Originally Published: July 6, 2010 10:05 p.m.
Steady monsoon rains are arriving later than usual in Arizona this year, but forecasters are not surprised.
They are learning more about the complex North American Monsoon in recent years, and that includes how far-away weather events can affect the arrival of monsoon rains.
For example, when it's unusually wet in the Midwest like it is so far this summer, that tends to push back the monsoon rains, said Eric Pytlak, science and operations officer at the National Weather Service office in Tucson.
Right now the jet stream is too far south below Canada, and that's keeping us in a dry westerly wind pattern instead of the southeasterly winds associated with the monsoon, Pytlak added.
Adding to the possibility of a drier-than-usual monsoon is the fact that after an El Niño winter, Arizona's monsoon tends to be a little weak. El Niño heats up the equatorial Pacific Ocean.
On the bright side for monsoon moisture, the cooling tropical Pacific Ocean trend known as La Niña is moving in. That should help increase precipitation in the latter half of the monsoon, although it also can help produce a dry winter later.
When La Niña takes hold this summer as forecasters expect, it will help push the jet stream north, along with the subtropical high pressure system (aka "monsoon ridge") that tends to hover over the Four Corners area during a good monsoon flow pattern.
Subtropical monsoon moisture usually moves into southeastern Arizona first, on average by the first week in July, then moves north.
Prescottonians have been seeing some thundercloud buildup and even scattered showers last Thursday, but otherwise it's been bone-dry since April.
Arizona's forecast is calling for a monsoon flow pattern to move in later this week, however.
Prescott has a 10 to 20 percent chance of thunderstorms Wednesday through Friday, peaking at a 30 percent chance by Saturday. Chances for Saturday rain are higher in Flagstaff (40 percent) and the White Mountains (50 percent).
As usual in the early monsoon pattern, such storms also could produce dry lightning and wind that could spark dangerous wildfires.
While the monsoon pattern is trying to set up, it's still relatively cool in the Gulf of California and forecasters expect it to dry out again early next week.