With the hot and dry conditions we’ve seen over the past several days in Colorado, the wetting rains of the North American Monsoon pattern may seem like a long-lost, distant memory. July and August are our most active months for monsoon-enhanced precipitation, but it’s not unusual to see the first glimpses of pattern emergence during the month of June. This summer is no exception, as it looks like we’ll get our first taste of subtropical moisture this weekend. But first, here is a little background information to put things into perspective.
North American Monsoon Pattern (NAM)
Most simply, the NAM is a prolonged, intra-seasonal shift in the wind pattern over the southwestern United States during the mid and late-summer months. This wind shift allows moisture-rich subtropical air to stream north out of the Gulfs of Mexico and California, and move over the Desert Southwest and eventually Colorado. The influx of moisture can contribute to showers and thunderstorms producing heavy rainfall over areas with typically-arid climates.
The key to diagnosing the NAM lies in the presence and strength of large-scale weather features over the southwestern United States. Here’s a nice schematic of the pattern, courtesy of Mike Baker (NWS Boulder).
During the summer months in the northern hemisphere, subtropical high pressure positions over the middle part of the United States. Clockwise flow around the high efficiently transports moisture northward over a good portion of the country. The second key feature typically seen with the NAM is an area of low pressure known as a “heat low” or “thermal low.” This feature arises as temperatures hit their seasonal peak over the Desert Southwest. Counterclockwise flow around the low also ushers moisture-rich subtropical air northward. The result is (2) persistent large-scale weather features working in concert to pump deep moisture into our region.
One of the keys to understanding the NAM is recognizing that it is an intra-seasonal process, and its spatial scope is regional. The NAM is not a storm, nor is it an exceptionally-rainy afternoon. In this sense, the NAM is more similar to a process like ENSO (El Nino Southern Oscillation). Nothing occurs very quickly with the NAM. It takes weeks for large-scale features to strengthen and position correctly. Similarly, the process does not arrive overnight. Typically, the influx of monsoonal moisture slowly ramps up during late June/early July, peaks for a few weeks, then slowly relaxes as we move into September.
And just because the NAM pattern is in place, it doesn’t mean you’re going to get rain. Thunderstorm coverage is highly-sensitive to the position of the subtropical moisture plume. This plume can position as far west as California/Oregon/Washington, or as far east as the southern plains. Typically, the moisture plume slowly oscillates from east-to-west as the large-scale feature’s shape and strength evolve.
So, how do we know know when the NAM officially arrives?
The NAM pattern is declared once the daily-average dewpoint in Tuscon, Arizona reaches 54°F or higher for three-consecutive days. The 54°F threshold is relatively arbitrary, but it does typically signal a large-scale influx of subtropical moisture into the mid latitudes associated with the NAM. As we would expect this time of year, surface moisture is slowly increasing over Arizona, but we still have a lot of work to do to reach the NAM threshold. As the red curve indicates, NAM conditions typically arrive during early-to-mid July and persist for about two months.
We’ll need to wait a few more days to iron out the finer details, but it looks like we’ll see a preview of NAM conditions over Colorado by this weekend.
This is GFS model imagery, courtesy of TropicalTidbits. The map shows precipitable water normalized anomaly. Basically, a map of integrated moisture in the atmosphere, and how unusual the values are for this date in June. Anomalous moisture streaming north is clearly evident, and the plume appears to position over much of Colorado this weekend. We’ll get an extra boost from Hurricane Bud in the E Pacific (not pictured), which will move north and phase with the large-scale circulation.
This is not the North American Monsoon per se, but it is a very monsoon-esque pattern concerning moisture trajectory and magnitude.
So, is the weekend ahead a wash for outdoor activities in Colorado? Not necessarily. We still need the same daily ingredients at the local scale for thunderstorms to form and eventually capitalize on the increased moisture.
For now, we’ll keep our eye on the weekend ahead, recognizing that a return to wetter conditions appears likely. We’ll also watch the dewpoints and storm activity over Arizona during the coming weeks as evidence the NAM has arrived and is here to stay.