central equatorial Pacific remained below average, with temperatures ranging from 2°C to 5°C below average at thermocline depth. The recent SST forecasts (dynamical and statistical models) for the Niño 3.4 region indicate a continuation of La Niña conditions into Northern Hemisphere during spring 2008. Over half of the models predict a moderate strength La Niña to continue through February-April, followed by weaker La Niña conditions. Current atmospheric and oceanic conditions and recent trends are consistent with a likely continuation of La Niña into the Northern Hemisphere spring 2008.

The “Great Coastal Gale”

Wolf Read, George Taylor and Cadee Hale

A large and very damaging storm affected all of the Pacific Northwest during the first week in December. Winds exceeding 100 mph were accompanied by intense rains, which led to flooding. Damage was severe in many locations, mostly near the Oregon and Washington coasts.

The storm began as a large, but not particularly strong, mid-latitude storm in the mid-Pacific. Moisture from several decaying typhoons moved eastward and were absorbed by the storm, causing rapid enhancement. The storm deepened and grew in size, eventually reaching a diameter of several thousand miles; at one point, it stretched from western Idaho to the International Date Line, or about the size of the continental United States.

The moisture-enhanced deepening is what led to the very strong winds. The sheer size of the storm allowed it to reach well down into the tropics and tap abundant tropical moisture, which formed an “atmospheric river” – warm, moist air from the southwest, the kind of situation that brings our biggest winter flood events. And that’s exactly what occurred.

An animation showing enhanced infrared satellite images (courtesy Mauna Kea Weather Center) appears on the Oregon Climate Service web site:

(http://www.ocs.oregonstate.edu/page_links/whats_new/wind_storm.html.

In analyzing the images we were impressed with the sheer size of the storm, which was approximately the size of the continental U.S. — and, in fact, comparable to the diameter of the Moon! See below.

The Rain Storm

Major flooding occurred in SW Washington and NW Oregon. Data are still coming in. We will report fully next month.

The Wind Storm

Wolf Read’s analysis of the wind storm:

The damage exceeded anything I've seen from a significant windstorm, especially among the northern communities. But even in the southern part of my tour, the damage stood out: At least 48 trees fell across Hwy 20, most of those in the last 15 miles. That started things off. Once at Newport, toppled trees became a common sight, and the further north I traveled, the worse the destruction became. Just when I thought things couldn't get any worse, I'd roll into a region with even heavier damage. After many miles, I ceased to be surprised. Damaged structures, not just toppled trees, eventually became commonplace. Based on what I could glean during my travels through fog and periodic rain showers, the storm's impact breaks into four fuzzy sections:

1) The southern stretch through Depoe Bay and into Lincoln City. Characterized by numerous treefalls, both due to root-failure and stem-breaks. Many tree-downed powerlines, including some large transmission lines. The breakage of some highway signs, typically those with 4' x 4' posts.

2) Lincoln City north to about Cloverdale. Characterized by local swathing of coastal forest with many trees across the road in places, some quite large. Downed large branches as numerous as twigs in a more normal storm. Light damage to house roofs, usually shingle loss. Some window breakage. Plastic store signs punched out. Some wooden fences downed. Obvious loss of electrical service.

3) Cloverdale north to about Tillamook. Characterized by: Trees as in 2). Many outbuildings and some houses with sheet-metal roofs and walls peeled off. Structures thrown out-of-kilter. Fences down. Structural debris strewn across farm fields. Frequency of broken highway signs is high enough that the sight of such damage seems as commonplace as snapped boles.

4) Tillmook north to Rockaway, and especially Bay City. Characterized by: As in 3). Broken windows more numerous, both on store fronts and houses, and usually boarded up by the time I arrived. Large percentage of houses had at least some shingle loss, with some down to the bare boards. Siding peeled off of homes and businesses. Fences of all types frequently laid flat. Many signs, especially Stop signs, broken. Especially around Bay City: Some structures, such as a wood-frame garage, and a large metal-framed public storage building, blown apart. Some houses lightly damaged, especially eave elements. Church steeple broken off and crashed in street. Masonry chimneys broken. Tree damage in nearby forestland incredible, with perhaps 20% to 40% trees lost or damaged.

After seeing the extent of damage, I have to agree that the hurricane-force wind warning was justified, though the warning didn't verify in many areas, save for a few exposed headlands