How Much Rain is Needed to Fill Lake Lanier?

Wednesday, December 17th, 2008

The rain last week raised the water level of Lake Lanier about a foot, which likely means that Atlanta’s water supply won’t reach a record low in 2008. As usual, the mainstream media issued stories which said that the rain is good, but the drought isn’t over yet, we’ve got a long way to go, etc. &ct.

Now comes the Southeast River Forecast Center with the best estimate (PDF) I’ve seen for what it will take to get Lake Lanier to full pool — or 1071 feet above sea level, about 20 feet higher than where it is today.

Surprisingly, it’s only 10% above normal rainfall for a year.

The forecast center measured annual precipitation against Lake Lanier’s level since the lake began to fill back in 1952.  It determined that, on average, 60 inches of rain fell in the lake’s drainage basin per year, providing about 1.36 million acre feet of water.  Using those figures, and accounting for the fact that the Corps of Engineers has restricted the outflow from Lanier over the past year, the Forecast Center says,

Lake Lanier’s pool was at 1054 feet msl at the beginning of this water year (1.37 million acre-ft). To get to full pool at elevation 1071 feet msl, an additional 585,000 acre-ft of inflow is needed. However, this assumes no outflow. If we apply the annual average outflow of last year for this year (1070 cfs), 775,000 acre-ft are released downstream. So the total inflow volume needed to fill the reservoir and sustain a release similar to last year is 1.36 million acre-ft.
…[T]his would put the total annual precipitation at around 65 inches (following the lower end of the trend). This is only about 10% above normal.

Of course, part of the equation depends on when the water comes. If we get a lot of excessive rain in the winter, it will count more than if we get it in the summer, since the heat and absorption by plants prior to the water reaching the lake will be higher in the summer months. The calculations used in the analysis are based on rain during the water year, which starts on October 1, and there has already been six inches of rainfall. That means there are only 59 inches to go.

I listened to Lieutenant Governor Casey Cagle (who is clearly running to become governor in 2010) addressing the Gwinnett Chamber’s monthly meeting this afternoon. He dumped most of the problems with metro Atlanta’s water supply on the Corps of Engineers favoring mussels in Apalachicola Bay over the water needs of Georgia. He also offered an innovative idea for increasing the water available to metro Atlanta. He estimated 10% of the lake’s capacity had been lost due to silt (something I’m not going to argue with). By dredging the lake, Cagle says we could not only expand the lake’s capacity to hold more water, we could recycle and resell any valuable materials that were dredged up.

In my mind, Cagle’s plan will only work if Georgia gets some formal rights to the lake’s water. Based on court decisons and the official use plan for the lake, we aren’t there yet.

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Say Goodbye to Indian Summer

Thursday, November 6th, 2008

North Georgia has been blessed this year with a gorgeous few days of Indian Summer, traditionally defined as warm weather following a frost. Following a chilly end to October, November has been a delight, with temperatures more than five degrees above normal, which should be around 66. Of course, Indian Summer has its downside: a lack of rainfall. It’s been dry for the past two weeks, although that’s likely to change soon, as you’ll see below.

The dry weather hasn’t had much impact on the drought situation. The latest drought monitor doesn’t show much change from what we’ve seen since Labor Day. The latest drought outlook, released this morning, calls for some improvement in the short term, but paints a grimmer picture as winter settles in. The silver lining might be that the Lake Lanier drainage basin may get more precipitation than metro Atlanta proper.

As indicated by this post’s title, the mild weather we’ve had recently is going to come to an end, beginning tomorrow, when a cold front brings rain, and then cooler weather for the weekend. Following that, we’ve got a series of storms that are likely to bring rain several times over the next ten days or so. Temperatures are likely to seesaw during the period, with warmer weather during the week, and cooler weather on the weekends.

It’s all part of the battle between summer’s warmth and winter’s cold, with winter inevitably winning out as we move into December. The tropics are making a last stand as well. Hurricane Paloma has formed in the Gulf of Mexico, and appears to be heading towards Cuba as a category two storm, before recurving back out into the Atlantic, with no threat to the United States. Despite 17 Atlantic storms this year, Florida State University is reporting that overall, tropical activity in the Northern Hemisphere is down over the last two years, primarily due to a lack of storms in the Pacific. This is the lowest level of northern hemisphere storms seen in 30 years.

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Things We Haven’t Seen in a While

Saturday, October 4th, 2008

I almost hate to talk about it, in fear of creating a jinx, but it looks like the Atlanta area could get some rain starting late next week.  The last time we had any rainfall at all, either here or Atlanta Hartsfield was back on September 11th, when .01 inches was recorded.  That means we are at 23 days and counting without rain. Now, models are indicating a trough will begin to move from west to east, and could bring rain sometime Wednesday or Thursday.  It may not be one of these one hour wonders either.  Accuweather says that the rain will stick around through next weekend, bringing almost two inches by Columbus Day. The CPC extended outlook calls for above normal rainfall in the 6-10 day period and weather.com is less optimistic, with rain only on Wednesday or Thursday.

I did the annual fall aerating and overseeding for my fescue lawn two weeks ago, which means that I get a pass from the watering restrictions, and can turn on my irrigation system on the appropriate odd/evn schedule.  Even with watering, when I’ve dug a few holes to plant some new bulbs and shrubs, I’m amazed at how dry the ground is. The rain, if it comes, will be much appreciated.

Another thing we haven’t seen in a while is freezing temperatures.  While North Georgia typically doesn’t get its first frost until the second week of November, Michigan, Vermont and Maine are seening freezing temperatures over the next few days.  Nationwide, the lowest temperature this morning was 17 in Stonington, Michigan.  There’s a frost advisory tonight starting in Western Michigan and moving east through northen Ohio, much of northern Pennsylvania and southern New York.  And, snow has begun to fall in the Rocky Mountains, including areas of Utah and Colorado, where a few inches are predicted through tomorrow evening.

You can keep up with the approach of winter on our Winter Weather page.  It’s just a matter of time before we begin to feel winter’s effects here in Georgia.

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While I Was Gone…

Monday, September 29th, 2008

It’s been almost two weeks since I’ve posted… partially because of some well-deserved vacation, partially because, at least here in North Georgia, a fairly quiet weather pattern, and partially because of some other interesting events taking up my web time.

So, what did we miss. After a few weeks of quiet on the tropical front, last week brought Hurricane Kyle, which was notable in that in the first time in 17 years or so, a hurricane made landfall in Canada. Right now, Subtropical Storm Laura is on her way to England, apparently, bothering no one, really. And then there was the storm that didn’t get named despite showing some tropical characteristics—including an eye, but brought rain to the Carolinas and up into the Mid-Atlantic states.

During all this turmoil, weatherwise and otherwise, the Atlanta area stayed dry.  I’ve recorded less than 2/3 of an inch of rain this month, and the official rain gauge at Hartsfield airport shows 3/4 of an inch, with no meaningful precipitation since back on the 12th.  That’s about 3 inches less than a normal September, although one only has to go back to 2005 to find less, when we only had a quarter of an inch.  The final total could change, since there are some storms in the area tonight, but it’s highly likely that we’ll go through another month with below-normal rainfall.

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What a Difference a Storm Makes

Thursday, August 28th, 2008

The two maps below show the tremendous difference a tropical system can make in dampening the effects of a drought. The map at left shows drought conditions in Georgia as of Tuesday, August 19th, when almost ten percent of the state was in exceptional drought conditions and only two percent was drought free. The right hand map shows conditions as of Tuesday morning the 26th at 7 AM. Now, none of the state is in exceptional drought, and 16% is drought free.

Remember, the cutoff for the drought maps was 7 AM Tuesday, which was before a good bit of the rain hit North Georgia, so I expect to see continued improvement with next week’s drought monitor as well. Lake Lanier has also risen just over two feet from the rainfall brought on by Fay.

The weather service has investigated the severe weather brought about by Fay, and has issued a report saying that there were six tornadoes in Georgia brought on by the storm. Most of the damage was in Hall and Jackson counties. Three EF1 tornadoes with 90 MPH winds were verified in Hall, where trees were knocked down and damage was reported to an elementary school.

Another tornado with 100 MPH winds touched down near Commerce in Jackson County, while two additional EF0 tornadoes with 70 MPH winds were reported in Monroe and Wilkes counties.

(more…)

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