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Seasonal Forecast October – December 2021 Issued: September 16, 2021

Contact: ODF Lead Meteorologist Pete Parsons 503-945-7448 or [email protected]

Oregon Department of (ODA) - Oregon Department of Forestry (ODF) Production support: Diana Walker; Jacob Cruser; Andy Zimmerman; Julie https://www.flickr.com/photos/canonshott/sets/72157654242713168 El Niño Southern Oscillation (ENSO) Current Status and Forecast n The June – August 2021 Oceanic Niño Index (ONI) of -0.4°C is on the border between ENSO-Neutral & weak La Niña. n The ONI is a 3- running mean and lags real- SSTS, which are cooling and very near La Niña conditions. n NOAA’s Climate Prediction Center (CPC) expects ENSO-Neutral conditions to transition to La Niña during the next couple of , with La Niña continuing through this coming .

IMPORTANT NOTE: Beginning with the October 2017 update, ONI values use ERSSTv5 data (Huang et al. 2017, J. Climate, vol. 30, 8179-8205).

https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf Forecast Highlights n The analog (1971, 1996, & 2008) have been performing well the past couple of month and remained unchanged from last month. They have fairly-diverse solutions early in this forecast period but show a more consistent signal (relatively cool and wet) by December. n Expect alternating cool and mild periods in October and November, but the chances for below-average , -air outbreaks, and significant valley snowfall increase dramatically in December. n Above-average should bring relief from the severe-to- exceptional drought conditions covering most of Oregon, including near-to-above average snowfall.

Disclaimer: This forecast is based on past and current data and is not associated with CPC predictions (see “Forecasting Methods…” at: https://oda.direct/Weather) nor the official CPC “Three-Month Outlooks,” which are available here: https://www.cpc.ncep.noaa.gov/products/predictions/long_range/seasonal.php?lead=1 Pacific Animated (PowerPoint only) SSTs (top) / Anomalies (bottom)

Real-time tropical Pacific SSTs have cooled close to La Niña range

Courtesy: https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_update/gsstanim.shtml Tropical Pacific Ocean Cool SST anomalies are approaching La Niña

SSTs are near-to-below average across most of the tropical Pacific

Courtesy: https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_update/sstweek_c.gif Tropical Pacific Ocean

(1970-71; 1995-96; 2007-08) Jun. – Aug. ONI (-0.4°C) Strong El Niño was in the Moderate ENSO-Neutral range. Weak

ENSO-Neutral

Weak Moderate Jun. – Aug. Strong La Niña analogs ranged from La Niña to ENSO-Neutral Tropical Pacific Ocean

(1970-71; 1995-96; 2007-08) August La Niña analogs in weak La Niña range

ENSO-Neutral

August SOI (+0.6) borderline El Niño ENSO- Neutral / weak La Niña North Pacific Ocean (Poleward of 20°N )

July PDO not (1970-71; 1995-96; 2007-08) available August 2021 PDO (-1.12) Warm was in the “Cool” zone

Neutral

August analogs ranged from Cool “Neutral” to “Cool.” SST Anomalies Comparison August Analog Composite August 2021

n The August analog composite (left) has a similar SST anomaly pattern compared to the August 2021 plot (right) but is generally cooler. n Those differences could reflect the overall warming of the climate during that period, which adds error to an analog-based forecast.

October 2021 Forecast Mean Upper-Air Pattern Upper-Air Anomalies

n October 1971 and 1996 had anomalous troughing over Oregon, while 2008 had anomalous ridging. That reduces forecast confidence. n The blend (above) yields near average conditions, which is somewhat misleading, due to the wide range in analog solutions. October 2021 Forecast Temperatures Precipitation

n Analogs favor slightly below-average temperatures. n Precipitation is expected to range from near-to-above average north and west to slightly below average south and east. November 2021 Forecast Mean Upper-Air Pattern Upper-Air Anomalies

n 1971 & 1996 had near-average conditions aloft over Oregon, while 2008 maintained anomalous ridging. n Although this pattern favors relatively mild conditions, it may also bring much-needed precipitation to the state. November 2021 Forecast Temperatures Precipitation

n Near or above average temperatures likely. n 1971 & 1996 were relatively wet and 2008 was dry. A blend suggests a transition from above-average precipitation north and west to near average precipitation south and east. December 2021 Forecast Mean Upper-Air Pattern Upper-Air Anomalies

n The analog years were consistent in showing anomalous troughing over Oregon, with the strongest negative anomalies in 1971 and 1996. n 1971 and 2008 had more ridging in the Gulf of Alaska than 1996, but confidence is high in the general anomaly pattern shown above. December 2021 Forecast Temperatures Precipitation

n Near-to-below-average temperatures. 1971 and 2008 had low-elevation cold-air outbreaks with significant valley snowfall. n Precipitation and mountain snowfall near or above average. n Heightened chances for cold outbreaks & low-elevation snowfall. October – December 2021 Forecast Mean Upper-Air Pattern Upper-Air Anomalies

n 1971 & 1996 had anomalous troughing over Oregon, while 2008 maintained anomalous ridging until December. n A blend of the analogs (above) shows anomalous troughing over the Pac NW. Highest confidence for anomalous troughing is in December. October – December 2021 Forecast Temperatures Precipitation

n A blend of the analogs yields minimal 3-month departures, but December favors colder-than-average conditions. n Above-average precipitation and mountain snowfall should begin to bring drought (next slide) relief. U.S. Drought Monitor National Drought Mitigation Center (NDMC) https://droughtmonitor.unl.edu/ Forecast Resources n CPC Official US Three-Month Forecasts (Graphics): https://www.cpc.ncep.noaa.gov/products/predictions/long_range/seasonal.php?lead=01 n CPC US 30-Day & 90-Day Forecasts (Discussions): https://www.cpc.ncep.noaa.gov/products/predictions/long_range/fxus07.html n CPC Weekly & Monthly ENSO Discussions: https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory n Australian Government Summary: http://www.bom.gov.au/climate/model-summary/#region=NINO34&tabs=Overview n Australian Government ENSO Wrap-Up: http://www.bom.gov.au/climate/enso n IRI ENSO Quick Look: https://iri.columbia.edu/our-expertise/climate/forecasts/enso/current/ n ODA Seasonal Climate Forecast Home: https://www.oregon.gov/ODA/programs/NaturalResources/Pages/Weather.aspx Supply Information n NDMC U.S. Drought Monitor: https://droughtmonitor.unl.edu/ n NIDIS North American Drought Portal: https://www.drought.gov/nadm/content/percent-average-precipitation n NRCS Water Equivalent Oregon Map: https://www.wcc.nrcs.usda.gov/ftpref/data/water/wcs/gis/maps/or_swepctnormal_update.pdf n NRCS Snow Water Equivalent Products: https://www.wcc.nrcs.usda.gov/snow/snotel-wereports.html n NRCS Weekly Water and Climate Update: https://www.wcc.nrcs.usda.gov/cgibin/water/drought/wdr.pl n NRCS Western Data & Water Supply Forecast: https://www.wcc.nrcs.usda.gov/cgibin/westsnowsummary.pl n WRCC WestWideDroughtTracker: https://www.wrcc.dri.edu/wwdt/ Updated Monthly (Around the 20th)

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