Extreme weather patterns in US

Blocking Patterns: Rivers of Moisture to Converge in Major Rain Event For Eastern US?

(Image source: GOES/NOAA)

Robertscribbler,

2 July, 2013

Today, a very muscular blocking high pressure system located over the Western US flexed. After having set off record fires and heat waves it reached deep into the Arctic and drew moisture and more unstable air down from over Hudson Bay across the Rockies and over top of Arizona and New Mexico, igniting powerful thunderstorms which blanketed large areas in hail, heavy rainfall and lightning. A second moisture stream drawn into the high’s circulation from the Pacific also fed these storms.

You can see the bright, high, cold cloud tops now firing over Arizona and New Mexico.

To the east, an upper level low pressure system is just now starting to draw this concentrated moisture into two other feeding, damp air flows. The largest draws straight up from the Caribbean over Florida and then rushes up the US East Coast. The second, pulls moisture from tropical storm Dalila in the Pacific, draws it over Mexico, then pulls it over Mississippi and up the back side of the Appalachians.

The action of this powerful blocking high over the US West and associated upper level  low over the East is likely to result in very moist, rainy conditions for a large section of the country east of the Mississippi River. On the Gulf Coast, as much as 5-8 inches of rain is expected. But a wide swath shows potentials for 1-5 inches over the coming week.

This persistent wet and extremely moist flow raises the risk of flash flood conditions where major storms light off. Record rainfall over many areas has already left the ground saturated and atmospheric conditions are very unstable, setting off the potential for powerful storms.

The broad sweep of these convergent moisture flows also sets up the possibility that even more violent conditions may emerge. Large blocking highs were associated in all the major flood events that have occurred around the world so far this year. In one example, noted by commenter Colorado Bob, Pakistan suffered 120 degree heat under a blocking high during May and June. The high then swept a massive flood of moisture up over India and into the Himalayas. The result there was devastating floods that left hundreds dead in a virtual tsunami of mud and water.

The strength of the current upper level pattern, drawing moisture from the Arctic, the Pacific, a tropical weather system, and from the Caribbean sets in place the components for major instability to meet with four rivers of moisture over the Eastern US. It’s a dangerous set of circumstances that may result in current rainfall forecasts under-shooting long-term totals. This week has already seen a number of torrential downpours over broad sections of the US East Coast. But this flow taps even more moisture than what was previously in place, drawing from multiple sources across an area spanning more than 4,000 miles to link rivers of moisture with unstable air. Let’s hope these convergent flows don’t set off flood events similar to those seen in Europe and India this year.

Fair warning: the mangled Jet Stream now has the Eastern US under the gun.

(Hat tip to X-Ray Mike for his comments on strange storms in Arizona today).

String of Deadly Wildfires Ignited By Record Heat Wave, Extreme Jet Stream over US Southwest.

Robertscribbler,

2 July, 2013

This Sunday, Death Valley hit a new all-time June record high temperature of 129 degrees Fahrenheit. It was the most recent high mark in a record-shattering heat-wave that baked the western US over the weekend, pushed power grids to the limits, resulted in numerous heat injuries and one death, and sparked multiple dangerous and deadly wildfires.

Though the forecast is for high temperatures to slightly abate over the coming days, the long-range outlook is for heat to return with a vengeance by about July 11th. Overall, temperatures are expected to drop from the 110s and 120s into the still excessive, 100s and 110s, then jump back toward record-setting levels.

A large, high amplitude blocking pattern and associated heat dome high pressure system is expected to persist. So conditions remain for temperature spikes as July progresses. With no long-range changes in this Jet Stream configuration expected, the Southwest will stay under the gun for record heat and dryness probably at least until the end of summer. So this initial volley of record heat can best be described as the first in a very vicious pattern of heat pulses which are likely to continue through the hot months of July, August and September.

This extreme Jet Stream induced hot pattern is plainly visible in the latest ECMWF model runs.

(Image source: ECMWF)

Here we see the record heat predicted for tomorrow with a broad area of 95 degree average 5,000 foot temperatures expected. Average temperatures of 95 degrees at the 5,000 foot level can often result in 110 or greater daytime high temperatures at the surface. As noted above, these very hot temperatures are expected to fade somewhat over the coming week before rising again toward record highs by around July 11th.

More than 20 Large and Dangerous Fires Ignite in US Southwest

This heat wave is just the crowning blow in a long string of warm, dry weather that has settled over the US Southwest since early 2012. The extraordinarily long duration heat and drought is the direct result of a near-permanent blocking pattern. A high amplitude wave in the Jet Stream and associated blocking high pressure have kept heat and drought conditions continuously in place. Such a stationary pattern is not normal and is brought about by a loss of sea ice and snow cover over the Arctic which is a direct result of human-caused climate change. These changes have both slowed down the Jet Stream and made weather patterns more persistent. Beneath the up-slope of these large blocking patterns heat waves, droughts and fires become a near-constant hazard. In the troughs, cooler and much stormier conditions predominate.

In the west, fears were that, eventually, massive and damaging wildfires would result from the epic heat and drought. At first, it appeared that Colorado would be ground zero for this year’s fire season. There, multiple blazes devoured tens of thousands of acres and hundreds of homes. A single devastating fire, the Black Forest Fire, destroyed about 500 homes. Now, the current heatwave has set off numerous large blazes. Over the weekend and into Monday more than 20 massive fires sprang up throughout this drought and heat stricken region.

As the worst of these fires bore down on Yarnell, Arizona, a team of 19 firefighters lost their lives while attempting to defend the town from the rapidly spreading blaze. A quick shift in the wind forced the firefighters to resort to the use of emergency shelters as the flames turned toward them. Sadly, only one of the team survived. The blaze quickly engulfed the town, destroying at least half of the 700 buildings there.

Dangerous fires also raged in California, Nevada, New Mexico, Utah and Colorado. Unfortunately this might be the beginning of a very long, dangerous, and costly fire season. Jason Sibold, an ecologist at Colorado State University reported to Business Journal:

“That area is experiencing extreme drought, low snowpack, late onset of winter, early snowmelt and warmer springs and summers. A lot of those places, in the old climate scenario, would still be under snow. There’s no way they would be burning.”

So the long period of heat, drought and ever-expanding wildfires for the US Southwest, predicted by climate scientists and brought about by human caused global warming, appears to have started. Unfortunately, it only gets worse from here. Even with very aggressive reductions in greenhouse gas emissions, conditions will continue to worsen over the next few decades. Without a rapid reduction in greenhouse gas emissions, the drought which is now devastating the US Southwest will steadily expand north and eastwards until almost half of the United States is impacted. In Europe, the Sahara Desert eventually jumps into Spain, Italy, Greece and southern France and Germany.

An expanding band of heat-bleached and eventually uninhabitable lands eventually engulf the equatorial regions and grow polewards. If we wish to slow and then stop the growth of these damaged zones, then we must stop emitting carbon into the atmosphere as soon as possible.

Sadly, the most recent droughts, heatwaves, and wildfires in the US Southwest are just some of the milder first assaults of global warming. Things get worse from here. Far worse if we don’t drastically reduce carbon emissions.

Links:

America is Burning

ECMWF