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Climate experts discuss winter and spring forecasts

TitleClimate experts discuss winter and spring forecasts
Publication TypeFeature Articles
2005
AuthorsBrandon, D, Hartmann, H, Polasko, E, Smith, J, Wolter, K, Lenart, M
JournalSouthwest Climate Outlook
Volume4
Issue11
Start Page2
Pagination2-3
Date Published12/2005
Full Text

The winter and spring seasonal forecasts issued on November 17, 2005 by the National Oceanic and Atmospheric Administration Climate Prediction Center (NOAA-CPC) showed the Southwest as having “equal chances” of above-average, near-normal or below-average precipitation (i.e., there’s no forecast). Similarly, the International Research Institute for Climate and Society (IRI) had made no Southwest precipitation forecast for the coming winter and spring. Both the CPC and IRI have more recently forecast an increased likelihood of aboveaverage temperatures in the Southwest.

On November 18, CLIMAS sought the input of experts to contribute their insight to a roundtable discussion on how the region’s snowpack and water supply might fare this winter and spring based on the forecasts at the time. The CPC and IRI have since adjusted their forecasts to project dry conditions for the Southwest region in the coming months, an outlook that reflects comments made by our climate experts. Some definitions and explanations are included within the discussion. Please see the CLIMAS online glossary (http://www. ispe.arizona.edu/climas/forecasts/glossary. html) for terms that are not defined here.

Roundtable Participants

Dave Brandon: Hydrologist in Charge, NOAA Colorado Basin River Forecast Center

Holly Hartmann: PhD Assistant Research Scientist, UA Department of Hydrology and Water Resources; Investigator, CLIMAS

Ed Polasko: Senior Service Hydrologist, NOAA National Weather Service, Albuquerque

Jeff Smith: Senior Hydrologist, NOAA Colorado Basin River Forecast Center

Klaus Wolter: PhD Meteorologist, Climate Diagnostics Center, Boulder; Research Associate, University of Colorado

Melanie Lenart: PhD Roundtable Moderator and Research Associate, CLIMAS

Lenart: With the forecasts that just came out for winter precipitation, there’s not much to say for the Southwest, but maybe you have some ideas on what we can expect. Any comments?

Brandon: We put out more of an outlook than a forecast this time of year, since this early there’s a lot of error involved… One of the things we look at is the antecedent streamflow [the total quantity of water that flows through river systems] of the system—what are the flows of the river in the fall compared to what they are usually? We also have a soil moisture model that we continuously run, which is probably the most important factor. There’s not much snowpack this early, but we have 116 SNOTEL [snowpack telemetry] sites over Lake Powell that we look at. We combine those and compare them to last year and other years’ average. Obviously, it’s very early in the season, but we’re about a hair below average right now, and last year at this time we were a little bit above average. When we run these forecasts, the main thing we find is that we can be about 10 to 16 percent more accurate than we would be just using the averages for the last 30 years. A lot of that comes from the moisture model. If you’ve been in a very dry or wet period, the models reflect that well. We also look at ENSO [El Niño Southern Oscillation] signals. We now have an operational procedure in which we look at CPC forecasts for the season and translate those into a shift in precipitation or temperature. We’ve found that in the last 15 La Niñas, 14 were dry in Arizona. There isn’t a strong signal right now…but that’s something we’re starting to look at, is a trend towards a La Niña. Using these variables, we come up with an ensemble streamflow prediction and then run previous years through our model to check it.

Lenart: From what you’re saying, it sounds like you have some bad news for us in terms of your streamflow outlook this year.

Brandon: Well, bad news is in the eyes of the beholder. There’s a lot of error this early, but Lake Powell streamflow looks like it’s going to be around 80 percent.

Smith: That’s around 6.5–6.7 million acre-feet from April to July. The average is about 7.9 million.

Brandon: That’s the Upper Colorado River and Lake Powell. In 2002, we had 1.1 million acre-feet, so it’s relatively much better. When we ran the model last year at this time, the prediction was a little higher, but we’d had a wet fall and early snow in the San Juan Mountains. That’s coming off a very dry period, and we were still predicting a little below normal.

Wolter: But that was the forecast, what actually happened? Didn’t we get a lot more?

Brandon: We ended up just a bit above normal for the whole basin.

Lenart: The San Juans are an area serving New Mexico from the Colorado River, so how would things look for the rest of the state [i.e., the areas not in the Colorado River watershed]?

Polasko: Before I get into projections, let’s take a step back and look at where we were in June 2004 in two of our major reservoirs: Navajo Reservoir, in the northwest part of the San Juan system, and Elephant Butte, which is our major reservoir on the Rio Grande down near Truth or Consequences. At the end of June 2004, the storage in Elephant Butte was around 228,000 acre-feet and the storage in Navajo was a little over a million. At the end of June 2005, Navajo had 1.5 million acre-feet, so in one year the increase in storage in Navajo was 50 percent. In Elephant Butte, it went up to about 560,000 acre-feet, so that’s about two and a half times what we started with. The 2004–05 winter was extremely good for us, especially coming off the extremely dry period of the last four years.

Lenart: Do you think the lucky streak might continue, or are we going back to drier times?

The winter and spring seasonal forecasts issued on November 17, 2005 by the National Oceanic and Atmospheric Administration Climate Prediction Center (NOAA-CPC) showed the Southwest as having “equal chances” of above-average, near-normal or below-average precipitation (i.e., there’s no forecast). Similarly, the International Research Institute for Climate and Society (IRI) had made no Southwest precipitation forecast for the coming winter and spring. Both the CPC and IRI have more recently forecast an increased likelihood of aboveaverage temperatures in the Southwest.

On November 18, CLIMAS sought the input of experts to contribute their insight to a roundtable discussion on how the region’s snowpack and water supply might fare this winter and spring based on the forecasts at the time. The CPC and IRI have since adjusted their forecasts to project dry conditions for the Southwest region in the coming months, an outlook that reflects comments made by our climate experts. Some definitions and explanations are included within the discussion. Please see the CLIMAS online glossary (http://www. ispe.arizona.edu/climas/forecasts/glossary. html) for terms that are not defined here.

Roundtable Participants

Dave Brandon: Hydrologist in Charge, NOAA Colorado Basin River Forecast Center

Holly Hartmann: PhD Assistant Research Scientist, UA Department of Hydrology and Water Resources; Investigator, CLIMAS

Ed Polasko: Senior Service Hydrologist, NOAA National Weather Service, Albuquerque

Jeff Smith: Senior Hydrologist, NOAA Colorado Basin River Forecast Center

Klaus Wolter: PhD Meteorologist, Climate Diagnostics Center, Boulder; Research Associate, University of Colorado

Melanie Lenart: PhD Roundtable Moderator and Research Associate, CLIMAS

Lenart: With the forecasts that just came out for winter precipitation, there’s not much to say for the Southwest, but maybe you have some ideas on what we can expect. Any comments?

Brandon: We put out more of an outlook than a forecast this time of year, since this early there’s a lot of error involved… One of the things we look at is the antecedent streamflow [the total quantity of water that flows through river systems] of the system—what are the flows of the river in the fall compared to what they are usually? We also have a soil moisture model that we continuously run, which is probably the most important factor. There’s not much snowpack this early, but we have 116 SNOTEL [snowpack telemetry] sites over Lake Powell that we look at. We combine those and compare them to last year and other years’ average. Obviously, it’s very early in the season, but we’re about a hair below average right now, and last year at this time we were a little bit above average. When we run these forecasts, the main thing we find is that we can be about 10 to 16 percent more accurate than we would be just using the averages for the last 30 years. A lot of that comes from the moisture model. If you’ve been in a very dry or wet period, the models reflect that well. We also look at ENSO [El Niño Southern Oscillation] signals. We now have an operational procedure in which we look at CPC forecasts for the season and translate those into a shift in precipitation or temperature. We’ve found that in the last 15 La Niñas, 14 were dry in Arizona. There isn’t a strong signal right now…but that’s something we’re starting to look at, is a trend towards a La Niña. Using these variables, we come up with an ensemble streamflow prediction and then run previous years through our model to check it.

Lenart: From what you’re saying, it sounds like you have some bad news for us in terms of your streamflow outlook this year.

Brandon: Well, bad news is in the eyes of the beholder. There’s a lot of error this early, but Lake Powell streamflow looks like it’s going to be around 80 percent.

Smith: That’s around 6.5–6.7 million acre-feet from April to July. The average is about 7.9 million.

Brandon: That’s the Upper Colorado River and Lake Powell. In 2002, we had 1.1 million acre-feet, so it’s relatively much better. When we ran the model last year at this time, the prediction was a little higher, but we’d had a wet fall and early snow in the San Juan Mountains. That’s coming off a very dry period, and we were still predicting a little below normal.

Wolter: But that was the forecast, what actually happened? Didn’t we get a lot more?

Brandon: We ended up just a bit above normal for the whole basin.

Lenart: The San Juans are an area serving New Mexico from the Colorado River, so how would things look for the rest of the state [i.e., the areas not in the Colorado River watershed]?

Polasko: Before I get into projections, let’s take a step back and look at where we were in June 2004 in two of our major reservoirs: Navajo Reservoir, in the northwest part of the San Juan system, and Elephant Butte, which is our major reservoir on the Rio Grande down near Truth or Consequences. At the end of June 2004, the storage in Elephant Butte was around 228,000 acre-feet and the storage in Navajo was a little over a million. At the end of June 2005, Navajo had 1.5 million acre-feet, so in one year the increase in storage in Navajo was 50 percent. In Elephant Butte, it went up to about 560,000 acre-feet, so that’s about two and a half times what we started with. The 2004–05 winter was extremely good for us, especially coming off the extremely dry period of the last four years.

Lenart: Do you think the lucky streak might continue, or are we going back to drier times?