National Weather Service Office Charleston May , 1997 Volume 1, Issue 4
5777 S. Aviation Ave. Charleston, SC 29406
803-744-0303

Dr. Gray's Atlantic Basin Hurricane forecast for 1997

Hurricane season is here! Dr. William Gray, a noted hurricane researcher from Colorado State University predicts another active season (June 1-November 30) with a total of 11 tropical storms. Seven will become hurricanes, and of these, three are forecast to become major hurricanes (i.e. Cat 3, 4 or 5 with winds greater than 110 mph). Seasonal averages for the Atlantic Ocean are 9.3 tropical storms, 5.8 hurricanes, and 2.1 major hurricanes.

How accurate was Dr. Gray's 1996 prediction for Atlantic hurricanes? His forecast of 11 tropical storms / 7 hurricanes / 3 major hurricanes fell short of the 13 storms / 11 hurricanes that formed. Dr. Gray makes his preliminary forecasts for each season in November, with updates in April, June, and August to reflect observed West African rainfall, Pacific Ocean sea surface temperatures, upper level winds, and surface pressure patterns over the Atlantic Ocean and Gulf of Mexico. When interpreting these forecasts, please note that there is no mention of where or if the tropical systems will make landfall; only their formation is predicted.

If 1997 proves to be another above average year, then the years 1995-97 would become the most active three-year hurricane span in history...a switch from the quiet years of 1991-94. Actually, we may be heading into a period of increased activity, according to researchers at Colorado State University. A look back at the last 25 years (excluding 1995-96) shows a relatively quiet period in time, with only a few intense hurricanes (Cat 3 or greater) affecting the US east coast, Caribbean Islands, or the Gulf of Mexico coast.

Now is the time to make your hurricane plan.

* Know the hurricane risks in your area.

* Become familiar with your local hurricane evacuation route(s).

* Know where local emergency shelters are (even if you plan on staying).

* Devise a family plan of evacuation to a safe area...if ordered to by authorities (plan on staying at a motel/shelter).

* Obtain necessary materials to secure your home.

* Keep rain gutters and down spouts clear of debris.

* Determine a safe inland place to store your boat.

(cont. page 2)

In This Issue:

1. 1997 Hurricane forecast / Hurricane preparedness tips.
2. Discussion of Probability of precipitation in forecasts.
3. Heat and Ultra-Violet (UV) Index / Spotter reporting guidelines.
4. Rip currents / The ideal beach water temperature.
5. Tide info / Cooperative observer news.

6. Spotter news & Area weather roundup.

Visit our web page site
on the internet:
http://wchs.csc.noaa.gov

Includes local weather observations/forecasts & local historical/ climatological data for southeastern SC & GA.

 

Hurricane preparedness cont...

* Review insurance policy & find a safe place for important legal documents.

* Have a good supply of cash on

hand since ATMs will not be

accessible, if there is no power.

* Fill gallon jugs with enough water per person (5 gallons) for several days. The local water supply will likely be polluted.

* Do not forget a hurricane plan

for your pet. Include places of shelter and a supply kit.

* Review the needs and working condition of emergency equipment such as flashlights,

batteries, and portable radios.

* Do not wait until the Hurricane /

Tropical Storm Watch is posted to do important chores such as

filling up your automobile's gas

tank. There may not be service at

area stations until you get well inland.

* After a watch is posted for your area, insurance companies will not sell policies due to the increased risk...Insure your property/valuables before
Hurricane season.

* Always listen to authorities regarding evacuations. They know the dangers of staying in harm's way.

What does the probability of precipitation in the weather forecast mean?

The probability of precipitation is the likelihood of measurable precipitation (0.01 inches or greater) for a specified forecast time period, and occurring at any point for which the forecast is valid. The probability is expressed as a percent and can be misleading to the public when decid-ing if scattered rain will occur, or if the day will be a complete washout.

The meteorologist assigns a precipitation probability that shows his/ her confidence in the forecast, the areal coverage, and duration of the event. Precipitation probabilities are misinterpreted when thunderstorms are forecast, as there is some degree of uncertainty in the areal coverage and event duration. An example used in the summer follows:

"Partly cloudy. A 30 percent chance for showers and thunderstorms".

A forecast contains these terms when specific probabilities are used:

(NWS Operations Manual) POP=Probability of Precipitation.

POP (%) Expression of Uncertainty Thunderstorm Areal Coverage
20 Slight chance Isolated/Few/Widely Scattered 30-50 Chance Scattered
60-70 Likely Numerous (or none used )
80-100 (none used...may already be happening...expected to continue)

If you are one of the three in 10 people to experience typical summer-time seabreeze thunderstorms that form near the Georgia and South Carolina coastlines, then you have probably thought that quite a bit of rain fell for a long period of time (with only a 30% POP in this fore-cast example). Remember that the forecast may cover an area of several counties. The forecast has to represent an average of weather that the meteorologist expects to occur throughout the period. A higher probability would mean more areas will receive rainfall amounts of at least 0.01 inches for a given time period.

When planning outdoor activities, always check the latest short term forecast (Nowcast). It covers a smaller geographic area and time frame, and can be more site specific with the arrival and departure times of rainfall. Winds, temperature, cloud cover, and significant weather are included in these forecasts, too.

Meteorologists rely on computer model guidance to make a precipitation probability forecast. If the models depict similar outcomes, a forecaster is likely to follow guidance. If the models are showing different scenarios, then the forecaster must consider many elements such as past model performance with recent precipitation events. Local effects such as the sea breeze must also be considered when assigning a precipitation probability in the forecast.

Page 2.

 

The Heat Index

Heat index (HI), also referred to as the apparent temperature, is a measure of the contribution humidity makes when high temperatures reduce the body's cooling ability. The HI is the temperature the body senses based on normal humidity levels. For example, if the actual temperature is 100 degrees with 40% relative humidity, the effect of this on the body would be the same as 109 degrees with normal humidity for that temperature. Refer to the HI chart at right. The National Weather Service (NWS) generally issues heat advisories when the index reaches 105 degrees.

In an effort to alert local authorities and the general public, the NWS will highlight excessive heat /humidity conditions in its forecasts and statements through the release of appropriate safety rules and civil emergency messages.

Ultra-Violet Index

The National Weather Service issues this daily index for selected cities. It describes the next day's peak exposure to UV rays (scale 1 - 10+ ). This value is broadcast on NOAA weather radio daily.

Index: Exposure
0-2 Minimal
3-4 Low
5-6 Moderate
7-9 High
10+ very high

Page 3.

 

Rip Currents...What are they?

 

Rip currents are narrow channels of water flowing seaward from the beach and through areas of breaking waves. Though extremely localized, they result in numerous deaths every year.

A rip current...sometimes mistakenly called an undertow...is a strong but narrow current of water flowing from the beach through the surf zone. It can rapidly carry a swimmer into deeper water and exhaust an individual trying to swim against it.

If you are swimming at the beach and find yourself caught in a rip current, wade sideways parallel to the beach until you are out of its pull. By doing this you will be exerting less energy and also getting out of the current's way. Another means of escape for good swimmers is to ride the current beyond the surf zone where the rip current dissipates. Then swim to shore outside the effect of the narrow current. Strong onshore winds for an extended period will contribute to the development of rip currents.

Since rip currents are caused by a combination of meteorological and oceanographic conditions, the National Weather Service can not act as an authority in determining the likelihood of these events. But when storms are forecast to form or track off the southeastern US coast (including tropical systems), the mention of the headline "Dangerous rip currents are possible" is incorporated into the recreational beach forecast and broadcast on NOAA weather radio.

 

 

What is the ideal temperature for swimming in the ocean?**

When you hear that the water temperature at the beach is 85 degrees, you might think that it's a great day to head to the ocean. But most people find water of this temperature to be uncomfortably warm for swimming. Generally, swimming is best when the water is from 70 to 80 degrees.

Since water takes longer than air to heat and cool, it is less influenced by the short term day to day temperature fluctuations. Ocean temperatures will fluctuate when surface weather systems produce extended periods of wind. Winds from nearby storms can change the temperature of the water at the surface by as much as ten degrees depending on direction, and where the water originated. An unusually cold winter will have a greater effect on cooling the shallow coastal water than further offshore in deeper water.

From time to time, ocean currents, or prolonged winds from a certain direction can cool or warm the water. For example... during the winter, an extended period of southeasterly winds along the Georgia and South Carolina coast can cause the western edge of the Gulf Stream current to move further west. Seawater temperatures will increase a few degrees in a short time period along the coast. In the late summer and early fall, an extended period of northeasterly winds will abruptly cool the water temperatures, and sometimes the readings do not rebound to their previous levels.

Here are some guidelines: (As with everything else, personal preference dictates the ideal water temperature)

Below 60....Dangerous for swimming.
60-70..........Uncomfortable for prolonged time period.
70-80..........Generally comfortable. Best temperatures for swimming are                        readings in the upper 70s.
80+.............Uncomfortable for vigorous swimming.

Warning: Even in summer, prolonged immersion in water below 70 degrees can produce hypothermia, a subnormal internal body temperature that can be deadly. Physicians have recently learned that divers, even in tropical water, can develop hypothermia slowly without the usual symptoms of shivering. This can happen when they do not rest and warm up between dives.

**the above information is courtesy USA TODAY.

Page 4.

 

The rise and fall of the tides...

Tides are the rising and falling of waters on large bodies of water, which occur on a regular time schedule. They are only substantially noticed along the edge of the conti-nent, or on an island in the ocean. The region between a high tide and a low tide is called a tidal range.

Spring and Neap tides...

A spring tide occurs when the moon and earth are aligned, and the gravitational pull on the earth results in extremely high tides.

A neap tide occurs when the moon and sun exert gravita-tional forces on the earth at right angles. This causes an unusually low tide.

Both spring and neap tides occur about every fourteen days. A spring tide is at the new and full moon, and neap tides are at the first and third quarters. In addition, if the moon is in perigee or nearest the earth at the time of the spring tide, the tidal currents will be substantially higher.

Shallow coastal flooding will likely occur when astronomically high tides (spring/neap) coincide with onshore winds and/or heavy rainfall.

Cooperative Observer News...

In addition to our network of severe weather spotters, cooperative observers are volunteers who contribute to our forecast and climate program by calling in weather reports on a daily basis. Some of them have collected daily temperature and rainfall data for decades, and in many cases it has become a family tradition.

We salute Mr Chester Stewart, our cooperative observer of the quarter who has collected weather data for the last 25 years. Mr. Stewart was presented a certificate for dedicated service and received a letter of appreciation from South Carolina Senator Ernest "Fritz" Hollings.

Mr. Ronald Wiggins, river and rainfall observer for Givhans Ferry, South Carolina received a 15-year pin for dedicated service. Mr. Wiggins is the backup observer for the automated river gage, and recently began reporting temperature by remote observation system automation.

Thanks to all cooperative observers for daily weather reports that you provide. We appreciate any extra weather information that will help to improve our forecasts.

Page 5.

 

Spotter News...

Charleston County Warning Area Spotter Reports.

1. Marcus A. Brewer - Colleton
2. Elda Butts - Charleston
3. Herbert Cummings - Dorchester
4. Eric R. Davis - Berkeley
5. Dorothy Fennell - Chatham
6. Ed Frank - Berkeley
7. Crystal Harley - Screven
8. John Henson - Liberty
9. Bobby R. Hilderbrandt- Jenkins
10. Clyde H. Jones Jr. - Hampton
11. Shelley Lewis - Colleton
12. Scott M. Luedtke - Berkeley
13. R. M. Small - Jenkins

Thanks to the weather spotters who contacted us during severe weather events! We rely on the "ground truth" that could only be provided by YOU, the trained weather spotter.

On March 14th , two F1 tornadoes touched down in northern Charleston County. On Sullivans Island and in Awendaw, several homes sustained minor damage. Severe weather affected the Carolinas and Georgia late in the evening on Easter Sunday, but no damage was reported throughout Southeast Georgia and South Carolina. A more significant event occurred in the early morning hours of April 23rd. A small tornado touched down in Dorchester County...just west of St. George. A roof was partially blown off a trailer and many trees were uprooted in the tornado's path. Candler County officials notified us of golf ball size hail in the town of Metter. Many counties reported wind damage and uprooted trees with this storm system. In Evans County, 60mph winds were associated with a thunderstorm in Claxton. Numerous warnings were broadcasted on NOAA Weather Radio on April 23rd. On May 3rd, conditions were ripe for severe weather as a cold front combined with the sea breeze, producing a line of thunderstorms that moved east to the coastline. Many warnings were issued due to timely weather spotter reports of uprooted trees, and pea/marble/golf ball size hail!

Again, thank you for your help. These reports that you provide us often help in deciding to issue a warning. We look forward to hearing from you in the future. Remember... Do not hesitate to give us a call, even if you think someone has already done so in your area.

 

 

Severe weather reports to the National Weather Service

1. Call Us!!!

2.What did you observe? Tornado/Funnel cloud? Damaging Wind? Hail?

Significant rains/snows? Other pertinent info?

3. When did you observe it?

4. Where did you observe it (Direction and distance from a known location)?

5. Where is it headed?

6. Other useful info...What is happening? Damage/Destruction?

 

National Weather Service

5777 S. Aviation Ave

Charleston, SC 29406

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