Overview

The really unsettling thing about the winter of 2005-06 is this: it keeps getting drier and drier! Most of Oregon reported less than 50% of normal monthly precipitation for February, and the statewide seasonal total is hovering around 60% of normal. This will go down as one of the driest winter ever. We still hold out hope for AT LEAST an average spring, but it’s becoming clear than we’re going to hear a lot of the “D” word this year – drought!

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 and seasonal precipitation totals throughout the state are listed.

Basin Summary

Here is a summary of precipitation, water supply, and snow pack as of the end of the month, by river basin:

	Precipitation			Snow	Stream Flow		SWSI
BASIN	(1)	(2)	(3)	(4)	(5)	(6)	(7)
OWYHEE	23	90	76	67	28	51	-1.5
MALHEUR	25	86	64	42	15	47	-2.1
GRAND RONDE, POWDER, BURNT	47	65	61	45	44	64	-2.4
UMATILLA, WALLA WALLA, WILLOW	23	50	56	29	25	54	-2.3
UPPER JOHN DAY	84	71	64	49	28	43	-1.7
UPPER DESCHUTES, CROOKED	51	62	54	51	30	45	-1.5
LOWER DESCHUTES, HOOD RIVER	25	53	50	21	30	48	-2.6
WILLAMETTE	23	51	51	32	26	46	-2.4
ROGUE, UMPQUA	26	65	63	37	22	44	-1.4
KLAMATH	20	66	61	43	47	62	-1.9
LAKE COUNTY, GOOSE LAKE	22	102	77	79	26	41	-1.0
HARNEY	45	92	69	53	37	67	-1.1
NORTH COAST	24	59	47	0	23	54	-2.1
SOUTH COAST	35	68	n.a	n.a.	23	55	-1.3

n.a. Not available
(1) Percent of normal February 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 February 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 March-May appear below. Temperatures for Oregon (and all of the West) are likely to be above normal, while precipitation probabilities are below normal.

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

ENSO Update (Australian Bureau of Meteorology, March 10, 2005)

Summary

February was an exceptional month in the western to central equatorial Pacific: Trade winds were strongly suppressed for virtually the entire month, record low atmospheric pressure was observed at Tahiti and with a value of –29, the Southern Oscillation Index (SOI) plummeted to its lowest value for any month since February 1983 (–33). Accompanying the reduction in the Trades (known as a westerly wind burst - WWB), has been a marked increase in subsurface temperatures near the dateline.

These recent developments have increased the chances of a basin-wide El Niño occurring later this year, as opposed to the more restricted warming which occurred in 2004. The tropical Pacific has now been warmer than average for most of the past three years (including during the 2002/03 El Niño), and was close to the El Niño threshold during the recent southern spring and early summer. As autumn is the critical time for El Niño development, any further changes over the coming months will be monitored closely.

Historical data and the latest information indicate that:
* The sub-surface warmth in the western Pacific will gradually move to the east, reaching South America in about two to three months (April/May). This movement is called a "Kelvin wave".

* Surface temperatures in the eastern to central Pacific are likely to rise in response to the arrival of the Kelvin wave.

* Most computer models haven't included the current WWB and its associated strong subsurface warming. The Bureau of Meteorology's POAMA model has included these, and it has the chance of warming above El Niño thresholds at close to 40%, which is a little less than double the long-term average for this time of year.

* March to June is known as the "predictability" barrier and model skill is at its lowest predicting across this span of months.
There remains uncertainty about:

* The intensity and duration of the warming in the eastern to central Pacific in response to the Kelvin wave.

* Whether any follow-up westerly wind bursts will occur during autumn - if an El Niño event is to occur this year there will almost certainly need to be more WWBs in the next few months, as it is unlikely the current one will be sufficient in itself to trigger an event.

In Brief
* Sea surface temperatures throughout the tropical Pacific have cooled since the beginning of the year. Temperatures remain slightly warmer than average in western to central areas but are cooler than average in the east.

* Subsurface data for February show strong warming in the central Pacific, and moderate to strong cooling in the east.

* The SOI dropped from +2 in January to –29 in February. The approximate SOI for the 30 days ending 7th March was –26.

* A westerly wind burst near the dateline has weakened Trade Winds in the western tropical Pacific since the beginning of February. During March, the region of suppressed Trades has retreated a little to the west.

* Cloudiness around the dateline has generally been well above average since the beginning of February.

* Nine of twelve computer models predict neutral eastern Pacific conditions in July 2005.

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The Driest Winters
George H. Taylor, March 2005

The Five Driest Years at NWS Oregon Stations.

Below are tables of this year’s Water Year compared with other dry years at selected Oregon stations (sites of first-order National Weather Service stations). Maybe not the driest ever – but it’s close!

Portland Airport

Salem

Astoria

Medford

Pendleton

Oregon Climate Service

George H. Taylor, Oregon State Climatologist

Wayne P. Gibson, Programmer/GIS mngr.

Eileen Kaspar, Research Assistant/Manager of Data Services

Melanie Mitchell, Undergraduate Assistant

Wolf Read, Undergraduate Assistant

Emily Gibson, Student Assistant

Cadee Hale, Publications Assistant

Kelsey Kuykendall, Undergraduate Assistant

Sara Joos, Student Assistant

Sean Daly, Student Assistant

Kirk Borgerding, Undergraduate Assistant

Oregon Climate Service, Strand 326,                             	     Phone: (541) 737-5705  
Oregon State University                                                      Fax: (541) 737-5710   
Corvallis, Oregon  97331                                                     E-mail: oregon@coas.orst.edu   
                                                                             Web: http://www.ocs.oregonstate.edu