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Where's the Monsoon?

It's been bone-dry in the Albuquerque metro. After some beneficial rain and snow in February and March, precipitation in the metro area has been hard to come by since then.

Now that we're officially in the summer monsoon season, folks in Albuquerque are asking, "Where's the rain?"

Four Types of Monsoon Patterns

Prior research on the North American Monsoon have revealed there are four distinct patterns, each having different impacts for the American Southwest.

Type I - "Classic" pattern

This is your typical monsoon pattern with moist southerly flow over New Mexico, leading to diurnal rounds of showers and thunderstorms statewide. Under this pattern, the upper-level high develops over the southern Plains (Texas/Oklahoma), and a thermal trough develops over the Mojave Desert. Sometimes a weaker high forms near the Four Corners, but the Texas/Oklahoma high center remains dominant. Daily rounds of afternoon and evening thunderstorms can be expected, with locally heavy rain and flash flooding. Occasional "monsoon bursts" of widespread heavy rain and flash flooding can occur with the Type I pattern as the remnants of tropical cyclones and disturbances get pulled north around the Texas/Oklahoma high center. Storm motion is usually from south to north.

Type II - "Reverse" pattern

The Type II pattern develops when high pressure over the Great Basin or Four Corners becomes the dominant feature. Meanwhile, an area of low pressure or longwave trough develops over the Mississippi Valley. Shortwave troughs often clip the northern and eastern portions of New Mexico, sending "backdoor" cold fronts southward down the eastern plains. Sometimes those fronts push westward through the central mountain chain and into the Rio Grande Valley, often resulting in strong easterly canyon winds in the Albuquerque and Santa Fe metro areas. Low-level east/southeasterly flow draws moisture northwestward into New Mexico, while strong north/northwesterly winds aloft create ample vertical shear to favor the development of severe thunderstorms, particularly over the eastern plains. Storm coverage is not as widespread, although storms tend to be strong to severe with the Type II pattern. Additionally, storms tend to favor the eastern plains where richer moisture is present, while central and western New Mexico tend to experience more isolated storm coverage due to drier air being closer to the upper high center. The Type II pattern is called the "reverse monsoon" since storm motion is typically from north to south.

Type III - Trapping pattern

As its name suggests, the Type III pattern keeps the deep moisture trapped to our south, as the high center is situated over or just to to the south of New Mexico. The circulation around the high center keeps very dry air in place over much of New Mexico, although enough moisture may occasionally seep into the state for isolated storms favoring the higher elevations. More significantly, most of these storms are mainly virga (rain not reaching the ground), resulting in a greater threat for dry microbursts. With the close proximity of the high center to New Mexico, storm motions are slow and erratic. Localized flash flooding may be of concern if sufficient moisture is present for the development of "wet" storms.

Type IV - Transitional

The Type IV pattern is only usually seen near the beginning and end of the North American Monsoon season as the upper-level westerlies retreat northward in the late May and June timeframe, and when the westerlies advance southward in September and October. As its name suggests, the Type IV pattern is a transition between one of the other three monsoon patterns and the non-monsoon pattern. Typically an upper-level low or trough that may or may not be progressive in nature digs along the West Coast. Meanwhile the high center shifts eastward into Texas and builds into the Great Plains. Deep moisture is advected northward into New Mexico in strong southerly flow between the two features. As the upper low/trough moves inland from the West Coast, a Pacific cold front advances eastward across the American Southwest. As the advancing cold front interacts with the deep moisture plume ahead of it, widespread heavy with embedded thunderstorms precedes the front. Some of those storms can become severe with sufficient upper-level shear. While individual storm motions may be from south to north, the overall band of rain will propagate from west to east as the cold front advances eastward.

Delayed start to summer means delayed monsoon

When it comes to the weather, it seems like everything is running behind schedule this year. We had a late start to winter, with substantial snow and rain arriving in the Albuquerque metro in February and persisting into March. Moreover, it was a very good winter season up north with well above normal snowfall in northern New Mexico and southern Colorado. While that was extremely beneficial for the parched Rio Grande Basin, that late winter snowfall also meant a delayed arrival of spring, and likewise, summer and the monsoon.

The average first day of 90-degree weather in Albuquerque (at the Sunport) is around May 27th. This year, our first 90-degree day was June 18th, when the Sunport reached 93 degrees. That's three weeks behind schedule. We usually have three weeks to a month of really hot and dry weather before the monsoon kicks into full gear. So if that schedule were to hold, I wouldn't expect the threat of significant monsoon moisture to arrive in the Albuquerque metro until mid to late July.

Another factor to consider is that past research has shown there is a correlation between the temperature and soil moisture content over the American Southwest to the strength of the monsoon circulation. Given the monsoon circulation is caused by temperature differences between the eastern Pacific Ocean and the southwestern United States, a greater temperature contrast (warmer and drier over the southwest) contributes to a stronger monsoon circulation and thus greater rainfall potential during the monsoon season. The opposite would be the case if the preceding winter and spring are cool and wet. Given the relatively cool and wet spring for many areas in New Mexico, if you're looking for a wet and stormy monsoon season, you'll likely be disappointed.

A few miles makes a huge difference

The placement and the strength of the major synoptic features also plays a vital role in who gets soaked and who is left high and dry during the monsoon season. For example, a slight shift of the Four Corners high--50 to 100 miles--can make a huge difference on what parts of New Mexico get rain during the Type II "reverse monsoon" pattern. As we saw during the 2018 monsoon season, there was a sharp cutoff through central New Mexico, where well above normal rainfall occurred to the east of that line, and bone dry and scorching temperatures were recorded to the west. The transition zone itself was extremely sharp--only a few miles in some places. An example of that sharp transition was observed in the Albuquerque metro area, where Rio Rancho and the west side of Albuquerque generally received 3 to 4 inches of rainfall; meanwhile, rainfall totals over the east side of Albuquerque and the adjacent foothills ranged from 7 to 14 inches.

Even this year so far, there have been a couple of rounds of storms in the Albuquerque metro area where one area received one-quarter of an inch of rain, and it was completely dry a couple blocks away. Again, that's something we often see during monsoon season: one place gets pummeled, and another place a short distance away gets nothing but sunshine.

Finally, the last factor to consider is the strength of the synoptic features in determining how wet the monsoon will be. Weaker high centers, regardless of their placement, will typically allow for greater moisture penetration into our region, and thus increase the likelihood of thunderstorms and rainfall. Precipitation will typically be more widespread and rainfall amounts will generally be greater with weaker high pressure centers. Conversely, stronger high centroids will act to suppress thunderstorm activity, resulting hotter and drier conditions, more isolated thunderstorm activity, and lower precipitation amounts.

Closing Remarks

Trying to guess what ZIP codes will win out in the rain lottery during monsoon season is like playing a slot machine at the casino. There are a lot of variables in play that make determining when and where storms will develop, and how much rain will fall on any given day. If the location and strength of the synoptic features properly align, your ZIP code just might end up hitting the rainfall jackpot this monsoon season. Otherwise, this season looks to be a bust.

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