NOAA increases chances of La Nina to 75% this winter!!

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La Niña conditions were present in August, and there’s a 75% chance they’ll hang around through the winter. NOAA has issued a La Niña Advisory. Just how did we arrive at this conclusion, and what does a La Niña winter portend? Read on to find out!

Checking the boxes
Let’s revisit our La Niña decision tree.
La Nina Diagnostic Flowchart

Flowchart showing decision process for determining La Niña conditions. Figure by Fiona Martin, adapted by Climate.gov.

The answer to the first question, “Is the monthly Niño3.4 sea surface temperature anomaly equal to or less than -0.5°C?” is an easy “yes.” August’s value was -0.6°C according to our most consistent sea surface temperature dataset, the ERSSTv5 (though that is not the only SST dataset we monitor). For a quick refresher, the Niño3.4 sea surface temperature anomaly is the difference from the long-term average temperature of the surface of the Pacific Ocean in the Niño3.4 region. In this case, the long-term average is 1986-2015.

ENSOblog_202009_EDD_ERSSTv5_620.png

Monthly sea surface temperature in the Niño 3.4 region of the tropical Pacific for 2019-2020 (purple line) and all other years starting from neutral winters since 1950. Climate.gov graph based on ERSSTv5 temperature data.

The second step is “Do you think it will stay more than half a degree cooler than average for the next several months?” and again, the answer is “yes.” Most of the dynamical computer models predict that the sea surface temperature will remain below the La Niña threshold of -0.5°C through the winter.

NMME_plume_graph_202009_620.png


Climate model forecasts for the Niño3.4 Index. Dynamical model data (black line) from the North American Multi-Model Ensemble (NMME): darker gray envelope shows the range of 68% of all model forecasts; lighter gray shows the range of 95% of all model forecasts. NOAA Climate.gov image from University of Miami data.

Now, on to the critical third step: “Is the atmosphere showing signs of a response to the cooler-than-average sea surface?” Another “yes!” La Niña intensifies the contrast between the warm far western Pacific and much cooler eastern Pacific, and so La Niña’s atmospheric response is a strengthening of the Walker circulation. This large-scale circulation pattern is characterized by air rising over the very warm waters of the far western Pacific and Indonesia, traveling eastward high in the atmosphere, sinking over the eastern Pacific, and traveling back westward near the surface. (Creating the trade winds—more on those in last month’s post.)

When the Walker circulation is stronger than average, the trade winds are stronger, which we observed in the end of August and early September. More rising air over the far western Pacific means lower air pressure, while descending air over the eastern Pacific means higher air pressure; the contrast between these two arms of the Walker circulation is measured using the Southern Oscillation Index and the Equatorial Southern Oscillation Index. Both indexes were positive in August, at 1.1 and 1.0 respectively. These values, which are in the top 20% of the 1950–present record, indicate a stronger-than-average Walker circulation.

La Niña impacts
La Niña’s altered atmospheric circulation over the Pacific Ocean affects global weather and climate. While every ENSO event (and every winter!) is different, La Niña can make certain outcomes more likely. This includes more rain than average through Indonesia, cooler and wetter weather in southern Africa, and drier weather in southeastern China, among other impacts.

LaNina_impacts_global_Dec-Feb_620.jpg

Typical winter (December–February) temperature and precipitation impacts from La Niña. Map by climate.gov.

One important global impact of La Niña is its effect on the Atlantic hurricane season. La Niña reduces wind shear—the change in winds between the surface and the upper levels of the atmosphere—allowing hurricanes to grow. The likelihood of La Niña was factored into NOAA’s August outlook for the Atlantic hurricane season, which favored an “extremely active” season. As of September 8th, we have seen 17 named storms so far this season, and the forecast is for a total of 19-25 named storms (the hurricane season ends on Nov. 30th).

La Niña affects US weather through its impact on the Asia-North Pacific jet stream, which is retracted to the west during a La Niña winter and often shifted northward of its average position. Tom wrote a great explanation of the La Niña/jet stream mechanics and impacts here. Generally, La Niña winters in the southern tier of the US tend to be warmer and drier, while the northern tier and Canada tend to be colder. Official seasonal outlooks are available from the Climate Prediction Center, and Nat will be writing about CPC’s winter outlook for the blog in November.

Nina_impacts_flat_620_0.png

Average location of the jet stream and typical temperature and precipitation impacts during La Niña winter over North America. Map by Fiona Martin for NOAA Climate.gov.

We have a bunch of information on La Niña impacts here on the ENSO Blog! In the early stages of our last La Niña, 2018–2019, Tom mapped out the temperature and precipitation during every La Niña winter on record.

Also, guest posts have covered La Niña’s effect on snow in the US, a potential link with tornado season, and how La Niña can interact with other climate variability patterns. And of course, we’ll be here every month, updating you on how ENSO conditions are evolving.
 

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What about snow during La Niña winters?
Author:
Stephen Baxter



This is a guest post by Dr. Stephen Baxter who is a NOAA Climate Prediction Center (CPC) meteorologist and does applied research on subseasonal-to-seasonal climate variability. In particular he specializes in understanding how the middle-to-high latitude circulation is influenced by the tropics versus other processes. He also has a lot of opinions on Siberian snow cover and the role of the western tropical Pacific in forcing seasonal climate over North America.

Recent cold air outbreaks over the north-central and northwestern U.S., along with record cold on Veterans Day in parts of the Northeast and Mid-Atlantic, should have people excited about (or dreading) the upcoming winter. My colleagues and I at the Climate Prediction Center have just issued our final outlook for the upcoming “meteorological winter,” that is, December through February. Right now, our official outlook covers only temperature and total precipitation, with the latter combining both liquid and frozen precipitation. However, what about the frozen stuff? What about snow?
Because many people remember winters that were either exceptionally snowy or not snowy at all, we get a lot of questions about what the winter forecast portends for seasonal snowfall accumulation. In many parts of the country, snowfall also has major economic and societal ramifications, including water resource management and winter tourism, among others.

Snow Way!
Tackling this problem is not easy, though. Part of the issue boils down to the technical difficulties of snowfall measurements—a real “problem child” as Deke Arndt (NCEI) puts it. The other issue is related to the difficulties with prediction. As many people in the Northeast corridor are aware, snowfall with any given storm system is a function of the dreaded rain-snow line that separates air masses that are below or above freezing.

For any given storm system, the exact boundaries between rain and snow can be hard to predict even days in advance. Luckily, at CPC, we aren’t trying to predict specific events, but the climate instead. We take a step back and see how seasonal temperature and precipitation forecasts might play a role in determining seasonal snowfall accumulation.

In regions that receive a large percentage of their cold-season precipitation in the form of snow, increased seasonal precipitation is intuitively related to increased snowfall accumulation. In more temperate areas that receive a relatively small percentage of frozen precipitation, temperature becomes important. Anomalously cold temperatures are, more or less, a necessary condition for snow in those areas. Therefore, a region with a relatively cold winter may find itself on the cold side of storm systems more often.

In more mountainous areas, where temperature varies as a function of elevation, colder systems result in snow falling at lower altitudes and more total snowfall coverage over a given region. This is where the long-term warming trends, recently discussed by Tom, become important over western North America. Drier and warmer climate signals will generally result in lower snow coverage.
Because a La Niña Advisory was recently issued, we will take a look at how La Niña, in general, affects snowfall across North America. This analysis is part of a broader effort at CPC to better understand and potentially predict seasonal snowfall, made possible in part by a new snowfall dataset (1).

La Niña = Skiers Delight over the Northern United States
In a nutshell, La Niña is associated with a retracted jet stream over the North Pacific Ocean. The retreat of the jet stream results in more blocking high pressure systems that allow colder air to spill into western and central Canada and parts of the northern contiguous U.S. At the same time, storm track activity across the southern tier of the U.S. is diminished under upper-level high pressure, which also favors milder-than-normal temperatures. The storm track is in turn shifted northward across parts of the Ohio Valley and Great Lakes (2).

Based on climate analysis (3) from this new snow dataset, we see that La Niña favors increased snowfall over the Northwest and northern Rockies, as well as in the upper Midwest Great Lakes region. Reduced snowfall is observed over parts of the central-southern Plains, Southwest, and mid-Atlantic.

All La Nina winter composite


Snowfall departure from average for all La Niña winters (1950-2009). Blue shading shows where snowfall is greater than average and brown shows where snowfall is less than average. Climate.gov figure based on analysis at CPC using Rutgers gridded snow data.

This La Niña footprint is pretty intuitive. Given the northward shift of the storm track, relatively cold and wet conditions are favored over the northern Rockies and northern Plains, resulting in the enhancement of snowfall. Warmer and drier winters are more likely during La Niña over more southern states, and this is exactly where seasonal snowfall tends to be reduced (4). The more vigorous storm track and slight tilt toward colder temperatures over the northern tier of U.S. during La Niña modestly increases the chance of a relatively snowy winter.

Snow and Strength
We can break up the snow pattern further and look at the weakest and strongest La Niña events. Splitting La Niña events into strength reveals some interesting differences worth investigating further. In this preliminary analysis below, there is a suggestion that weaker events are snowier over the Northeast and northern and central Plains on average.

Weak La Nina years


Snowfall departure from average for weaker La Niña winters (1950-2009). Blue shading shows where snowfall is greater than average and brown shows where snowfall is less than average. Climate.gov figure based on analysis at CPC using Rutgers gridded snow data.

On the other hand, stronger La Niña events (see below) are snowier across the Northwest, northern Rockies, western Canada, and the Alaska panhandle. Also, there is a tendency toward below average snowfall over the mid-Atlantic, New England, and northern and central Plains, which is not seen during weak La Niña.

Strong La Nina winters


Snowfall departure from average for stronger La Niña winters (1950-2009). Blue shading shows where snowfall is greater than average and brown shows where snowfall is less than average. Climate.gov figure based on analysis at CPC using Rutgers gridded snow data.

Overall, stronger La Niña events exert more influence on the winter climate pattern over western North America. Weaker events appear to be associated with more widespread above-average snow over the northern United States. Because a weak La Niña means that the forcing from the Pacific is weaker than normal, it may imply other mechanisms (e.g. Arctic Oscillation) may be at play and is worth further investigation.

The predictability of seasonal snowfall may be somewhat similar to precipitation in that one or two big events can dramatically affect the seasonal average. Thus, in general, the expected prediction skill is likely to be lower than for temperature. However, because temperature also plays an important role in snowfall, some predictability is likely nonetheless. And like for seasonal temperature and precipitation, knowing the state of ENSO is a pretty reasonable place to start.
Lead editor: Michelle L’Heureux
 

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What about snow during La Niña winters?
Author:
Stephen Baxter



This is a guest post by Dr. Stephen Baxter who is a NOAA Climate Prediction Center (CPC) meteorologist and does applied research on subseasonal-to-seasonal climate variability. In particular he specializes in understanding how the middle-to-high latitude circulation is influenced by the tropics versus other processes. He also has a lot of opinions on Siberian snow cover and the role of the western tropical Pacific in forcing seasonal climate over North America.

Recent cold air outbreaks over the north-central and northwestern U.S., along with record cold on Veterans Day in parts of the Northeast and Mid-Atlantic, should have people excited about (or dreading) the upcoming winter. My colleagues and I at the Climate Prediction Center have just issued our final outlook for the upcoming “meteorological winter,” that is, December through February. Right now, our official outlook covers only temperature and total precipitation, with the latter combining both liquid and frozen precipitation. However, what about the frozen stuff? What about snow?
Because many people remember winters that were either exceptionally snowy or not snowy at all, we get a lot of questions about what the winter forecast portends for seasonal snowfall accumulation. In many parts of the country, snowfall also has major economic and societal ramifications, including water resource management and winter tourism, among others.

Snow Way!
Tackling this problem is not easy, though. Part of the issue boils down to the technical difficulties of snowfall measurements—a real “problem child” as Deke Arndt (NCEI) puts it. The other issue is related to the difficulties with prediction. As many people in the Northeast corridor are aware, snowfall with any given storm system is a function of the dreaded rain-snow line that separates air masses that are below or above freezing.

For any given storm system, the exact boundaries between rain and snow can be hard to predict even days in advance. Luckily, at CPC, we aren’t trying to predict specific events, but the climate instead. We take a step back and see how seasonal temperature and precipitation forecasts might play a role in determining seasonal snowfall accumulation.

In regions that receive a large percentage of their cold-season precipitation in the form of snow, increased seasonal precipitation is intuitively related to increased snowfall accumulation. In more temperate areas that receive a relatively small percentage of frozen precipitation, temperature becomes important. Anomalously cold temperatures are, more or less, a necessary condition for snow in those areas. Therefore, a region with a relatively cold winter may find itself on the cold side of storm systems more often.

In more mountainous areas, where temperature varies as a function of elevation, colder systems result in snow falling at lower altitudes and more total snowfall coverage over a given region. This is where the long-term warming trends, recently discussed by Tom, become important over western North America. Drier and warmer climate signals will generally result in lower snow coverage.
Because a La Niña Advisory was recently issued, we will take a look at how La Niña, in general, affects snowfall across North America. This analysis is part of a broader effort at CPC to better understand and potentially predict seasonal snowfall, made possible in part by a new snowfall dataset (1).

La Niña = Skiers Delight over the Northern United States
In a nutshell, La Niña is associated with a retracted jet stream over the North Pacific Ocean. The retreat of the jet stream results in more blocking high pressure systems that allow colder air to spill into western and central Canada and parts of the northern contiguous U.S. At the same time, storm track activity across the southern tier of the U.S. is diminished under upper-level high pressure, which also favors milder-than-normal temperatures. The storm track is in turn shifted northward across parts of the Ohio Valley and Great Lakes (2).

Based on climate analysis (3) from this new snow dataset, we see that La Niña favors increased snowfall over the Northwest and northern Rockies, as well as in the upper Midwest Great Lakes region. Reduced snowfall is observed over parts of the central-southern Plains, Southwest, and mid-Atlantic.

All La Nina winter composite


Snowfall departure from average for all La Niña winters (1950-2009). Blue shading shows where snowfall is greater than average and brown shows where snowfall is less than average. Climate.gov figure based on analysis at CPC using Rutgers gridded snow data.

This La Niña footprint is pretty intuitive. Given the northward shift of the storm track, relatively cold and wet conditions are favored over the northern Rockies and northern Plains, resulting in the enhancement of snowfall. Warmer and drier winters are more likely during La Niña over more southern states, and this is exactly where seasonal snowfall tends to be reduced (4). The more vigorous storm track and slight tilt toward colder temperatures over the northern tier of U.S. during La Niña modestly increases the chance of a relatively snowy winter.

Snow and Strength
We can break up the snow pattern further and look at the weakest and strongest La Niña events. Splitting La Niña events into strength reveals some interesting differences worth investigating further. In this preliminary analysis below, there is a suggestion that weaker events are snowier over the Northeast and northern and central Plains on average.

Weak La Nina years


Snowfall departure from average for weaker La Niña winters (1950-2009). Blue shading shows where snowfall is greater than average and brown shows where snowfall is less than average. Climate.gov figure based on analysis at CPC using Rutgers gridded snow data.

On the other hand, stronger La Niña events (see below) are snowier across the Northwest, northern Rockies, western Canada, and the Alaska panhandle. Also, there is a tendency toward below average snowfall over the mid-Atlantic, New England, and northern and central Plains, which is not seen during weak La Niña.

Strong La Nina winters


Snowfall departure from average for stronger La Niña winters (1950-2009). Blue shading shows where snowfall is greater than average and brown shows where snowfall is less than average. Climate.gov figure based on analysis at CPC using Rutgers gridded snow data.

Overall, stronger La Niña events exert more influence on the winter climate pattern over western North America. Weaker events appear to be associated with more widespread above-average snow over the northern United States. Because a weak La Niña means that the forcing from the Pacific is weaker than normal, it may imply other mechanisms (e.g. Arctic Oscillation) may be at play and is worth further investigation.

The predictability of seasonal snowfall may be somewhat similar to precipitation in that one or two big events can dramatically affect the seasonal average. Thus, in general, the expected prediction skill is likely to be lower than for temperature. However, because temperature also plays an important role in snowfall, some predictability is likely nonetheless. And like for seasonal temperature and precipitation, knowing the state of ENSO is a pretty reasonable place to start.
Lead editor: Michelle L’Heureux

I SOOOOOOOOOOOOOOOOOOOOO want to be excited about this but they predicted La Nina last year too and our winter SUUUUUUUUUCKKKED!!! Fingers crossed, 2020 could use a killer winter coast to coast.
 
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