Article Submitted by Mark McDonnell, USN Weather Forecaster, Retired
WEATHER OUTLOOK FOR APRIL 2011
Below Average Temperature (Cooler), Normal Hi/Lo: 57F/31F
Above Average Precipitation (Wetter), Normal 0.91 inches
WEATHER OUTLOOK FOR MAY 2011
Slightly Below Average Temperature (Cooler). Normal Hi/Lo: 66F/40F
Slightly Above Average Precipitation (Wetter), Normal 1.78 inches
WEATHER OUTLOOK FOR JUNE 2011
Slightly Below Average Temperature (Cooler), Normal Hi/Lo: 75F/48F
Equal Chance of Average Precipitation: 1.82 inches
La Nina conditions are present across the tropical Pacific Ocean. According to the Climate Prediction Center, recent sea surface temperatures near the equator in the eastern and central Pacific Ocean have been at least 1 degree Celsius below normal in key parts of the ocean. On the flip-side, sea surface temperatures from the Central Pacific to the South American coast have warmed during the last two weeks. The contrast in warming and cooling is instrumental to the over-all wind pattern across the ocean which eventually is the driving force for our weather across the continental U.S., specifically in our case, the Northwest United States and Rocky Mountains.
The large scale atmospheric circulation over the Pacific Ocean in the general vicinity of the equator is consistent with La Nina conditions. The low-level easterly trade winds are enhanced over the western and central pacific. Changes in the westerly wind flow at upper levels (Jet Stream) are enhanced. The jet stream is basically a super highway for weather.
The April 2011 temperature and precipitation outlooks issued by the Climate Prediction Center were based entirely on computer driven forecasts. The resulting predictions indicated a consistent pattern with relatively cool temperatures along the west coast and along the northern tier of the states with increased chances of above median precipitation in the northern Rockies.
Forecasts from most computer programs (models) indicate the La Nina will diminish as we progress through spring and the El Nino will build-in through the summer months. So far it appears the strength of El Nino will be neutral, not strong nor weak; however, there is high uncertainty as to the reliability of that forecast at this point.
Although computer models were considered, forecasts for May and June are based more on trends of historical behavior coupled with current actual observations and data collected. With El Nino building-in and uncertainty high, The Climate Prediction Center has predicted below normal temperatures to prevail across the Rockies and slightly lower then normal precipitation across the northwest as we transition into the summer season.
