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January-February 2015

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Weather Queries

I have been collecting daily surface weather maps valid at 5:00 a.m. Eastern Standard Time since the 1960s when I was in high school. During July 2014, we experienced five distinct cold fronts along the mid-Atlantic Coast. Particularly noteworthy is that each of these fronts ushered in an unusually strong polar air mass from Canada. Several of these fronts even reached the Gulf Coast and Florida. On the morning of August 2, 2014, a stationary front extended from Mexico, south of Arizona eastward off the coast of New Orleans, across Florida and off the East Coast into the North Atlantic, with two or three waves along it. Technically, it appears that the only area in the Lower 48 in a tropical air mass was South Florida and possibly parts of the Desert Southwest with a monsoonal flow there. And this in early August?

I have never seen a pattern like this in all my years. Even though the typical pattern for early August features a well-established subtropical ridge over the Western Atlantic—the Bermuda High—now and then I have observed ridge-in-the-West, trough-in-the-East in mid- to late summer. But the pattern is always fleeting, and sooner or later the flow of humid tropical air resumes over the eastern United States But not this time! It seems that the strong, long-wave trough over eastern North America that prevailed for so much of last winter never went away and has continued through this summer thus far. Just what is going on?

Scott Tester

Sperryville, Virginia

A few illustrations will confirm your impressions. Figure 1 color codes the Lower 48 states according to mean July temperature in 2014. For each state, all years from 1895 through 2014 (120 years) are ranked from coldest (1) to warmest (120). The rank of July 2014 in this list is printed for each state. Note that the Western states were much warmer than average. Oregon, in particular, had the second warmest July on record. In marked contrast, states bordering the Mississippi and Ohio Rivers were much below average. Indiana and Arkansas had their coolest July on record. Three other states, Illinois, Missouri, and Mississippi, had their second coolest July on record.

Figure 1. Rank by state of the mean temperature for July 2014 among all months of July from 1895 through 2014, from coldest to warmest. The color code indicates magnitude of the departure from normal.

Figure 1. Rank by state of the mean temperature for July 2014 among all months of July from 1895 through 2014, from coldest to warmest. The color code indicates magnitude of the departure from normal.

What could cause opposite extremes at two ends of the country? Figure 2, a depiction of the mean 500 mb flow pattern and the height anomaly for July 2014, gives the answer: a persistent ridge aloft in the West and a trough in the East. The light gray contours are drawn at 60 meter intervals. They depict the height above sea level of the 500 mb pressure surface. The wind at 500 mb follows the contour lines, and wind speed is proportional to the spacing between contours. The northward excursion of the wind in the Pacific Northwest is called a ridge. The southward excursion over the Ohio Valley is called a trough. Normally the wind flows roughly from west to east across the United States in summer, especially in the Northern states, but the large departures from normal of the 500 mb height in the west (positive 50–60 meters as shown by the innermost orange shading over Oregon, Washington, and Idaho) and in the east (negative 50–60 meters as shown by the innermost blue shading over the Great Lakes) cause a prominent wave pattern, which is very unusual for July. This pattern keeps warm and dry air over the West but brings periodic surges of cool air down into the northeast quadrant of the country from central Canada.

Figure 2. Mean height of the 500 mb pressure surface above sea level in meters for July 2014. Height contours are at 60 meter intervals. The departure from normal (in meters) of the 500 mb height is color-coded according to the color bar at right. Note the large positive anomaly in the Pacific Northwest and the large negative anomaly over the Great Lakes.

Figure 2. Mean height of the 500 mb pressure surface above sea level in meters for July 2014. Height contours are at 60 meter intervals. The departure from normal (in meters) of the 500 mb height is color-coded according to the color bar at right. Note the large positive anomaly in the Pacific Northwest and the large negative anomaly over the Great Lakes.

For the summer months June through August, the Lower 48 states were warmer than normal from the Rocky Mountains westward (except for Montana and Wyoming) and cooler than normal from the Great Plains to the Appalachians (except for Minnesota, Wisconsin, and Texas). This mirrored conditions in July, but the departures from normal were not so pronounced. Despite the cooler than normal summer weather over a broad area east of the Rockies, it is noteworthy that surface temperatures worldwide (land and sea), June through August 2014, were the highest ever observed. Global warming is indeed alive and well.

Perhaps the most newsworthy climate anomaly so far this year is the highly persistent United States pattern of warm-in-the-West, cool-in-the-East. I have not been able to obtain a map of mean 500 mb flow or the corresponding height anomaly for January-August 2014, but Figure 3 almost guarantees that it would show ridge-in-the-West, trough-in-the-East. Similar to Figure 1, it gives the state-specific ranking of mean temperature, January through August, from coldest to warmest. Thirteen states, all in the country's midsection, recorded temperatures that are in the lowest 10% for the first eight months of the year. Seven states, all in the West, recorded temperatures in the highest 10%, all in the West. Such a huge contrast over the contiguous 48 states over such an extended period is virtually unprecedented in 120 years. Your perception of a highly persistent anomaly is on the mark.

Figure 3. Rank by state of the mean temperature, January through August, 2014, from coldest to hottest for the 120 years ending in 2014. Note that California, with rank 120, had the hottest such period on record. The color code indicates the magnitude of the temperature anomaly. Very few states were near average. Such a large contrast over an eight-month span is virtually unprecedented.

Figure 3. Rank by state of the mean temperature, January through August, 2014, from coldest to hottest for the 120 years ending in 2014. Note that California, with rank 120, had the hottest such period on record. The color code indicates the magnitude of the temperature anomaly. Very few states were near average. Such a large contrast over an eight-month span is virtually unprecedented.

I've often heard meteorologists talk about upslope snow or rain. They say the higher the topographic barrier blocking the wind, the greater the expected precipitation. I wonder how high the mountain should be relative to the surrounding terrain to generate upslope precipitation.

Dima Smirnov

Chapel Hill, North Carolina

Precipitation caused by topographical barriers to the wind can be appreciable, and the barriers need not be especially high. First some background …

Water vapor is always present in the air, with the maximum amount strongly controlled by temperature. For example, near sea level, up to about 16 grams of water vapor can mix with each kilogram of dry air at a temperature of 70°F. This upper limit is called the saturation mixing ratio. Add more vapor at the same temperature, and some of it will condense into a cloud. At 32°F, the saturation mixing ratio is only 3.8 grams of water vapor per kilogram of dry air, more than four times less than at 70°F.

When unsaturated air (no clouds) is forced to rise over sloping terrain, it cools at a very specific rate: 5.4°F per 1,000 feet of lift. The mixing ratio of vapor in the air remains the same as it rises, but eventually, as the air continues to cool, the saturation limit is reached. At this point, a cloud forms. Continued lifting causes more condensation, and the cloud thickens. Eventually rain or snow may form in the cloud, and it falls to the ground. This is called upslope precipitation.

How much rain or snow can upslope flow generate? It depends upon how warm the rising air is and how much water vapor is initially present. From the information presented above, it's clear that saturated air at 70°F lifted 5,000 feet by a topographical barrier will generate more precipitation than saturated air at 32°F, because there is much more vapor to condense at the higher temperature. The intensity of rain or snow also depends upon the rate of uplift, which is controlled by the steepness the slope and the strength of the wind blowing up the slope.

During large-scale upslope events, the greatest precipitation sometimes falls at intermediate altitudes. Why? Air approaching the major topographical barrier may not become saturated until it has been lifted a few thousand feet. Partway up the barrier, if the air becomes saturated and is still relatively warm, considerable vapor condenses. Thousands of feet higher, because the air is much cooler, continued condensation yields less precipitation.

Where I live, adjacent to the foothills of the Colorado Rockies, this pattern occurs frequently. A deep layer of air advancing across the gently sloping plains toward the mountains generates a layer of stratocumulus clouds (perhaps beneath higher clouds). As this air approaches the foothills, the height of the cloud base lowers, and the clouds thicken. Precipitation starts a few tens of miles from the foothills as the topography begins to block the easterly flow. Once the air surges into the foothills, the air ascends and cools more quickly, and precipitation is more substantial. If the upslope flow reaches the Continental Divide, near 12,000 feet, precipitation continues that far west, but not as much falls at high altitudes because the saturation mixing ratio is much less at lower temperatures and the vapor supply is consequently much lower.

Examine the map of mean annual precipitation for the Lower 48 States. Several trends are apparent:

  • East of the Continental Divide, precipitation increases with proximity to large bodies of water, in particular, the Gulf of Mexico and the Atlantic Ocean. Moisture-bearing air from these warm-water sources (Gulf Stream off the East Coast) frequently streams inland and contributes to precipitation.

  • Within this area, precipitation is regionally higher along the Appalachian Mountains, stretching from northwest Georgia to western Maine. Upslope flow from the east or west is the cause.

  • Along and west of the Continental Divide, mountain ranges capture moisture carried by the wind, regardless of direction, and cause local maxima in precipitation. Note the areas colored green, where annual precipitation exceeds 20 inches.

  • Along the Pacific shore, coastal ranges from central California northward capture substantial moisture in onshore, upslope flow, and the inland ranges, the Sierra Nevada in California and the Cascades in Oregon and Washington, wring out much of the remaining moisture from the prevailing westerly flow, thus explaining the dry climate in much of the Intermountain West, high elevations excluded.

In summary, upslope flow of moist air explains most of the regional maxima in annual mean precipitation over the Lower 48 States.

As noted at the beginning, topographical barriers need not be very high to affect annual precipitation. In the July–August 2008 installment of “Weather Queries” in Weatherwise, I examined the mean annual precipitation of Wisconsin. A north-south ridge of hills rises 300–400 feet southeast of Lake Winnebago in east central Wisconsin. At the crest of these hills, mean annual precipitation is three to four inches greater than at surrounding lower elevations.

Weatherwise Contributing Editor THOMAS SCHLATTER is a retired meteorologist and volunteer at NOAA's Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author at weatherqueries@gmail.com, or by mail in care of Weatherwise, Taylor & Francis, 530 Walnut St., Suite 850, Philadelphia, PA 19106.


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