One more round of breezes is on tap for tomorrow as a weak trough moves through the Great Basin, strengthening the pressure contrasts across Arizona. This will decrease the high temperatures across Arizona slightly, but the breezes will lead to near critical fire weather conditions. However, later in the week, it appears we will have an early start to the monsoon! The word monsoon comes from Portuguese and Arabic origin, meaning “season”, and it used to refer to a seasonal shift in the prevailing wind. Generally, the upper-level and mid-level wind across Arizona is westerly most of the year. However, each summer between about mid July through mid September, the prevailing winds become southerly or southeasterly (although officially the monsoon ranges from 15 June – 30 September). This wind shift is caused by the buildup of high pressure to our north and east as the result of temperature contrasts between the terrain of North America and the surrounding ocean. Monsoon patterns are observed in other parts of the world, especially in southeast Asia.
During the latter half of this week, we will see a warming trend as strong high pressure builds into the Four Corners region, then shifts north into Utah by the weekend. The clockwise circulation around the high pressure will circulate southerly flow aloft across Arizona on Wednesday and Thursday, which will begin to bring subtropical moisture. Then, for Friday through at least next Monday, the flow aloft will become easterly. Weak disturbances within the easterly flow will arrive on Saturday – Sunday to help bring some additional moisture and lifting to the area. A surge of lower atmosphere moisture, which is most important for creating showers and thunderstorms, will most likely arrive on Friday and increase through the weekend, with dew points expected to rise well into the 50s. This increasing moisture, combined with hot and unstable air in place across the state, will lead to a slight chance of mainly afternoon and early evening thunderstorms on Friday and an increasing chance of showers and storms through the weekend. With the flow aloft becoming easterly, and the surface winds generally southwesterly, the shearing of the wind with height will provide favorable conditions for lines of thunderstorms on Friday – Monday to form over the Mogollon Rim and propagate westward or southwestward across Yavapai County at speeds of about 10 mph. The flow aloft will become weaker through much of next week, but the potential for thunderstorms should continue each day.
The greatest weather threats will be the possibility of microbursts and wildfire ignition. Microbursts are bursts of strong downward gusts of cooler air caused by the evaporation of precipitation, which create strong wind gusts and hazardous wind shears and turbulence for aviation. Also, the extremely dry vegetation combined with gusty wind could be a bad recipe for wildfire ignition.
Nevertheless, the possibility of wetting rainfall across our drought-parched state this weekend through next week is very exciting! We had a very dry winter, and extreme to exceptional drought conditions are indicated all across the area (http://www.cpc.noaa.gov/products/Drought/). However, some drought relief is expected with the impending monsoon. It’s very difficult to predict whether it will be a drier or wetter-than-normal monsoon, but we are looking at an early start to the monsoon this year (likely as a result of the lack of snow pack in the Rocky Mountains).
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