The Winter Ahead (Dec,2020 -Feb, 2021) - National Weather Service Little Rock, Arkansas September 15, 2020 - National Weather ...
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The Winter Ahead
(Dec, 2020 – Feb, 2021)
National Weather Service
Little Rock, Arkansas
September 15, 2020
Where Are We Now? The following four slides will show departure from normal temperature and precipitation so far in 2020 as well as existing drought conditions.
Temperatures in 2020
Departure From Normal through
September 14
Source: High Plains Regional
Climate Center
Arkansas (through August): +0.8°F
Precipitation in 2020
Departure From Normal through
September 14
Source: High Plains Regional
Climate Center
Arkansas (through August): +13.29”
Drought Monitor September 8, 2020
El Niño/La Niña Status
Through Spring, 2021
Source: International Research Institute
for Climate and Society through
Columbia University
We are in La Niña
territory and will likely El Niño
remain there through
the winter.
May/Jun/Jul, Apr/May/Jun,
La Niña
2020 2021
What is La Niña?
This is cooler than normal water (by at least 0.5 degrees C)
along the equator in the Pacific Ocean for five or more
consecutive three month periods (Sep/Oct/Nov,
Oct/Nov/Dec, Nov/Dec/Jan, etc).
Source: Climate Prediction Center
La Niña Climatology Climatologically, the following are textbook conditions with La Niña (as shown in the next two slides): If colder and wetter than normal conditions materialize, this is most favored across the northern states. If warmer and drier conditions occur, this is most likely to happen across the southern states.
La Niña Climatology
Temperature
Source: Brian Brettschneider
Note: Compiled Using an Analysis
of Past La Niña Years
La Niña Climatology
Precipitation
Source: Brian Brettschneider
Note: Compiled Using an Analysis
of Past La Niña YearsLa Niña Climatology
The Tropics
Hurricane Laura in the
Gulf of Mexico on August
25, 2020
Tropical activity in the Atlantic basin tends to increase
when La Niña conditions are present. As of September
14th, there were 20 named storms in 2020. The record is
28 named storms in 2005, which was a La Niña year.NMME Winter Outlook
Dec, 2020 to Feb, 2021
Long range models help forecasters determine what might
happen in the months ahead.
The NMME (North American Multi-Model Ensemble) from
the Climate Prediction Center averages temperature and
precipitation input from half a dozen models.
The latest winter forecast from the NMME (in the next two
slides) is close to what could be expected during a typical
La Niña.NMME Winter Outlook
Dec, 2020 to Feb, 2021
Temp Forecast Note: NMME is North American Multi-
(Dec/Jan/Feb) Model Ensemble from the Climate
Prediction CenterNMME Winter Outlook
Dec, 2020 to Feb, 2021
Rain Forecast Note: NMME is North American Multi-
(Dec/Jan/Feb) Model Ensemble from the Climate
Prediction CenterCPC Winter Outlook
Dec, 2020 to Feb, 2021
The CPC (Climate Prediction Center) winter outlook is a
confidence forecast.
The process starts with the same odds (33%) of above
normal, below normal, and near normal temperatures and
precipitation.
From there, the odds of warmer, colder, wetter, or drier
than usual conditions increase/decrease based on trends
and/or the expected pattern. If the odds remain unchanged
(33%), then the forecast is equal chances (EC) of
occurrence.CPC Winter Outlook
Dec, 2020 to Feb, 2021
Temp Forecast
(Dec/Jan/Feb) Note: CPC is Climate Prediction
CenterCPC Winter Outlook
Dec, 2020 to Feb, 2021
Rain Forecast
(Dec/Jan/Feb) Note: CPC is Climate Prediction
CenterDrought Outlook
Through December, 2020
Based on a drier than normal forecast across the southern
United States, drought conditions are expected to expand
eastward from the Pacific Coast and Rockies.Drought Outlook Through December, 2020
La Niña Extremes Several extreme and historic weather events have occurred during La Niña winters.
La Niña Extremes El Niño (Red) La Niña (Blue) January, 1999 Tornado Outbreak (Largest in State History) – 56 Tornadoes
La Niña Extremes El Niño (Red) La Niña (Blue) December, 2000 Ice Storms (2) in Central Arkansas – One of the Largest Natural Disasters in State History.
La Niña Extremes El Niño (Red) La Niña (Blue) February, 2008 – Longest Track Tornado (122 Miles) in State History. For the Year…81 Tornadoes (2nd Most On Record)
La Niña Extremes El Niño (Red) La Niña (Blue) January, 2009 – Catastrophic Ice Storm in Northern Arkansas (One of the Worst in State History)
La Niña Extremes El Niño (Red) La Niña (Blue) January to May, 2011 – Major Snowstorms (Including One to Two Feet of Powder in the Ozarks on February 9th) followed by 75 Tornadoes (4th Most on Record)
La Niña Extremes
Arctic Oscillation (AO) Variables such as El Niño/La Niña are used to make a winter forecast because they are fairly stable in the long term (lasting six to eighteen months). Other variables such as the Arctic Oscillation (AO) are not nearly as reliable, switching phases (positive/negative) more quickly (lasting two to three weeks). However, a strong and persistent AO can overpower the effects of El Niño/La Niña.
Arctic Oscillation (AO)
Negative Phase
Higher pressure toward
the North Pole and lower
pressure in the mid-
latitudes.Arctic Oscillation (AO)
Negative Phase
The pressure gradient
decreases/relaxes, leading
to a slower west to east
flow across Canada.Arctic Oscillation (AO)
Negative Phase
Given a slower and less
efficient flow, cold air
builds up and bulges into
more southern latitudes.Arctic Oscillation (AO)
Negative Phase
It’s supposed to be
mild across the south
given La Niña
conditions (early
2011), but a strongly
negative AO took
over and it became
cold and snowy in
January/February.Arctic Oscillation (AO)
Negative Phase
On February 9th, one
to two feet of snow
piled up in the Ozark
Mountains of
northwest Arkansas.Local Winter Forecast A warmer and drier winter is favored in Arkansas given La Niña conditions. This is a three month (Dec/Jan/Feb) average, with variability in temperature and precipitation likely. Historic winter events tend to happen during La Niña years. The state has experienced anything from heavy snow to ice storms and tornado outbreaks. Watch for variables such as the Arctic Oscillation. If such a variable becomes strong and persistent, it can either negate or enhance the effects of La Niña depending on the phase.
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