April 16, 2018 – Oklahoma's third megafire in three years – the Rhea Fire, which has torched some 242,000 acres in less than a week – may grow even worse on Tuesday, as horrific fire weather conditions sweep in from New Mexico and west Texas. Relative humidity may drop as low as 3 percent in northwest Oklahoma, with temperatures expected to be well above 90°F and winds predicted to gust above 40 mph.

The NOAA/NWS Storm Prediction Center has placed a huge swath of New Mexico, Colorado, Kansas, Texas and Oklahoma under an "extremely critical" fire weather threat for Tuesday. It's the most dire rating of fire threat used by SPC, and the second extremely-critical day in the past week, following Friday, April 13. "Given dry fuels and ongoing drought, the stage will be set for fast-moving fires exhibiting extreme behavior," warned SPC in the outlook issued at 1 a.m. CDT Tuesday.

The storm system driving this threat, now barreling toward the Plains, won't produce anywhere as much heavy precipitation as the behemoth that caused flash flooding along the East Coast and some of the heaviest snows on record in the Upper Midwest. Tuesday's storm could be every bit as dangerous a fire-stoker, though. Winds ahead of the surface cold front gusted to 55 mph in Salt Lake City, Utah, pushing visibility down to 6 miles in blowing dust. (Hours later, it was snowing.)

Weather, climate, vegetation: How they're conspiring to bring megafires to Oklahoma

It seems that every year in Oklahoma now brings a megafire – defined by the National Interagency Fire Center as a wildfire that consumes more than 100,000 acres.

In March 2016, it was the Anderson Creek Fire, which raced across the Oklahoma/Kansas border and torched a total of nearly 400,000 acres. (Even though only part of the fire was in Kansas, it still qualified as that state's largest on record.)

March 2017 brought the even-more-massive Northwest Oklahoma fire complex, which devoured more than 830,000 acres.

Of the five biggest wildfires (> 100,000 acres) in Oklahoma since records began in 1997, three have burned in the last three years.

The recent spate of Oklahoma megafires isn't the work of a single villain; it's more of a team effort. State climatologist Gary McManus helped me wrap my brain around this emerging threat to the state's land, people and economy. McManus identified several factors at work:

• Unusually serious fire weather.

The sun-baked, wind-whipped Oklahoma landscape can dry out quickly in late winter or early spring. All it takes at that point is a day or two of extreme fire weather, and the main components are simple: strong winds and very low relative humidity (RH). You'll often see very high temperatures accompanying these two, because dry air that flows downslope from the Southern Rockies, or that descends on the south side of a major spring storm system, warms up as it heads toward Oklahoma.

Each of the big three fires in 2016, 2017, and 2018 has featured some variant of this hot, dry, windy pattern. Record highs have been in the mix, but it's the very low RH (typically below 10 percent) and strong winds (usually gusting above 40 mph) that have the biggest impact on fire risk, just as they do in places like California.

"I'm not sure if you'd consider these conditions too far outside of the climatological norms," said McManus. However, he pointed to the "historic" fire-weather designation by the NWS/Norman office as a sign of the truly high-end conditions present on Friday, April 13, 2018, and predicted for Tuesday, April 17. Here's an explanation from NWS/Norman:

"The 'Historic' reference on the Fire Weather Outlook is in reference to the Historically Critical category of the Red Flag Threat Index (RFTI). This is a simple fire weather index with values from 0-10 that rates the climatological rarity of combined RH and wind combinations. Historically Critical values of 9 or 10 can only be reached when either RH or wind speed exceed a 10-yr observational data set."

The office began using the RFTI in its fire weather graphics in 2016. Last Friday, April 13, was the first time that forecast conditions reached the "historically critical" category (essentially a subset of the "extremely critical" designation by SPC). It showed up again in the forecast for Tuesday, April 17.

• Alternating wet and dry periods leading to a profusion of fire-prone vegetation.

Echoing a global trend that's associated with human-produced climate change, Oklahoma has seen signs of a ramp-up in hydrologic extremes over the past few years.

May 2015 was the state's wettest single month on record, and 2015 was its wettest year. "The November-December 2015 period was the wettest on record as well, and the sixth warmest. So the growing season extended into winter to some extent that year," said McManus. The result was an unusually lush landscape going into the first part of 2016 that dried out quickly in the weeks leading up to the Anderson Creek fire.

Likewise, the summers of 2016 and 2017 were on the moist side, said McManus. "We also had a pretty severe ice storm during January 2017 that left lots of big fuels on the ground waiting for that spark," McManus said. Later that year came the the state's second-wettest August on record. "August would normally be a time we'd get rid of some growth in our typical summer burn season," said McManus.

The landscape of the Southern High Plains has been extraordinarily dry over the last six months. The western third of Oklahoma has seen little more than 2 inces since October – only about 20 percent of average – and most of the Texas and Oklahoma Panhandles have received much less than 1 inch, making it the driest six months on record in some locations. Any moistening of the landscape has been all too brief, which has left the landscape highly vulnerable to a spell of fire-friendly weather.

Californians may hear some familiar notes in the refrain above. That state's catastrophic fires of October-December 2017 followed one of the wettest winters on record (2016-17) and one of the hottest summers (2017). The moisture helped cover the landscape with a blanket of fire-friendly grasses and shrubs that dried out in the typically parched California summer. The autumn rains arrived late – especially in Southern California – which made matters even worse.

• And then there's the red cedar.

A growing complication for fire risk across the Southern Plains is the expansion of eastern red cedar. These small, tough evergreen trees, native to nearly all of Oklahoma, are the nation's most widespread tree-sized juniper. Prior to European settlement, fires set by lightning or by Native Americans kept eastern cedar limited to small canyons or outcrops where the fires burned less intensely.

Trends in land ownership and management, especially in recent years, have allowed eastern red cedar to spread more widely across the landscape. A state brochure noted that infestations of at least 50 red cedars per acre grew fourfold in the second half of the 20th century. It added: "The effects that the exploding populations of red cedar is having on the state might be compared to the soil erosion that occurred during the 'Dust Bowl' era of the 1930s-40s. It is becoming a problem in almost all counties, and will take years and millions of dollars to bring the spread of cedars under control."

According to the Nature Conservancy, "Perhaps the greatest threat to the productivity of Oklahoma's native landscapes and wildlife is the spread of eastern red cedar." Similar warnings have been issued in southern Kansas.

The same volatile oils that give cedar its appealing scent – think of cedar air freshener, or cedar plank salmon – can make the tree a virtual firebomb when it's dried out. A fire that's racing across grasslands in hot, dry, windy conditions may get a sudden boost when it hits a bank of eastern cedar. (See the photo at top.) The species' thick branching near the ground adds to the threat.

The terrain burned by the Anderson Creek Fire in 2006 had tens of thousands more cedar trees than it did during a similar fire in 1996 that burned less intensely.

Finally, Some Moisture for the Drought Zone?

The unforgiving fire-weather pattern of late may finally break in favor of the Southern High Plains on Friday and Saturday, thanks to a sprawling, slow-moving Pacific storm that's predicted to move into the Southern Rockies. Ahead of this system, moisture will be funneled across the drought-stricken plains, and there should be enough lift in the atmosphere to produce the most thirst-quenching rain in at least six months.

The latest five-day outlook from the NOAA/NWS Weather Prediction Center shows a solid 1 inch of rain for the Southern Plains from Midland, Texas, northward and eastward, including the region of exceptional High Plains drought noted in the most recent U.S. Drought Monitor (issued April 12). At least one more moist large-scale storm system may follow next week.

Looking Further Out: The 'Meridian Shift'

A fascinating pair of studies recently published in Earth Interactions (see Part 1 and Part II) discusses the eastward progression of the arid-to-humid boundary across the Southern Plains – historically associated with longitude 100°W, or the 100th meridian – as a result of climate change. This gradual shift may become an increasingly important factor in the fire-and-climate mix in and near Oklahoma as the century unfolds. Columbia University and weather.com have more on these studies.

Percentage of average rainfall (based on the 1981-2010 climatological period) for the six months ending April 15, 2018. Image credit: Oklahoma Climatological Survey.