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La Nina Impacts, Long Range Forecasts, Spring Flood Forecast February 20, 2011

Posted by mikeheard in Uncategorized.

A strong La  Nina was predicted for the winter season of 2010/2011. The colder than normal pacific equatorial waters has influenced global weather patterns beginning in November of 2010 and continues to do so today.

Typically, La Nina produces a colder than normal and wetter than normal winter season across Montana and the stronger the episode the higher the probability Montana will endure a very cold and snowy winter, the weaker the La Nina the lower the probability of a severe winter season. This La Nina is one of the stronger ENSO events we have seen in many years.

Here is some data to compare the La Nina events of 2008/2009 and the current event for 2010/2011 winter seasons.

Arctic outbreaks November to March:

Gallatin Field – Bozeman in the 2008/2009 season saw a total of 8 extreme cold events during a weak to moderate La  Nina season. 2010/2011 season, as of February 20th, 2011 Bozeman has seen 11 extreme cold episodes.

Butte, the data is very similar, with 7 total extreme cold events in the 2008/2009 weak to moderate La Nina season vs. 11 so far for the 2010/2011 strong La  Nina season.

Extreme cold data to date for the winter season 2010/2011 Strong La Nina.

Gallatin Field – Bozeman monthly coldest lows November to February (as of Feb. 20, 2011)

-21 Nov. 24, 2010

-19 Dec. 31, 2010

-13 Jan. 10, 2011

-21 Feb. 9, 2011

Butte monthly coldest lows November to February (as of Feb. 20, 2011)

-22 Nov. 24, 2010

-18 Dec. 31, 2010

-21 Jan. 1, 2011

-21 Feb. 1, 2011

Looking at the 7 to 10 day forecast it appears more Arctic air will be digging into Montana by Thursday through the weekend with forecast lows -5 to -20. Also, a pacific storm and Low will move into NW Montana and track through the state Tuesday through Thursday producing a chance for more widespread snow. As this storm exits the region then the Arctic air drops into the state. This is a classic example of the La Nina influenced pattern.

Valley Precipitation Trends on Water Yeard (Oct. 2010 to Jan. 2011)

Water Year Precipitation Trends % of Average

The graphic above shows mixed results on precipitation trends for the lower elevations. Overall, the water year has been good with % of average values vary from 88% of average in Butte to 117% of average in Ennis, MSU 93%, Dillon 120%, Boulder 82%. These values are near to slightly above normal.

Snowpack for area river basins:

Gallatin River - Snowpack as of Feb. 20, 2011

Jefferson River Basin - Snowpack as of Feb. 20, 2011

Madison River Basin Snowpack - as of Feb. 20, 2011

Upper Clark Fork River Basin Snowpack - as of February 20, 2011

Upper Yellowstone River Basin Snowpack - as of Feb. 20, 2011

From the above graphics showing the individual river basins in SW Montana shows snowpack conditions are running slightly above the 30 year mean and vary locally between 105% to 115% of average on snow/water data.

Snowpack conditions remain healthy and are well above conditions from last year, recall that El Nino was the dominate weather pattern in the 2009/2010 season with below normal snowpack and slightly warmer than normal winter season.

Long Lead Forecasts:

National Climate Prediction Center computer models for long range forecast continues to show a strong La Nina pattern to continue into the early half of the upcoming Spring months. That means more of the same, colder than normal and wetter than normal conditions for Montana.

30 Day Forecast for Temperature and Precipitation

30 day forecast Temperature

30 day forecast Precipitation

90 Day Forecast for Temperature and Precipitation

March, April, May 90 Day Forecast Temperatures


March, April, May Precipitation 90 day forecast


The Great Falls National Weather Service issued an early Spring Flood Forecast last week and we reported local findings on the 5:30pm and 10pm newscast back on Thursday, however, here is a link to their report: NWS Hydrologic Outlook

Some of the highlights for SW Montana are based on current snowpack conditions. Gallatin River has a better than 60% chance of minor flooding due to snowpack melting this spring and the Big Hole River has a greater than 80% chance for minor flooding. SW Montana rivers regionally has less than a 30% chance for moderate flooding. Again, this forecast is based on current data.

In my opinion flooding risks will go up for SW Montana if current weather trends continue, numerous snow storms and continued colder than temperatures, as snowpack numbers continue to go up and the colder than normal weather keeps the snowpack in the mountains and before you know it the spring rainy season hits. The worst case scenario is a rapid warm up with heavy rain on top of higher than normal mountain snowpack. That scenario is what I will be watching for in the months to come.





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