"2024-2025 Winter Outlook" by Arcfield Weather
Paul Dorian
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 region from southern California-to-Florida with persistent high pressure ridging 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, much of 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’s 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 Ocean with colder-than-normal sea surface temperatures (SSTs)
2. Colder-than-normal SSTs across the northern Pacific Ocean
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 Signal)
b. increasing autumnal snowpack in Siberia and other parts of the Northern Hemisphere (Positive Signal)
c. high solar activity (Negative Signal)
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 sea surface temperature anomalies, it is important in the longer-range forecasting of the upcoming winter season 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 kind of pattern is generally more favorable for colder-than-normal weather conditions across the central and eastern US and an increased chance of 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) overall levels of solar activity. The overall message is mixed from these three factors as described below with one being neutral, one positive, and one negative with respect to the chances this winter season of high latitude blocking events.
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 AO and NAO teleconnection indices refer to temperature and pressure patterns in the 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. As an example, persistent negative AO index values during October typically translate to consistent negative values during the subsequent winter season. The trend for the NAO and AO teleconnections indices this fall season has been quite mixed with some periods on the positive side and some stretches being negative.
North Atlantic Oscillation (NAO) index (top), Arctic Oscillation (AO) index (bottom) from 01 July to October 21st; Plots courtesy NOAA/CPC
2) Increasing autumnal snowpack across Siberia and other parts of the NH (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 subsequent 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 areas on maps).
Elsewhere in the northern hemisphere, snow cover has increased substantially in early-to-mid October across portions of Alaska and northwestern Canada. This can be an important source region for cold air during La Nina winters in that the polar jet stream is usually quite active and it can help transport cold air masses from Alaska/northwestern Canada to the Northern Plains/Upper Midwest.
Noticeable expansion of Siberian snow cover (in white) from 9/30 (left) to 10/22 (right); Graphics courtesy NOAA/NOHRSC
3) High Solar Activity (Negative Signal for HLB)
Research and empirical observations have shown that solar activity tends to play an important role in the formation of high latitude blocking during winter seasons. The 500 millibar height anomaly plots (below) show a clear tendency of high latitude blocking in low solar activity years (left) with higher heights (and pressure) on average over Greenland/Canada (orange/yellow) and the opposite holds true during high solar activity years. As we are approaching or actually in a solar maximum phase of solar cycle 25, solar activity is expected to remain on the high side this winter season which is an unfavorable signal for high latitude blocking events.
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. These analog years can provide us a few clues as to what kind of weather we may experience across the nation during the upcoming winter season. 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 average anomalies across the US was given to the winter seasons of 2007-2008 and 2013-2014 as - of the selected five - these two featured the most similar sea surface temperature patterns to what we are currently experiencing.
Average November-to-March temperature anomaly pattern for the five analog years; Graphic courtesy NOAA/NCEI/PSL
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 was a range of drier-than-normal conditions across southern sections to wetter-than-normal in parts of the north.
Average November-to-March precipitation anomaly pattern for the five analog years; Graphic courtesy NOAA/NCEI/PSL
The bottom line
Several factors have been analyzed in the preparation of this year’s “Winter Outlook” to provide us with some clues about the upcoming winter season. These factors include such metrics as global sea surface temperatures, prospects for high latitude blocking, and a look at temperature and precipitation anomalies during selected “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 in the range of 0 to -1.0°C. There should indeed be some impressive Arctic air outbreaks that make their way from northwestern Canada and Alaska all the way into the Mid-Atlantic region riding in on what should be a strong polar jet stream during the upcoming La Nina winter.
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 in the Mid-Atlantic region with some early season accumulating snow possible during the latter part of November and in December. It is not all that unusual for a winter season to get off to a quick start in the eastern US with respect to snowfall if the tropical season has a late ending compared to normal.
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 the coldest temperatures relative-to-normal concentrated on the Upper Midwest and Northern Plains.
Meteorologist Paul Dorian
Arcfield
arcfieldweather.com