ECMWF mean 500mb jet stream flow 168-240hr mean ending 11/23 Left panel. Right panel: GFS
Penn State University Dept. of Meteorology

Not the most typical weather on a nationwide basis since September. The long-range models have on several occasions over the past two weeks suggested a re-alignment of the hemispheric flow that would bring more typical November conditions. However, they have repeatedly had to back off those projections.

The latest operational variants of the the American GFS and the European ECMWF do show some changes but also have familiar hints of the split flow, a strong and active Pacific Jet and a preference for a Mid-continent favored storm track with a distinct lack of strong signals for any widespread or lasting cold. The Rockies, Great Basin of the Southwest and the Western and Northern Plains still seem favored for snow. Bouts of warmth and rainy spells with some risk of strong thunderstorms favors the East and Dixie with some back and forth on temperatures. The model suggestions for a more wintry turn for the end of November are present true enough, but are suspect by me for now due to previous "false starts or false alarms".

In general a fairly progressive type pattern looks to continue and I have to look with eyebrows raised at model hints of a pattern change until AT LEAST after the 24th of this month. I say at least because I will suspect no major pattern shift until sometime in December unless I see changes in teleconnection points or strong model convergence to a new pattern in the consensus of equations. The long-range outlook in general for Georgia the next 30-Days is for temps to average below-normal with rainfall above-normal. The 90-Day outlook temps to average below-normal with precipitation near-normal.

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.