Seasonal Climate Forecast September – November 2015

Issued: August 20, 2015

A cooperative product between the Oregon Department of Agriculture (ODA), and the Oregon

Department of Forestry (ODF).

Contact: ODF Meteorologist Pete Parsons at 503-945-7448 or peter.gj.parsons@oregon.gov

Get related Seasonal Climate Forecast information at: http://www.oregon.gov/ODA/programs/NaturalResources/Pages/Weather.aspx

El Niño Southern Oscillation (ENSO) Current Status and Forecast

n  The Climate Prediction Center (CPC) has issued an El Niño Advisory. Equatorial Pacific Ocean SSTs have continued to warm and have entered the moderate El Niño range. n  The current Oceanic Niño Index (ONI) is +1.0°C. The CPC thresholds for a weak, moderate, and strong El Niño are an ONI of +0.5°C, +1.0°C, and +1.5°C respectively. n CPC forecaster consensus unanimously favors El Niño becoming strong and peaking this late-fall/early-winter.

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf

Forecast Method Notes…

n  The top analog years (1987; 1991; 2004) used to create this forecast were all years with developing El Niño (like this summer), following a winter with positive ONI anomalies (like last winter). n  The best analog “match” to this year is 1987, but all three years were given equal weight in the forecast graphics. n  This forecast is based solely on historical weather data and does not utilize dynamic modeling (see Forecasting Methods).

Forecast Notes of Interest…

n  2015 has proven to be quite rare, in terms of Pacific Ocean SSTs. The corresponding weather patterns have been “extreme,” with Oregon experiencing record or near-record warm and dry conditions since last winter. n  SST records, going back to 1950, show only one other year (1987) with an ONI for the May-June-July period of +1.0 or greater (like this year) following a winter with positive ONIs (like last winter). 1987 also exhibits the best “match” to the incredibly high PDO values of the current year. n  The predicted strong El Niño for this fall/winter favors a continuation of warmer and drier-than-average weather, especially across the northern zones.

Pacific Ocean Animated (in PowerPoint only) SSTs (top) / Anomalies (bottom)

Courtesy: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_update/gsstanim.shtml

Well above average SSTs extend across the eastern Gulf of Alaska and most of the tropical Pacific Ocean.

Tropical Pacific Ocean El Niño has strengthened into the “moderate” range...

Courtesy: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_update/sstweek_c.gif

SSTs are well above average across most of the tropical Pacific Ocean

ENSO-neutral

Tropical Pacific Ocean

El Niño

La Niña

(2003-04; 1990-91; 1986-87)

Weak El Niño “Modoki” last winter El Niño is currently

strengthening...

ENSO-neutral

El Niño has strengthened into the

moderate range

El Niño

Tropical Pacific Ocean

(2003-04; 1990-91; 1986-87)

La Niña

Weak El Niño “Modoki” last winter.

Weak

Moderate

Strong

Average

North Pacific Ocean

Warm

Cool

(2003-04; 1990-91; 1986-87) Last Winter Had The Highest PDO Values Since 1997

PDO is rising again…

Model forecasts favor ENSO-neutral conditions through spring 2014; followed by El Niño development.

ENSO Predictive Models El Niño is predicted to become strong this fall and winter…

“Base” Graphic Courtesy: http://iri.columbia.edu/our-expertise/climate/forecasts/enso/current/

La Niña

El Niño

ENSO-neutral

Chart Updates

n  A time-series graphical comparison of “PDO” values was recently added. n  Predicted upper-air patterns and anomalies have also been added to help explain the reasons for the corresponding temperature and precipitation forecasts. n  Precipitation forecasts have changed from “departure from average” (in inches) to “percent of average” (with 100% being average). n  The color palettes have been updated for both the temperature and precipitation anomaly graphics.

September 2015 Forecast Mean Upper-Air Pattern Upper-Air Anomalies

n  Upper-level ridging is likely to be stronger than average over the NW US and SW Canada.

n  A strengthening “split-flow” westerly jet stream is likely, which will tend to weaken storm systems, as they approach the Oregon coast.

September 2015 Forecast Temperatures Precipitation

n  Expect a continuation of above average temperatures and below average precipitation.

n  However, there is a tendency, during a moderate-to-strong El Niño, for at least one cool and possibly damp period in September.

October 2015 Forecast Mean Upper-Air Pattern Upper-Air Anomalies

n  Stronger than average upper-level ridging is likely over the NW US. n  Early-autumn storms will have a tendency to “split,” as they move

onshore, with most of their precipitation being directed north, into Canada, and south, into California.

October 2015 Forecast Temperatures Precipitation

n  Analog years suggest a continuation of generally mild and dry weather. n  This “El Niño-driven” climate signal typically becomes more evident in

the fall and winter months. n  Northern zones will likely see the driest conditions (relative to average).

November 2015 Forecast Mean Upper-Air Pattern Upper-Air Anomalies

n  Anomalous upper-level ridging is predicted to be centered over extreme SW Canada, which would weaken approaching fall storms.

n  A “split-flow” jet stream pattern will likely direct considerable storm energy south of Oregon, into California.

November 2015 Forecast Temperatures Precipitation

n  No major change - continued generally warmer and drier than average. n  It is possible that the number of days with rain/snow will be near

average but with decreased precipitation totals. n  The likelihood of valley fog episodes is elevated.

September – November 2015 Forecast Mean Upper-Air Pattern Upper-Air Anomalies

n  Strong anomalous upper-level ridging is predicted to be centered just off the southern BC coast.

n  Expect a “split-flow” jet stream pattern to emerge and strengthen, with storm systems spreading apart, as they approach the Oregon coast.

September – November 2015 Forecast Temperatures Precipitation

n  An El Niño in the tropical Pacific Ocean typically yields warmer and drier-than-average weather for Oregon during the autumn season.

n  However, significant cool periods are possible, including brief early-season Arctic outbreaks.

Forecast Resources n  CPC Official US Three-Month Forecasts (Graphics): http://www.cpc.ncep.noaa.gov/products/predictions/long_range/seasonal.php?lead=01

n  CPC US 30-Day & 90-Day Forecasts (Discussions): http://www.cpc.ncep.noaa.gov/products/predictions/long_range/fxus07.html

n  CPC Weekly & Monthly ENSO Discussions: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory

n  Australian Government Weekly Tropical Climate Note: http://www.bom.gov.au/climate/tropnote/tropnote.shtml

n  Australian Government ENSO Wrap-Up: http://www.bom.gov.au/climate/enso

n  IRI ENSO Quick Look: http://iri.columbia.edu/our-expertise/climate/forecasts/enso/current/

n  ODA Seasonal Climate Forecast Home: http://www.oregon.gov/ODA/programs/NaturalResources/Pages/Weather.aspx

Water Supply Information n  NDMC U.S. Drought Monitor: http://droughtmonitor.unl.edu/

n  NIDIS North American Drought Portal: http://www.drought.gov/nadm/content/percent-average-precipitation

n  NRCS Snow Water Equivalent Oregon Map: http://www.wcc.nrcs.usda.gov/ftpref/data/water/wcs/gis/maps/or_swepctnormal_update.pdf

n  NRCS Snow Water Equivalent Products: http://www.wcc.nrcs.usda.gov/snow/snotel-wereports.html

n  NRCS Weekly Water and Climate Update: http://www.wcc.nrcs.usda.gov/cgibin/water/drought/wdr.pl

n  NRCS Western Snowpack Data & Water Supply Forecast: http://www.wcc.nrcs.usda.gov/cgibin/westsnowsummary.pl

n  WRCC WestWideDroughtTracker: http://www.wrcc.dri.edu/wwdt/

Updated Monthly (around the 20th)

Contact: Pete Parsons, ODF Meteorologist at 503-945-7448 or peter.gj.parsons@oregon.gov

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