Originally by Jason Samenow on October 31, 2004
I'll preface this by cautioning readers that this is an outlook, as opposed to a forecast. It conveys the most likely set of conditions that may affect our region this winter. It is not intended to be as detailed or precise as a next-day forecast. But based on the prevailing climate patterns and lessons learned from the past, it is my best assessment of what the winter may entail.
This year, we're under the influence of a weak El Nino. During strong El Nino events, anything can happen. For example, during the strong El Nino winters of 1983 and 1987, we received much above average snowfall, but very little snow fell during the significant El Nino event of 1992.
Weak El Ninos, on the other hand, tend to have more predictable effects for our area. In general, they're on the wintry side. The last winters with weak El Ninos occurred in 2003 (very snowy and cold) and 1994 (very cold and icy).
During weak El Ninos, we tend to see split flow over the U.S.- where you have a strong southerly jet stream that brings plenty of disturbances across the southern tier of the U.S. This will probably be conducive to overrunning events, where disturbances slide to our south, producing a shield of light to moderate precipitation that overspreads our area and interacts with cold air at the surface. Depending on the depth of the cold air, we'll see our share of snow and/or ice, which may change to rain depending on the prevailing storm track.
The depth of the cold air and the prevailing storm track will be largely dependent on the dominant phase of the North Atlantic Oscillation (NAO). Throughout the late summer and early fall, the NAO was largely in the positive phase (creating favorable conditions for hurricanes). But since mid September, the NAO has become negative. When the NAO is negative, the east coast experiences more cold air outbreaks and generally snowy conditions. If the NAO stays largely negative, it will be quite a winter for snow hounds. But we don't have much skill in forecasting the phase of NAO.
Temperatures will be largely dependent on the dominant phase of the NAO. If it's positive, it will be milder than average. If it's negative, it will be colder than average. As I said, I really have no idea which phase will dominate. However, weak El Ninos do tend to be on the cold side in our area, so I will forecast slightly colder than average temperatures. The graph below shows a probabilistic forecast indicating I believe there is a 70% chance temperatures will be at or below average levels this winter.
Given the likelihood of a strong southern jet stream (related to the weak El Nino), it's probably going to busy tracking all of the disturbances moving across the South. It's not clear how much of the moisture from those disturbances will make it into our region. I am fairly confident that area to our south (southern Maryland, southern Virginia and the Carolinas) will see above average precipitation. For us it could go either way. Snow lovers may be frustrated as storm after storm slides to our south and out to sea. By the same token, it would just take one or two southern stream disturbances to 'hook-up' with the northern stream producing major snow and/or ice storms. The graph below shows my snowfall forecast. Our average snowfall is about 18". I think there's a 60% chance of average or below average snowfall. But the chance of above average or much above average snowfall is still considerable. I'll call for 15" of snow to fall at National Airport, but it will seem like more than that due to the number of days with snow and/or ice/sleet.
My forecast is not dissimilar to the National Weather Service's and AccuWeather's. We're all looking at and interpreting the same information. But hopefully I've provided more detailed information, tailored to our area.