Predicting what will happen day-to-day in upcoming seasons is incredibly difficult, and the monsoon is no different.
Here are a few key factors we look at in determining what kind of monsoon we're in for in southern Arizona:
> Sea temperatures in the Pacific
> Snow pack in the Rockies
> Location of the subtropical, or "monsoon", high
Let's start from the beginning.
The monsoon is a seasonal shift in winds. We usually have a dry northwesterly flow in June. It's dry, and it's hot.
By July, high pressure begins to move northeast toward Colorado and the central plains. Since the wind direction around high pressure is clockwise, winds now shift out of the south and southeast for us in southern Arizona. That wind transports tropical moisture from the Pacific, Gulf of California, and Gulf of Mexico.
Usually it takes a while (through mid-to-late June) for the moisture to move through Mexico beyond the Sierra Madre Mts. Once the moisture reaches Arizona, our rising hot air can now cool and condense into thunderstorms.
By September, our "monsoon high" begins to break down and move farther away to the east. By then, we usually have to rely on tropical systems from the east Pacific to bring in more rain.
Now here's the forecast.
There is currently an above-average snow pack over the Rockies, which can suppress the monsoon high to the south. This could attribute to a later than usual start to the monsoon, and more frequent breaks from rain throughout July and August.
A weak El Nino is forecasted for the Pacific, which means the water could be warmer than normal. There is very weak correlation with El Nino leading to a drier than normal monsoon, but it's not enough evidence to tell if we'll be drier this year. However, the warmer water could lead an above-normal tropical season. More tropical systems increases the chances of us getting tropical surges (more rain), especially late in the monsoon.
So it's going to be hit or miss storms per usual in July and August, and a possibly wetter than normal September. It's not a perfect forecast, but there just aren't any major influences on our day-to-day weather in the monsoon. We'll have rainy days, and we'll have dry days. Some people could get 1" of rain one day, while another area gets none.
One thing we can be sure about is the heat. Be ready for humidity levels to rise by July, and be ready for at least 40 to 50 more days above 100° in Tucson.
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