I have added a new look to the homepage that incorporates real-time weather from the National Weather Service and a web cam overlooking the Great Smokies. The page also changes during the day to reflect the amount sunlight. The goal here is to inspire the city-bound hiker to get on the trail this spring.
Warm weather is approaching and I thought it would be great to have more web cams and animations to watch the unfurling of Spring and severe weather.
The animation to the left is the visible light spectrum from the GOES satellite and shows a cold front passing over Kentucky this past Saturday. I have assembled a few scripts to output a daily animation from dawn until dusk at four frames per hour.
Each frame is 640 pixels square, so each daily animation is about 8-16 megabytes, depending on the length of daylight. I’m working on automating the output to a .flv movie which would be easier on bandwidth. View a large-version snippet of this of this animation here. As I get the file sizes smaller, these animations will be incorporated into the weather page.
What I find most exciting about these animations is that they are photographs. We know weather and clouds from ground level of course, but to see a corollary from space I think helps a photographer understand the movement of clouds and how they filter sunlight. Though it’d be hard to use these animations to predict future light conditions beyond a few hours, they are helpful in understanding how the atmosphere moves and, in retrospect, what cloud cover is associated with which weather conditions.
How about a full day of photographs?
The animation to the right is a full-day digest from Sunday. It shows the snow cover and lake effect snow machine impacting the Appalachians. The full animation is 9 megabytes and can be view here. Note the clearly defined line of cloud cover and snow over the mountains that straddle the TN-NC state line. To the east you can see Mt. Mitchell and its orographic uplift.
Two New Web Cams
Our current NPS web cams Look Rock and Purchase Knob both look east. I incorporated two forest service web cams, Joyce Kilmer and Cold Mountain, which both look west. Though they are not in the park, they are useful in getting a panoramic real-time view of the Great Smokies. All of these web cams can be found on our Great Smokies weather page: http://www.outrageGIS.com/weather/grsm.
Winter storm warning, Dec 15-16, 2008.
“The National Weather Service will issue a winter storm warning when a dangerous combination of heavy snow, with sleet and/or freezing rain, will occur or has a high probability of occurring within the next 12 hours.”
Severe winter weather events are notorious to forecast. Our TV weathercasters had a range of predictions from a half-inch of ice accumulation to 5 inches of snow. The National Weather Service (NWS) had a similar forecast. After the event, we had a lot of variability across the region, so in part, all of the predictions were somewhere correct. I think the NWS made a good prediction on where the bad weather would happen, but the severity was hard to calculate.
The above map (shown in greater detail below) shows low temperatures across the U.S. and highlights the problem with forecasting such a large storm. The huge temperature change from -20° F in the northern states to 60° F in the southeast indicates two vastly different air masses in close proximity. The greater the difference, the more difficult the predictions. When the air masses begin to interact and do their dance, that’s the problem.
It’s easy to forecast calm conditions, but when the weather becomes bad, the severity is determined by how different the air masses are and how much they interact. Two similar air masses doing a “big dance” will have little impact. Two vastly dissimilar air masses just interacting slightly will produce enormous weather. Though we can measure the characteristics of air masses, the problem is that we can’t predict how they’ll interact.
The basic setup we had here was that cold polar plunged southward, and displaced the warmer, more moist tropical air. As the warmer, less dense air rode over the advancing colder, denser air, precipitation fell as rain. As it traveled through the colder, ground layer of air, it froze on surface contact. That’s freezing rain. However when upper layer of air approaches the freezing point, precipitation could fall as snow or sleet. As this boundary zone between air masses changes, the vertical temperature profile changes and so does the type precipitation that hits the ground.
Can you feel it for the weathercaster trying to predict this stuff?
Watches and Warnings Map, Dec 16
Above map is the Watches and Warnings for the U.S., December 16, 2008. Compare this map to the below map, which forecasts low temperatures for this morning. Note the axis of warnings from Texas to Pennsylvania which overlays a sharp temperature contrast between the polar air to the northwest and tropical air to the southeast. This steep temperature gradient is almost 90° F and is zone of interaction.
Low Temperature Map, Dec 16
The below map shows snow and ice totals for the Bluegrass region.
Wind advisory, Sunday Dec 14, 2008
The National Weather Service will issue a wind advisory for sustained winds 31 to 39 mph for at least 1 hour or any gusts 46 to 57 mph. However, winds of this magnitude occurring over an area that frequently experiences such winds would not require the issuance a wind advisory.
Lexington recorded sustained winds of 39 mph with gusts around 45 mph. This created difficult walking conditions at times around downtown. Wind speeds can be faster in isolated parts of the built environment because of turbulence. Though cities typically decrease average wind speeds near ground level, during periods of high wind speed tall buildings interrupt flow causing them to ‘pile up’ on windward sides and then rush off with increased velocity in channels between buildings. Wind eddies on the leeward side of buildings will also have strong gusts.
A comparison to a simple stream might be helpful. Water flows fastest in the stream’s center. An obstruction blocking the stream will divert the flow into two channels, each with increased velocity. Unlike water though, air can be highly compressed. When air piles up on the windward or upstream side, it will move in a more dynamic manner around the obstruction. Turbulence.
The mechanics of how all this plays out is beyond my understanding, but it is sufficient to say that when a wind advisory is issued for Lexington, we can experience strong and erratic winds downtown. I noticed many flocks of pigeons remaining on their protective perch, and no soaring hawks or vultures, so that’s probably a good gauge of strong turbulence.
The last time the area was under a wind advisory was during the passing of the remnants of Hurricane Ike. I think winds gusted to near-hurricane strength. Winds this weekend are associated with a strong low that passed to our north. This low pressure system created blizzard conditions and wind chill warnings across the northern states. The system also brought the cold punch of air that created our first major winter storm a day later.
The chart below is from the Beaufort Scale of wind speed. So next time you’re wondering, “What’s that wind speed?” here’s your chart.
| Description | Speed | Visual Clues and Damage Effects |
|---|---|---|
| Calm | Calm | Calm wind. Smoke rises vertically with little if any drift. |
| Light Air | 1 to 3 mph | Direction of wind shown by smoke drift, not by wind vanes. Little if any movement with flags. Wind barely moves tree leaves. |
| Light Breeze | 4 to 7 mph | Wind felt on face. Leaves rustle and small twigs move. Ordinary wind vanes move. |
| Gentle Breeze | 8 to 12 mph | Leaves and small twigs in constant motion. Wind blows up dry leaves from the ground. Flags are extended out. |
| Moderate Breeze | 13 to 18 mph | Wind moves small branches. Wind raises dust and loose paper from the ground and drives them along. |
| Fresh Breeze | 19 to 24 mph | Large branches and small trees in leaf begin to sway. Crested wavelets form on inland lakes and large rivers. |
| Strong Breeze | 25 to 31 mph | Large branches in continuous motion. Whistling sounds heard in overhead or nearby power and telephone lines. Umbrellas used with difficulty. |
| Near Gale | 32 to 38 mph | Whole trees in motion. Inconvenience felt when walking against the wind. |
| Gale | 39 to 46 mph | Wind breaks twigs and small branches. Wind generally impedes walking. |
| Strong Gale | 47 to 54 mph | Structural damage occurs, such as chimney covers, roofing tiles blown off, and television antennas damaged. Ground is littered with many small twigs and broken branches. |
| Whole Gale | 55 to 63 mph | Considerable structural damage occurs, especially on roofs. Small trees may be blown over and uprooted. |
| Storm Force | 64 to 75 mph | Widespread damage occurs. Larger trees blown over and uprooted. |
| Hurricane Force | over 75 mph | Severe and extensive damage. Roofs can be peeled off. Windows broken. Trees uprooted. RVs and small mobile homes overturned. Moving automobiles can be pushed off the roadways. |
One of the most obvious and striking winter weather conditions is the atmospheric influence of the Great Lakes. When high pressure builds to the west of the Great Lakes and there’s a prominent northwest-to-southeast surface wind flow, weather is significantly different downwind of the lake compared to upwind. This often produces lake effect snow that can truly bury southeastern shore cities. However, the same influence can reach far south into Kentucky and the Appalachians.
The difference in weather can be outstanding, as people who live near the lakes know very well. Observe weather on both sides of Lake Michigan. The red number indicates the temperature in °F and the circle indicates cloud cover. A solid circle is completely cloudy, and an empty circle represents clear skies. The barb indicates wind speed and direction (see below for legend).
Compare Lake Michigan’s west shore with its east shore. The west shore of Lake Michigan is cold, below zero, and cloudless. Weather on the east shore of Lake Michigan is 20°F warmer, it is cloudier, and snow is falling, as represented by the green asterisks.
The cause of these conditions is simply the great reservoir of heat that the lakes contain and transfer to the atmosphere through evaporation. Though the lakes are slowly cooling through winter, they are significantly above freezing across a large area. When a colder, dry air mass blows over the lakes, evaporation will transfer heat and moisture into the air mass. As the rising, warmer air cools, if often condenses resulting in cloudiness and heavy snow; the lake effect snow machine. Also note the how far south the warmer air has penetrated compared to other locations at the same latitude.
For example this station here: 
is reporting that the current temperature is 55 degrees F, the sky is clear, and the winds are blowing from the north at 18 miles per hour.
And this station 
is reporting completely overcast conditons, winds from the north at 24 miles per hour and the temperature is 58 degrees F.
