Overview

March was quite warm and rather dry; only a few stations had above-average precipitation, and even fewer had below-average temperatures. The combination of warm and dry weather cuased the mountain snowpack to decline significantly through the month.

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 water indicators at the end of the month, by river basin:

n.a. Not available
(1) Percent of normal March 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 March 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 April-June appear below. Temperatures for Oregon show a slight tendency toward above normal temperatures, while precipitation probabilities show equal chances of above-, near-, and below-normal.

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

ENSO Update
Climate Prediction Center (CPC), April 4, 2007
The pattern of anomalous sea surface temperatures (SSTs) during March 2007 was consistent with ENSO-neutral conditions in the tropical Pacific, with average to slightly below-average SSTs extending from the date line to the west coast of South America. The latest weekly SST departures are near 0†C in the Niño 3.4 region, and below -0.5†C in the Niño 3 and Niño 1+2 regions. An area of anomalously warm SSTs persisted well west of the date line (near 165†E), and an area of SSTs exceeding 30°C was centered between 150°E and 165°E.

The upper-ocean heat content (average temperature departures in the upper 300 m of the ocean) remains below-average across the central and east-central equatorial Pacific, with temperatures at thermocline depth generally 3°-5°C below average. Consistent with the surface and sub-surface temperature patterns, stronger than-average low-level easterly winds persisted throughout the month of March over the central equatorial Pacific, and convection was enhanced over the western equatorial Pacific and Indonesia and suppressed near the date line. Collectively, these atmospheric and oceanic conditions are consistent with a trend towards a Pacific cold (La Niña) episode.

Most of the statistical and coupled model forecasts, including those from the NCEP Climate Forecast System (CFS), indicate additional anomalous surface cooling during the next several months. Some forecast models, especially the CFS, indicate a transition to La Niña during May-July 2007. This forecast is consistent with the observed trends in atmospheric and oceanic conditions. However, the spread of the most recent statistical and coupled model forecasts (ENSO-neutral to La Niña) indicates considerable uncertainty as to when La Niña might develop and how strong it might be.

ENSO Summary: Australia Bureau of Meteorology (BOM), March 28, 2007

Central to eastern Pacific sea-surface temperatures have remained close to average during March, following the rapid cooling that took place during January and February at the end of the 2006/07 El Niño event. The SOI, Trade Winds and Pacific cloud patterns are other ENSO indicators which are currently in a neutral phase. There appears to be little chance of a return to El Niño conditions in 2007, with a continuation of neutral, or a switch to La Niña conditions, the more likely outcomes.

A La Niña in 2007?

The chance of a La Niña developing in 2007 is thought to be higher than the long-term average (which is about one in five or 20%) because (a) they have a tendency to follow an El Niño; (b) the 2006/07 El Niño decayed somewhat earlier than normal thereby giving time for a La Niña to begin developing during the critical March to June period; and (c) a large pool of cold sub-surface water remains in the central to eastern tropical Pacific Ocean and is starting to affect surface temperatures in the region. La Niña events are generally associated with wetter than normal conditions across much of the eastern half of the country from about autumn.

The 2007 Atlantic Hurricane Season
George H. Taylor

Every year since 1984, Dr. Bill Gray and his colleagues at Colorado State University (CSU) have issued forecasts of tropical storms for the upcoming Atlantic hurricane season. Last week they issued their latest forecast, for the summer-fall of 2007.

Gray’s current co-author is Philip Klotzbach, a young atmospheric scientist who has been working with Gray for the last 6 years. Klotzbach is now the primary forecaster and writes most of the publications and technical reports.

2006 was an unusually bad year for forecasters, including the CSU team. As Bill says, “Our 2006 seasonal hurricane forecast was not successful. We anticipated a well above-average season, and the season had activity at slightly below-average levels. We did catch this downward trend beginning with our early August update. We attribute a large portion of this forecast over-prediction to a late-developing El Niño and increased mid-level dryness in the tropical Atlantic.”

In a forecast, you always compare your forecast to “persistence” and “climatology,” and over the long term you want to beat both. Otherwise, why bother? For persistence, you predict that tomorrow (or next year) will be the same as today (or this year). A climatology prediction uses the long-term average. Gray’s forecasts have done significantly better than persistence and climatology. However, last year he went with persistence (2005 had been a very active year – that was the “Katrina year”) and overpredicted. A climatology prediction would have been more accurate.

For 2007, Gray’s group has “increased our forecast for the hurricane season, largely due to the rapid dissipation of El Niño conditions. We are now calling for a very active hurricane season. Landfall probabilities for the 2007 hurricane season are well above their long-period averages.”

El Niño comes into play because during such events Atlantic hurricanes are suppressed, even though El Niño is a Pacific phenomenon. An El Niño triggers stronger winds in the middle atmosphere which “knock the tops off” hurricanes, thus preventing them from getting really strong.

For 2007, the forecast is for a season that is “much more active than the average 1950-2000 season. We estimate that 2007 will have about 9 hurricanes (average is 5.9), 17 named storms (average is 9.6), 85 named storm days (average is 49.1), 40 hurricane days (average is 24.5), 5 intense (Category 3-4-5) hurricanes (average is 2.3) and 11 intense hurricane days (average is 5.0). The probability of U.S. major hurricane landfall is estimated to be about 140 percent of the long-period average.”

Gray’s group offers the odds of a hurricane landfall in specific US regions as:
1 Entire U.S. coastline - 74% (average for last century is 52%)
2) U.S. East Coast Including Peninsula Florida - 50% (average for last century is 31%)
3) Gulf Coast from the Florida Panhandle westward to Brownsville - 49% (average for last century is 30%)
4) Above-average major hurricane landfall risk in the Caribbean

The 2004-2005 seasons were unusually destructive not because of the high frequency of major hurricanes, but because a high percentage of major hurricanes made landfalls on the US coastline. This was caused by favorable upper-air wind patterns present during those years.

Those are the same conditions, by the way, that appear to be shaping up right now. My daughter, Annie, is about to move from Pensacola, Florida to southern California. After seeing Gray’s forecast, I’m really glad she’s moving.