I came across an interesting winter-weather item. According to NOAA’s Climate Prediction Center, they’re predicting a 50-55% chance of a La Nina conditions arriving this fall and lasting throughout Winter 20-21.
If this in fact is the case, what does this mean for North American snow and ice management professionals
I’m a little rusty on my Nina/Nino conditions, so I reached out to my weather buddies at Thermodynamic Solutions to get the scoop on what La Nina could influence weather-wise.
Here's what TDS's Beth Carpenter had to say:
On July 9, the Climate Prediction Center (CPC) issued a La Nino Watch, for a 50-55% chance of La Nina development in the Northern Hemisphere this Fall. Meteorologically speaking, Fall is defined as September through November. So, what does this mean?
You’ve heard of the terms El Nino and La Nina, which are two of the three phases of the El Nino Southern Oscillation (ENSO). The third is ENSO-neutral. ENSO is characterized by changes in the oceanic and atmospheric circulations of the Earth along the equator -- specifically the Equatorial Pacific.
During El Nino conditions, warmer than normal sea surface temperatures (water temperatures) are observed in the aforementioned region. We also see a weakening of the traditional easterly trade winds (winds that blow from east to west), or even a reversal to westerly winds (winds that blow from west to east).
During La Nina conditions, the opposite occurs. Sea surface temperatures cool to
below normal in the Equatorial Pacific and the typical easterly winds along the equator strengthen even more. To be defined as El Nino or La Nina, both the atmospheric state and the oceanic state must be in agreement of these conditions. In ENSO neutral years, neither El Nino or La Nina conditions are present and sea surface temperatures are near average, or the oceanic and atmospheric conditions are not in agreement. For example, the sea surface temperatures appear to feature El Nino conditions, but the equatorial winds are unchanged). Typical El Nino and La Nina oceanic conditions are shown below.
This Summer, we’ve been in ENSO-neutral conditions, where we aren’t seeing
strong influences from El Nino or La Nina. However, the strongest effects from these phenomena are typically felt in the United States during the winter months. Sea surface temperatures are expected to continue to cool through the latter portions of Summer and through the Fall months, hinting at the possibility of La Nina development into winter. The below chart shows a plume of climate model data and the trend of decreasing sea surface temperatures.
During La Nina years, the United States sees a very active Polar Front Jet (PFJ).
This is the northern branch of the jet stream that sags southward over the United States during the wintertime. A more active jet stream means more troughs and ridges, resulting in more active weather at the surface and more storm systems. It also results in more variations in temperatures from cool to warm and back again. Sustained cold (below normal temperatures) typically settles in across the north-central United States, while warmer air settles in south. The Great Lakes, Midwest, and Northeast get caught somewhere in between!
La Nina years typically bring a wetter pattern to the Great Lakes and Mid-Atlantic as
well, but this doesn’t always result in snowier years. So, what does this mean for snow contractors? In weaker La Nina years, above average snowfall typically occurs across most of the Snow Belt. In stronger La Nina years, the above normal snowfall is focused primarily across the Pacific Northwest, the Upper Midwest, and New England.
Outside of these initial observations, Beth says its still too early make an accurate prediction as to what Winter 2020-21 will end up looking like. TDS starts putting together its outlook in mid-October for a preliminary report around Nov. 1 and its final winter outlook around Dec. 1.
Mike Zawacki is editor of Snow Magazine. You can reach him at firstname.lastname@example.org