Video and text provided by Eric Snodgrass, Nutrien Ag Solutions, Principal Atmospheric Scientist
The Fall and Winter jet stream pattern for 2020 has been a bit wild. October was the 14th coldest on the 126-year record but it was quickly followed by a very warm November that ranks as the 8th warmest. As we transition into winter, the greatest concern will be over drought expansion from the western US. The bottom of Figure 1 shows the US Drought Monitor as of December 10, 2020, and the expansive drought across the western US has spread into the Great Plains. The lack of a strong sub-tropical branch of the jet stream has kept the southwestern US away from an active weather pattern and if this pattern doesn’t change throughout January, February, and March, we must be prepared for drought expansion to the east in Spring 2021. Heavy snows at times in early December that formed in Southern Kansas and Oklahoma have helped the drought situation there, but much of the Great Plains is still very dry compared to normal.
Figure 1. Statewide Average Temperature Ranks for October 2020 (top left) and November 2020 (top right). Source: https://www.ncdc.noaa.gov/temp-and-precip/us-maps/ (Bottom) US Drought Monitor on Dec 10, 2020 Source: https://droughtmonitor.unl.edu/
La Niña conditions, marked by cold water in the equatorial Pacific Ocean, developed quickly this fall and are about to reach a peak as we approach the New Year. As the La Niña begins to fade, we can expect the coldest air to be anchored over Alaska and the Canadian Prairie as more mild air (compared to normal) will likely persist along the southern tier of the US. Nearly all long-range weather forecast models are consistent with the temperature and precipitation patterns forecast by the IRI Seasonal Forecast Group from Columbia University. Notice in Figure 3 how drier conditions are forecast for January-March along the Southern US while an active winter storm track will likely develop from Missouri through the Ohio River Valley. The temperature forecast for the remainder of winter is projected to be closer to average with the coldest air coming from the northwest throughout the winter months. The Polar Vortex is quite strong in early winter which tends to keep the arctic air tucked away over Alaska, Greenland, and Northern Canada, but we need to be watching carefully for any disruptions in the Polar Vortex for January and February.
Figure 2. Dec 15, 2020 Sea Surface Temperature Anomalies. Source: https://www.ospo.noaa.gov/Products/ocean/sst/anomaly/
Figure 3. IRI Multi-Model temperature (left) and precipitation (right) anomaly forecast for January-March 2021. Source: https://iri.columbia.edu/our-expertise/climate/forecasts/seasonal-climate-forecasts/
Parts of South America have had a very dry start to the growing season as the Brazilian Monsoon was late in developing and then inconsistent in intensity. Three very notable dry time periods at the beginning of November, end of November, and again in Mid-December have regions like Mato Grosso in Central Brazil experiencing a 20+ inch rainfall deficit. More normal rains are forecast for January but there are concerns that with the late-planted crop and the potential for heavier rains in January and February, there could be delays in the harvest. It is not yet known how large of an impact the drier weather has had on yields and total production, but the potential for a large crop still exists due to a sizeable increase in planted acres. Figure 4 shows the recent rainfall deficit data for South America as well as regional rainfall analysis over the last 180-days for Mato Grosso. Figure 5 shows the Jan-Mar 2021precipitation anomaly forecast from the ECMWF. Notice how dry it could potentially be in Argentina.
Figure 4. (Right) 30-day precipitation anomalies from Nov 13 – Dec 12, 2020. On the left are the 180-day cumulative precipitation amounts (top), and daily precipitation totals over the last 180-days (bottom). Source: NOAA CPC Created by Snodgrass
Figure 5. ECMWF January-March Seasonal Precipitation Anomaly forecast for South America. Source: ECMWF and www.weatherbell.com
