La Nina advisory means warmer and drier conditions in the central Plains
for The Fence Post
A La Nina Advisory has been issued for the U.S. (and globally) by the Climate Prediction Center, which basically means a 75 percent chance or greater of La Nina occurring through the winter months.
As of Sept. 10, 2020, weather computer models were in agreement for a La Nina from October through January, and possibly continuing into February. There are variations in strength of the important sea surface temperatures (SSTs) with anomalies (deviations from the average) dropping to -1.2 Celsius to -1.7C which puts La Nina in a low end moderate event. Sea surface temperatures are generally used to determine strength of La Nina (or El Nino).
La Nina is a natural climate phenomenon when sea surface temperatures are cooler-than-average across the central and eastern Pacific Ocean near the equator. It is the opposite of El Nino. (El Nino would develop when water at the surface/upper part of the central and eastern Pacific Ocean becomes significantly warmer than usual.)
So, what does this La Nina mean for eastern Colorado? And the central Plains states?
“La Nina usually leads to warmer than normal and drier than normal conditions as a whole. But there are some things to watch out for that may impact eastern Colorado through the winter months,” said Kyle Mozley, meteorologist, National Weather Service in Pueblo, Colo.
Number one is the strength of La Nina which impacts the storm track. The storm track tends to be from the Pacific Northwest, southeast into the Missouri Valley.
“That leaves Colorado in dry northwesterly flow throughout much of the winter,” Mozley said. “The exception would be the central mountains where northwesterly flow has favorable orographics (when an airmass is lifted up over rising terrain; a mountain).
This prediction does not bode well for the current drought conditions in the central Plains.
“Across the eastern Colorado Plains and into western Kansas, drier than normal conditions would prevail,” Mozley said. “That does not speak well for the current drought conditions, and I would expect drought to continue into spring on the Plains.”
Also of concern for a La Nina, would be strong winds across the region. While cold fronts usually bring precipitation to the Plains, the northwest wind behind cold fronts produces a downsloping effect, drying out the air, and limiting precipitation across the Plains.
Agricultural impacts could be significant due to drought.
“Most counties across the eastern Plains are running (for the March to August period) around 55 percent to 65 percent of normal precipitation,” Mozley said. “I do not see much improvement going forward. The Grass-Cast is also pretty dismal when looking at the expected below normal precipitation as well as the CPC (Climate Prediction Center) outlooks through spring.”
Meanwhile, in the short term for autumn, above normal temperatures are favored across all of the central Rockies and central Plains in October. Below normal precipitation is forecast from the southwestern states across the southern Plains. Equal chances of having either average, below or above average precipitation are forecast for the central and northern Plains. This is consistent with model guidance, long-term trends and La Niña.
WINTER CLIMATE DRIVERS
As we get closer to winter, several factors are expected to drive winter climate, some forecastable at the seasonal time frame, and some not. In addition to La Nina this winter, other large-scale patterns are also players in the forecast including the Madden Julian Oscillation (MJO), the Arctic Oscillation/North Atlantic Oscillation (AO/NAO), trend and natural variability.
Other factors that influence our weather are, what’s called the Pacific Decadal Oscillation (PDO), and Pacific/North American Oscillation (PNA).
“The PDO is currently negative, and when a negative PDO phases with a La Nina event, it enhances the drying effect, making the dry conditions worse on the Plains,” Mozley said.
Eastward is favored for cold air outbreaks, when the AO/NAO is in the negative phases, especially east of the Rockies. Think polar vortex out east, which happens every winter. “And then I would expect that we could have cold outbreaks across eastern Colorado. Unfortunately, they would (also) likely be dry, favoring precipitation well to the east,” Mozley said.
Of these, only ENSO and trend can be anticipated beyond ~ (approximately) two weeks.
So the keys here are La Nina, negative PDO, positive PNA and negative AO/NAO, and the wind, all translate to dry conditions in eastern Colorado, with a very dry trend in western Kansas.
Since we expect a La Nina this year, bottom line is that whether we are drier or wetter, will depend on the strength of La Nina.
CENTRAL PLAINS IMPACT
Also, the La Nina for Nebraska and Kansas usually means colder and drier weather, although winter storms can still develop in the central Plains depending on shorter wave weather systems (troughs) and how much they intensify.
“If an upper level low pressure system tracks from the southern Plains up into Missouri, then Kansas can get heavy snow depending on vertical temperature profiles from the surface to 3,000 feet above the surface, which would determine whether the precipitation resulting would be snow, sleet or freezing rain. If that temperature is greater than 3 degrees Celsius (37 degrees F) and if temperatures at the surface are below freezing it could be freezing rain,” said Bill Gargan, meteorologist at the National Weather Service in Topeka, Kan., noting, “It’s still too far to tell.”
“However, if the low tracks from Colorado to Wisconsin, then eastern Kansas and south central Nebraska would usually be on the warmer side of these blizzards which more likely travel then to the Dakotas and Minnesota,” Gargan said. The central Plains would then get a cold rain.
From Kansas Climatologist Mary Knapp, “The current outlook for La Nina is for a moderate event, with a 75 percent chance of continuing through the winter (December through February). There is a 50 percent chance of it lingering through April 2021. The usual pattern with a La Nina is for a shift in the jet streams keeping the colder air to the north,” said Mary Knapp; Assistant State Climatologist, located at Kansas State University in Manhattan, Kan.
For the northern plains, this usually signals a colder and wetter fall and winter. For the southern plains, it generally brings a warmer and drier pattern.
“As Kansas is sandwiched between the two regions, the impacts may be more variable,” Knapp said. “Most commonly, it brings a hot, dry fall with a mild, dry winter.” And the current CPC seasonal outlook has the warmer/drier pattern in play for Kansas for the October to December period, with an even more pronounced La Nina pattern in the December to February time period, Knapp added.
The official National Oceanic and Atmospheric Administration winter outlook is expected to be available in late October, with an updated winter outlook issued a quick month later, in November. ❖
— Hadachek is a freelance writer who lives on a farm with her husband in north central Kansas and is also a meteorologist and storm chaser. She can be reached at email@example.com.
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