2018-2019 Winter Storm Summary Part I

Overview
The winter of 2018-19 was pretty rough across parts of the U.S. while other parts were lucky to even get a taste of winter. As an example, the Northern Plains and Upper Midwest went through weeks of exceptional cold and saw record snowfall for the month of February. Meanwhile, along the East Coast, there was nary a Nor’easter and locations such as Boston, MA went through much of the season with very little snowfall.

If I had to highlight the winter, here are some of my main points:

  • Exceptional cold and snow in the Plains through Upper Midwest in February and early March leading to record flooding in the Midwest.
  • Mountains of snow for the West and copious overall precipitation that led to complete erasure of the California drought.
  • Large differences in snowfall and wintry impacts from Coastal New England to Interior locations.
  • Very little snow and cold weather for the Ohio Valley and South.

Below are the tracks for named winter storms this season, 24 in all. Some interesting patterns are revealed.

  • Many coast-to-coast storms re-developed as Colorado Lows in the area to the lee of the Rockies. Those systems took favorable tracks to produce heavy snow in the Upper Midwest, while keeping much of the Ohio and Tennessee Valleys in the warm sector.
  • Not many storms tracked through the South or Southeast.

Storms tracking up the East Coast hugged the shoreline limiting significant snowfall for the coastal areas.

Winter Severity Rankings
The Accumulated Winter Season Severity Index (AWSSI) was developed to objectively quantify and describe the relative severity of the winter season (Boustead et. al 2015). AWSSI uses a set of parameters that are readily available throughout the U.S. observation network to rank the severity of winter including the intensity and persistence of cold weather, the frequency and amount of snow, and the amount and persistence of snow on the ground.

This past winter’s rankings revealed big differences across the nation and those differences reflect the named storm tracks. At a glance, the Northern Plains and Upper Midwest stick out for a Severe to Extreme winter. Meanwhile, much of the Ohio and Tennessee Valley as well as Southeast experienced a Mild winter. Another area that had an Extreme winter was Interior New England including places like Caribou, ME, while not too far away, Southeast New England had Mild to Moderate rankings. Finally, although not necessarily reflected in the AWSSI map, the West enjoyed deep snowpack, to the delight of skiers, and as noted, the wet pattern was responsible for completely erasing a significant California drought. The reasons for these differences operate on several scales that I will address below.

Figure 2: Automated Winter Season Severity Index (AWSSI) through April 29, 2019. The Northern/Central Plains had a Severe to Extreme winter, while the Ohio and Tennessee Valleys saw  Mild conditions.

Even before this winter season began, there was a lot of talk about a weak El Nino developing and the implications that pattern might have on the winter season. There was also a lot of discussion on other features that could contribute to modulating the winter season in the Northern Hemisphere, including the potential for Sudden Stratospheric Warming Events, the early season build-up of Eurasian snow cover and the lack of Arctic ice cover. Some seasonal forecasts suggested that the back end of the winter might be exceptionally cold for the US. How did those forecasts verify? Well, it took a while, and cold weather did not develop across as big an expanse of the U.S. as some of those forecasts predicted, but where it got cold, it got REALLY cold.

Below are 4 maps showing the mean temperature profiles for the entire 3 month meteorological winter (Dec-Jan-Feb) as well as each individual month. The season began on a warm note and that held over through the second month of winter for much of the U.S. However, during February temperatures plummeted across the across the western 2/3 of the nation as a large trough developed over the west and a ridge persisted over the Southeast. In fact, in what is typically the coldest part of the Lower 48, many locations in the Northern Plains and Rockies had their Top 3 coldest February on record with average temperatures for the entire month as much as 30 degrees below normal!

Figure 3: Mean temperature percentiles for: 2018-19 Winter season, December, January, February.
Figure 4: Average Temperature departure February 2019.

Exceptional Cold and Snow For Parts of The Plains and Upper Midwest

As the season headed into February, an arctic high pressure system in Central Canada maintained very cold air over a deep snowpack. That cold air was poised to move down across the U.S. The large scale weather pattern across North America set the stage for a strong, southwest-northeast oriented jet stream, resulting in storm tracks that would produce Colorado Lows that often tap abundant moisture from the Gulf of Mexico. As those storms track northeast along the jet stream they also will bring arctic air from Canada southward deep into the U.S. In fact, that is exactly what happened, not once, but several times in February as no less than 8 named winter storms produced rounds of high-impact winter weather including blizzards for the Plains as well as Upper Midwest. Many locations in the Northern and Central Plains to the Upper Midwest posted record February snowfall.

Figure 5: Named storm tracks February 2019.
Figure 6: February record snowfall across the Northern Plains and Upper Midwest.

The relentless cold temperatures, as noted earlier, allowed all of that snow from successive storms to deepen and maintain that deep snow cover maintained itself into March. In addition, those cold temperatures caused ice cover on rivers and streams to thicken from week to week. By early March, with all of this snow cover and thick ice in place, a catalyst named Winter Storm Ulmer (March 11-15) brought warmer temperatures and heavy rainfall to that huge area covered by deep snowpack. The results were catastrophic when the runoff from rainfall and snowmelt swelled waterways well beyond their banks. To add insult to injury the thick ice cover broke up and huge chunks travelled in the swift currents to destroy everything in their path. Ulmer was without doubt, the highest impact storm on a national scale for the 2018-19 winter season.

Figure 7: Photo of extensive flooding at Offutt Air base in Nebraska, where as much as one-third of the base was under water. (Courtesy Facebook / 55th Wing Commander)
Figure 8: In Niobrara, Nebraska, the Country Cafe became inaccessible as the slabs of ice, some up to 2 feet thick, stacked up at least 10 feet high against the building, the Omaha World-Herald reported.

A Bounty of Snow In The West

The semi-permanent trough in the West also led to exceptional snowfall for the mountains, to the delight of skiers of course. More importantly however, that copious precipitation completely erased the drought in California and significantly reduced it in other parts of the West. Several episodes of heavy precipitation occurred in association with Atmospheric Rivers (AR) of moisture. That is reflected in the Snow Water Content, measured in late March, for the river basins of the West with only the far Pacific Northwest and portions of the northern Rockies a bit below normal.

Figure 9: Comparison of drought conditions across the West from November 27, 2018 (pre-winter) to March 19, 2019 (post-winter).
Figure 10: Basin Average Snow Water Content (% of average) in late March across the West.

One local story of interest in the West was snowfall in Seattle. Of course, this location in not noted for snowfall, because the population sits right on the coast of the Pacific Northwest at sea level. They get a lot of precipitation in the form of rain, but it is rare to get that combination of cold air and moisture to produce snowfall at elevations close to sea level. However, two separate winter storms, Maya and Nadia, brought nearly a foot and a half of snowfall to Seattle in less than a week in February. Seattle averages 6.8” of snow for an entire season. By the end of the month Seattle recorded 20.2” of snow, making it their second snowiest February on record and their snowiest month in almost 50 years.

Lack of Significant Northeast Snowstorms Along Coastal New England

For much of the first half of the winter, if you lived in New England, it was either feast or famine regarding snowfall. There is normally quite a difference from the coast to inland locations when it comes to snow because coastal areas have more temperate conditions. But through mid-February much of Southeast New England had yet to see much more than a few snowflakes because the storms that typically bring wintry weather tracked too far inland to allow cold air to make it down to the coast. As a result, while the coast was getting a cold rain, more interior locations in New England were getting significant snowfall. In fact, a snapshot of the snow depth in mid-February showed bare ground in Southeast New England while Interior locations had over 4 ft. of snow on the ground. Under the heading of “better late than never” however, places like Boston rebounded with 24.3” of snow in 21 days between Feb 12th and Mar 5th.

Figure 11: Snow depth across the Northeast Feb 11, 2019. (NOAA)

Lack of Wintry Weather Across The South

Other than a unique heavy snow event for Lubbock TX in December, there was very little wintry weather across the South this past season. In fact, as you can see in the AWSSI map, most of the South had a Mild to Moderate winter.  Temperatures for the season were well above normal. At southern latitudes any temperatures above normal almost guarantee no snow or ice. In fact, Nashville had a record Mild winter as measured by AWSSI. They only received 0.5” of snow for the entire season when they average 6.4”. In addition, their temperature averaged nearly 5 degrees above normal.

Figure 12: Temperature departure across the U.S. for the winter season (December through February)

Other Notables
A very warm December across the Great Lakes Region kept those waters from cooling to the freezing point so that by early January less than 5% of the Lakes had ice cover. By late January however, the gates opened for cold air to drain south from Canada and the result was a rapid increase in ice cover. Through the cold February mentioned above, that ice cover continued to build so that by early March 74% of the Lakes were ice covered, nearly 30% higher than normal for that point in the season.

Figure 13: Weekly Ice Coverage for the season (blue bars) vs. average (green line)
Figure 14: Snapshot of the ice cover on the Great Lakes from the NASA MODIS polar orbiting satellite on March 8, 2019

Overall, the worst winter location in the U.S. according to AWSSI rankings was Caribou ME, who according to the ranking system experienced their most severe winter on record. Their winter essentially started in mid-November and once they got going, they never looked back. Look at these numbers, 164.7 inches of snow through April 29th, over 4.5 feet (55.7 inches) above normal. Four of the five months between November and March featured below normal temperatures and January was only 0.3° F above normal. The combination of heavy snowfall and an extended period of cold weather allowed snow cover to build to 45” at one point. In fact, this past winter Caribou endured 163 consecutive days with 1” or more of snow on the ground from November 10th to April 21st, a new record for the site!

Figure 15: AWSSI chart for Caribou ME through April 29, 2019

Summary

As with most U.S. winters, the winter of 2018-19 produced a variety of high impact winter weather. Record snow and cold in February across the Upper Midwest set the stage for what will likely be a billion dollar disaster from flooding in places like Nebraska and Iowa. Several Atmospheric Rivers set their sights on California and by the end of winter, the extensive drought conditions across the state were effectively eliminated. Meanwhile from the Ohio and Tennessee Valleys through the South got off pretty easy with no significant large scale winter storms or extensive, prolonged cold weather. With winter 2018-19 now in the books, it is time to look to Spring and Summer, enjoy! Part II of the seasonal summary looks at each named storm in detail.


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