Western US weather
Monsoon season gets off to a muted start across the Southwest
By Brian Thompson , AccuWeather meteorologist
7/01/2019, 6:43:38 AM
As we turn the calendar into the month of July, monsoon season is fast approaching in the Southwest. That being said, there isn't a whole lot of moisture on the way in the short term before things may try to ramp up a bit around the middle of the month.
There has been some moisture in the past couple of days across the Southwest that was leftover from what was Hurricane Alvin, but that had pretty minimal impacts. There were high clouds around on Friday and into the weekend, and Las Vegas even picked up a brief shower on Sunday even though the relative humidity never got above 20%. It was still not enough to give Las Vegas any measurable rain for the month of June.
That moisture is now on the way out, and a drier pattern is going to set up shop. There will be some spotty afternoon thunderstorms in the high terrain of far eastern Arizona and New Mexico on Monday and Tuesday, but much of the time will be dry.
By midweek, the thunderstorm chances will wind down even in these areas, with most of the monsoonal activity being held down in Mexico. The image above shows all the dry air at ~5,000 feet on Thursday.
Looking at the averages, though, that is not that uncommon for this time of year.
Below is a graphic from a CPC report, which is a PDF file that you can access here - it's a good read for those of you interested on the Southwest monsoon. It shows how the monsoon spreads from Mexico up into Arizona and New Mexico from June into July. Eventually, other portions of the Southwest get in on some of the showers and thunderstorms as well.
Notice that we're still a week or so away from when the monsoon typically gets going in the Southwest. This year, it may take a little longer than that to get going.
As our long-range team discussed in the summer forecast, signs are pointing to a below-average monsoon season, so the slow start to the season may be a sign of things to come.
There will be a bump in moisture after July 4th. I'm going to use "precipitable water" to show this, which is a variable we use to quantify how much moisture is in the atmosphere. Here's what the precipitable water is forecast to be late this weekend off the GFS.
Notice those 1-inch values are creeping into Arizona, which is a big change from midweek when that bluish color is all over the area. This increase in moisture will lead to afternoon thunderstorm chances increasing in the mountains of eastern Arizona (mostly east of Tucson) and into New Mexico, but not so much in the deserts.
Looking forward, the moisture may become more widespread toward the middle of the month. I hate showing single model runs two weeks out, but here's what the GFS has on Sunday, July 13th.
Those orange areas near Phoenix are quite high for the Desert Southwest. This surge would likely send the first wave of monsoonal thunderstorms into places like Tucson and Phoenix along with increased humidity. The timing of this isn't too far off the average for that area.
Instead of relying on just a single run of a model, we also use ensembles, which are made up of many runs of the same model with different starting conditions, since our assumptions of what the atmosphere looks like initially are not perfect. These yield different results, especially over longer periods of time, and give us an idea of how confident we should be in a solution.
With that in mind, here is the precipitable water anomaly from the GFS ensembles valid around that same time - close to the middle of the month.
That orange area in southern Arizona is where the cluster of models shows, on average, abnormally high precipitable water values. This gives us some confidence that a surge of moisture will occur around midmonth.
We'll watch over the next two weeks and see if the monsoon does indeed start to ramp up then.
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