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Seasonal to Interannual Forecasts

North American Extreme Weather Risk Forecasts

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Experimental short lead-time (7-14 day) extreme weather risk assessment forecasts are now available for the United States and Canada. The forecasts are based on observed relationships between the probability of certain extreme weather events in North America and variations in the Pacific North American (PNA), Northern Annual Mode (NAM), and El Niño/Southern Oscillation circulation patterns. These pages focus specifically on the relationship between extreme weather risk and the PNA pattern.

Extreme events forecasted by this method are:

An event is characterized as “extreme” if it exceeds ± 1.5 standard deviations from the long-term mean on a particular day.

Current PNA Conditions

See the National Weather Service's Climate Prediction Center PNA web page for current PNA conditions.

PNA Forecast

Determining the current forecast for the risk of extreme weather events in the Pacific Northwest (PNW) (or other parts of the country) is a two step process:

STEP 1: Obtain the most recent forecast of the phase of the PNA.

STEP 2: Obtain information about the likelihood of the extreme event of interest under the forecasted phase of the PNA.

You can look up general information about the likelihood of different extreme events in the PNW in the table below. Click on the links in the table to obtain information about a specific location in the region. Alternatively, you can look up information on all types of extreme events for a specific location. Similar products have been developed for the entire United States and for Canada.

Relative probability of extreme weather events in the Pacific Northwest
Extreme Weather Event Positive PNA Days
(PNA index > +1)		 Negative PNA Days
(PNA index < -1)		 Definitions and Thresholds

Snow
For information about the historical occurrence of snowfall for Pacific Northwest (PNW) locations, view statistics on maps.

For more details about the historical distribution of snowfall events, select an individual station from a list or from a map.

Snow has never occurred (since 1948) in the Puget Sound lowlands on a positive PNA day (see map).

High snow events are 2-4+ times more likely during negative PNA than positive PNA, depending on location (see map).

Snow is defined as total daily snowfall, measured as liquid water equivalent.

Extreme snow events are days on which the snow amount was in the top 20% of the 1948-1997 November-March days with measurable recorded snow.

Maps showing threshold values of extreme high snow events can be found here.

Daily high temperatures
For information about historical PNW daily maximum temperatures, view statistics on maps.

For details about the historical distribution of daily high temperatures, select an individual station from a list or from a map.

PNW locations are 2-4 times more likely to experience an extremely high daily maximum temperature on positive vs. negative PNA days, depending on location (see map).

PNW locations are 2-4 times more likely to experience an extremely low daily maximum temperature on negative vs. positive PNA days, depending on location (see map).

Extreme high daily maximum temperature events are defined as days on which the daily recorded maximum temperature was in the highest 1/15th (top 6.7%) of all the 1948-1997 November-March TMAX observations.

Extreme low daily maximum temperature events are defined as days on which the daily recorded maximum temperature was in the lowest 1/15th (bottom 6.7%) of all the 1948-1997 November-March TMAX observations.

Maps showing threshold values of extreme high and low daily maximum temperatures can be found here.

Daily low temperatures
For information about historical PNW daily minimum temperatures, view statistics on maps.

For details about the historical distribution of daily low temperatures, select an individual station from a list or from a map.

PNW stations are 2-4+ times more likely to experience an extremely high daily minimum temperature on positive vs. negative PNA days, depending on location (see map).

PNW stations are 2-4+ times more likely to experience an extremely low daily minimum temperature on negative vs. positive PNA days, depending on location (see map).

Extreme high daily minimum temperature events are defined as days on which the daily recorded minimum temperature was in the highest 1/15th (top 6.7%) of all the 1948-1997 November-March TMAX observations.

Extreme low daily minimum temperature events are defined as days on which the daily recorded minimum temperature was in the lowest 1/15th (bottom 6.7%) of all the 1948-1997 November-March TMAX observations.

Maps showing threshold values of extreme high and low daily minimum temperatures can be found here.

High precipitation
For information about historical PNW precipitation, view statistics on maps.

For details about the historical distribution of precipitation events, select an individual station from a list or from a map.

Small variable effect on extreme high precipitation events depending on location (see map).

Variable effect on precipitation events depending on location (see map).

Precipitation (PRCP) is defined as total daily liquid water equivalent.

Extreme high PRCP events are defined as days in which the PRCP amount was in the top 20% of the 1948-1997 November-March days with measurable recorded PRC.

Maps showing extreme high PRCP threshold values can be found here.

Recent conditions

NOAA's National Center for Environmental Prediction shows the values of the PNA index for the past 120 days here (the black line in the first panel of the figure).