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Second Warmest U.S. February on Record: Chalk It Up to Greenhouse Gases

By: Bob Henson 6:35 PM GMT on March 08, 2017

As suggested by plants budding and blooming several weeks ahead of schedule, last month placed second among all U.S. Februaries in records going back 123 years, according to NOAA’s National Centers for Environmental Information. In its quarterly climate summary, NCEI announced that the 48 contiguous United States saw its second warmest February and sixth warmest winter (December - February) on record.

The month’s warmth was remarkably widespread. Every contiguous state but Washington came in above average, and 16 states from Texas to New York had their warmest February on record (see Figure 2). The warmth and a fast-drying landscape helped pave the way for enormous wildfires that raged across the Southern Plains on Monday, killing at least seven people.

The most spectacular index of February’s warmth is the 28-to-1 ratio of daily record highs (11,743) to daily record lows (418) noted in Wednesday’s report. When it comes to all-time monthly records, the ratio was even more wildly skewed: 1151 to 2.

Figure 1. Tulip magnolia trees bloom in Washington, on Feb. 28, 2017. Crocuses, cherry trees, and magnolia trees were blooming several weeks early because of an unusually warm February. The National Park Service predicted on March 8 that the peak bloom of the famed cherry trees at D.C.’s Tidal Basin would occur on March 14-17. This would put it on par with the earliest bloom on record: March 15, 1990. Image credit: AP Photo/Cliff Owen.

Figure 2. Statewide rankings for average temperature during February 2017 as compared to each February since 1895. Darker shades of orange indicate higher rankings for warmth, with 1 denoting the coldest month on record and 123 the warmest. Image credit: NOAA/NCEI.

Figure 3. Average temperatures for the contiguous U.S. for each February from 1895 to 2017. Image credit: NOAA/NCEI.

A new norm evolving for late-winter temperature
As shown in Figure 3, NOAA data show that only February 1954 (average temperature of 41.41°F) topped February 2017 (average 41.16°F). The only other February to rack up an average above 40°F was in 1930. It’s noteworthy that February 2017 was much less of a leap from the “new normal” than its rivals. Against the long-term linear trend of the last century (the blue line in Figure 3), last month came in about 5°F above the trend line, whereas February 1954 was nearly 7.5°F above the trend line.

The message here is that it’s not as difficult to get this kind of February warmth as it used to be. Increased greenhouse gases are the main reason, according to a study released Wednesday by the World Weather Attribution project. The WWA, led by Climate Central with several partners worldwide, uses observations and in-depth climate modeling to provide prompt assessments of how recent weather and climate events fit into the context of a changing planet.

The new WWA report, led by Geert Jan van Oldenborgh (KNMI, the Netherlands center for climate research), analyzed NOAA data from 1900 to show that a month as warm as February 2017 could now be expected about every 8 years on average, whereas the odds of getting such a month in 1900 were about 160 to 1.

The second part of the study pinned this trend on greenhouse gases by examining output generated for the most recent IPCC report from 42 comprehensive climate models, as well as two higher-resolution weather-only models. A series of historical model runs spanning 1960 - 2015 found that solar variations and volcanic eruptions had no effect on the warming trend. Because natural climate variations cancel each other out across this 55-year ensemble of 15 model runs, human-produced greenhouse emissions are the only solid explanation for the long-term warming. Another striking finding: “Around 2050 temperatures like this are projected to be completely normal, occurring approximately every three years on average.”

Warm-weather lovers may rejoice at this news (while cross-country skiers may cringe), but a shift toward Februaries on par with 2017 would have major consequences for U.S. forests and agriculture, especially with the possibility of damaging late-winter freezes still in the mix.

Figure 4. Statewide rankings for average temperature during winter 2016-17 (December through February), as compared to each Dec-to-Feb since 1895. Darker shades of orange indicate higher rankings for warmth, with 1 denoting the coldest month on record and 122 the warmest. Image credit: NOAA/NCEI.

Sixth warmest U.S. winter on record
The average contiguous U.S. temperature for December through February of 35.90°F tied with 1997-98 for sixth place among the 122 years of recordkeeping, according to Wednesday’s NOAA report. All but one of the seven warmest U.S. winters on record have occurred since 1997, with three of those recorded in the 2010s. The only cooler-than-average pocket this past winter (see Figure 4 above) was the northwestern corner of the country, including Washington, Oregon, Idaho, and Montana. Mildness was overwhelming from the southern Rockies northward and eastward, with both Texas and Louisiana notching their warmest winters on record and every state east of the Mississippi having a top-ten warmest winter.

Figure 5. Relentless storms this winter have left a trail of damage on California roadways, including this section of westbound Highway 50 near Pollock Pines photographed on Feb. 21. By late February, the bill to repair California's roadways hammered by floods and rockslides in an onslaught of storms this winter is already at least $550 million, more than double what the state budgeted for such emergencies. Image credit: AP Photo/Rich Pedroncelli.

An unexpected deluge in California
The biggest precipitation news of the winter was the return of ample moisture to California after five years of withering drought. The state scored its second-wettest winter on record in 2016-17 after a near-average winter in 2015-16 (focused mainly on the state’s northern half) and four below-average years before that. The statewide average precipitation of 21.67” in 2016-17 compares to a total of just 4.81” in 2011-12! Sierra snowpack in Phillips Station south of Lake Tahoe was at 185% of the average for the date on March 1, putting it among the four biggest accumulations on record at this point in the year.

Figure 6 below makes it clear that wild swings in precipitation are the norm for California. What’s new in recent years is that dry periods are now much more likely to be paired with extreme warmth, as was the case in the early to mid-2010s. Higher temperatures tend to exacerbate the impact of drought by pulling moisture from already-parched soils and reservoirs.

Figure 6. Winter precipitation for California showed a phenomenal jump in 2016-17 compared to the previous few years, while the average precipitation has shown little trend over the last century. Image credit: NOAA/NCEI.

El Niño, La Niña, and West Coast precipitation
This was the second winter in a row to vex seasonal forecasters who leaned on signals from the El Niño-Southern Oscillation (ENSO) to predict precipitation across the West Coast. The very strong El Niño of 2015-16 failed to produce the expected downpours across southern California (which trends wet during El Niño) while giving Seattle (which tends dry during El Niño) its wettest winter on record. This year’s weak La Niña also ran against the grain of expectations, leaving the state of Washington slightly below average in precipitation while giving California a bumper crop of moisture all the way to San Diego.

Although strong El Niño events are very likely to produce wet winters in California, the relationship is not an ironclad guarantee. “The bottom line is that California can get wet during El Niño, but not always,” noted Jan Null (Golden Gate Weather Services). Likewise, a weak or borderline La Niña event tends slightly toward dryness across Southern California, but annual precipitation during these events has varied widely, from as little as 54% to as much as 127% of average, not counting this year’s deluge. There is sure to be fascinating research ahead on the winters of 2015-16 and 2016-17 and their wildly divergent West Coast precipitation outcomes.

This winter’s ample moisture extended from California across the Rockies and into the Midwest (see Figure 7 below). Both Nevada and Wyoming had their wettest winters on record, while the only significant pockets of seasonal dryness were across the mid-Mississippi Valley and the mid-Atlantic. Dry conditions intensified and expanded across both of these areas last month: Missouri, Illinois, South and North Carolina, Virginia, Maryland, and Delaware all saw a top-ten driest February.

We’ll be back with a new post on Friday at the latest.

Bob Henson

Figure 7. Statewide rankings for average precipitation during winter 2016-17 (December through February), as compared to each Dec-to-Feb period since 1895. Darker shades of green indicate higher rankings for moisture, with 1 denoting the driest month on record and 122 the wettest. Image credit: NOAA/NCEI.

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