Summer monsoon thunderstorm at Westcliffe, Colorado – late June 2016
Sunday – 20 Jun 2021 – 7:00
“True, the sun and the wind inspire. But rain has an edge. Who, after all, dreams of dancing in dust? Or kissing in the bright sun?” Cynthia Barnett
I want to dance in the rain! Where is the rain?
June is Pagosa Spring’s driest month. It certainly has felt like it this year. We’ve had a scant 0.11” of precip this month north of Pagosa Spring’s in O’Neal Park. June normally averages 0.95”. In July, precip nearly doubles to 1.88”. This is due to the North American Monsoon that impacts the southwest region from the early/mid July till the end of September. Some years it begins earlier and other years it lasts longer. In a few notable years it has lasted much longer when we have an active Pacific tropical storm season and tropical moisture is caught up in the monsoonal pattern.
August is our wettest month of the year with an average of 2.52” of precip. Since the monsoon tends to weaken in September due to temps in the upper levels cooling, the average precip for September drops to 1.85”.
What exactly is the monsoon?
Let’s review a little basic earth science to answer that question. If you believe the earth is flat, you’re not going to like what I have to say. The earth is round! Technically an oblate spheroid.
As the earth spins, forces of centrifugal, centripetal, and pressure gradient tug and pull at each other to give us our 6 primary circulation zones. We live under the Ferrel Cell and westerly winds. If the earth were uniform, our winds would be nice and steady out of the west and we wouldn’t have much to report to you each day.
Global circulation cells. We live under the Ferrel Cell
The Earth’s Airmasses
However, the earth is broken up by massive areas of ocean and land. These areas cool and heat differently. Weather airmasses form over the ocean or large land areas that have similar temperature and humidity stretching over hundreds to thousands of square miles.
Note: the San Juan Mountains are NOT an airmass source region.
A Continental Polar airmass forms over the high plains of Canada, a Continental Tropical forms over the desert southwest and, most important to our discussion, a Maritime Tropical airmass forms over the Gulf of Mexico.
When these airmasses migrate from their original location and interact with other airmasses, the fun begins and weather happens!
Throughout the year these airmasses strengthen and weaken. For example, the lack of sun during the winter strengthens the Canadian Polar airmass which is dry and cold. And during the summer, the long days and strong solar radiation strengthens the Gulf of Mexico Maritime Tropical airmass which is warm and moist. The Maritime Tropical airmass is an important piece of the North American Monsoon.
North American Airmasses
What is the monsoon?
“The term “monsoon” describes large-scale wind shifts that transport moist tropical air to dry desert locations, such as the southwestern United States. A monsoon pattern also affects several other locations around the world including Southeast Asia, Australia, Africa, and South America. The Indian Monsoon is the strongest in the world due to the height of the Tibetan Plateau.”
Ingredients for the North American Monsoon
1. “Intense heating of the land over Mexico and the southwestern United States in the early summer months creates the wind shifts in the low levels.” As you can see from recent and forecast high temperatures, it is plenty warm in the low levels over the Southwest into Mexico.
Hot Surface Temperatures for Today
2. Strong 500 millibar (mb) ridge. 500MB averages 18,000 feet in a standard atmosphere. Meteorologists like to look at this level to determine where the atmospheric highs and lows are (the surface can be deceiving). The larger the height, then the warmer that column of air is. It’s a great tool! We use the term “ridge” for a bubble of warm air.
We can see a bubble of air over the four corners area of 5,940+ (abbreviated to 594dm). This chart is valid on July 1st. “A height above 5950m is indicative of a strong monsoon ridge with hot temperatures underneath.” When this happens there are strong easterly winds in Mexico south of the ridge wrapping moisture from the gulf up into Arizona and the southwest corner of Colorado.
Strong Ridge in Place Over the 4 Corner on 1st of July
3. Position of the ridge is important too. We want it to stay over the southern plains. If it drifts to our west, over Arizona and Utah, our is moisture is cut off and the atmosphere is more stable. We want that ridge in the above graph to drift back to the east.
4. Next, we can look at the surface dewpoints across Arizona to see if moisture is slowly moving into our area to provide fuel for those afternoon thunderstorms. Moisture can wrap from the Gulf of Mexico towards the northwest and into the desert southwest or it can also move northeasterly out of the Baja area.
Dewpoints are still drier at this time. However, the 700MB relative humidity charts are showing a few fingers of moisture moving from Mexico into the desert southwest over the next few weeks. This is a good sign! This chart is valid June 30th which is a few days before we can expect monsoon season to begin here in southwest Colorado.
Lowlevel/Midlevel Moisture chart valid for 30 June 2021.
700MB averages 10,000 feet in a standard atmosphere. Since many of us in Archuleta County live at ~ 6,500-8,000 feet, 700MB is a good height to look at for moisture that is going to help build those monsoon rain showers and thunderstorms!
Typically late June through mid July will see sputters of the monsoon as the pattern tries to ramp up. If the pattern can set up properly by mid July and hold strong, we should see monsoonal showers about every day through mid August. Then as the sun angle weakens, the ridge weakens and so does the that easterly flow. So we’re back to the sputter and start events towards the end of August through the beginning of September.
Moisture advecting in from the Baja area can really enhance our afternoon storms and help them last well into the night. Also, as we get into September, a Pacific tropical system can feed into the flow and then the good times really take off with potential for widespread heavy rains and localized flooding.
Monsoon Season Outlook
There are a number of indicators we can look at for the long-range forecast for our monsoon season.
First, the National Weather Service Climate Prediction Center one-month precip forecast shows that the monsoon should be average in July for southwest Colorado. However, it indicates below average for the three-month period. This is a sign that August and September may be below average.
CPC forecast charts
Over oceanic areas, the way in which systems and air masses circulate is referred to as oscillation. In the central Pacific, the primary oscillation system is the Walker Circulation. This system assists moving pressure systems, associated fronts, and air masses along so that we are impacted by weather systems and receive precip on a regular basis. However, differing sea temperatures can throw this schedule off.
We can look at sea surface temperatures (SST) in the equator area or the El Niño-Southern Oscillation (ENSO). Most of our winter the SST were cooler – a La’Nina ENSO. That indicates drier conditions for us. Now those temperatures are normal! That means we should expect average precipitation.
The Albuquerque, New Mexico National Weather Service Office put together a great chart with explanations of the current ENSO.
Sea Surface Temperature anomalies valid June 5th, 2021
Circling the globe around the equator is the Madden-Julian Oscillation (MJO). It takes 30-60 days for this pattern of convection and drier areas to rotate around the Earth. Depending on where its storm areas sit, it can influence the Walker and therefor the systems impacting us.
We are currently in a dry area of the MJO or about stage 3. We want to transition to about stage 7 in the next few weeks. If we line up during a wet area of the circulation, when the ridge is in a proper alignment, with warm surface temps in Mexico and high dewpoints across Arizona, we should see some good monsoonal action.
MJO pattern set up valid 8 Jun 2021 and 13 Jun 2021
I expect us to have warmer than average temps and just a little less than average monsoon precip. However, that would be better than last year!
If you want to learn more, here are a few good links:
NWS Tucson Monsoon info: Tucson Monsoon info
NWS Albuquerque Monsoon brief: ABQ Monsoon Brief
NWS MJO info: MJO