PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE PEACHTREE CITY GA
200 PM EDT WED NOV 4 2009

...HISTORICAL RAINFALL FREQUENCY AMOUNTS OCCUR OVER PORTIONS OF
   NORTH GEORGIA IN SEPTEMBER 2009...

THE NATIONAL WEATHER SERVICE HAS BEEN REVIEWING THE EXTREME HEAVY
RAINFALL THAT OCCURRED IN SEPTEMBER 2009 WHICH CAUSED EPIC FLOODING
IN PORTIONS OF NORTH GEORGIA. NUMEROUS LOCATIONS IN DOUGLAS...EAST
PAULDING...WEST COBB...EAST CARROLL...CENTRAL GWINNETT AND SOUTHWEST
WALKER COUNTIES EXCEEDED 10 INCHES OF RAIN DURING A 24 HOUR PERIOD.
THE GREATEST AMOUNT WAS 16.7 INCHES JUST WEST OF DOUGLASVILLE.

THE FOLLOWING AMOUNTS ARE FOR 24 HOUR RAINFALL FREQUENCY PERIODS IN
NORTH GEORGIA AND THE CHANCES OF RECURRENCE IN ANY GIVEN YEAR...

7.2 INCHES IS 1.0 PERCENT...100 YEAR RAIN EVENT
7.7 INCHES IS 0.5 PERCENT...200 YEAR RAIN EVENT
8.2 INCHES IS 0.2 PERCENT...500 YEAR RAIN EVENT
8.7 INCHES IS 0.1 PERCENT...1000 YEAR RAIN EVENT
9.7 INCHES IS 0.05 PERCENT...5000 YEAR RAIN EVENT

USING RAINFALL FREQUENCY CALCULATIONS...IT HAS BEEN DETERMINED THE
CHANCES OF 10 INCHES OR MORE OCCURRING AT ANY GIVEN POINT ARE LESS
THAN ONE HUNDRETH OF ONE PERCENT. THIS MEANS THE ODDS ARE 1 IN 10000
OR MORE OF RECURRENCE IN ANY GIVEN YEAR...OR A 10000 YEAR RAIN EVENT.

WEATHER RADAR ESTIMATES COMBINED WITH AUTOMATED HOURLY RAINGAGE
REPORTS INDICATED THE 24 HOUR PERIOD WITH THE MOST INTENSE RAINFALL
FOR THE WHOLE STORM EVENT WAS FROM 800 PM EDT ON SEPTEMBER 20
THROUGH 800 PM EDT ON SEPTEMBER 21.

Looking at rainfall amounts since September 1, Athens and Macon was the wettest 61 day period for September and October on record, while Atlanta was number 2 and Columbus was number 3. It is interesting to note that the rainfall experienced this year is in no way attributable to a landfalling Tropical Storm or Hurricane. A look back through the National Hurricane Center's Archives shows that the remnants of Tropical Storm Irene impacted Columbus October 6-10th, 1959; Hurricane Dora affected Columbus September 9-12, 1964; and the remnants of Hurricane Opal impacted both Columbus and Atlanta October 3-6th, 1995. Unfortunately data on tropical storms in 1888 and 1937 was unavailable.

FEMA publishes flood insurance rate maps (F.I.R.M.) that show various categories of flood hazard zones. However, the calculations, engineering, and surveying needed to determine those zones are done by engineers working in the private sector. This work might be done under these circumstances:

1. An area-wide study (maybe an entire county) to update a set of F.I.R.M.s, such as for an urbanizing area. This will normally be contracted by the local community, often with Federal and State funding.
2. Study of a specific river or stream, within a single community, to assist with planning and orderly development. The community will normally contract with the engineer.
3. Study of a short stretch of a waterway that is to be developed (or has been developed) to determine the impact of the development on the flood-carrying capacity. This is typically contracted and paid for totally in the private sector.
4. Occasionally FEMA itself, or another branch of the Federal or State government will contract with private sector engineers for a major flood study. This is most typically true for levee systems or flood control dams and reservoirs.

Hydrology and Hydraulics

For this flood plain work, the engineer must complete certain calculations. Hydrology calculations predict rainfall and compute the resulting stream flow. This is determined by the physical characteristics of the drainage basin--area, slope, shape, soil type, amount of development--and the regional climate--probable rainfall pattern and intensity based on years of historical records.

FEMA procedures require that all flood mapping be based on what is called the 100-year flood. This is the rainfall amount and associated stream flow that have a 1 percent chance of occurring in any given year (and thus statistically should occur once every hundred years). FEMA calls this the base flood. Calculations are also done for the 500 year flood, although that is less important in flood insurance issues than is the base flood.

Hydraulic calculations take the run-off determined from hydrology and compute the depth and spread of flood waters. The physical characteristics of the stream, such as slope, main channel dimensions, overbank dimensions, roughness, obstructions, and development on the overbank, are all factored into the calculations.

The hydraulics of most interest are for the base flood. The calculations determine the height of flood waters from the 100 year rainfall, which is then called the base flood elevation (BFE). This is the elevation of greatest regulatory significance within the flood plain.

Flooding occurs when an existing stream (such as a river or creek) can't handle the waterflow. The cause of the high waterflow varies, but generally comes from high amounts of precipitation, or from snowmelt. The existing channel is overwhelmed, and the water "comes out of bank" to enter what is termed the flood plain.

The floodplain is a part the terrain adjacent to the channel where water doesn't normally flow; a floodplain is often not obvious to the inexperienced observer. A floodplain is simply a natural storage reservoir for flood waters, and has been created by nature, the master engineer, through thousands of years of water flow and floods.

When water leaves the normal flow channel, you have a flood event. The flood level is defined by the amount of water present. There are two general factors affecting the level of the flood:

• The amount of water in the channel.
• The shape and characteristics of the channel at a given point.

The amount of water is governed by local precipitation: snowpack, rainfall, and sometimes storage capacity in a reservoir.

The channel and shape characteristics control how fast the water flows. A narrow, steep channel tends to move water quickly, while a wide, flat channel moves water slowly. When water moves slowly, it tends to rise, or back up. This can cause over bank flooding. Other factors, such as vegetation and soil, will also affect water flow.

Flood events are defined by the probability that a certain amount of water is possible any one year. For example, the infamous "100-year flood" is in fact the level of water with a 1-percent chance (1 in 100) occurring any one year. The amount of water actually varies from river to river. In fact, that amount can vary along a river. The use of "n year flood" is technical jargon that has caused endless problems with the public. The term does not mean that a flood occurs every n years, but that it has a chance of 1/n of occurring any one year. Water volume increases as the probability decreases. The table below shows how this might affect you:

In short, the infrequent floods tend to high and violent water flows (and a good thing, too!). The common floods are much smaller, although damage is still possible.

"Great!" you say, "But how high is a 5-year flood?" The answer, as noted earlier, is not simple. That's because the climate varies, and conditions affecting water flow along a channel change. Therefore, each site must be examined to determine the potential water elevations! This has been done extensively across the country by FEMA, for 100 and 500 year flood plains in selected communities. Such a study is rarely made for lesser floods, although data from 100 flood plain studies can yield estimates for lesser floods.

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE PEACHTREE CITY GA
200 PM EDT WED NOV 4 2009

...HISTORICAL RAINFALL FREQUENCY AMOUNTS OCCUR OVER PORTIONS OF
   NORTH GEORGIA IN SEPTEMBER 2009...

THE NATIONAL WEATHER SERVICE HAS BEEN REVIEWING THE EXTREME HEAVY
RAINFALL THAT OCCURRED IN SEPTEMBER 2009 WHICH CAUSED EPIC FLOODING
IN PORTIONS OF NORTH GEORGIA. NUMEROUS LOCATIONS IN DOUGLAS...EAST
PAULDING...WEST COBB...EAST CARROLL...CENTRAL GWINNETT AND SOUTHWEST
WALKER COUNTIES EXCEEDED 10 INCHES OF RAIN DURING A 24 HOUR PERIOD.
THE GREATEST AMOUNT WAS 16.7 INCHES JUST WEST OF DOUGLASVILLE.

THE FOLLOWING AMOUNTS ARE FOR 24 HOUR RAINFALL FREQUENCY PERIODS IN
NORTH GEORGIA AND THE CHANCES OF RECURRENCE IN ANY GIVEN YEAR...

7.2 INCHES IS 1.0 PERCENT...100 YEAR RAIN EVENT
7.7 INCHES IS 0.5 PERCENT...200 YEAR RAIN EVENT
8.2 INCHES IS 0.2 PERCENT...500 YEAR RAIN EVENT
8.7 INCHES IS 0.1 PERCENT...1000 YEAR RAIN EVENT
9.7 INCHES IS 0.05 PERCENT...5000 YEAR RAIN EVENT

USING RAINFALL FREQUENCY CALCULATIONS...IT HAS BEEN DETERMINED THE
CHANCES OF 10 INCHES OR MORE OCCURRING AT ANY GIVEN POINT ARE LESS
THAN ONE HUNDRETH OF ONE PERCENT. THIS MEANS THE ODDS ARE 1 IN 10000
OR MORE OF RECURRENCE IN ANY GIVEN YEAR...OR A 10000 YEAR RAIN EVENT.

WEATHER RADAR ESTIMATES COMBINED WITH AUTOMATED HOURLY RAINGAGE
REPORTS INDICATED THE 24 HOUR PERIOD WITH THE MOST INTENSE RAINFALL
FOR THE WHOLE STORM EVENT WAS FROM 800 PM EDT ON SEPTEMBER 20.

A seasonal-scale climatological analysis correlating spring tornadic activity with antecedent fall-winter drought in the southeastern United States

Marshall Shepherd¹, Dev Niyogi² and Thomas L Mote¹

¹ Climate Research Laboratory, Department of Geography, University of Georgia, Athens, GA 30602, USA
² Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN 47907, USA

Received 10 February 2009
Accepted 16 June 2009
Published 24 June 2009

NOAA: El Niño to Help Steer U.S. Winter Weather

October 15, 2009

High Resolution (Credit: NOAA)

El Niño in the central and eastern equatorial Pacific Ocean is expected to be a dominant climate factor that will influence the December through February winter weather in the United States, according to the 2009 Winter Outlook released today by NOAA's Climate Prediction Center. Such seasonal outlooks are part of NOAA's suite of climate services.

"We expect El Niño to strengthen and persist through the winter months, providing clues as to what the weather will be like during the period," says Mike Halpert, deputy director of the Climate Prediction Center - a division of the National Weather Service. "Warmer ocean water in the equatorial Pacific shifts the patterns of tropical rainfall that in turn change the strength and position of the jetstream and storms over the Pacific Ocean and the U.S."

"Other climate factors are also likely to play a role in the winter weather at times across the country," added Halpert. "Some of these factors, such as the North Atlantic Oscillation are difficult to predict more than one to two weeks in advance. The NAO adds uncertainty to the forecast in the Northeast and Mid-Atlantic portions of the country."

Highlights of the U.S. Winter Outlook (December through February) include:

• 

  High Resolution (Credit: NOAA)

  Warmer-than-average temperatures are favored across much of the western and central U.S., especially in the north-central states from Montana to Wisconsin. Though temperatures may average warmer than usual, periodic outbreaks of cold air are still possible.
• Below-average temperatures are expected across the Southeast and mid-Atlantic from southern and eastern Texas to southern Pennsylvania and south through Florida.
• Above-average precipitation is expected in the southern border states, especially Texas and Florida. Recent rainfall and the prospects of more should improve current drought conditions in central and southern Texas. However, tornado records suggest that there will also be an increased chance of organized tornado activity for the Gulf Coast region this winter.
• Drier-than-average conditions are expected in the Pacific Northwest and the Ohio and Tennessee River Valleys.
• Northeast: Equal chances for above-, near-, or below-normal temperatures and precipitation. Winter weather in this region is often driven not by El Niño but by weather patterns over the northern Atlantic Ocean and Arctic, such as the North Atlantic Oscillation. These patterns are often more short-term, and are generally predictable only a week or so in advance.
• California: A slight tilt in the odds toward wetter-than-average conditions over the entire state.
• Alaska: Milder-than-average temperatures except along the western coast. Equal chances for above-, near-, or below-median precipitation for most areas except above median for the northwest.
• Hawaii: Below-average temperatures and precipitation are favored for the entire state..

This seasonal outlook does not predict where and when snowstorms may hit or total seasonal snowfall accumulations. Snow forecasts are dependent upon winter storms, which are generally not predictable more than several days in advance.

FEMA publishes flood insurance rate maps (F.I.R.M.) that show various categories of flood hazard zones. However, the calculations, engineering, and surveying needed to determine those zones are done by engineers working in the private sector. This work might be done under these circumstances:

1. An area-wide study (maybe an entire county) to update a set of F.I.R.M.s, such as for an urbanizing area. This will normally be contracted by the local community, often with Federal and State funding.
2. Study of a specific river or stream, within a single community, to assist with planning and orderly development. The community will normally contract with the engineer.
3. Study of a short stretch of a waterway that is to be developed (or has been developed) to determine the impact of the development on the flood-carrying capacity. This is typically contracted and paid for totally in the private sector.
4. Occasionally FEMA itself, or another branch of the Federal or State government will contract with private sector engineers for a major flood study. This is most typically true for levee systems or flood control dams and reservoirs.

Hydrology and Hydraulics

For this flood plain work, the engineer must complete certain calculations. Hydrology calculations predict rainfall and compute the resulting stream flow. This is determined by the physical characteristics of the drainage basin--area, slope, shape, soil type, amount of development--and the regional climate--probable rainfall pattern and intensity based on years of historical records.

FEMA procedures require that all flood mapping be based on what is called the 100-year flood. This is the rainfall amount and associated stream flow that have a 1 percent chance of occurring in any given year (and thus statistically should occur once every hundred years). FEMA calls this the base flood. Calculations are also done for the 500 year flood, although that is less important in flood insurance issues than is the base flood.

Hydraulic calculations take the run-off determined from hydrology and compute the depth and spread of flood waters. The physical characteristics of the stream, such as slope, main channel dimensions, overbank dimensions, roughness, obstructions, and development on the overbank, are all factored into the calculations.

The hydraulics of most interest are for the base flood. The calculations determine the height of flood waters from the 100 year rainfall, which is then called the base flood elevation (BFE). This is the elevation of greatest regulatory significance within the flood plain.

Flooding occurs when an existing stream (such as a river or creek) can't handle the waterflow. The cause of the high waterflow varies, but generally comes from high amounts of precipitation, or from snowmelt. The existing channel is overwhelmed, and the water "comes out of bank" to enter what is termed the flood plain.

The floodplain is a part the terrain adjacent to the channel where water doesn't normally flow; a floodplain is often not obvious to the inexperienced observer. A floodplain is simply a natural storage reservoir for flood waters, and has been created by nature, the master engineer, through thousands of years of water flow and floods.

When water leaves the normal flow channel, you have a flood event. The flood level is defined by the amount of water present. There are two general factors affecting the level of the flood:

• The amount of water in the channel.
• The shape and characteristics of the channel at a given point.

The amount of water is governed by local precipitation: snowpack, rainfall, and sometimes storage capacity in a reservoir.

The channel and shape characteristics control how fast the water flows. A narrow, steep channel tends to move water quickly, while a wide, flat channel moves water slowly. When water moves slowly, it tends to rise, or back up. This can cause over bank flooding. Other factors, such as vegetation and soil, will also affect water flow.

Flood events are defined by the probability that a certain amount of water is possible any one year. For example, the infamous "100-year flood" is in fact the level of water with a 1-percent chance (1 in 100) occurring any one year. The amount of water actually varies from river to river. In fact, that amount can vary along a river. The use of "n year flood" is technical jargon that has caused endless problems with the public. The term does not mean that a flood occurs every n years, but that it has a chance of 1/n of occurring any one year. Water volume increases as the probability decreases. The table below shows how this might affect you:

In short, the infrequent floods tend to high and violent water flows (and a good thing, too!). The common floods are much smaller, although damage is still possible.

"Great!" you say, "But how high is a 5-year flood?" The answer, as noted earlier, is not simple. That's because the climate varies, and conditions affecting water flow along a channel change. Therefore, each site must be examined to determine the potential water elevations! This has been done extensively across the country by FEMA, for 100 and 500 year flood plains in selected communities. Such a study is rarely made for lesser floods, although data from 100 flood plain studies can yield estimates for lesser floods.

MODELS INDICATE
WIDESPREAD 2-4 INCHES...AND 12Z MODELS COMING IN THIS MORNING HAVE
HIGHER QUANTITATIVE PRECIPITATION FORECASTS (QPF)THAN LAST NIGHT`S RUNS. 
WILL HAVE TO KEEP AN EYE ON
THINGS BUT MODELS TYPICALLY ARE UNDERDONE ON LARGE QPF EVENTS.
EXPECT WIDESPREAD MINOR FLOODING AT LEAST...WOULD NOT BE SURPRISED
TO SEE SEVERAL GAUGES RISE TO MODERATE FLOOD STAGE WITH PERHAPS A
FEW MAJORS. STAY TUNED FOR FURTHER UPDATES ON THIS SITUATION.

Minor, local flooding may stem from this latest rash of southern downpours. Wider impact will be that of rain-slicked roadways and the slowing of highway traffic. And travelers through the airport may find delayed flights.

The highest snowfall of 3 to locally 6 inches will stretch eastward from Wyoming near the South Dakota-Nebraska line. However, most amounts along this snow band will range from 1 to 3 inches.

The snow and unusual cold will make for conditions of wintry driving with road surfaces varying from slush and wet to slick, icy and hazardous. Interstate highways I-25, I-29, I-35, I-80, I-90 and I-94 will be among the key roadways subject to wintry driving conditions. Minneapolis-Saint Paul will get its first significant snowfall of the season from this storm.The first big Pacific storm is set to strike California and the rest of the West Coast as early as late Monday night. The monster storm will bring with it plenty of rain, some very heavy, along with strong gusty winds. Rain amount by Wednesday morning will top 6 inches in a few instance, amounts of 2-4 inches being widespread over the northern two-thirds of the state. These are connected to last week's powerful western Pacific Typhoon. Melor dissipated after a pounding of Japan, but its heavy load of tropical warmth and moisture has since been handed off down stream to a storm slated to reach the California coast on Tuesday. Southern California north and west of Los Angles is much more likely to get a soaking than areas from the city on south and east.

Weather buzz in the East is likely to include talk of early snow late in the week. The storm will have tropical warmth and moisture to one side and some unusually early cold to the other.

The storm center will track across the mid-Mississippi Valley by early Thursday on to the Atlantic Seaboard on Friday. Cold soaking rain will fall north of the immediate storm track over the Ohio Valley to the mid-Atlantic.

Enough cold air will settle in place over the north-central Appalachian region to trigger a little wet snow. Should all the essential factors fall into place, a heavy wet snowfall would be the result in the higher elevations of Pennsylvania and New York. High temperatures Wednesday through Friday will be held in the 40s for the first time this season across the interior Northeast, including Buffalo, N.Y., Boston, Mass., and State College, Pa.

Temperatures at night will drop into the 20s across the higher elevations and 30s in most other areas to the north of the Mason-Dixon Line. If you haven't done so already, you may want to pull out your heavier coats for the cold mornings that lie ahead later this week. Monday night into Tuesday will bring some snow to the Adirondack Mountains of New York and the mountains of northern Vermont, New Hampshire and Maine. Accumulations between 1 and 3 inches are possible.

Further south, outbreaks of thunderstorms will unleash downpours, some heavy enough to spark flooding. This in a corridor that has seen at least two-times the normal rainfall since the start of September.The primary suite of medium-range models (GFS, ECMWF, GEM) have suggested that in parts of Georgia we will have to monitor potential for some flooding, some severe weather and at the tail end of the event MAYBE an early frost or freeze and maybe just maybe some curiosity flurries as the seasons first Nor 'Easter brings winter from the Great Lakes region to New England and then sends a sliver South into the heart of Dixie or the Southeast. As always I remind you the models are simulations not the real weather and even if they all agree they can all be wrong, and the devil is in the details and I will take care of that on a daily basis on the radio.

Leaving America, An outbreak of cold more fitting of December than October will knife southward out of Scandinavia into the heart of central and eastern Europe early this week. Rain and snow from Germany and Poland south into the middle of the Balkan Peninsula. Significant snowfall will keep mostly to hilly peeks and, especially, to the mountains such as the Carpathian, Alps and Balkan highlands. Locally, there will be heavy mountain snow.

Soaking rain will break out near the Adriatic Sea on Monday, then will shift east to Romania, Bulgaria and western Ukraine by mid week.