Tuesday, November 20, 2007
2007-08 CapitalWeather.com Winter Outlook
Matt Ross @ 11:00 AM
As the days grow colder and shorter, it comes time for our annual Winter Outlook. While great advances have been made in seasonal forecasting, there is still a large deal of uncertainty in forecasting in the long range. Please keep in mind that this is a low confidence forecast, especially the monthly breakdowns. Enjoy!
The following factors were considered in making this forecast:
The Pacific Ocean
After a weak/moderate El Nino event last winter we are currently in the midst of a moderate La Nina event, the 1st in 8 years. This event shows no real signs of weakening and could further strengthen through the end of the year. Best indications are that a moderate/strong event will persist through much of the winter with some weakening possible in February/March.
The Atlantic Ocean
The Atlantic this winter will be a battle of 2 competing elements, the North Atlantic Oscillation (NAO) and the Southeast Ridge, a strong ridge of high pressure off the southeast coast often centered over Bermuda. Although there will be plenty of variability throughout the winter, we will have extended periods of high latitude blocking which supports a colder/stormier pattern for the east. Aided by a strong easterly Quasi-biennial Oscillation (QBO), the NAO should finish the December - March period near normal or somewhat negative. However the moderate La Nina pattern also correlates strongly with a southeast ridge. A southeast ridge can be helpful in bringing storms up the coast when combined with other factors, but it is typically associated with warmer weather in the Carolinas and Mid-Atlantic.
Summer/Fall Pattern
This summer saw very warm anomalies centered over the western part of the nation which moved to the Ohio Valley and Mid Atlantic for the end of summer and start of autumn. Here in DC we averaged about 1.3 degrees above normal for the summer, but the August through October period featured average temperatures over 4 degrees above normal in DC. Currently it looks like November will finish near or below normal. While we have made great strides in countering our own drought conditions, the summer/fall has seen severe drought conditions persist in the southeast. Hurricane season featured 14 named storms (slightly above average) with most recurving or tracking south of the US.
Based on the above factors, the following winters emerged as analogs for this upcoming winter.
Primary: 1970-71
Secondary: 1942-43, 1954-55, 1973-74, 1998-99
Others Considered: 1892-93, 1949-50, 1988-89
This winter will be one of huge variability. While we definitely will see colder/stormier periods, the traditional heart of winter could be very warm. March will likely be the coldest month with respect to normal with February the warmest. December and/or March is likely to see above average snowfall with below average snow for January/February. Snow lovers may find solace in the fact that several of the above analogs featured 8"+ snowstorms for DC and we believe that the risk of a moderate/big snowstorm is higher than usual, though still less than 50%. The bust potential for this forecast is also higher than usual as we will often be caught between competing weather regimes, with dry, warmer conditions in the Carolinas and what may be a cold and snowy winter for Southern New England and the Northeast.
Overall 2007-08 Winter Temperatures: Normal to +1 (degrees F)
December: Normal to +1 (degree above normal)
January: Normal to +1
February: +1 to +2
March: -1 to -2 (degrees below normal)
Snowfall: Below Normal, but near the median
National Airport (DCA): 12"
Dulles Airport (IAD): 18"
Baltimore-Washington Airport (BWI): 18"
Fairfax/Loudoun/Montgomery Counties: 15-20"
DC/Arlington/Alexandria/Prince George's County: 12-16"
Factors Considered
The following factors were considered in making this forecast:
The Pacific Ocean
After a weak/moderate El Nino event last winter we are currently in the midst of a moderate La Nina event, the 1st in 8 years. This event shows no real signs of weakening and could further strengthen through the end of the year. Best indications are that a moderate/strong event will persist through much of the winter with some weakening possible in February/March.
The Atlantic Ocean
The Atlantic this winter will be a battle of 2 competing elements, the North Atlantic Oscillation (NAO) and the Southeast Ridge, a strong ridge of high pressure off the southeast coast often centered over Bermuda. Although there will be plenty of variability throughout the winter, we will have extended periods of high latitude blocking which supports a colder/stormier pattern for the east. Aided by a strong easterly Quasi-biennial Oscillation (QBO), the NAO should finish the December - March period near normal or somewhat negative. However the moderate La Nina pattern also correlates strongly with a southeast ridge. A southeast ridge can be helpful in bringing storms up the coast when combined with other factors, but it is typically associated with warmer weather in the Carolinas and Mid-Atlantic.
Summer/Fall Pattern
This summer saw very warm anomalies centered over the western part of the nation which moved to the Ohio Valley and Mid Atlantic for the end of summer and start of autumn. Here in DC we averaged about 1.3 degrees above normal for the summer, but the August through October period featured average temperatures over 4 degrees above normal in DC. Currently it looks like November will finish near or below normal. While we have made great strides in countering our own drought conditions, the summer/fall has seen severe drought conditions persist in the southeast. Hurricane season featured 14 named storms (slightly above average) with most recurving or tracking south of the US.
Analogs
Based on the above factors, the following winters emerged as analogs for this upcoming winter.
Primary: 1970-71
Secondary: 1942-43, 1954-55, 1973-74, 1998-99
Others Considered: 1892-93, 1949-50, 1988-89
The Forecast
This winter will be one of huge variability. While we definitely will see colder/stormier periods, the traditional heart of winter could be very warm. March will likely be the coldest month with respect to normal with February the warmest. December and/or March is likely to see above average snowfall with below average snow for January/February. Snow lovers may find solace in the fact that several of the above analogs featured 8"+ snowstorms for DC and we believe that the risk of a moderate/big snowstorm is higher than usual, though still less than 50%. The bust potential for this forecast is also higher than usual as we will often be caught between competing weather regimes, with dry, warmer conditions in the Carolinas and what may be a cold and snowy winter for Southern New England and the Northeast.
Overall 2007-08 Winter Temperatures: Normal to +1 (degrees F)
December: Normal to +1 (degree above normal)
January: Normal to +1
February: +1 to +2
March: -1 to -2 (degrees below normal)
Snowfall: Below Normal, but near the median
National Airport (DCA): 12"
Dulles Airport (IAD): 18"
Baltimore-Washington Airport (BWI): 18"
Fairfax/Loudoun/Montgomery Counties: 15-20"
DC/Arlington/Alexandria/Prince George's County: 12-16"
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