Thunderstorm over Pagosa Peak during monsoon last year. Photo Credit: Mark Langford
Sunday – 29 May 2022 – 7:30pm
“Rain is not only drops of water. Its the love of sky for earth. They never meet each other but sends love this way.” unknown
I’m feeling neglected by my love, the sky.
It’s been an exceptionally dry spring. We are not seeing the beautiful wild flowers like we did last year after beautiful May rain. This April we had about a half inch of precip when the average is 1.4″. May has been worse. We’ve only had a abysmal .01″, when the average is 1.2″. June normally averages only 0.95”, the driest month all year.
In July, precipitation 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”.
So the question we all have is, will the monsoon bring us any relief this summer from this drought?
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 it is an oblate spheroid.
As the earth spins, Coriolis, 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.
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 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 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! 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.”
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 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, also, 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.
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.” The chart below shows northern Mexico, Arizona and New Mexico heating up with sizzling high temps at approximately 5,000 foot MSL near 100 degress by the 13th of June.
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. Another description is a High in the upper levels with anticyclonic or clockwise turning.
We can see a bubble of air over Mexico of 5,880+ (abbreviated to 588dm). This chart is valid on June 13th. “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.
Based on the low level temperatures forecast for this time frame, I would expect this ridge to continue to build as it gets into the third week of June.
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 last year. Southern Arizona had a record monsoon season! But the ridge was too far west. 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 in the graph above is centered over northern Mexico, southern Arizona and southern New Mexico. Based on the low level temperatures forecast for this time frame, as the ridge builds, it will also move farther north to center above the bubble of warmest air.
4. High Surface Dewpoints
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.
Into the second week in June, the southwest is still dry in the low levels. We want relative humidity levels of at least 60% for afternoon rainshowers and thunderstorms.
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!
So those are the ingredients for the North American Monsoon. What are some of the indicators we can look for the monsoon season outlook?
Indicators for the North American Monsoon Season over Pagosa Springs
There are a number of indicators we can look at for the long-range forecast for our monsoon season.
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. “A positive AO favors increased rainfall across Arizona and decreased across far eastern New Mexico.”
The AO index is trending upwards into June.
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 La Niña is going to persist, that means our monsoon will be weak, 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 La Niña the waters in the western Pacific are warmer than normal. Due to the warmer waters than average, the air columns are enhanced. This reverses the ocean currents which causes upwelling in the easterly Pacific waters. This upwelling pulls up cooler ocean waters to the surface.
We’ve had 2 winters of La Niña where the Sea Surface Temperatures are colder than average. During such a winter the polar front jet and the associated storm track stays farther north and the Pagosa area see less snow than average. The waters had started to warm in January and everyone, including the long range models, were optimistic about a neutral ENSO system by now. However, there was a drastic reversal in February and by May, the waters were colder then they have been all year. So, the last few months have been a very typical La’Nina pattern with Idaho and Montana under the polar front jet gun and continuing to see snow system after system while we have remained very dry.
The forecast is to stay predominantly in a La Niña pattern though there is a good chance that we will be neutral during the summer months as well. Keep in mind, that often the data that is not available till June will help statistical long range weather models as well as forecasters determine what the trend will be for the upcoming winter. It is possible there will be changes in the chart below for the winter months once we get into July.
The good news is that a La Niña pattern can have a positive impact on the North American Monsoon. The cooler sea surface temperatures in the Pacific impacts the upper level shear in the eastern Pacific and the Atlantic ocean areas. It increases the shear in the eastern Pacific which leads to decreased tropical storm formation. Meanwhile in the Atlantic there is weak shear which enhances tropical storm formation.
NOAA predicts a 65% chance of an above-normal season for Atlantic hurricane storms. “The increased activity anticipated this hurricane season is attributed to several climate factors, including the on going La Niña that is likely to persist throughout the hurricane season, warmer-than-average sea surface temperatures in the Atlantic Ocean and Caribbean Sea, weaker tropical Atlantic trade winds and an enhanced west African monsoon.”
The Atlantic tropical storms help to feed moisture into the North American Monsoon! We just need that ridge to keep its position near or to the east of the four corners area.
3. Low Snow Pack/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.”
Sadly, New Mexico’s snowpack has been low all winter. Based on the Albuquerque projections based on this study alone our monsoon precipitation should be above average.
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 ten 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 10 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 forecast shows that the monsoon should be average for July, August, and September for southwest Colorado.
The long range seasonal models are even more optimistic. Both the Euro and the Canadian long range seasonal models shows more than average precipitation for southwest Colorado.
Pagosa Weather’s Monsoon Forecast
We are forecasting an active North American Monsoon Season with above average precipitation for Pagosa Springs. Sadly, we have to make it through June first before we see the very welcome rains.
If you would like to read more about the subjects I have discussed, I have included some links for your reading pleasure.