Monsoon 2015 forecast

Published: May. 4, 2015 at 10:14 AM MST|Updated: Feb. 28, 2018 at 5:22 PM MST
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TUCSON, AZ (Tucson News Now) - Monsoon 2015 is looking like it could be a good one. That is IF we can tap into the moisture from what is forecast to be an active tropical season off the west coast of Mexico. Below is a breakdown of the factors that influence the monsoon, plus if they have a good or bad impact this year.

Note: this story will be updated as new data comes into the Tucson News Now weather center.

BAD: El Niño.

The one big 'if' in the monsoon forecast this year is El Niño. The connection between El Niño and the monsoon is weak, meaning the data doesn't show a strong relationship between the two. That being said, simply the presence of El Niño could weaken the onshore flow of tropical moisture needed for monsoon storms.

El Niño means there is warmer than average water off the west coast of Mexico and towards the equator. This results in a smaller land-sea temperature difference, which is the opposite of what we want for stronger onshore moisture flow. The hot summer land temperatures create a low pressure system over the Mohave Desert. The presence of the low means there is rising air over the desert. That air is then replaced by tropical moisture flowing inland from the East Pacific and Gulf of California. With warmer ocean water temperatures, this onshore flow is weakened.

Early in the monsoon El Niño also has a negative impact because it generally means cooler Canadian air drops down the west coast into late spring and early summer. That pattern is what brought Arizona the cooler May temperatures and helped bring rain to the state in what is, on-average, the second driest month of the year. These dips of cool air push back the monsoon moisture and the hot temperatures we need to set up the monsoon. That dip of cool air also offsets the monsoon ridge. In early summer the heat is generally brought into Arizona by an area of high pressure migrating northward out of Mexico, over Arizona, and into the Four Corners. As this high moves north, it opens the door for tropical moisture to move in from the south. During El Niño years, this ridge is usually weakened and offset south and east of Arizona. That has a negative impact on the tropical moisture flow into the state. 

The El Niño forecast is for stronger conditions into the summer and remain strong through the rest of 2015. The below graphic shows the current forecast. The thinner, rainbow-colored lines are the various computer models used to forecast El Niño, while the thicker lines are the average of the model groups. The forecast points on the bottom x-axis show the forecast grouping in three month time frames. For example, JJA equals June, July, and August. As you can see the lines generally go up through the summer months, which means the current El Niño could become stronger. 

GOOD: El Niño's influence on the tropical storm forecast. However, the presence of El Niño is good for the tropical storm forecast for the East Pacific. These storms can send a surge of tropical moisture northward over the Gulf of California and into Arizona, spiking monsoon storm chances for a few days. The warmer than average water temperatures means there is a better chance of tropical storm and hurricane development off the west coast of Mexico this year. This can increase monsoon storm chances, especially in late August and September which is typically the peak of hurricane season. Hurricane season in the East Pacific runs from May 15 through November 30.

Track the tropics by clicking here.

GOOD: Wet conditions in Sierra Madres.

Then there are the moisture conditions in the northern Mexico mountain ranges. Typically the monsoon starts in Mexico in mid-June with the moisture and storm forecast expanding northward as we head into July. For Tucson the better chances for monsoon storms generally arrive around the July 4th holiday. Starting with dry conditions in Mexico can delay the start of the monsoon in Arizona. But on the flipside, the greener the mountains are at the start of the monsoon, the better chance the monsoon downpours will move into Arizona faster or on time. This process is nicknamed the 'greening of the mountains'. This year the conditions are looking wet in Mexico and that means the mountains are possibly already green. The below image shows the percent of normal precipitation over the last 90 days. The blues and greens show where above average rain came down. This includes northern Mexico.

SWITCHED AFTER SPRING FLOODS- GOOD TO BAD: Drought to flooding in Southern Plains.

While the moisture in Mexico is good for the monsoon in Arizona, we want the opposite in the Plains states to our east. Drought conditions favor the set up of the Bermuda High, which is a broad area high pressure that forms over the the eastern U.S. as the temperatures heat up into summer. This High pressure is generally centered near the eastern U.S. coastline or into the Atlantic near where Bermuda is located. The clockwise circulation around this high pushes moisture inland from the Gulf of Mexico. This provides the upper level moisture that helps to increase clouds and high-based storm chances early in the monsoon. Drought in the Plains states means that land will heat faster, bringing hot summer temperatures to the area quicker than it would if the land were moist. If the moisture is in place, some of the sun's energy would go into evaporating that moisture instead of heating the land.

The below graphic shows the latest U.S. Drought Monitor update. A severe drought that was covering much of the southern Plains in April, mostly disappeared as heavy rain fell in this area during May. However, it must be noted that this connection to the monsoon is relatively weak in the data. While it may have a small impact, it is not a defining factor of the monsoon year-by-year.

SWITCHED AFTER WET SPRING IN COLORADO - GOOD TO BAD: Dismal spring snowpack in Colorado.

Lack of spring snow in the Rocky Mountains of Colorado is also good for the monsoon. This is for the same reason that drought is good in the Plains. If snowpack is still deep in the mountains in Colorado, the sun's energy goes to melting that snow instead of heating the land. We want the hotter temperatures in the Four Corners area to allow the Monsoon High to migrate into that area from Mexico. Early in the summer an area of high pressure moves out of Mexico, over Arizona, and sets up in the Four Corners area for the monsoon. This Monsoon High, as it is nicknamed, is partly what brings the very hot weather to Arizona in June. But as that high moves north, it opens the door for tropical moisture from the south to move into Arizona. The faster it moves north into the Four Corner's area, the faster the monsoon moisture has a chance to get into place over Arizona.

Mountain snowpack across the entire west was, in general, through May. The below graphic shows the snowpack as of May 1, 2015. The red, yellow, and orange dots in indicate below average snowpack.  However, note the grouping of green dots in Colorado. A stream of tropical moisture that brought rain to Texas also helped increase snowpack on top of the mountain peaks of Colorado. Just as with the drought in Texas and Oklahoma, this connection with the monsoon is generally weak.


And finally there is the Madden-Julian Oscillation, known shortly as the MJO. The MJO sends a wave of atmospheric energy that starts thousands of miles away in the Indian Ocean. This travels across the Pacific near the equator and towards the Central and South American coastlines. As this energy approaches the coastline, it generally ramps up tropical activity in the East Pacific. This translates to increased monsoon activity days to about a week later in Arizona. While this influence is only temporary, a more active MJO during the summer can lead to better monsoon storm chances overall.

However during the length of an El Niño summer, generally there are fewer areas of MJO energy crossing the ocean, plus the ones that do move into the Pacific Ocean are weaker as compared to non-El Niño years. This means overall, the effect may not be as prominent through this summer as compared to other years.

Forecast models are bringing a wave of MJO energy across the Pacific in early June. This could ramp up the monsoon later in the month.  

Click here for the latest MJO forecast. 

But then again, this is the monsoon. We all know it can be finicky. Flooding downpours one day with weeks of heat in between. The downpours will arrive. The big question is how much are we going to get. Check rain totals for your neighborhood by clicking here.

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