"2024-2025 Winter Outlook" by Arcfield Weather

La Nina will likely be the dominant player this winter season in the equatorial Pacific Ocean with colder-than-normal sea surface temperatures. Typically, this type of pattern leads to an active polar (northern) jet across Canada and much of the northern US and colder-than-normal conditions from Alaska to the Northern Plains. In addition, La Nina winters are often warmer and drier than normal in much of the southern US from southern California-to-Florida with persistent ridge in the southeastern states. Map courtesy NOAA

Overview

Last winter was generally warmer-than-normal in the Mid-Atlantic region with below-normal snowfall and 2024 began with a rather strong El Nino event in the tropical Pacific Ocean. However, those warmer-than-normal water temperatures have since flipped to below-normal and this upcoming winter season is quite likely to feature weak La Nina (colder-than-normal) conditions. Typically, La Nina winters feature a more active polar jet stream that helps to transport cold air masses from northwestern Canada into the Northern Plains while, at the same time, the southern US often experiences warmer and drier conditions. La Nina winters are somewhat random in the Mid-Atlantic region with respect to temperatures and precipitation with some years featuring more snow than normal and others less.

Normal Seasonal Snowfall for the Mid-Atlantic I-95 Corridor

Key factors

There are several factors listed below that were used in the preparation of this year’s Winter Outlook: 

1.       Weak La Nina conditions in the tropical Pacific with colder-than-normal sea surface temperatures (SSTs)

2.       Colder-than-normal SSTs across the northern Pacific

3.       Generally warmer-than-normal SSTs in the Atlantic Ocean

4.       Mixed signals for the prospects of “high latitude blocking”:

a.     Arctic Oscillation/North Atlantic Oscillation trends (Neutral)

b.     autumnal snowpack in the Northern Hemisphere (Positive)

c.     solar activity (Negative)

5.       An analysis of “analog” years suggests colder-than-normal conditions in much of the nation and drier-than-normal conditions across much of the southern US

Current sea surface temperature anomalies across the globe with La Nina conditions dominating the scene in the equatorial Pacific Ocean; Plot courtesy tropicaltidbits.com

Pacific Ocean

The Pacific Ocean is the largest on the planet, covers more than 30 percent of the Earth’s surface, and is bigger than the landmass of all the continents combined. The warm waters of the equatorial Pacific Ocean store a great amount of latent heat when compared to cooler waters and breed a great deal of convection which impacts downstream ridging and troughing in the atmosphere. As such, its sea surface temperature (SST) pattern has a tremendous influence on all weather and climate around the world and the more anomalous the sea surface temperatures, the more the impact can be on the atmosphere around the world. The El Nino-Southern Oscillation (ENSO) is a recurring climate pattern involving changes in the temperature of waters in the central and eastern tropical Pacific Ocean. El Nino and La Nina are the extreme phases of the ENSO cycle; between these two phases is a third phase called ENSO-neutral.

Rolling 3-month averages of sea surface temperature anomalies as generated by a series of dynamical and statistical computer forecast models are shown in this plot for the central part of the Pacific Ocean. The compilation of model forecasts issued during October 2024 suggests a weak La Nina is in the offing for the upcoming winter season. Plot courtesy International Research Institute for Climate and Society

In the tropical Pacific Ocean, weak La Nina conditions have replaced the relatively strong El Nino conditions of early this year and the colder-than-normal sea surface temperatures are expected to last through the upcoming winter season. Numerous forecast models (dynamic and statistical) support the notion of a weak La Nina event this winter in the central Pacific Ocean with an El-Nino Southern Oscillation (ENSO) index likely to fall somewhere around -0.5°C below neutral. Another notable feature as we head towards the winter season is the large area of colder-than-normal water seen off Alaska’s coastline in the northern Pacific Ocean.

Prospects for “High Latitude Blocking (HLB)”

In addition to the analysis of oceanic sea surface temperature anomalies, it is important in longer-range forecasting of winter season conditions in the Mid-Atlantic region to evaluate the prospects for “high latitude blocking” in the atmosphere. High-latitude blocking during the winter season is generally characterized by tenacious high pressure in northern latitude areas such as Greenland, northern Canada, and Iceland and this pattern is favorable to colder-than-normal weather in the central and eastern states and an increased chance for snow. Without this type of blocking pattern to the north, it is difficult to have sustained cold air masses in the Mid-Atlantic region and this is usually an important criterion for significant snowstorms in, for example, the big cities along I-95. 

Three signals that can provide clues as to the prospects for high latitude blocking this upcoming winter include: (1) recent trends in the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) teleconnection indices, (2) recent trends in snowpack build-up across Siberia, and (3) levels of solar activity.  The overall message is mixed from these three factors with one being neutral, one positive, and one negative with respect to the chances this winter season of high latitude blocking.

1)     Arctic Oscillation/North Atlantic Oscillation teleconnections trend (Neutral Signal for HLB)

High latitude blocking is tracked by meteorologists through teleconnection indices known as the Arctic Oscillation (AO) and its closely related cousin called the North Atlantic Oscillation (NAO). The Arctic Oscillation refers to opposing atmospheric pressure patterns in middle and high latitudes.  When the AO is positive, for example, surface pressure is low in the polar region, and this helps the mid-latitude jet stream to blow strongly and consistently from west-to-east keeping Arctic air locked up in the polar region. When the AO index is negative, there tends to be high pressure in the polar regions (i.e., high-latitude blocking), weaker zonal winds, and greater movement of polar air into the middle latitudes such as the Mid-Atlantic region. While the AO and NAO indices are primarily used during by forecasters during the winter season, trends in the fall season can provide important clues about the ensuing winter season. Specifically, negative AO index values in October typically translate to negative values during the following winter season and the trend this October has been on the negative side.

North Atlantic Oscillation (NAO) index (top), Arctic Oscillation (AO) index (bottom) through October 21st; Plots courtesy NOAA/CPC

2)     Autumnal snowpack across Siberia (Positive Signal for HLB)

In addition to the Arctic Oscillation signal, snowpack in the Northern Hemisphere during the autumn season has also been found to be an important predictive factor with respect to high latitude blocking patterns during subsequent winter seasons. In fact, research studies have pinpointed the region in Siberia below 60°N during the month of October as critical with respect to the likelihood of high latitude blocking atmospheric patterns during the following winter season.  If snowpack is above-normal and consistently expanding during October in that part of Siberia, research studies and empirical observations suggest there is an increased chance for more frequent high-latitude blocking configurations in subsequent winter months.  In fact, there has been a significant increase in snowpack across Siberia during the first few weeks of October including in the region south of 60°N (white area in maps).

Noticeable expansion of Siberian snow cover (in white) from 9/30 (left) to 10/22 (right); Graphics courtesy NOAA/NOHRSC

3)     Solar activity (Negative Signal for HLB)

Research and empirical observations have shown that low solar activity tends to be correlated with frequent high-latitude atmospheric blocking events, and we are now experiencing high activity with numerous sunspots as solar cycle 25 nears its solar maximum phase. The “analog” years plot (below) shows upper-level height anomalies in previous low solar activity years (i.e., during solar minimum phases) and high pressure tended to dominate near Greenland and Iceland (orange, red regions) which typically leads to sustained cold air outbreaks into the eastern US.  Based on the actual results of these previous low solar activity years, odds would favor high latitude blocking scenarios this upcoming winter season given the strong likelihood for low solar activity to continue.

Typical 500 mb height anomaly patterns during low solar activity periods (left) and high solar activity years (right); Graphics courtesy NOAA/NCEP

A look at temperature and precipitation patterns during “analog” years

In addition to the evaluation of current oceanic conditions, it is quite useful for longer-range forecasting purposes to find analog years in which there was a comparable El Nino-Southern Oscillation (ENSO) state (i.e., El Nino, La Nina or neutral) and similar overall sea surface temperature patterns around the world. In addition, in the selection of my analog years, I focused on winters in the past that were preceded by an El Nino winter as will be the case this year. Indeed, I believe the following five winter seasons of 1966-1967, 2005-2006, 2007-2008, 2013-2014, and 2020-2021 have met these thresholds featuring similar sea surface temperatures in the tropical Pacific Ocean (i.e., weak-to-moderate La Nina) and around the globe and they all followed an El Nino event during the prior winter. The highest weighting in the generation of analog year temperature and precipitation averages across the nation was given to the winter seasons of 2007-2008 and 2013-2014 as they featured the most similar sea surface temperature patterns to what we are experiencing at the current time.

Average November-to-March temperature anomaly pattern for the five analog years; Graphic courtesy NOAA/NCEI/PSL

These analog years can provide us a clue as to what kind of weather we may experience across the nation during the upcoming winter season. In fact, these five analog winter seasons resulted in temperatures that averaged below-normal across much of the nation including slightly-below average readings in the Mid-Atlantic region for the November through March period.  As far as precipitation is concerned, these five analog years featured nearly normal to slightly wetter-than-normal conditions across much of the northern US with drier-than-normal conditions throughout the southern US. In the Mid-Atlantic region, there were generally nearly normal overall precipitation amounts in these five analog years.   

Average November-to-March precipitation anomaly pattern for the five analog years; Graphic courtesy NOAA/NCEI/PSL

Conclusions

Several factors have been analyzed to provide us with clues about the upcoming winter season including expectations on sea surface temperatures around the world, prospects of high latitude blocking events, and temperatures and precipitation averages of analog years. Based on this analysis, I believe the Mid-Atlantic region from DC-to-Philly-to-New York City will experience nearly normal to slightly below normal temperatures with departures from normal ranging from 0 to -1.0°C. There should indeed be some impressive Arctic air outbreaks that make their way from northwestern Canada all the way into the Mid-Atlantic region riding in on a frequently strong polar jet stream.

Snowfall amounts should be in the nearly normal range with 13-17 inches in DC, 20-24 inches in Philly, and 23-27 inches in New York City (slightly higher amounts in the northern and western suburbs of these three big cities). The winter could very well get off to a quick start with some early season accumulating snow possible during late November and December.

Elsewhere, the dominating weather pattern this winter should feature drier-than-normal and warmer-than-normal conditions across much of the region from southern California-to-Florida and colder-than-normal temperatures concentrated on the Upper Midwest and Northern Plains.

Meteorologist Paul Dorian
Arcfield
arcfieldweather.com

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