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Old 04-15-2012, 10:19 PM
 
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This question from prospective transplants comes up so often that I thought I would write a comprehensive post about it (and, for those who do not know, I am an amateur climatologist who has studied Western climate for over 40 years).

First and foremost, the climates of California and Colorado are fundamentally different. Anyone who says the climates are similar really knows little about climate.

California first. California and the West Coast are unique in the US for having a "Mediterranean" climate--that is, a climate that is generally wet in winter and dry in summer. This is because California's climate is influenced primarily by its proximity to the Pacific Ocean and the various northerly-southerly-lying mountain ranges that grace the state. During the winter, the combination of air and water currents facilitate moisture-laden winds that blow in from the Pacific nearly constantly. As those moisture-laden air currents are forced upward by the various mountain ranges, they shed large amounts of moisture. Northern Coastal ranges can see upwards of 80" of precipitation, while many locales in the Sierra Nevada can see over 50". Even some Central Valley areas may see up to 12" of rain during the winter months. The only areas of California that do not see significant winter moisture are areas of California east of the Sierra Nevada or in far southern California. Even in those areas, though, what little moisture does come usually comes in winter.

The moist Pacific winds in winter also moderate winter temperatures considerably. California's lowlands see freezing temperatures only infrequently. Even the higher areas of the Sierra Nevada west of the Sierra Crest are generally warmer than similar elevation areas in the Rockies during winter. California's coldest winter locales--such as Bridgeport, Bodie, and the Truckee River canyon are all east of the Sierra Crest. Incursions of Polar air are very rare west of the Sierras in California.

The moist winds off of the Pacific essentially end in California by mid-spring, when the Pacific storm track recedes north. As a result, California is essentially rainless during the summer months, save for an occasional "pop-up" convective thundershower that may occur in the higher reaches of the state's mountain ranges. Often, these put out little precipitation, but enough lightning to start forest fires. The Central Valley of California gets quite hot during the summer, and the warm temperatures often extend well into the surrounding mountains--the High Sierra can often be considerably warmer during the day in summer than a similar elevation area in Colorado will be. Only the areas in the very high Sierras, and immediately on the Coast are reliably cool in summer in California. Drought do occur in California and can be severe if, as they often do, wind up affecting the whole state.

Unlike Colorado, true "desert" in California is confined to its southern third and areas east of the Sierra. Despite its rainless summers, most of the Central Valley would be considered "semi-arid."

Now, contrast all of that to Colorado. In winter, Colorado west of the Continental Divide is influenced by many of the same Pacific storm systems that affect California. However, those systems have had to pass over several mountain ranges and travel across hundreds of miles of arid desert before they reach Colorado. As a result, the storm systems contain much less moisture and their frequency and duration is much less reliable. Areas east of the Continental Divide only see moisture from Pacific storm systems sporadically--usually when a particularly large or strong Pacific system manages to cross the Continental Divide and collide with Polar air moving south along the Front Range from Canada. More often, winter precipitation east of the Continental Divide occurs when moist warm air from the Gulf of Mexico (and, sometimes, to a lesser extent, from the Gulf of California) moves north and collides with cold Polar air over the area east of the Continental Divide. These type storms can occur anytime during the winter, but are most common in March. These winter storms, particularly if they occur in the coldest winter months, can be Colorado's most deadly and dangerous. Newcomers and travelers are often taken by surprise by these monsters because the storm is often preceded by unseasonably warm (50's and 60's temperatures) caused by the incursion of warm, moist Gulf air. If a large Polar incursion barrels south and collides with this warm air, a savage blizzard ensues, with heavy, blinding snow, severe winds, and temperatures that can drop from the 50's and 60's to below zero within a few hours. Epic storm events like this occur every few years--and have resulted in loss of life, as well as setting the record for Colorado's heaviest 24-hour snowfall.

Despite these occasional large storms, Colorado's lower elevation areas east of the Continental Divide are generally dry and brown from mid-October until early May, with snowcover coming and going several times during those months.

Polar air incursions can occur at anytime all winter in all locations in Colorado, so, unlike California, every locale in Colorado will generally see temperatures below zero at some point during the winter. Colorado's mountain areas, unlike California's, regularly see below zero temperatures, with some interior high mountain valleys--such as the Yampa valley, upper Colorado River valley, North Park, South Park, upper Gunnison Valley, and the San Luis Valley--regularly seeing low temperatures well below zero during winter. Some places regularly record low temperatures below -30 F. in winter, with Maybell in northwestern Colorado holding the state's all-time record low of -65 F. Because of these cold air incursions, no place in Colorado has a growing season longer that around 160 days--again, much different from California. Because of Colorado's high average elevation (6,800 ft.--the highest of the mainland US states), most areas of Colorado west of the Eastern Plains have a growing season of 140 days or less, with most mountain areas having a growing season of 90 days or less--the exception being the west-central valleys on the Western Slope with a growing season of 150-170 days (the latter occurring at Palisade east of Grand Junction). Some mountain towns have an average growing season of 3 weeks or less.

Unlike California, Colorado does receive precipitation in the summer months, but it is sporadic and somewhat unreliable from month-to-month and year-to-year. Most of this moisture from around the first of April until the end of September is the result of the westward expansion of the Bermuda High that begins in spring and recedes in fall. The clockwise rotation of the High fetches moisture from the Gulf of Mexico and brings it northward to Colorado. The Eastern Plains and Front Range are favored with this first, beginning usually in March (this actually is the cause of some of the epic spring blizzards that occur on the Front Range and eastern Colorado, when this moist air collides with colder, dryer air moving east and south from Canada; those collisions are also what cause a lot of tornadic activity in spring farther east on the Great Plains). This northward movement of air is what makes May and June typically the wettest months on Colorado's Eastern Plains. As the Bermuda High continues its westward expansion, it will begin influencing western and southern Colorado by early July. At this point, it will also fetch moisture from the Gulf of California and thus begins the "Southwest Monsoon" that brings nearly daily thundershowers to the Southwest mountains of Colorado. By September, the Bermuda High retreats to the east and the state becomes more influenced by Pacific storm systems by mid- to late fall.

Colorado's lower elevation areas do regularly see hot daytime temperatures in summer--mid-80's to high 90's, with occasional days over 100. Colorado's hottest areas are the lower Arkansas River Valley and the western valleys. As one moves east across Colorado's Eastern Plains, humidity increases, becoming occasionally muggy during the summer. Unlike California, areas over 7,500 ft. elevation in Colorado seldom surpass 90, with many summer days not exceeding the high 70's.

Spring moisture is critical to dryland farming in eastern Colorado. "Monsoonal" moisture does little to improve the streamflows upon which Colorado depends for agricultural and domestic water, but it does preserve soil moisture and it is critical for many mountain plants, especially in the middle elevations of Colorado. Either a dry winter or lack of summer moisture can lead to severe fire danger in Colorado. If both occur in succession, that is when Colorado can have catastrophic forest fires--a stark example was 2002 (and one that could repeat this year). Unlike California, most lower elevation areas west of the Continental Divide are true desert, with annual precipitation of less than 10". Nearly all of Colorado's area of lower elevations east of the Continental Divide are semi-arid to arid. While some California mountain areas can see in excess of 60" of precipitation per year, even Colorado's highest mountain areas seldom see an average of more than 30", many not much more than 20".

Unlike Colorado, California's typical summer drought means that wildland fires occur there with some regularity. "Time between fires" in some California chaparral plant communities may typically be as little as five years. As a result, many areas burn relatively frequently, but will regenerate quickly. In Colorado, by contrast, the typical presence of both at least some summer and winter precipitation in its mountain areas means that fires are less frequent--"time between fires" being sometimes a century or more in a given spot--but when fires do occur they can be large, extensive, and devastating--the area often taking over a century or more to recover. A good example of this exists around Cumbres Pass in far southern Colorado. The area burned in 1879, and, in many spots, trees are only now beginning to re-colonize the burned over areas. Many visitors assume the open parklands there are because the area looks like "timberline," but, in fact, timberline is actually nearly a thousand to 1,500 ft. higher in elevation. Over a century of fire suppression has only aggravated the potential for "megafires" and much of Colorado is now "prime to burn" in a dry year such as we are experiencing at the moment.

There are many nuances and microclimate in both California and Colorado, but this should at least give some basis for comparison. As always, I recommend checking these two sites for climatic averages:

Weatherbase

Historical Climate Information - Western Regional Climate Center
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Old 04-15-2012, 11:02 PM
 
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Very interesting, thanks. Colorado definitely has more in the way of convective or "pop up" storms in the summer. Nearly everyday there could be a storm but they are always isolated, and are usually in the mountains and occasionally on the Plains (including over Denver and Colo Springs). Almost everyday in the summer brings clouds and storms in the late afternoon.
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Old 04-16-2012, 10:02 AM
 
Location: Western Colorado
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Great post Jazz!
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Old 04-16-2012, 10:55 AM
 
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Excellent Posting / Thread; now listed in the Index of Key Threads.
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Old 04-16-2012, 09:46 PM
 
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One other note: Colorado is subject to flash flooding and the Big Thompson flood of July 31, 1976 (ironically, the day before Colorado celebrated its centennial as a state) was one of the deadliest single weather events in US history. Colorado's flash floods are typically caused by severe summer thunderstorms. Particularly dangerous are large thunderstorms which develop from a strong incursion of Gulf moisture destabilized by a cold front or extreme convection. These type storms can cause significant flash flooding, but become truly dangerous if winds aloft allow them to sit for hours in one locale. This is what happened with the Big Thompson flood, when winds "parked" a severe thunderstorm for hours over the upper reaches of the Big Thompson river canyon, dumping around 12" of rain over a few hours in an area that normally would see only around 18" for a whole year. The Arkansas River flood that devastated Pueblo in June 1921 was caused by a similar storm event, as was the severe Plum Creek/South Platte river flood that devastated Denver in June of 1965 (I was a firsthand witness of that one).

Another cause of significant devastating floods occurs in Colorado when the remnants of a Pacific hurricane or tropical storm slams into the Southwestern United States. These relatively rare events generally occur in late August to mid-October and generally affect the southwestern portion of Colorado. Durango and numerous other areas of southwestern Colorado were devastated by such events in Ocotber 1909 and even more severely two years later in October 1911. The worst storm of that type to hit southwestern Colorado in relatively recent history was the Labor Day weekend event in 1970--I was also an eyewitness to that one. It was ranked between the 1909 and 1911 floods in severity. I find it mildly amusing that almost all of the tacky yuppie development that has occurred north of Durango in the last 30 years is located smack in the middle of the Animas River floodplain--those McMansions would all have been flooded out had they been there in 1909, 1911, or 1970. It's only a matter of time . . .
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Old 04-16-2012, 10:49 PM
 
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Originally Posted by jazzlover View Post
as was the severe Plum Creek/South Platte river flood that devastated Denver in June of 1965 (I was a firsthand witness of that one).
I lived in Englewood at the time. There were several things I remembered about the flood, the first being the nearly pitch black sky overhead at 10 in the morning. At 11 in the morning sheriffs deputies cruised the streets with the loudspeaker blaring "Stay in your homes. Belleview westbound, Bowles Ave. westbound and Santa Fe drive have been closed until further notice!" At least there was a few hours notice as the flood roared through in the afternoon. One of my neighbors tried to get to one of the lumber yards on south Windermere St., not far from the South Drive In, and told me later he could see from a distance the crest and hear it as well.

Here are the sobering stats. Twenty eight people killed. Twenty six bridges between Denver and the suburbs got knocked out, including every bridge from Bowles avenue to the Colfax viaduct, essentially cutting the south part of Denver, Englewood, Sheridan, and Littleton in half.

Centennial Race Track was just a few days from starting their season. So much for that season. Overland Golf Course was inundated on the north side of Santa Fe and Columbine Country Club to the south end. Back then there were several brick mom and pop motels on the west side of santa Fe drive from Oxford to Evans. All of that and a couple trailer parks were washed away. And for miles and miles the Platte River was packed with chairs, beds, mattresses, lumber, cars, and other debris. And lots of humans who didn't heed the call to leave their homes.

That day was simply horrific. And the cleanup took a long, long time. All told, the estimation of damage in the dollar amount came to over $540 million. I can't imagine that figure in today's dollars.
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Old 04-17-2012, 12:00 AM
 
Location: Southeastern Colorado
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Default While We're Dating Ourselves...

[quote=jazzlover;23893205]One other note: Colorado is subject to flash flooding and the Big Thompson flood of July 31, 1976 (ironically, the day before Colorado celebrated its centennial as a state) was one of the deadliest single weather events in US history.

I lived in Estes Park at the time of the Big Thompson Flood, in a litle house perched on a hillside along Fall River Road up toward the entrance to RMNP. I remember the endless booming thunder, lightning, and rain all night long, and the eerie stillness that I awoke to the next morning. I made my way into town, and had to drive beyond Lyons into Longmont in order to find a working pay phone (remember those?) that didn't have 10 or 15 people standing in line.

Back on the East Coast, my father marked up the front page of The Washington Post, which covered the national story of the flood and the horrific story of those holiday campers who were trapped in the Big T Canyon. He doodled the name of the gas station where I worked, with a row of question marks following, while he waited to hear from his adventurous 20-something daughter who had headed West just about a month and a half prior.

When the Canyon was finally opened to traffic, I, like many others, slowly snaked through on the way to Loveland, our mouths hanging open in what could truly be considered "shock and awe" at the unrecognizable landscape and the infinite power of Mother Nature, never to be underestimated, never to be trifled with.

The Denver Post did a look-back in 2006, the 30th anniversary of the flash flood:

Big Thompson Flood memoirs - The Denver Post
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Old 04-17-2012, 05:59 AM
 
Location: Everywhere and Nowhere
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This may be why when you read the bios of many early CO settlers you notice how many later moved to CA. Otto Mears, who was mentioned on here recently as responsible for the construction of railroads in Southern CO as well as the Million Dollar Highway ended up in Pasadena. I'm sure there's more than one CO governor or U.S. senator now residing in a CA cemetery. 100 years ago there could been someone like Jazz complaining about all the CO transplants moving in to So Cal, in many cases for the better weather but also for jobs. Coastal So Cal actually has better weather than the Mediterranean in my opinion as it's generally cooler in the Summer and warmer in the Winter. The one flaw is the "June Gloom" that often hangs over the Coast during much of the day in late Spring and early Summer. There are also flash floods, mudslides, and brush fires. No place is perfect.

So why does Eastern CO not see deadly tornado activity to the same extent as its neighbors to the East?

Last edited by CAVA1990; 04-17-2012 at 06:07 AM..
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Old 04-17-2012, 06:19 AM
 
Location: Everywhere and Nowhere
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Originally Posted by jazzlover View Post
The only areas of California that do not see significant winter moisture are areas of California east of the Sierra Nevada or in far southern California. Even in those areas, though, what little moisture does come usually comes in winter.
Actually in Southern California most of the rain occurs in late Winter and early Spring. March always seemed to be the rainiest time of year there. For most of the Winter the weather is beautiful with sunny skies and mild temperatures. We often went boating off the Coast in January while you were buried under snow.
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Old 04-17-2012, 09:40 AM
 
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Originally Posted by CAVA1990 View Post
So why does Eastern CO not see deadly tornado activity to the same extent as its neighbors to the East?
Easy answer. Tornadic activity requires an especially "juicy" atmosphere, with warm air capable of holding a lot of moisture. Typically, eastern Colorado is still cool enough in April, May, and early June (because of both air currents and its higher elevation) that those conditions simply don't occur as frequently as farther east. By contrast, plains areas of Oklahoma, Kansas and Nebraska east of Colorado, and areas of central to northern Texas to the south and east of Colorado get warm, humid air frequently in spring. When that collides with cold air moving east from the Rockies, the explosive and large tornadoes that occur in "Tornado Alley" can develop. The statistical evidence of this pattern is that most severe tornadoes occur in the Great Plains states in April, May, and June (with many places having a May peak), while the peak month for tornadoes in eastern Colorado is June--when both warm temperatures and higher atmospheric moisture content are available for tornadic development. One also has to remember that areas west of the 100th Meridian (running north to south through central Nebraska and Kansas) is the dividing line between more wet and humid areas to the east and the semi-arid plains to the west. Eastern Colorado is well into that semi-arid area.

By the way, the collision point between cold, dry air moving south and east of the Rockies and humid, warm air moving north and west from the Gulf of Mexico is known as the "dry line." Storm chasers carefully monitor the location of the dry line on any given day because that is the point where most severe "mesocyclone" thunderstorms develop--those are the kind of thunderstorms that can produce big tornadoes.

On a separate note, great post from DoubleH about the Denver flood and bovinedevine about the Big Thompson flood. I was deep in the South Platte flood zone only hours after the flood. For those familiar with Denver, the high water mark of the Platte River was about 16' up the side of the Gates Rubber factory building next to S. Santa Fe Drive. Chatfield Reservoir was built to protect Denver from another such flood, but Plum Creek, which devastated downtown Castle Rock and Sedalia, has no such protection and could easily flood again under similar storm conditions as existed in the 1965 event.

Back in 1976, I dated a young lady a few times whose parents had a summer cabin up in Big Thompson Canyon near Drake. She and her parents were there when the storms started on July 31st. Her Dad, who had been going to that cabin since he was a small boy, had a bad feeling about the severity and length of the storm. He insisted that they leave the cabin and return to Denver, though they had planned to stay up there all that week. About and hour and a half after they left was when the flood roared down the canyon. Their close to 70-year old cabin was destroyed--they never found even a teacup from it, but they survived. Several of their neighbors who stayed died in the flood.
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