Virginia tornadoes

Yesterday a series of at least three tornadoes tore through southeastern Virginia, with the largest impact being in Suffolk.

Interestingly, yesterday was also the six year anniversary of the catastrophic La Plata, MD tornado. Although tornadoes are not exactly rare in the mid-Atlantic states, an F4 tornado like La Plata had is extremely rare along the Atlantic coast or so far north. The tornado was one of an outbreak of 48 confirmed tornadoes over several states on April 27-28, 2002. The National Weather Service has produced a special assessment report, in addition to a web page created by the local forecast office. More photos are available on this page.

Wind Roses

No, they are not flowers that grow in windy places. A wind rose is the most common way to display statistical information about the relationship between wind speed and direction at a particular site. Basically the frequency of occurrence of winds from different directions is calculated from a dataset, and then a polar plot is constructed (similar to a bar graph) with length out from the center proportional to relative frequency. This allows a simple visual portrayal of the most common (or rare) wind directions. Just remember that meteorological convention always designates wind direction according to the direction that the wind comes from. Within each of the resulting "pie slices" of the wind rose, the relative frequency of different wind speeds may be indicated by areas of different color, similar to a "stacked" bar chart.

Wind roses are used in many applications, such as wind energy development, sailing, and air quality management.

Here is an example of a wind rose showing the winds observed at Miscou Island, New Brunswick, Canada during June 2007:



Software for creating wind roses from raw data files is available from Lakes Environmental (free) and EnviroWare. Precalculated wind roses for many U.S. stations are available online through the Natural Resources Conservations Service. More discussion and examples are available here.

Science fair projects

For students wanting to do science fair or class projects relating to the weather, there are many possibilities. Some of the most interesting however are ones that make use of actual observed weather data. The Oklahoma Climatological Survey maintains a website that lists some sample projects that have been done using data from the Oklahoma Mesonet observing network, as well as several related pages dealing with K-12 education and science fairs.

Several online data sources are listed on the Synoptic Meteorology class webpage, with wunderground.com and the NOAA weather server probably being the most user-friendly.

The Dryline

Especially in the springtime, it's common for a well-developed "dryline" to be found in West Texas. Similar to a front, the dryline is a discontinuity that separates the hot, dry (continental Tropical; cT) air of the Desert Southwest and Mexican Plateau from the warm, moist (maritime Tropical; mT) air that flows off of the Gulf of Mexico. Typically the sudden transition occurs in the vicinity of the 50°F isodrosotherm (line of constant dewpoint temperature). Whether it stays quasistationary or progresses eastward in advance of a low pressure system and cold front, the dryline holds special interest for tornado chasers and other people interested in severe storms since the area northeast of the dryline is often very favorable for initiation of severe convective storms. More than any other atmospheric feature, this is responsible for the existence of "Tornado Alley" in North Texas and central Oklahoma. The presence of dry air contributes to the generally greater visibility of tornadoes compared to tornadoes in the eastern U.S.

This afternoon there is a well-defined dryline extending as far north as western Kansas, with dewpoints in the mT air mass typically between 65-70°F and dewpoints of 10-40°F in most of the cT air mass. Although temperatures to the west of the dryline are generally warmer in the 20 Z observations, this is primarily a reflection of a larger diurnal cycle of temperature in the drier air. In early morning, the temperature gradient will often be reversed from the afternoon pattern. The dewpoint discontinuity is often maintained and strengthened by converging surface winds within a weak lee/thermal trough. When a dryline intersects a cold front, the point where they meet is commonly referred to as the triple point.

Weather Modification?

So is there actually any way that we can change the weather? While it's become increasingly well established that human activity can cause systematic climatic changes by modifying the land surface and contributing air pollution, such unintentional effects normally seem to make things worse instead of better. Of course, for every loser there's usually a winner. For example, if a drought causes crop failures in one area, then farmers in other areas may reap profits from higher prices.

The two applications that people usually think of for intentional weather modification are increasing rain over dry areas and minimizing the impact of hurricanes. Science-based rain-making has been attempted since at least the 1940s, although only fairly recently have researchers been able to show a clear improvement, under ideal circumstances. Known as cloud seeding, this normally involves injection of silver iodide or other specialized salts to help cloud droplets grow faster. Currently the big news is how much money China has invested to try to produce favorable weather for the Beijing Olympics. In general their effort is viewed with substantial skepticism.

Attempts to modify the strength or path of hurricanes have largely either been dismal failures or figments in the imagination of conspiracy theorists. For several years, the U.S. government supported research on Project StormFury, before determining that the potential for useful modification was exceeded by the inherent risks. Although federal research funding for hurricane modification has dried up, new (and old) ideas continue to be proposed. These have included moisture-absorbing gels, atomic bombs, and oil slicks.

Other areas of interest in weather modification are hail suppression (e.g. in Alberta) and clearing fog at airports.

A Big Flood on the Lower Mississippi

The current flooding along the lower Mississippi River has reached historic levels, bringing back memories of 1927, 1973, and other major flood years. So far the levees have held back major disaster though. Within the state of Mississippi, the river has now crested, barring any rises due to additional rainfall upstream. Estimates are that a couple hundred homes have been impacted. With about half of the floodgates on the Bonnet Carre Spillway now open above New Orleans, the huge amount of river water flowing into Lake Pontchartrain is now giving concerns that small boats may collide with floating debris entering the lake.

In the Mississippi Delta area to the east of the big river above Vicksburg, the current flooding has heightened the debate about a Corps of Engineers pumping scheme that appears headed for a veto by the Environmental Protection Agency (EPA).

The Father of Chaos

Yesterday Dr. Edward Lorenz died of cancer at age 90. He was a respected meteorology professor at MIT, who was the first to significantly develop chaos theory and apply it to the atmosphere. Out of that came our current concepts of atmospheric predictability and the need for ensemble methods in numerical forecasting.

Some outdoor recreation weather dangers

In New Zealand, six college students and their instructor drowned on Tuesday in a flash flood while hiking in the Mangetepopo Gorge.

Climbers in Scotland are concerned about the seasonal ending of the avalanche forecasting service while large amounts of snow remain on the mountains. Up in Juneau, Alaska, the problem is a major avalanche that knocked out transmission lines supplying power to the city.

Thunderstorms and Snow

Although the thunderstorms that rumbled through the South today were tame in comparison to last week's batch, the large storm system dominating the middle of the country still caused some problems. In particular, Minnesota was subjected to blizzard conditions that shut down mail delivery in some areas.



Other areas dealing with late season snow included Calgary, Alberta and even all the way down to western Kansas. And then there are the extreme seasonal snowfall totals that have accumulated, such as in Spokane, WA, Vail, CO, and Concord, VT.

High Water on the Big Muddy

With levels continuing to rise on the lower Mississippi River, the most extensive river flooding in11 years is now developing. Although the probable opening of the Bonnet Carré Spillway would give protection for New Orleans, other river towns are beginning to feel the impact. The Corps of Engineers has heightened monitoring of levees, and there has been a breached private levee today in the area of the Morganza Spillway.

Mississippi Cleaning Up

The cleanup from Friday's storms in Jackson and central Mississippi continues, and will for a while.

Initial storm reports are now available from the National Weather Service.

A Stormy Day in the South

Last night Little Rock, Arkansas had major damage from a tornado.

Today the squall lines marched across Mississippi. All of this stormyness was in the warm moist air a few hundred miles ahead of a cold front. Even at 12Z (7:00 AM) this morning, the upper air soundings were showing unstable air across the region, with CAPE of 666 J/kg at Jackson and 1375 at Shreveport (which was already near the squall line). The severe winds of the squall line came through the Jackson Metro area around 12:30 this afternoon, resulting in widespread damage. The ASOS at Hawkins Field in Jackson recorded the following report of 47 kt wind gusts before apparently losing power:

KHKS 041729Z 28035G47KT 1 1/4SM HZ SQ FEW036 BKN046 BKN070 24/17 A2994 RMK AO2 PK WND 26047/1728 PRESRR P0002

Earlier this morning, the surface charts provided a good example of potential problems in frontal analysis due to squall lines, outflow boundaries, and rain-cooled air.


For the most part, the fronts and air masses are easily identified in the RUC Theta-e analysis, including the relict cool air mass to the east of the Appalachians. The important point to note is that the low Theta-e air mass extending from northern Louisiana to Ohio is due to low-level rain-cooled air behind the squall line.
In east-central Texas the visible satellite imagery showed a dramatic shadow from the anvil of a complex of severe storms. Overshooting tops were also fairly pronounced in the morning sunlight.



Here is the radar reflectivity showing the squall line approaching Jackson:


And a close-up of the Doppler velocity just before the strongest winds reached JSU, showing storm-relative winds in excess of 50 kt:

Weather Roundup

Strong winds associated with a southerly buster (cold front) caused widespread damage in much of Australia. There was a dramatic rescue from the snow in Keswick Ridge, New Brunswick (Canada). And snow removal costs in northern Maine have prompted calls for federal disaster declarations.

NOAA Air Resources Lab

NOAA's Air Resources Lab (ARL) is the primary NOAA branch dealing with research on air quality and chemistry issues. Offices are located in Maryland, Tennessee, Idaho, North Carolina, and Nevada. Particular emphasis in recent years has been on urban meteorology and applications for homeland security. In recent years, JSU has worked with ARL on atmospheric dispersion studies and data collection, in concert with the UrbaNet project. Another prominent project of ARL has been the East Tennessee Ozone Study (ETOS). The NOAA/ATTD DataViewer is an effort to dynamically mesh meteorological data with a Geographic Information System (GIS).

A Mighty Long Rope

Tracking a cold front with satellite imagery is easiest when there's a rope cloud. Formed by the sudden uplift of moist air to the Lifting Condensation Level (LCL) by the hydraulic head of a surface density current, rope clouds are linear features that rarely produce significant precipitation. In some cases they may also be associated with thunderstorm outflow boundaries or mesoscale fronts. Usually their position will correspond very well with the location of the surface discontinuity, although this can vary some in cases like "tipped over" fronts where cool water surface temperatures inhibit the surface front. In some cases kinks or intersections of multiple rope clouds can help to localize uplift sufficiently to initiate thunderstorms. Generally the low cloud top height and narrow scale of rope clouds make them much easier to see with visible imagery than with infrared. An example over Florida was studied by Seitter and Muench in 1985.

Today there is an unusually long rope cloud feature, spreading in a pretty much continuous line from Ontario to Texas. The position of the cold front can be identified also in New Mexico, by the presence of upslope stratus. Comparison with the RUC analysis of equivalent potential temperature confirms the strong density discontinuity. The decreased gradient across the front in West Texas is related to the presence of a short dryline feature in northeast Mexico. A weak outflow boundary from the prefrontal convection extends across from San Antonio to southern Mississippi.