Monsoon thunderstorms ready to roll across the Southwest
Monday, July 10, 2017, 3:55 - Southwest summer monsoon showers are expected to continue through the work week as moisture from both the Gulfs of Mexico and California flows into the Four Corner states.
Thunder showers will affect most of Arizona and New Mexico, and extend well into Utah and Colorado. This very active weather pattern will appear off and on at least between now and next weekend, and is expected to intensify in the region later in the week.
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Two main synoptic features drive the Southwest monsoon, a thermal low developed by intense daytime heating over the southwestern U.S. and Northwestern Mexico deserts, and an upper level high pressure ridge normally located east of this low.
With this pressure pattern set up, moisture from both the Gulfs of Mexico and California flows into the region, which in conjunction with the local orography, can lead to vertical cloud development. The evening and nighttime thunderstorms are capable of producing spotty but intense precipitation accompanied by an incredible lighting display.
Below: Rainfall totals Monday (July 10) to Saturday (July 15)
Summer monsoon to deliver full week of rain across the Southwest
Model predictions are anticipating that this will be a very active weather week, with frequent evening showers from Arizona and New Mexico into Colorado and Utah. Some of these showers and thunderstorms could be especially intense in mountain areas of southeast Arizona, southwest and northern New Mexico, and southern Colorado.
Click play to watch below: Rainfall timing (GFS model)
The National Weather Service is forecasting precipitation amounts of 1 to 2 inches between Sunday and Thursday across central portions of Arizona, west central New Mexico and south and central Colorado. This may not sound like much, but keep in mind that precipitation in some of these areas may be intense and fall in a short time period.
For some, the monsoon is a period of relief as it suppresses much of the intense summer heat that affects the southwest deserts. It also replenishes water resources in the region and nourishes the vegetation. The other side of the coin is that this hit and miss type of precipitation can also be dangerous, causing frequent flash flooding, crop damaging hail and intense gusty winds with occasional dust storms in the Four Corner states during the summer months.
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During the monsoon months, another important phenomenon associated with this frantic thunderstorm activity is lightning. In the Southwest, lightning on average ignited more than 2,300 fires annually since 2001, burning close to 277,000 acres per year.
Below: New Mexico Flash Flood c/o NWS
How will this year's El Niño affect the Southwest Monsoon?
Most computer models continue predict a weak El Niño (0.5 to 1.0oC) will form during and after the summer months, but, it's not easy to fully understand the variability of summer time convective activity in the Southwest U.S. and Northwest Mexico.
The complex interaction between synoptic and mesoscale mechanisms can overshadow the larger scale El Niño signal, especially when it is a weak one as expected in 2017.
Below: El Niño Forecast graph
The main effect of El Niño on the quality of the summer monsoon rains appears to be related to the moisture availability and transport into the region.
Research connecting the onset of the Southwest Monsoon and El Niño suggests that only during strong El Niño events, the subtropical ridge struggles to form early on, and delays the onset of the season. It is also relevant to follow-up and on the activity of the Julian-Madden Oscillation during the summer months, as it could affect the impact of the El Niño phenomenon when present in the Pacific.
Below: Map SST Anomalies
This July, the two main sources of moisture for developing Southwest monsoon thunderstorms, the Gulfs of Mexico and California, are showing cooler than average surface water temperatures. These negative SST anomalies could mean less moisture available and thus less precipitation. But as discussed earlier, despite a possible reduction in the amount of potential water vapor, the dynamics of the Southwest monsoons are complex, and can be affected by a variety of mechanisms.