Winter 07-08 Steve Todd Meteorologist In Charge National Weather Service Portland, OR
Overview Winter Weather Outlook How to stay informed
Winter Outlook LaNina conditions are present across the tropical Pacific basin. Model forecasts indicate a weak to moderate LaNina will continue well into 2008. Recent data and model forecasts indicate LaNina conditions will likely strengthen during the next several months.
Current Conditions Sea Surface Temperature Departures ( o C) Equatorial Pacific SSTs are -2ºC to -3ºC below average between 120ºW and the South American coast, and -0.5ºC to -1.0ºC below average near the Dateline. Sea Surface Temperature Anomalies (Oct 7 Nov 3)
Recent Evolution of Equatorial Pacific SST Departures ( o C) DEC 2006 El Niño disappeared rapidly in January 2007. Time Negative SST anomalies now extend from the Date Line to the west coast of South America. Oct 2007 Longitude
Niño Region SST Departures ( o C) Recent Evolution The latest weekly SST departures are -1.6ºC in the Niño 3 region and -2.2ºC in the Niño 1+2 region. SST departures are -1.2ºC in the Niño 3.4 region and -0.5ºC in the Niño 4 region.
Tropical Circulation during LaNina
The Effects of La Niña La Niña episodes are associated with three prominent changes in the wintertime atmospheric flow across the eastern North Pacific and North America. An amplification of the climatological mean wave pattern and increased meridional flow across the continent and the eastern North Pacific. Increased blocking activity over the high latitudes of the eastern North Pacific. A highly variable strength of the jet stream over the eastern North Pacific, with the mean jet position entering North America in the northwestern United States/ southwestern Canada.
Global Effects of La Niña The effects of La Nina are most evident in each hemisphere s cold season when the jet stream is strongest. Some of the most reliable effects are suppressed rainfall and drought over the central Pacific, Peru, Ecuador, and the southern United States and enhanced rainfall in northwestern U.S., Indonesia, Australia, southern Africa, and northeastern Brazil. These effects, while looking to be the mirror image of those associated with El Niño, do have some slight differences.
Historical El Niño and La Niña episodes Warm Episodes max Cold Episodes min JAS 1951 - NDJ 1951/52 0.7 MAM 1957 MJJ 1958 1.6 JJA 1963 DJF 1963/64 1.0 MJJ 1965 MAM 1966 1.6 OND 1968 AMJ 1969 1.0 ASO 1969 DJF 1969/70 0.7 AMJ 1972 FMA 1973 2.1 ASO 1976 JFM 1977 0.8 ASO 1977 - DJF 1977/78 0.8 AMJ 1982 MJJ 1983 2.3 JAS 1986 JFM 1988 1.6 AMJ 1991 MJJ 1992 1.8 FMA 1993 JJA 1993 0.8 MAM 1994 FMA 1995 1.3 AMJ 1997 MAM 1998 2.5 ASO 1949 FMA 1951-1.8 MAM 1954 DJF 1956/57-2.1 ASO 1961 MAM 1962-0.6 MAM 1964 JFM 1965-1.1 SON 1967 MAM 1968-0.9 JJA 1970 DJF 1971/72-1.4 AMJ 1973 JJA 1974-2.0 ASO 1974 AMJ 1976-1.8 ASO 1983 DJF 1983/84-0.9 SON 1984 MJJ 1985-1.1 AMJ 1988 AMJ 1989-1.9 ASO 1995 FMA 1996-0.8 JJA 1998 MJJ 2000-1.6 SON 2000 JFM 2001-0.7 AMJ 2002 FMA 2003 1.5 JJA 2004 JFM 2005 0.9
Pacific NW LaNina Events (1950-2002) Precipitation Temperature Percentage of: above normal normal below normal
U.S. Precipitation Departures for Oct-Dec and Dec-Feb Oct-Dec La Niña Dec-Feb Weak Moderate/ Strong All episodes
So, What s the Forecast?
Factors Influencing Forecast ENSO Trends (10 yr temp, 15 yr precip) NAO & PNA Tropical 30-60 Day Oscillations (i.e. Madden Julian) Surface Conditions (snow, wet or dry soils, etc.) Statistical Forecast Tools Dynamic Forecast Models
Winter Outlook 2007/2008 Probability Forecasts -- how to use Above 33% Normal 33% Below 33% Example: 40 contour for above normal precipitation = 40% probability of above normal temperatures EC = equal chances of any 3 categories
Seasonal Outlook (December 07- February 08) Precipitation Temperature
Seasonal Outlook (March May 2008) Precipitation Temperature
Winter Forecast for Pacific NW La-Nina conditions lasting into 2008 Wetter than normal No clear signal on temperature departures (equal chances of below, normal, above normal temps) Precipitation Temperature (max) Temperature (min) Normal (PDX) 36.30 50 37 1995-1996 54.54 50 38 1998-1999 45.93 49 39 1999-2000 33.89 50 39 2000-2001 22.99 48 35
Staying Informed & Being Prepared NWS Weather Operations & Role OUTLOOK Weather patterns show potential for a weather event in the next 2 to 5 days WATCH Conditions are favorable for a severe weather event in the near future 12 hours up to 72 hours WARNING life or property threatening weather is occurring or imminent ADVISORY Weather effect is occurring or imminent that will cause significant inconvenience and if caution not taken may be life or property threatening
National Weather Service Product Stream READY SET GO Weather/Flood Outlook Weather/ Flood Watch Weather/ Flood Warning Time to onset of event (hours) 72 48 24 0 Increasing confidence that event will occur
Weather on the Web NWS Front Page weather.gov/portland Your link to real-time weather
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