PHOENIX -- Back in April, the Climate Prediction Center came out with their long range forecast calling for a chance of a wetter than average monsoon season for the eastern half of the state.
In May, the long range forecast changed from a wetter than average forecast to a drier than average forecast for the monsoon season.
And just a few days ago, the Climate Prediction Center changed the forecast once again.
It calls for a slightly elevated chance that rainfall will be above average for the 2009 season over eastern Arizona and western New Mexico.
There is an elevated chance of above average temperatures through September, although that chance has diminished slightly since last month's forecast.
The monsoon high expanded north from Mexico into the southern US a full two weeks earlier than usual, which was expected last month.
It is also tending a little stronger than usual.
The northward advancement of the monsoon ridge, along with rapid thunderstorm development over central Mexico two weeks ago, and an increase in tropical cyclone activity off the southwest Mexican coast, all strongly suggest that the monsoon will transition into full onset over southeast Arizona this week, which is about a week earlier than usual.
Because of continuing drought in Texas and eastern New Mexico, this high should remain stronger than usual and farther north than usual for much of July, which should keep the monsoon active for several weeks.
The forecast for the second half of the monsoon remains uncertain due to what appears to be a rapidly-developing El Niño.
An El Niño, which is a warming of water temperatures across the tropical Eastern Pacific Ocean, causes significant changes in both monsoon and jet stream patterns, as well as the number of tropical cyclones in both the eastern Pacific and Atlantic Oceans.
It is unusual for an El Niño to develop this early and rapidly in the summer, but it has happened before: in 1972, 1982, 1997 and 2004.
It is too early to tell how strong the El Niño will be, or when it will begin to affect the atmosphere.
Based on recent water temperature trends in the tropical Pacific, it appears that the atmosphere won't begin to fully respond to the developing El Niño until mid August or September, which should allow the monsoon high to remain generally north and east of Arizona, and keep things active for much of the summer.
However, when an El Niño develops rapidly in the summer months, the monsoon high can be pushed away from us into Mexico during the second half of the season as the jet stream strengthens and dips farther south along the west coast of the United States.
This in turn can result in a significant rainfall decrease in August and September. However, the same jet stream changes, coupled with an increase in tropical cyclones along the Mexico coast due to the warmer than normal water temperatures, can draw considerable tropical moisture into Arizona during September and October.
Thus, the El Niño development, and any jet stream changes it may cause, will be watched closely in the coming weeks.
