Wednesday – 1 Jul 2020 – 05:00pm
“I miss being little… I miss playing in the mud… but most of all I miss the mud puddles. Where is the rain?” Renee Nunley
It looks like our weather pattern may start to hint at a change over the next few weeks. Is this the start of the “monsoon”? 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 a spherical ball!
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.
However, the earth is broken up by massive areas of ocean and land. These areas cool and heat differently. Weather air masses form in ocean and land areas that have similar temperature and humidity stretching over hundreds to thousands of square miles. Note: the Southern San Juans is NOT an area where an air mass would form. A Continental Polar air mass forms over the high plains of Canada, a Continental Tropical forms over the desert southwest and, most important to our discussion, a Maritime Tropical air mass forms over the Gulf of Mexico. When these air masses move from their original location and interact with other air masses, 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 air mass which is dry and cold. And during the summer, the long days and strong solar radiation strengthens Gulf of Mexico Tropical air mass which is warm and moist. This is just one piece of the North American Monsoon.
The first indicator to look at is the 500 millibar (mb) ridge. 500MB is at 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.
You can see on the 500MB chart below, we can see a bubble of air over Texas and northeast Mexico of just over 5,880 meters (abbreviated to 588dm). This chart is valid today.
This is still a weak monsoon. By the middle of July the ridge has strengthened to just over 5,940 meters. “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.
The 24-Hour Precipitation Chart below valid the 15th of July is showing areas of monsoon showers.
Position of the ridge is important too. We want it to stay over the southern plains. If it drifts west over us and Arizona, moisture is cut off to us. We want that ridge centered over the inter-mountain west to drift back a little bit to the east in the 500MB chart below.
Next, we can look at the surface dewpoints across Arizona to see that moisture slowly moving into our area providing fuel for those afternoon thunderstorms. Today, looking at a low level relative humidity chart its pretty dry across Arizona into our area.
Relative humidity in our area is 15-20%. If we look at a chart from the 15th of July, relative humidity across Arizona is still not great but we are in a bubble of 40-50% an improvement.
Typically late June through mid July will see sputters of the monsoon as the pattern tries to ramp up. That’s what we will see this weekend. 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 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.
National Weather Service out of Tucson tracks the monsoon very closely. Today they can see some monsoonal moisture just starting to nose into southeast Arizona.
Initial long range precipitation outlook in May from the Climate Prediction Center indicated that this year would be a weaker monsoon for southwest Colorado. However, such systems can be impacted by a number of factors. We’ll have a better idea how strong the monsoon will be by the middle of July and will update you at that time.