Looking ahead to the monsoon, Arizona needs the rain - Tucson News Now

Looking ahead to the monsoon, Arizona needs the rain

Spring in Arizona is the driest time of the year.  

In April, May, and June the city of Tucson averages only 0.75" of rain. 

That is only about 6% of the yearly average, which is 11.59".

With over over 88% of Arizona currently experiencing some level of drought, the already dry conditions will likely lead to a bad wildfire season for the state as we head through the rain-deprived spring months.  

U.S. Drought Monitor update for Arizona  


  D0 - Abnormally Dry

 D1 - Moderate Drought

 D2 - Severe Drought

 D3 - Extreme Drought

 D4 - Exceptional Drought

An increase of monsoon moisture ends the wildfire season in Arizona.  

Unfortunately the start of the monsoon is also a dangerous time of year for wildfires.  

As the atmosphere slowly sets up for monsoon downpours to begin, the transition from dry spring air to muggy monsoon air sometimes produces 'dry thunderstorms'. 

These are storms that form early in the monsoon, generally in mid to late June.  

The rain that falls from these storms can evaporate before it reaches the ground. That rain is called 'virga'.  Even without the rain, these storms can still produce lightning and gusty winds.  

Lightning strikes commonly spark wildfires early in the monsoon, plus gusty storms outflow can quickly blow these fires out of control. 

A lightning strike started the deadly Yarnell Wildfire at the end of June last year. 

Plus, it was another monsoon storm that formed days later which produced the unpredictable gusty winds contributing to the rapid change in the Yarnell fire movement. 

It was those storm winds that drove the fire toward the 19 wildland firefighters killed fighting the flames.  

This year, as we look forward to the monsoon for relief from drought and wildfire risk, the news is both good and bad.  

Collectively, the major factors that influence the monsoon may be leaning towards a wetter year at this point in the forecast.  As more data is gathered over the next few months, we will have a more detailed monsoon forecast. 

The start of the monsoon is highly influenced by soil-moisture conditions in the Plains here in the U.S. and the mountains in Mexico.  

The monsoon first pours down in central Mexico with the rains moving north through the month of June, generally arriving full-force in Southeast Arizona in early July.  

The 'greening' of the mountains in northern Mexico once the rains begin is an indicator of when Arizona will see the downpours reaching across the border.  

Drought in northern Mexico can delay the start of the monsoon. 

The latest North American Drought Monitor (below) shows northern Mexico has very little drought. 

That is good news for an on-time start to the monsoon.  It is also good news for the monsoon to sustain its soaking power through the summer months. 

Also good for the monsoon in Arizona is the current drought in the Plains states to the east of the Continental Divide. 

This drought favors an early set-up of the Bermuda High, which pushes upper level moisture from the Gulf of Mexico into Arizona.  

On the flip-side, if the upper level moisture is in place long before low level moisture flowing inland from the Gulf of California and the East Pacific Ocean arrives in Arizona, this set-up favors dry thunderstorms and therefore raises the risk of lightning sparked wildfires.  

Deep spring snow pack in the Rocky Mountains of Colorado can offset the positive effects of these drought conditions.  

As of the start of March, the snow pack is still relatively deep in Colorado (see below). I'll be watching the snowpack levels closely over the next few months because warm spring weather can melt the snow quickly, changing that factor by June. 

Wet and dry periods within the monsoon are influenced by tropical storms in the East Pacific Ocean and a weather phenomenon called the Madden-Julian Oscillation (MJO).

Tropical storms off the west coast of Mexico can trigger deep moisture surges into Arizona. The National Hurricane Center won't release the tropical storm forecast for the East Pacific Ocean until we get close to the start of the season for that basin, which is May 15th.  However, the ocean surface is warming in the East Pacific Ocean, possibly heading into an El Niño event by summer.  Warm ocean water provides the energy needed to sustain tropical storms and hurricanes.

The MJO is a wave of atmospheric energy that travels from west to east across the Pacific.  When the MJO reaches the west coast of Latin America, within days to a week later Arizona generally sees an increase in monsoon activity.  The MJO forecast extends out to 40 days, which means it is too early to determine if an MJO event will help to kick off the monsoon and sustain the storm action. 

I'll be looking more closely at how all of these factors could influence the start of the monsoon closer to the June 15th date when monsoon data records begin each year.  

Then there is El Niño. 

The Climate Prediction Center is forecasting a 50/50 chance of an El Niño forming this summer.

However, over the last few weeks a stronger El Niño signal has appeared in the data.  

Below is move below shows a warm pool of water approaching the surface of the ocean in the central and eastern Pacific Ocean, which is the right side of the image. 

The reds and browns indicate the warm water, and the upward tilt shows the warm pool is approaching the surface.  


Although the data doesn't support a strong connection between El Niño and monsoon downpours, there is some evidence that El Niño can weaken the monsoon.  

At present the Climate Prediction Center models show about a 50/50 chance of a weak El Niño forming this summer.  

However, the current warm pool of water in the image above is showing up stronger than the numbers in the latest forecast CPC forecast.  

That difference between the forecast and reality may indicate a stronger El Niño is forming, which could influence weather patterns for the latter half of the year.

While the El Niño coinciding with the monsoon is not great news, a strong El Niño going into the winter could be very good news for Arizona. 

El Niño years tend to produce cool, wet winters. 

To wrap things up, drought in the Plains and a lack of it in Mexico favor an on-time start to the monsoon, but keep in mind this start can be highly influenced by the stability of the atmosphere in mid-June. 

If an MJO event ramps up or a tropical storm forms off the west coast of Mexico that would mean good news for the start of the monsoon, but if none of this happens and a stronger El Niño forms in the ocean, the news could turn bad for the monsoon.  

Plus, we all know, living in Arizona, sometimes the monsoon has a mind of its own.

I'll have an update on the monsoon forecast as more data comes into the newsroom.  

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