An evening monsoon shower over the Plumtaw area – Pic taken 8/23/2022. Photo Credit: Shawn Prochazka
Monday – 29 May 2023 – 7:46pm
Let the rain kiss you. Let the rain beat upon your head with silver liquid drops. Let the rain sing you a lullaby. – Langston Hughes
Last summer was lovely with a great deal of silver drops beating down on us. This spring has been glorious with the liquid drops turning our Pagosa Country a vibrant green. This past winter tied as the 3rd wettest winter in 47 years with feet of beautiful drought ending snow!
In July, average monthly precipitation doubles to 1.88”. This is due to the North American Monsoon that impacts the southwest region from early 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”.
So, the question we all have is, will we see a repeat of the great monsoon we saw last year when we had 9.32″ rather than the average 6.25″?
A review of Earth Science to understand 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 it is an oblate spheroid.
As the earth spins, Coriolis, Centrifugal, Centripetal, and Pressure Gradient Forces tug and pull at each other to give us our six 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.
The Earth’s Airmasses
However, the earth is broken up by massive areas of ocean and land. These areas cool and heat differently. Ocean areas are slower to warm and cool. Weather airmasses form over the ocean or large land areas that have homogeneous temperature and humidity stretching over hundreds to thousands of square miles.
Note: The San Juan Mountains are NOT an airmass source region due to our topography.
A Continental Polar airmass forms over the high plains of Canada, a Continental Tropical airmass forms over the desert southwest and, most important to our discussion, Maritime Tropical airmasses form over the Gulf of Mexico and the Baja area.
These airmasses migrate from their original location due to the circulation zones and oscillations. When they interact with other airmasses, the fun begins! Areas of low pressure and fronts form, 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 warm and moist Maritime Tropical airmass interacting with the hot and dry Continental Tropical airmasses are important pieces of the North American Monsoon.
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.”
As the earth tips towards the sun, North America warms, the polar front jet and associated pressure systems and fronts move north and weaken. At the same time in southern Colorado, the upper-level winds switch from the west to the east and the area falls under the Hadley cell for about three months.
Lifecycle of the North American Monsoon
Typically, late June through mid-July there will be 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 easterly flow. So, we’re back to the sputter and start events towards the end of August through the beginning of September.
As we get into September, the Pacific waters are warm enough for tropical storms to really intensify. A Pacific tropical system can feed into our monsoonal flow and then the good times really take off with potential for widespread heavy rains and localized flooding. These Baja area storms can last well into the night.
Note: It takes longer for Pacific storms to get going compared to the Atlantic due to how deep it is and that the east Pacific is cooled by northerly ocean currents whereas the Atlantic is warmed by southerly currents.
Ingredients for the North American Monsoon
1. Intense Heating
“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.” This chart shows for the months of July, August, September, the temperatures in the southern Rockies will be warmer than normal. Temperatures across the southern plains will also be warmer than average but not as much as to the west.
Note: Model data are algorithms based on current data from observations and satellite data built into a grid and run through a supercomputer. See this link for a more in depth explanation. https://pagosaweather.org/weather-101/what-are-weather-models/
The farther the model forecasts into the future the less reliable it is as depicted in the graphic below. Based on this a one month to four-month forecast is going to look like a squiggle. But it is a great place to start!
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, the warmer that column of air is. It’s a great tool! We use the term “ridge” for a bubble of warm air. Another description is a High in the upper levels with anticyclonic or clockwise turning.
The chart below shows the seasonal forecasted 500MB heights for the months of July, August and September, 2023. To be a strong ridge, it should be at least 595 decameters. This chart is indicating that a ridge will build but it will be a weak ridge of less than 594 decameters. That is not to say at times it won’t be stronger and then weaker at other times. But overall, it will be a weak ridge.
3. Position of the Ridge
Position of the ridge is important too. We want it to stay over from the southern plains to the four corners. If it drifts to our west, over Arizona and Utah, our moisture is cut off and the atmosphere is more stable. That is exactly what happened two years ago. Southern Arizona had a record monsoon season! But the ridge was too far west for Archuleta County. We saw some afternoon showers but it was from moisture wrapping up and over the ridge and approaching us from the northwest rather than the southwest.
The ridge position on the forecasted seasonal 500MB height chart is just to our west stretching from central Alberta to the southwest corner of New Mexico.
Below are a few studies from the Albuquerque NWS office. In August, 2006 the ridge was centered over the southern plains. For the month, the Archuleta country area had 3 to 8” of rain.
As compared to July 2008 when the ridge was centered over the four corners area and we had 1 to 3” of rain.
4. High Surface Dew Points
Next, we can look at the surface dew points 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.
Below are charts showing the dew points at several locations to our south in the summer of 2021. The red lines indicate the average for those dates. The blue line is the current years dew point. As you can see, all locations were fairly dry in June and saw an uptick into July. We look closely at the steering flow to determine which site to track.
So those are the ingredients for the North American Monsoon. What evidence can we examine to determine the monsoon season outlook?
Evidence of the North American Monsoon Season over Pagosa Springs
There are several parameters we can examine to determine what sort of monsoon season we will have.
1. Correlation to Arctic Oscillation
The Arctic Oscillation (AO) is a “back-and-forth shifting of atmospheric pressure between the Arctic and the mid-latitudes of the North Pacific and North Atlantic”. The daily AO index is figured by forecasting the daily (00Z) 1000mb height anomalies poleward of 20°N compared to the year-round monthly mean anomaly data.
A positive AO index means higher pressure in the midlatitudes of the northern hemisphere due to a stronger jet stream.
The AO index is trending downwards into June. This means the polar front jet will tend to remain stronger for this time of the year and could interfere with the subtropical ridge setting up.
2. Tropical Cyclone Activity
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.
A common query I have received lately is, “If El Nino is coming back, that means our monsoon will be strong, correct?”
In order to answer that question, we have to look at what has been happening with and at the current El Niño-Southern Oscillation (ENSO) forecast.
First, what is the ENSO? According to the National Oceanic and Atmospheric Administration (NOAA), “El Niño and La Niña are the warm and cool phases of a recurring climate pattern across the tropical Pacific ENSO. The pattern shifts back and forth irregularly every two to seven years, and each phase triggers predictable disruptions of temperature, precipitation. These changes disrupt the large-scale air movements in the tropics, triggering a cascade of global side effects.”
The Sea Surface Temperatures are measured across the Pacific near the equator. They have found a correlation between North American effects and area 3.4 in the middle of the Pacific.
During El Nino the waters in the eastern Pacific are warmer than normal. Due to the warmer waters than average, the air columns are enhanced and storm systems across the hemisphere are enhanced, and subtropical jet is stronger across the Atlantic.
This stronger subtropical jet causes additional shear across the Atlantic which can be a breaking mechanism for tropical storms and hurricanes. At the same time, the warm waters are favorable for storm formation. However, El Nino can enhance tropical storm activity in the eastern Pacific.
Earlier in the monsoon season we can expect to see a decrease in deep level moisture advection from tropical systems migrating into the Gulf of Mexico. However, later in the season, we should see more activity from the eastern Pacific as that area warms up.
3. Low Snowpack/Above average Monsoon Correlation
The Tucson and the Albuquerque National Weather Service Offices have done studies where they have found, “A significant correlation exists in the observed data such that wet winters in the Southwest are generally followed by dry monsoons, and dry winters are generally followed by wet monsoons.”
The logic is that with a deep snowpack it is hard for the deep warm temperatures to set up. Those warm temperatures help turn the winds to the east to facilitate the formation of the monsoon.
Like Colorado, New Mexico’s snowpack looked great this spring. It follows based on these studies that those areas should expect less than average monsoon. At the very least it indicates there should be a slow onset of the monsoon.
I was curious as to whether there was a correlation to our snowpack and to our monsoon moisture so I looked a back at the last eleven years of data for snowpack in April and in May as compared to the summer monsoon. I also looked at southern Arizona’s snowpack. The inference is that with a low snowpack, the surface temperatures will start heating sooner and the ridge will start building sooner leading to any early onset monsoon season.
Unfortunately, based on the data I analyzed, I could not find a correlation for Pagosa Springs. I thought I would still share the data with you. Another possibility is that it might be that 11 years is not big enough of a sample size. This is something I will continue to study and track.
Official Monsoon Season Outlooks
First, the National Weather Service Climate Prediction Center three-month precip forecast shows that the monsoon should less than average for July, August, and September for southwest Colorado.
Keep our horse graphic in mind.
The Euro Model seasonal forecast is also showing less than average precipitation for southwest Colorado for the months of July, August, and September.
However, the Canadian seasonal forecast is expecting ABOVE average precipitation for southwest Colorado for the months of July, August, and September.
My theory is that the Euro model has the ridge to our west and it is weak throughout the monsoon season. Whereas, the Canadian model moves the ridge to our east at some point and also sees the potential for the increase in tropical storm activity in the eastern pacific impacting the southwest Colorado area into August and September.
Pagosa Weather’s Monsoon Forecast
The Pagosa Weather Forecast for the ’23 Monsoon season:
– There will be a slow onset.
– July will be less than average precipitation as the subtropical ridge struggles to build and when it does, it is too far west. Most of our monsoon activity will come from the northwest and be anemic as compared to our normal monsoon activity.
– August, our wettest month of the year, will also have less than average precipitation. Hopefully by the middle of the month, the southern plain areas start to really heat up and we see the ridge move to the east. And we could be impacted from a tropical storm mid to late in the month. The impact from one tropical storm can bring a month’s worth of precipitation in a few days’ time.
– September will have above average precipitation as the ridge stays to our east and we are impacted by a few tropical storms advecting their deep moisture into the Colorado southwest.
Mark Langford graciously recorded our presentation at the San Juan Outdoor Club on May 31st, 2023. My explanations throughout the video are similar to information I have presented here but in some areas I expand a bit. In addition Mark reviews lightning safety and presents an update to Doppler radar in our area. At the end Shawn, Mark, and I answer a number of questions from the audience. Please feel free to send us any questions you may have.
If you would like to read more about the subjects I have discussed, I have included some links for your reading pleasure.