Good afternoon everybody! I hope everybody is doing alright today. I’m sure that many readers of this blog are dismayed by the results of the election Tuesday night. Like many others, I am ready to move on from this brutal political season and start rebuilding. Although we have many questions looking forward, I believe we can at least project whether we’ll need to don snowboots or sunglasses for the months ahead.
What’s the weather going to be like this winter?
Over the past two winters, warmer-than-average sea-surface temperatures in both the Northeast and Tropical Pacific have had a large impact on our weather. During the snow-starved winter of 2014-2015, we had a massive “Blob” of warm, nutrient-poor water in the Northeast Pacific, giving us much warmer-than-average temperatures and wreaking havoc on a variety of marine life. Then came the “Godzilla” El Nino of 2015-2016, vanquishing Mr. Blob and giving the Pacific Northwest one of its wettest winters on record. El Nino dramatically weakened once we sprung into April, and La Nina, El’s little sister, started to make her presence known in the Tropical Pacific.
However, as the summer months rolled on, our burgeoning La Nina began to stumble, and for a while, it appeared as though neither El Nino nor La Nina would lay claim to the Tropical Pacific. All the while, the dreaded Blob began to grow once again in the Northeast Pacific with the help of a strong and resilient ridge of high pressure. But one of the stormiest Octobers in recent memory helped smite the Blob, and La Nina has regained her footing over the past couple weeks. So here we are, with a dead Blob and a weak La Nina in store for the winter of 2016-2017.
The state of the tropical Pacific is one of the biggest factors meteorologists consider when constructing a seasonal outlook. Therefore, I thought it would be worthwhile to briefly review what has happened during weak La Nina events of years past before I give an outlook for the winter ahead.
During La Ninas, we often see a ridge of high pressure develop on the North Pacific. This bumps the jet stream northward into higher latitudes. The jet then often sinks down from the NW into the Pacific Northwest, bringing us cooler-than-average temperatures with above-average rainfall. On occasion, the ridge weakens and the jet stream comes straight across the Pacific into the Pacific Northwest.
The chart below shows the height anomalies in meters of the 500 mb pressure level of the atmosphere (approximately 18,000 feet) from November-February of all “weak” La Ninas since 1950. Where the 500 mb pressure surface is higher (oranges), you have a “ridge,” and where the 500 mb surface is lower (purples), you have a trough. As you can clearly see, there is a ridge in the Pacific, with a trough over much of North America.
Closer to the surface, weak La Ninas tend to bring above average rainfall to the Pacific Northwest, the central/southern Great Plains, the Lower Mississippi Valley, and the Ohio Valley while bringing below-average rainfall to the Gulf Coast, Atlantic Seaboard, and California. They also tend to bring cooler weather just about everywhere due to more frequent outbreaks of cold, arctic air. Additionally, these arctic outbreaks often occur a tad further west than usual, meaning that Montana and North Dakota get hit with the brunt of cold air rather than the Upper Midwest.
Although the existence of a weak La Nina gives us some guidance for a winter weather outlook, weak La Ninas offer much less for forecasters to go off of than strong La Ninas and tend to have far more variable atmospheric effects year-to-year. Seasonal climate models hint at us entering a typical La Nina pattern by January with a ridge forming over the Pacific, but because there is still a lot of “leftover” warmth in the atmosphere and some areas of the ocean from the crazy strong El Nino of 2015-2016, we could still experience a warmer-than-average winter for much of the country.
Now, without further ado, the 2016-2017 Winter Weather Outlook!
Southeast
La Niñas are correlated with a higher probability of a more northern storm track. As such, the southeast is predicted to be calmer and drier than normal, especially compared to last year’s extremely strong El Nino event, which brought flooding rains in December to much of the Southeast and kept most of the region wetter than normal through January and February. As the Palmer Drought Severity Index below shows, some of the same areas that experienced heavy rains last winter are currently in an “Extreme Drought,” and though this designation may be a bit hyperbolic, there is no denying that parts of the Southeast have been very dry lately, even with the surge of rain in early October due to Hurricane Matthew.
This isn’t to say that we won’t see stormy periods, but overall, this year has a greater probability of being warmer, sunnier, and drier than normal.
Northeast
Last winter was exceptionally warm for the Northeast. Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, and Connecticut all experienced their warmest December-February period on record. Central Park in New York City hit 72 degrees on Christmas Eve. Despite this, there were still several major snowstorms throughout the season – including “Snowzilla” in mid-late January, which set all-time daily snowfall records for New York City, Philadelphia, Washington D.C., and many other highly-populated areas.
One thing we didn’t see a lot of last year was lake-effect snow, which is snow that occurs due to cold, arctic airmasses picking up moisture as they travel across the Great Lakes. During La Nina, it is more common to see arctic outbreaks, particularly earlier in the winter. Though temperatures are still well above average over much of Canada, the airmass is cooling rapidly, and there is a possibility that places like Syracuse and Buffalo could experience a hefty lake-effect dump or two, particularly in late November and December when the lakes are still largely unfrozen.
El Nino years are up to 5 times more likely to bring “major” snowstorms to the Northeast (Grimaldi, 2008), so I expect that we will see fewer massive, regionwide blizzards than we saw last year. However, temperatures will likely be cooler than last year, especially during December.
We’ll have to keep a close eye on the “polar vortex,” which has become somewhat of a buzzword over the last couple years. The polar vortex is a persistent area of low pressure and cold air near the pole. When the polar vortex is weaker than normal, it sags south and delivers extremely cold, arctic air to the Northeast. This happened throughout the winters of 2013-2014 of 2014-2015, and some forecasts are saying a similar pattern could be in store for this winter.
Ohio Valley
The Ohio Valley is often wetter-than-normal during La Ninas due to a northward shift in the storm track. Also, as the 500 mb height chart posted before shows, there is often a trough over this region during La Nina events, opening up the door for cooler air pouring in from Canada. The aforementioned polar vortex could wreak havoc on the Ohio Valley as well, so that is something we will have to watch for.
Upper Midwest
Much of what was said for the Ohio Valley can also be said for the Upper Midwest. They will be wetter than normal, particularly in the latter half of the season, and experience cold shots throughout the winter. At this point, we have moderate confidence that this winter should be colder than average, and are very confident that it will be a much colder winter for the Upper Midwest than last year.
South
Keeping in line with a more northern storm track, the South will likely be warmer and drier than normal. This is especially true for Texas and the Gulf Coast. More northern areas, such as Kansas, will be more exposed to shots of cold air surging southward, and could experience dramatic swings between springlike warmth and bone-chilling cold.
Northern Rockies and Plains
Like areas further east, the Northern Rockies and Plains experienced record warmth last year due to a lack of outbreaks of arctic air sweeping down from Canada. The story here is the same as many other areas – there is moderate confidence that this winter will be cooler than normal, and high confidence that it will be cooler than last winter.
Southwest
The southwest has been consistently warmer and drier than normal over the past couple winters, and this winter looks to be no exception as the storm track stays further north. Of course, much of this area is a desert to begin with, and the winter is the dry season for this part of the U.S.
West
Western Nevada and much of California are still in the midst of a drought, and unfortunately, it doesn’t appear that these areas will get much relief this winter. Last year’s El Nino was forecast to bring massive amounts of rain to Southern California and make huge progress in reducing the drought there, but unfortunately, the storm track was further north than expected, giving record rains to the Pacific Northwest and leaving Southern California warmer and drier than normal. Expect a warm and dry winter for much of the region, save perhaps Northern California.
Alaska and Hawaii
The ridge in the Northern Pacific that tends to form during La Nina events tends to help “nudge” storms approaching from the west to the north. As a result, Northwest Alaska is often warmer and wetter during La Nina winters due to increased onshore flow. Areas further to the southeast like the Kenai Peninsula and Alaskan Panhandle have a higher probability of experiencing cool, northwesterly flow, which brings them cooler-than-average temperatures and near-normal to slightly-below-normal precipitation. Hawaii is further removed from the effects of La Nina in the winter, so there is significantly less confidence in the weather there. Sorry Hawaiians!
Northwest
After an incredibly stormy and record-wet October, November has gotten off to a calmer start around the Pacific Northwest as a large ridge over the Western U.S. has sent the storm track further north into British Columbia. Current models show the ridge breaking down, opening up the door for weaker systems to come through every couple days or so over the next week. However, the weather for the foreseeable future looks relatively benign, especially when compared to October.
La Ninas generally bring cooler and wetter-than-normal weather to the Pacific Northwest. However, these anomalies are far more pronounced during strong La Ninas than during weak ones. While weak La Ninas are on average warmer and drier than strong ones, they have much more variability. Moreover, our biggest windstorms, snowstorms, and floods have generally occurred during Neutral years, so a weak La Nina increases the chances of a major weather event compared to a strong La Nina.
Strong La Ninas tend to bring well above-average snowfall to the mountains, but weak La Ninas generally bring closer-to-average snowfall amounts. With residual warmth from last winter’s El Nino still in the atmosphere, it seems like we could expect a near-average snow year for the mountains. However, the moderate-strong La Nina winter of 1998-1999 following the massive El Nino of 1997-1998 brought record snowfall to North Cascades, with Mt. Baker ski resort notching 95 feet of snow for the season. Such a result doesn’t appear likely this winter due to our La Nina being weaker, but I would be happy to be proven wrong!
I’m not going to make any specific projections about snowfall for the lowlands, as forecasters Trump only political pundits with regard to accurately forecasting the correct outcome. That being said, our lowland snow chances look better than the past two years due to the dissolution of the Blob and our most recent El Nino. Fingers crossed.
Sources:
Grimaldi, R. (2008, January). Climate teleconnections related to El Niño winters in a lake-effect region of west-central New York. Atmospheric Science Letters, 9(1), 18-25. doi:10.1002/asl.166
Holliday, M. (2015). Polar Vortex: What Is It and Why Is It Important? Retrieved November 9, 2016, from http://firsthandweather.com/516/polar-vortex-important/
Mass, C. (2008). The weather of the Pacific Northwest. Seattle: University of Washington Press.
NOAA’s Climate Prediction Center. Retrieved November 10, 2016, from http://www.cpc.ncep.noaa.gov/
NOAA’s Physical Sciences Division. (n.d.). Retrieved October 26, 2016, from http://www.esrl.noaa.gov/psd/cgi-bin/data/getpage.pl
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