WASHINGTON - For those who want us to get right to the point, here are the top 5 things you need to know about our winter forecast for the upcoming season:
#1: Overall, we favor a COLDER winter in the Washington, DC region compared to last year
#2: Overall, we favor an ABOVE NORMAL year for snowfall in Washington, DC (16-24 inches)
#3: Odds of a major snowstorm this season are not zero, but are BELOW NORMAL
#4: We believe the majority of this winter's snow will come from multiple enhanced CLIPPERS
#5: We do favor a PROLONGED winter with a cold start to spring
For those who wish for a more detailed explanation, then buckle up and let's dive in to how we came up with this forecast this year.
Before we do though, let's check and revisit out summer outlook and see just how we did. We posted out summer outlook on May 10, 2016 (which you can read here) and made the following predictions:
#1: A cool June but very hot July & August (Nailed it! After only 5 days above 90 degrees in June, July and August, each featured 23 days above 90 degrees and August was the second hottest in D.C. history!)
#2: 45-55 days above 90 degrees (Very close! While at one time worried we would fall short, we ended up too low with 58 total days above 90 degrees this year)
#3: 1-3 days above 100 degrees (Again very close as we ended up with 4)
#4: 12-15 Named Tropical Storms/Hurricanes (As of October 27, 2016: 14 have been named)
#5: 8-12 Hurricanes (As of October 27, 2016: There have been 6)
#6: 3-5 Major Hurricanes (As of October 27, 2016: There have been 3)
All things considered, we didn't do so badly this summer! Ask any forecaster though, and they will tell you that summer predictions are much easier to make then winter predictions. So without further ado, let's dive into what we expect from the upcoming winter.
Generally speaking, every winter forecast starts by looking at sea surface temperatures in the equatorial Pacific. This is because warm waters here can lead to more convection (thunderstorms) over the region, which can lead to an increase in winds over the region, which can interact with a feature we call the subtropical jet stream. This feature can be a major role in the formation, track and intensity of winter storms as the track across the eastern half of the country. The National Weather Service has been tracking sea surface temperatures in this region of the pacific since 1950. Snow records for Washington D.C. exist well before then. So looking at the top 25 snowstorms in D.C.'s history, only 13 of them come after 1950. Of these 13 storms, 11 of them (or 85 percent) came during El Niño winters, including last winter when a January blizzard dropped 17.8 inches or 80 percent of our 22.2 inches of snow last year in a 36-hour period.
This year, however, will not be an El Niño year. Current ocean temperature trends in the region and ocean models suggest a neutral to possibly weak La Niña winter coming our way. Simply put, La Niña means cooler waters in this region, which means a less active subtropical jet, which typically means a significantly lower probability of a big storm. This being the case,WE DO NOT favor a storm of the magnitude that hit the region last January to happen again this year. As we all know though, just because the weather person says something is not likely to happen, does not mean it will not happen. In fact, two of the region's biggest snowfalls came during La Niña winters. February 18-19, 1979 brought 18.7 inches of snow to Washington (third most on record) and January 6-8, 1996 brought 17.1 inches of snow.
By the numbers, of all winters since 1950 (when El Niño and La Niña records began), there have been 20 years of varying intensities of La Niña. Seventy percent of La Niña winters average less than normal snowfall for the D.C. region. As a whole, they average 12.4 inches of snow, 3 inches below the 30-year average of 15.4 inches. When we take this grouping and factor it down to just weak La Niña years (expected this winter), snowfall numbers do come up a bit with only 63 percent of those winters seeing below normal snowfall in D.C., with the overall average snowfall during those 11 winters being 14.5 inches of snow. When you dig a bit deeper into those numbers, you see that the majority of the individual snow events during those years were not caused by the "big" snow events, but more the small to medium size snow events.
Here are some more snow numbers for your consideration. In those 11 weak La Niña years, measurable snow fell on a total of 96 days. Only on 5 of those 96 days did snowfall exceed 4 inches for one snow event, and more than half of those were only from one winter: 1995-1996. In fact, the average measurable snowfall per event was just 1.5 inches of snow. This is the primary reason why we are not favoring a major individual snowstorm for the District of Columbia for the upcoming winter. Snow during years with this type of pattern tends to be more the "northern clipper" variety than the big, southern snow monsters that we occasionally get. It is not, however, the only factor that goes into our overall forecast. We are favoring above normal snowfall after all! So let us move on to the next big factor...
Gulf of Alaska & Coastal Waters
The biggest factor in our winter outlook this year is the presence of abnormally warm waters in the Gulf of Alaska. Commonly referred to as "the blob" due to the extent of the warm waters in the region, this feature has appeared from time to time and has been present for the previous three winters. The school of thought on this feature is that when other seasonal driving features are weak, such as El Niño or La Niña, that it can drive the winter pattern a little more. It does this by periodically promoting warm air influxes into Alaska and northwest Canada, leading to abnormally higher pressure in the upper levels of the atmosphere over the region. As these higher pressures "pulse" northward, they displace colder air from the polar regions of northern Canada and the Arctic and force them southward into the eastern half of the United States.
The presence of this feature during the winters of 2013-2014 and 2014-2015 are believed to have been a major cause of the cold experienced during those winters - recall the term "polar vortex" being often used. While the blob was present during the last winter, 2015-2016, the presence of a rare extreme El Niño event overwhelmed the pattern and led to well above normal temperatures for our region. However, its influence was still felt in January when cold air traveled down from Canada just as a storm system came up from the south, giving us our historic blizzard last January. Going back to 1950, I've found 17 cases with abnormally warm waters in this region. Of these 17 cases, 14 of them had colder than normal winters in the D.C. region. That's 82 percent - very high as far as seasonal forecasters are concerned. When we filter these cases down for the expected neutral to weak La Niña cases, 7 of 8 of them were cold in our region. While there are other reasons for the cold winter outlook, this is the biggest.
Does the blob play any role in our snowfall though? Back in my college days, a wise professor of mine once told me "forecast the cold and the snow will come." Cold is the first ingredient in a recipe for snow after all! However, the analogs for snow are not as strong. Only 60 percent of all 17 years show above normal snow, and when filtered down to the 8 cases expected to me most similar to this year, the results are a 50 percent coin flip. So we had to take other features into account, including warmer waters along the West Coast, East Coast and in the Baja region of Mexico. What we found is that years where these features are all present tend to see a little more snow then when cooler waters are present. Generally speaking, clippers that cross the country tend to have a little more moisture present and can potentially strengthen as they get reach the warm Atlantic. For these reasons, we are favoring above normal snowfall, but not extreme snowfall.
Other factors that were considered when creating our forecast that I will not go into extreme detail in here include:
#1: Faster than normal expansion of snow across portions of Asia during the month of October. This should lead to a negative "Snow Advance Index" number, which means increased likelihood of Arctic blocking during the winter months
#2: Above normal snowfall during the month of October in northern Canada
#3: Periodic, but not prolonged blocking around Greenland
#4: The spread of sea surface temperature anomalies across the northern Atlantic
#5: The current sunspot cycle
What We Expect
We anticipate the first taste of winter cold will likely come our way during the second half of November, but cold likely to be juggled with some back-and-forth warmth even as we progress through the first half of December. More prolonged periods of cold are anticipated by the second half of December and throughout January and February. We do anticipate winter cold extending into early spring as well.
The average date for first measurable snowfall in Washington D.C. is December 18. We do anticipate some accumulation during the month of December, but believe the majority of snowfall in the District will come during the typical mid-January to early March time period.
As mentioned before, we believe the La Niña pattern will suppress storms tracking through the Gulf States which is necessary for the big blizzards around our region. More favored is the clipper track through the Midwest, which are typically moisture-starved and do not drop too much snow. However, we expect a greater number of them than normal.
We also believe that a few of these storms could be enhanced my additional as they approach the Atlantic, several of which could undergo rapid strengthening, commonly known as "bombogenesis." Generally speaking, we expect something in between the winters of 2013-2014 (32 inches of snow) and 2014-2015 (18.3 inches of snow). We anticipate a greater number of snow events in the region than last winter, but also anticipate the winter storms will lack the abundant moisture available for storms during the strong El Niño winter of last year.
What Can Go Wrong
Several things can go wrong that could dramatically change the winter outcome. These are things which are not favored, but were seen in winters with a similar setup in the past and are therefore risks to the forecast.
#1: Stronger La Niña - Our forecast hinges on the coming winter being only a weak La Niña. A few climate models are diverting from the average and trying to bring colder water into the equatorial Pacific. If they end up being correct, it could mean a winter not as cold as our forecast, and below normal snowfall.
#2: Lack of Alaska Ridge - Our forecast calls for higher pressures in Alaska and Northwest Canada to be a predominant feature during the winter months. Should this fail to be the case, a winter that is not as cold and not as snowy would be the result.
#3: Stronger Greenland Ridge - Some climate models have been hinting at the return of stronger ridging across Greenland during the winter months. Should the feature become predominant, it could lead to a more southern storm track and therefore, heavier and higher snowfall amounts.
Bring It On!
Well there you have it, the FOX 5 Weather Team's official outlook for the winter of 2016-2017! Of course, seasonal forecasts are what they are - not very accurate. The fact of the matter is no two winters are exactly the same and despite all our research and best efforts, we will in some way, shape or form, be wrong about the winter to come.
All we can really do it take the hints the planet is offering us, interpret them as best we can using historical data and then hope Mother Nature does not throw us a wrench, which she almost always done. Snowfall forecasts in particular are nearly impossible to nail right on the head.
So take what you see and what you read about the winter to come with some enjoyment, but also with a grain of salt. The FOX 5 Weather Team will be here watching for you, and when storms do threaten, you know you can rely on us to bring you the latest information all winter long.