La Niña & Winter Forecasts – Explained ~ OpenSnow

On September 10th, 2020, NOAA issued a La Niña Advisory, meaning that “La Niña conditions are observed and expected to continue.” In this case, the forecast gives a ~75% chance of La Niña persisting through the winter (see the blue bars below).

Source: CPC/IRI ENSO Forecast

As we have written about in the past, La Niña refers to cooler than average ocean temperatures in the Equatorial Pacific Ocean.

Why do we care about the temperature of the Equatorial Pacific in North America?

Small changes in the ocean temperature cause changes in the location of tropical convection (thunderstorms) which in turn changes the location and strength of storms around the world.

La Niña conditions cause tropical thunderstorms to be concentrated and intensified on the far Western edge of the tropical Pacific Ocean. Since convection is upward motion in the atmosphere, imagine that during La Niña, there is a strong disturbance to the atmosphere in the Western pacific that causes ripples of atmospheric waves to shoot off into the Northern Pacific.

Most importantly for North America, these atmospheric waves line up in such a way that a high-pressure region forms off the coast of California. This high pressure deflects the jet stream and winter storms northward, causing generally wetter conditions in the Pacific Northwest and dryer conditions in the Southern Rockies.

Source: NOAA Climate

How predictable are the impacts of La Niña? 

There are two major sources of uncertainty in long-range forecasts predicting weather months in advance. 

The first source of uncertainty is how accurately we can predict La Niña or El Niño. As mentioned above, the current forecast gives about a 75% chance of La Niña occurring this winter. Those odds are just about as good as it gets.

The second source of uncertainty is the atmospheric response to La Niña or El Niño. That is, if La Niña occurs, does it always affect weather in North America in the same way?

To determine how La Niña impacts weather in North America, we can look at precipitation records during past La Niña events and compare them to a typical year. 

The graphic below shows the average amount of rain/snow during La Niña winters compared to all winters. Green colors mark areas of above-average rain and snow. We see that La Niña leads to drier-than-average weather in the Southern US and wetter/snowier than-average weather in the Northern US.

Source: Brian Brettschneider

The graphic above looks very favorable for above-average snowfall in the Northwest and the Northern Rockies. 

But remember, the graphic above simply shows the average precipitation for 15 La Niña winters compared to all winters, and the average can be skewed by just a few big years.

Another way to look at the data is to look at the probability of wetter/snowier or drier conditions, which is what is shown in the graphic below. Again, green regions relate to above-normal precipitation.

Most relevant to our powder dreams, most of the Central Rockies and Northern Rockies show no (statistically) significant tendency towards above- or below-normal precipitation (white color) and there are many areas with decent probabilities for above-average precipitation.

Something to keep in mind, though, is that the probabilities for above-average and below-average precipitation max out at around 50%, so while we can hope and perhaps expect wetter and snowier conditions in the Northwest and drier conditions in the far Southwest, it’s far from a sure bet.

Source: Nathan Lenssen

So where does that leave the winter forecast?

A long-range climate forecast contains both the uncertainty of predicting the oceanic conditions of La Niña and the uncertainty in the atmospheric response over the USA. NOAA’s most recent precipitation climate forecast for the same December-February period looks quite similar to the two historical analyses of La Niña shown above.

Source: NOAA CPC

These long-range probabilities for snowier-than-average or drier-than-average weather conditions might be useful for ski areas making decisions over the course of an entire season. To find the best powder days, however, we have to stick with the 1-10 day weather forecast to dial in the specifics of each storm.

Download the OpenSnow app and stay tuned to our forecasts for the latest weather updates. 

This analysis was written by Nathan Lenssen, a lover of powder as well as a Ph.D. student at the International Research Institute for Climate and Society (IRI) at Columbia University.



La Niña patterns favor enhanced hurricane activity over the Atlantic.

Current sea surface temperature anomalies in degrees Celsius. Note the cool waters in the eastern Pacific commensurate with La Niña. (Tropical Tidbits) 

By Matthew CappucciSeptember 10, 2020 at 2:15 p.m. MDTAdd to list

The National Oceanic and Atmospheric Administration declared Thursday that a La Niña pattern had become established, having bearing on the remainder of the hurricane season and the upcoming winter. La Niña conditions are likely to continue through at least wintertime, potentially returning to a more relaxed “neutral” state by spring.

Seven tropical systems to watch, including longer term U.S. threat, on peak day of hurricane season

La Niña, which means “the girl” in Spanish, is the opposite of an El Niño. La Niña features unusually cool ocean waters in the equatorial tropical Pacific Ocean and can influence weather patterns beyond the Pacific.

The expectation of a La Niña pattern was a contributor in NOAA’s early August forecast of an “extremely active” hurricane season.

La Niñas and El Niños, which represent opposite phases of ENSO, or the El Niño Southern Oscillation, are major drivers of weather and climate trends in North America.

NOAA's estimated likelihood of La Niña to continue in the coming months. (NOAA/NWS)
NOAA’s estimated likelihood of La Niña to continue in the coming months. (NOAA/NWS) 

Much of late 2018 through early 2020 had skewed a bit more toward the weak El Niño side. Now, the pendulum is swinging in the other direction, which will have major implications in the months ahead.

What is a La Niña?

A historical chart of La Niña events (blue) and El Niño events (red). (NWS Tampa Bay)
A historical chart of La Niña events (blue) and El Niño events (red). (NWS Tampa Bay) 

A La Niña pattern is characterized by anomalously cool sea surface temperatures in the eastern Pacific. That’s the opposite of an El Niño, during which the east tropical Pacific is atypically toasty.AD

With a significant change in ocean temperatures ongoing and further in the offing, a La Niña alters several key circulation patterns in the atmosphere and can influence weather across the globe.

Most La Niña events last for at least several months but can occasionally stretch for years. Strong La Niñas occurred between 1973-1976, 1988-1989 and from 1998 to early 2001.

Winter weather

A typical wintertime La Niña pattern. (NOAA/NWS)
A typical wintertime La Niña pattern. (NOAA/NWS) 

NOAA forecasters have stated there is a 75 percent chance that La Niña will stick around for the entirety of winter. Generally speaking, La Niña typically increases the odds of above-average snowfall in the Pacific Northwest, northern Plains, Great Lakes region and northern New England. However, every La Niña is different, and other weather patterns can overwhelm its effects.AD

In the Mid-Atlantic, South, southern/central Plains and Southwest, snow may be more scarce. 

The impending La Niña is bad news for Central and Southern California, where potentially reduced snowpack at the higher elevations and fewer winter storms may prolong this year’s fire season, already a record, and set the stage for a challenging 2021.

Severe weather impacts 

A look at typical wintertime storm tracks during La Niña versus El Niño years. During La Niña years, the jet stream is typically farther north, with fewer storm systems passing directly over the South. (NWS Tallahassee)
A look at typical wintertime storm tracks during La Niña versus El Niño years. During La Niña years, the jet stream is typically farther north, with fewer storm systems passing directly over the South. (NWS Tallahassee) 

Because La Niña patterns shift the location of the jet stream farther north, there is generally less unsettled weather in the wintertime across the southern United States. This means drier, and subsequently warmer, conditions are likely for the Desert Southwest, South and Southeast.

It also limits the number of wintertime severe weather events in Florida and the South, commonly known as “Dixie Alley” by storm chasers. Disturbances in the jet stream can cause severe weather and tornado activity across the Interstate 10 corridor, particularly from Louisiana and Mississippi to Alabama, Florida and Georgia, between December and February. But with the jet stream retreating farther north, those chances are significantly diminished.

In fact, tornadoes in this region are only half as common during La Niña winters as compared with El Niño winters, welcome news for residents who live there.

A look at severe weather tendencies from March through May during El Niño and La Niña years. (
A look at severe weather tendencies from March through May during El Niño and La Niña years. ( 

Between March and May, however, a La Niña pattern can increase tornado and severe thunderstorm incidence for portions of the central and southern Plains, as well as into portions of Arkansas, northern Louisiana and the central Mississippi Valley.

It is unclear whether this La Niña will still be in place by then, though.


Do you ever wonder what meteorologists mean when they mention “models”, and how these models are used to forecast the weather? Here’s the breakdown.

What are weather forecast models?

Weather forecast models are computer programs that can help predict what the weather will be in the future, any time in the future from an hour to ten days out and even months ahead.

These forecast models take current weather observations collected from thousands of locations (such as wind speed, wind direction, air temperature, pressure, etc.), make an estimate about the current weather for locations where no actual data exists, and then use math and physics equations to predict what will happen in the future.

Below is an image from the “GFS” forecast model showing areas of high and low pressure as well as precipitation. We can use an image like this to know where storms may be at a point in the future.

There are many forecast models that cover the globe or smaller regions, and each model is developed with its own formulas in an attempt to be the most accurate.

Models that cover the entire globe

Two of the more well-known/used weather models are the European Center for Medium-Range Weather Forecast (ECMWF) a.k.a. the “Euro” model, and the United States’ Global Forecast System (GFS) model. Both of these models cover the entire globe.


  • Global Forecasting System
  • Produced by the US Government
  • Covers the entire globe
  • Forecasts out to 384 hours (16 days)
  • Updates 4x per day
  • Model resolution of 13km
  • Average accuracy score lags the ECMWF (but every storm is different)
  • Cost = freely available to anyone


  • European Center for Medium-Range Weather Forecasts
  • Produced by a group of European Governments
  • Covers the entire globe
  • Forecasts out to 240 hours (10 days)
  • Updates 2x per day
  • Model resolution of 9km (more detailed than the GFS)
  • Average accuracy score makes it the best model (but every storm is different)
  • Cost = $250,000 for commercial license to host data. Personal license is available to access data with an individual subscription (see Paid Forecast Model Websites below).

Models that cover a smaller area

Then there are mesoscale (fine-scale) models, which hone in on more specific regions and tend to be able to forecast really small weather features better than the global models, like thunderstorms or snowfall within steep mountains.

The two most popular U.S. mesoscale models are known as the North American Mesoscale Forecast System (NAM) and the High-Resolution Rapid Refresh (HRRR) model.


  • North American Model
  • Produced by the US Government
  • Covers the United States
  • Two versions: Standard and High-Resolution
  • Standard = 12km resolution for North America out to 84 hours (3.5 days)
  • High-Resolution = 3km resolution for the United States out to 60 hours (2.5 days)
  • Cost = freely available to anyone


  • High-Resolution Rapid Refresh
  • Produced by the US Government
  • Covers the United States
  • Forecasts out to 18 hours
  • Updates once every hour
  • Model resolution of 3km
  • Cost = freely available to anyone

Examples model forecasts

Weather models provide gigabytes of forecast data each time that they run. What we often show, and what you often see, are graphics and charts that are created from this underlying data. Here are some examples. 

A 16-day snow forecast for the United States and southern Canada from September 24 – October 10, 2019, from the American GFS model.

A 16-day precipitation forecast for the United States and southern Canada from September 24 – October 10, 2019, from the American GFS model.

A 16-day forecast for the weather pattern at about 18,000 feet. Blue colors help to identify storm systems. This is for the United States and southern Canada from September 24 – October 10, 2019, from the American GFS model.

Why do different models provide different forecasts?

First, forecast models differ in how they collect the current weather conditions across the globe. Even with a sophisticated measurement network including satellites, radars, weather balloons, ground-based weather stations, planes and ships, forecast models must make assumptions to fill in the gaps between actual weather observations, in places like oceans, large forests, deserts, etc.

Second, forecast models differ in the math and physics equations that they use to move from the current condition of the atmosphere and turn that into a weather prediction. Small changes in these equations can lead to rather substantial differences in forecasts.

And third, forecast models have different levels of detail (resolution) and can struggle to properly account for steep terrain like the mountains where we ski and ride. This is where a local forecaster can help because they can adjust the model forecasts based on their experience of seeing when the model does a good job versus times when the model is less accurate.

All of these factors play a roll in creating different forecast outcomes even though the models are starting with mostly the same information about the current state of the atmosphere.

Which forecast model is the most accurate?

Below is a chart showing the accuracy scores for 5-day forecasts for the northern hemisphere from several of the commonly used forecast models over the past 23 years.

The ranking from most skillful to least skillful is based on the one-year average accuracy of five-day forecasts. A higher number means that the model is more accurate. All four of these models cover the globe. While any model can more accurately predict a single storm, the European model has been and continues to be the most accurate.

  1. European Model (ECM = 0.920)
  2. British Model (UKM = 0.902)
  3. American Model (GFS = 0.888)
  4. Canadian Model (CMC = 0.883)
model scores

What are ensemble forecast models?

Forecast models provide imperfect predictions because we do not know the current weather for every place on earth and because we do not know the perfect math and physics equations to use in these models.

To account for both of these shortcomings, models are run many times with slightly different current weather conditions and equations. This produces a range, or ensemble, of many forecasts rather than a single forecast.

If the range of forecasts is small, we have greater confidence in the prediction. If, however, the range of forecasts is large, then the confidence in the forecast is much lower.

For example, the GFS ensemble forecast model is made up of 21 versions, each of which uses slightly different current conditions. And the ECMWF (European) ensemble forecast model has 51 versions. All of these model runs can be averaged together (a “mean” model) which can provide a more accurate forecast. 


Weather forecast models are a guide to the future, but forecast models are only one part of a weather forecasters toolkit. To create a prediction, forecasters use a combination of models, experience in understanding the model’s biases, and knowledge of the fundamentals of meteorology.

Each model has its own pros and cons. Most forecasters will look at several models and will take into account their own experience with the models as it pertains to their region when making a forecast. They should also tell you their uncertainty when the models disagree.

The forecasts made by the models are usually most accurate within 1-5 days, and then they lose accuracy the further out in time they go. This is because of the chaotic nature of weather, in which very small uncertainties in the current state of the atmosphere have a “butterfly effect” on the future.

Weather models have become more accurate over the last few decades, but are still far from perfect. As computer technology and scientific knowledge improve, the models will become more sophisticated continuing to lead towards more accurate forecasts.

Want to create your own forecast?

There are now many websites that allow you to look at the output from weather forecast models. Below are a few of the sites that we use. The free sites tend to have fewer details and options for viewing the models as compared to the paid sites.

Free Forecast Model Websites

NOAA – National Weather Service

NOAA Long Term Winter Forecast for 2020-21 ~~~ ENSO, El Niño, & La Niña – Explained ~ Open Snow

ENSO-neutral is favored to continue through the summer, with a 50-55% chance of La Niña development during Northern Hemisphere fall 2020 and continuing through winter 2020-21 (~50% chance). … Overall, the combined oceanic and atmospheric system is consistent with ENSO-neutral.Jul 14, 2020

NOAA Long Term Winter Forecast 2020-2021 | 55% Chance …

The US National Oceanic and Atmospheric Administration (NOAA) makes long-range forecasts each month.

We are going to show you their forecast for the upcoming winter, but first, a warning.

WARNING: Long-range forecasts are rarely accurate. These forecasts cover three months but we know that skiing quality improves and degrades with storm cycles that last a few days to a week.

Paying attention to the 1-10 day forecast (All-Access only) is the way that you’ll find powder and give you the best information for your weekend trip or destination ski vacation.

Temperature Forecast During December, January, & February

Warm. This outlook is based on a general trend of increasing temperatures during the past years and decades.

Precipitation Forecast During December, January, & February

The northern half of the United States is in the “Probability of Above” zone, which means that the odds of above-average precipitation is higher.

The middle of the country is in the “Equal Chances” zone, which basically means that the odds of average, above-average, or below-average precipitation is about the same.

The southern half of the United States is in the “Probability of Below” zone, which means that the odds of below-average precipitation is higher.

~~~~~~  ~~~  ~~~~~~~  ~~~~

But remember, 3-6 month forecasts have little to no value so don’t get too excited or upset by where your favorite areas fall on this map.

What about El Niño or La Niña?

A strong El Niño or La Niña (which refers to ocean water temperatures in the central Pacific Ocean) can help us predict snowfall patterns during the winter.

For the upcoming 2020-2021 winter season, there’s a ~50% chance that water temperatures will reach below-average (blue bar = La Niña), a less than 10% chance that water temperatures will be above-average (red bar = El Niño), and a ~40% chance that water temperatures will be near-average (grey bar = Neutral).

These model forecasts provide us with a signal for a La Niña event during the 2020-2021 winter season.

The official Climate Prediction Center outlook is similar to these model forecasts, calling for a 60% chance of La Niña for fall and a 55% chance for La Niña to continue through the 2020-2021 winter season.

Since long-range forecasts are rarely accurate or useful for finding great snow, here is a quick recap of our strategy for finding the deepest powder:

If you want the highest odds of deep powder, here are our six tips:

1) Live in a location that’s close to mountains with the deepest snow.

2) If you can’t live close to deep powder, wait until 7-10 days before booking your trip.

3) Even if you wait until 7-10 days before booking your trip, consider only booking to a general area.

4) If you have to book a trip far in advance, pick locations that statistics show have the deepest powder.

5) If you can’t execute any of the above strategies, change your expectations for your ski trip.

You often hear weather forecasters begin their discussion about the upcoming winter forecast by looking at the current state of the El Niño-Southern Oscillation (ENSO). So what is ENSO, El Niño, and La Niña?

What is ENSO?

El Niño-Southern Oscillation (ENSO) refers to the year-to-year variations in sea-surface temperatures, convective rainfall, surface air pressure, and atmospheric circulation that occur across the equatorial Pacific Ocean.

El Niño and La Niña represent opposite extremes in the ENSO cycle, while ENSO-Neutral refers to the state between El Niño and La Niña.

ENSO is one of the most important climate phenomena on Earth due to its ability to change the global atmospheric circulation, which in turn, influences temperature and precipitation across the globe.

What is El Niño?

The term El Niño refers to the large-scale ocean-atmosphere climate phenomenon linked to periodic warming in sea-surface temperatures across the central and east-central equatorial Pacific.

El Niño represents the warm phase of the ENSO cycle and means that the ocean water temperatures are warmer than average.

El Niño was originally recognized by fishermen off the coast of South America in the 1600s, with the appearance of unusually warm water in the Pacific Ocean.

The name El Niño, or ‘Christ child’, was chosen because these warm-water events happened to around the Christmas holiday.

What is La Niña?

The term La Niña refers to the large-scale ocean-atmosphere climate phenomenon linked to periodic cooling in sea-surface temperatures across the central and east-central equatorial Pacific.

La Niña represents the cool phase of the ENSO cycle and means that the ocean water temperatures are cooler than average.

What is ENSO-Neutral?

The term ENSO-neutral refers to periods when El Niño and La Niña are not present and typically occur during the transition between El Niño and La Niña events.

The ocean water temperatures over the equatorial Pacific Ocean are near the long-term average.

Images of El Niño & La Niña

The images below (courtesy of NOAA) show ocean water temperature compared to average in the Pacific Ocean.

The top image is La Niña, showing cooler than average ocean water temperature. The bottom image is El Niño, showing warmer than average ocean water temperature.

How do we measure ENSO?

The first ENSO indicator is the Southern Oscillation Index (SOI), which is calculated using the pressure differential between Tahiti and Darwin. The SOI is negative during El Niño years and positive during La Niña years.

The second ENSO indicator comes from the National Oceanic and Atmospheric Administration (NOAA). NOAA maintains monitoring buoys called the Tropical Atmosphere Ocean (TAO) Array across the equatorial Pacific Ocean.

The buoys record and transmit sea surface and sub-surface temperatures, atmospheric conditions, water currents, and wind data to scientists and researchers around the world in real-time.

El Niño & La Niña Alert System

On the second Thursday of each month, scientists with NOAA’s Climate Prediction Center, in collaboration with forecasters at the International Research Institute for Climate and Society (IRI), release an official update on the status of the El Niño-Southern Oscillation (ENSO).

Watch: Issued when conditions are favorable for the development of El Niño or La Niña conditions within the next six months.

Advisory: Issued when El Niño or La Niña conditions are observed and expected to continue.

Final Advisory: Issued after El Niño or La Niña conditions have ended.

Not Active: ENSO Alert System is not active. Neither El Niño nor La Niña are observed or expected in the coming 6 months.

El Niño Criteria

1) The average sea surface temperatures in the Niño-3.4 region of the equatorial Pacific Ocean were at least 0.5°C (0.9°F) warmer than average in the preceding month.

2) The average anomaly of at least 0.5°C (0.9°F) has persisted or is expected to persist for 5 consecutive, overlapping 3-month periods.

La Niña Criteria

1) The average sea surface temperatures in the Niño-3.4 region of the equatorial Pacific Ocean were at least 0.5°C (0.9°F) cooler than average in the preceding month.

2) The average anomaly of at least -0.5°C (-0.9°F) has persisted or is expected to persist for 5 consecutive, overlapping 3-month periods.

Why do we care about El Niño & La Niña?

Ocean water temperatures across the world influence the tracks of winter storms.

One area of ocean water temperature that has the biggest impact on winter storms in North America is the central Pacific Ocean. This is the location of El Nino and La Nina.

Thanks to decades of research, scientists now have a decent ability to predict the strength of El Nino and La Nina months in advance.

This means that even now in late summer, we are looking ahead to the temperature of the ocean during the upcoming winter, and it’s these water temperatures that can influence the tracks of our winter storms.

Typical El Niño & La Niña Winter Weather Patterns

The following maps (courtesy of NOAA) illustrate the typical impacts of El Niño and La Niña on the United States during the winter.

During La Niña, the Pacific jet stream often meanders high into the North Pacific and is less reliable across the southern tier of the United States. Southern and interior Alaska and the Pacific Northwest tend to be cooler and wetter than average and the southern tier of U.S. tends to be warmer and drier than average.

During El Niño, the Pacific jet stream will often dip further south across the southern tier of the United States. The southern states tend to be cooler and wetter than average, while the northern half of the U.S. becomes warmer and drier than average.

Keep in mind that one or more of these climate patterns have occurred during many El Niño and La Niña events in the past. That doesn’t mean that all of these impacts happen during every episode as every event is different.

Final Thoughts

You will come across numerous winter forecasts. These 1-6 month forecasts are rarely accurate.

If your main goal is to enjoy deep, fresh powder, you’ll need to watch the forecast closely through the season. Forecasts out to 7-14 days can guide you to areas where the weather is trending cold and snowy, and then you can nail down the exact day and location of the best snow about 1-3 days in advance.

Hint of fall early next week


polar jet dips south,

cooling temperatures and rain

mixed with snow in the alpine



First snow –

head clear,

I wash my face.


ETSUJINN (1656-1739)


Monsoon Season Kicks Off This Week in the Southwest, But It Won’t Bring Relief Until Later This Summer

Monsoon forecast from Joe Ramey and NOAA for southwest Colorado



By Linda Lam

June 16 2020 10:32 AM EDT

Southwest Monsoon Set to Arrive as Wildfires Rage

At a Glance

  • The monsoon in the Southwest runs from June 15 through Sept. 30.
  • Fire danger is high in June before the monsoon season starts.
  • Heavy rain, flash flooding, lightning, downburst winds and dust storms are threats during monsoon season.

Monsoon season in the Southwest is here, but relief from the hot and dry conditions are not expected until later this summer.

Monsoon season starts June 15 in the southwestern U.S., according to NOAA and lasts through Sept. 30. However, in most of the region, it doesn’t really kick in until late June or early July.

So what do we mean by monsoon season?

A monsoon is a change in seasonal winds. This occurs in the Southwest each summer when a thermal low develops due to intense heating of the land, but large bodies of water nearby do not warm as quickly. In addition, a ridge of high pressure builds over the Rockies or Plains.

Eventually, pressure differences between the warm land and cooler water cause more humid air from the Gulf of California and the Gulf of Mexico to be drawn toward the Southwest. This flow from moist ocean waters is a change from the usual flow from land areas to the ocean waters.

The result is thunderstorm development.

Monsoon Setup in the Southwest

Rain from these thunderstorms causes humidity to increase which triggers more storms. This cycle continues until early fall when the land finally cools and water temperatures reach their peak warmth, which reduces the pressure difference. As a result, the onshore moisture flow lessens. Occasional breaks in this pattern do occur during the season and the rainfall is not continuous.

Pre-Monsoon Concerns

Hot and dry conditions, with low humidity, prevail in June before monsoon season really gets going.

Fire danger is high due to these conditions. In addition, dry thunderstorms can cause dry lightning and gusty winds. Dry lightning can spark fires and gusty winds can spread them and make them difficult to contain.

Wildfire activity is expected to be above average in much of the Southwest into July before monsoonal rain lowers the risk.

June Average Temperatures

What Does the Monsoon Bring?

In Phoenix, the average onset of monsoon is July 7 or when the dew point is 55 degrees or higher for three consecutive days.

In addition to Arizona, portions of six other states are typically impacted by the Southwest monsoon: New Mexico, Nevada, Utah, Colorado, Texas and California.

The increase in monsoonal moisture ignites scattered showers and thunderstorms across the region during the summer.

This moisture reduces the risk of wildfires and brings much-needed rain to the region for reservoir replenishment and vegetation. Most of northwestern Mexico and the southwestern U.S. receive over half of their annual precipitation from the monsoon.

The monsoon pattern also brings concerns including, heavy rain, flash flooding, lightning, downburst winds and dust storms.

Flash flooding is the leading thunderstorm-related killer and most flash flood deaths occur in vehicles. Never drive through flooded roadways. Lightning also causes injuries and fatalities every year in the United States.

Downbursts are another concern that can result in an outward burst of damaging winds at the ground. Outflow boundaries created by strong thunderstorm downburst winds can cause blowing dust and potentially dust storms, which can significantly and quickly limit visibility.

The amount of relief from the dry and hot conditions the monsoon brings varies from year to year.

For example, last year the monsoon was the ninth driest and third hottest on record for the Southwest, since 1895. This was due to an unfavorable mid-level weather pattern, a decrease in tropical activity in the eastern Pacific and a lack of moisture spreading northward.




Southwestern Monsoon




Arizona and New Mexico receive up to half of their annual rainfall during the summer monsoon (Figure 1). The monsoon season suppresses much of the hot summer temperatures, replenishes water resources, and nourishes the vegetation. The monsoon arrives with much flash and fanfare, with its trademark thunderstorms and flooded streets in southwestern cities. Monsoon rainfall events tend to be short and spotty, with intense, local storms drenching some neighborhoods but not others. The water the storms bring quickly flows off the landscape into streets and rivers, with much of the remnant moisture evaporating in the summer sun.

The monsoon is driven by the sun heating up the land and the Pacific Ocean at different rates, with land surfaces warming more quickly than the ocean. The warm land creates low-pressure zones as hot air rises. Once this pattern establishes across the region, the winds shift to fill in the vacuum. With this shift in the winds, the monsoon begins in northern Mexico in May. The moisture-laden monsoon air travels north to Arizona and New Mexico, encouraged by the pressure difference between the hot, parched southwestern air and the cooler Mexican air. In 2008, National Weather Service officials decided to consider June 15–September 30 as the U.S. Southwest monsoon season in Arizona, although the thunderstorms that bring the rain may form in different times and places across the region.

The monsoon’s driving rains are most dramatic in southeast Arizona and in western New Mexico, tapering off in Phoenix and Yuma, AZ (Figure 2). In addition to being variable in space, some years produce weak monsoons, while others provide ample rain. There are no evident trends in annual monsoon strength, making it difficult to predict. This variable tendency has been consistent over the past 100 years when record keeping generally began.

A number of factors affect the monsoon. El Niño–Southern Oscillation (ENSO) can have a strong impact, with El Niño events often bringing more moisture up from the Gulf of Mexico. Changes in tropical Pacific convection and temperature patterns in the Midwest are also factors that can influence the summer monsoon.

Along with the relief that the summer monsoon provides to the region, it also brings flash floods, dust storms, strong winds, lightning, and dangerous fires that can harm people and property. In the Southwest, lightning has ignited more than 2,300 fires annually since 2001, burning approximately 277,000 acres per year. For much of the Southwest, the monsoon season sparks the fire season but also can bring the fire season to a close after the first few weeks of rainfall.

The monsoon brings rains that can help reverse the downward draw from reservoirs; it is also important to farmers who depend on natural rainfall. The monsoon fuels the growth of summer grasses, which can make or break southwestern ranchers struggling to eke out a living in harsh desert lands. Southwest summers can be especially harsh in years when the monsoon falters, leaving shriveled plants and baked soil in its wake.


Was Saturday’s wind storm Colorado’s first-ever derecho? ~ The Denver Post

A rare widespread, damaging wind storm took place on Saturday, and it was quite likely Colorado’s first-ever derecho on record.

Screen Shot 2020-06-08 at 12.08.53 PM

Colorado weather is a lot of things, but one thing it’s not: boring.

What appeared to be a fairly typical Colorado afternoon of wind-producing storms Saturday morphed into possibly the most widespread wind-specific thunderstorm event in the state’s recorded history.

Russ Schumacher, Colorado’s state climatologist, made that contention Sunday while pointing to a number of data points.

“In the storm data record from 1955-2018, the most severe wind reports on a single day in Colorado is 30, on May 22, 2006. (Saturday), there were 91.The most significant wind gusts (75 mph or greater) on a single day was 7, on a couple different occasions. (Saturday) there were 17,”  Schumacher said. “I think it’s fair to say we’ve never had such a widespread damaging thunderstorm wind event in Colorado, at least since reliable records have been collected.”

One thing that appears abundantly clear: Saturday’s story was likely Colorado’s first-ever significant derecho on record.

A derecho is a wide and long-lived line of damage-producing severe thunderstorms, but they’re far more common in the eastern half of the country. While individual thunderstorms produce damage all the time in Colorado, it’s rare to get a long, nearly uniform line of damaging storms like the one the Denver area saw Saturday afternoon.

Colorado-based meteorologist Dakota Smith captured a nearly 12-hour satellite and lightning loop showing the powerful line of storms as it raced through Utah, Colorado and eventually moved into Nebraska and the Dakotas on Saturday night.

Dakota Smith@weatherdak

Rare derecho stretching from Utah to the Dakotas today.

A Rocky Mountain special.

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The line raced through Colorado at speeds of 100 mph at times, but it also covered a wide swath of real estate. For example, a severe thunderstorm warning issued by the National Weather Service office in Boulder was the biggest geographical warning that office had ever issued — and it wasn’t even close.

Russ Schumacher@russ_schumacher

Based on @akrherz’s archive, these are the two largest warning polygons issued by @NWSBoulder by quite a margin

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That all appears to add up to the definition of a derecho. According to the Storm Prediction Center (SPC), the national governing body for severe weather, a derecho needs to have a swath of wind damage extending at least 250 miles in length, wind gusts that regularly exceed 58 mph, and a few 75 mph wind gusts or higher mixed in as well.

According to Elizabeth Leitman, a meteorologist with the SPC, Saturday’s line of storms more than fit the SPC’s official definition of a derecho. Leitman said the storms produced damage for “at least” 750 miles (three times the official criteria for a derecho), producing wind gusts that fit the criteria along the way as well.

Elizabeth Leitman@WxLiz

I put together some quick info on Saturday’s derecho event. Only two other derechoes in the Great Basin are well noted in literature. I’ve included storm reports for those events, as well as a derecho frequency map for comparison. Pretty remarkable event!

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In her tweet, Leitman also contends that far northwestern Colorado was clipped by a derecho in 1994, but that’s believed to be the only other derecho to impact the state in recorded history, or at least since reliable weather records began. Saturday’s derecho was unique in the fact that it covered a big geographical chunk of the state, unlike the 1994 event that merely clipped a small sliver of Colorado.

And, of course, getting any sort of a derecho is rare in the much drier western third of the United States. Dr. Sam Ng, a professor of meteorology at Metro State University, showed the climatology of derechos, and specifically how they tend to concentrate in the Midwest.

Sam Ng@DocWX

Indeed they are. Here’s a Derecho Climatology from SPC. Derechos for the High Plains are rare. 

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Chris Bianchi@BianchiWeather

Today’s Colorado windstorm was likely a derecho- a very rare, prolonged/widespread/strong wind event for the Front Range.