Science and Technology
Welcome to the Wilmington North Carolina National Weather Service science and technology page. On this page you will find comprehensive explanations of local and regional weather phenomena that make forecasting for the area a challenge. Use the quick links below to navigate to the section you are interested in.
Sea Breeze | Sea Fog | Water Spouts | Rip Currents | Tropical Weather Forecasting
The classic sea breeze occurs when a weak pressure gradient is present. The shape closely resembles the coast. There is always an inflection point with the classic sea breeze. It is often accompanied by weak showers and thunderstorms. The classic sea breeze can propogate well inland.
Classic Sea Breeze
The Southerly Resultant Sea Breeze with Inflection is one of the most common types of seabreezes in our area due to the prevailing flow. This feature always has an inflection point which is displaced to the north or northeast. It generally stays confined to the coastal counties and can be accompanied by weak pulse storms.
Southerly Resultant Sea Breeze With Inflection
The Southerly Resultant Segmented Sea Breeze partially mimics the coastline but stays close to the coast across the southern areas. There is no inflection point. This sea breeze often has little in the way of convection because the atmosphere is capped.
Southerly Resultant Segmented Sea Breeze
The Southerly Resultant Hybrid Sea Breeze remains confined to the coast across the northern and southern areas while moving inland toward the center.
Southerly Resultant Hybrid
The Northeast Resultant Sea Breeze with Inflection slightly mimics the coast. This feature always has an inflection point that is displaced to the west. The inland propogation is limited...and with the prevailing Northeasterly flow...convection is usually limited because of poor instability.
Northeast Resultant Sea Breeze with Inflection
The Northeast Resultant Segmented Seabreeze partially mimics the coast and remains confined near the coast along the northernmost segments. This feature does not have an inflection point and usually inland propagation is limited.
Northeast Resultant Segmented Sea Breeze
Sea fogs are advection fogs that form in warm moist air cooled to saturation as it moves over colder water. The colder water may occur as a well-defined current, or as gradual latitudinal cooling. The dewpoint and the temperature undergo a gradual change as the air mass moves over colder and colder water. The surface air temperature falls steadily, and tends to approach the water temperature. The dewpoint also tends to approach the water temperature, but at a slower rate. If the dewpoint of the air mass is initially higher than the coldest water to be crossed, and if the cooling process continues sufficiently long, the temperature of the air ultimately falls to the dewpoint, and fog results. However, if the initial dewpoint is less than the coldest water temperature, the formation of fog is unlikely. Generally, in northward moving air masses or in air masses that have previously traversed a warm ocean current, the dewpoint of the air is initially higher than the cold water temperature to the north, and fog will form, provided sufficient fetch occurs. The rate of temperature decrease is largely dependent on the speed at which the air mass moves across the sea surface, which, in turn, is dependent both on the spacing of the isotherms and the velocity of the air normal to them. The dissipation of sea fog requires a change in air mass (a cold front). A movement of sea fog to a warmer land area leads to rapid dissipation. Upon heating, the fog first lifts, forming a stratus deck; then, with further heating, this cloud deck breaks up into a stratocumulus layer, and eventually into convective type clouds or evaporates entirely. An increase in wind velocity can lift sea fog, forming a stratus deck, especially if the air/sea temperature differential is small. Over very cold water, dense sea fog may persist even with high winds.
Fair-weather waterspouts are small tornado-look circulations extending from the ocean surface up into a towering cumulus or cumulonimbus cloud. Fair-weather waterspouts occasionally occur during the warm season across North and South Carolina coastal waters and are a hazard to boaters and beachgoers. Wind speeds in large waterspouts can rival that of small tornadoes and sometimes reach over 100 mph. Fair-weather
waterspouts can sometimes move onshore as a tornado, but normally weaken and dissipate within a minute of reaching shore due to increased surface friction and turbulence.
Tornadic waterspouts (those associated with a severe thunderstorm) are formed through classic tornado processes involving extreme wind shear and a strong parent mesocyclone. Fair-weather waterspouts are a separate entity. As a result, tornadic waterspouts are not considered in this forecasting scheme.
The Waterspout Risk forecasting scheme cannot forecast the occurrence of an individual fair-weather waterspout. Due to their small size and transitory nature, even advanced Doppler weather radar is of little use in this regard. We count on real-time reports from mariners and spotters on the beach to alert us to the actual development of waterspouts. The purpose of this waterspout risk scheme is to forecast the broad atmospheric conditions that are favorable for the eventual formation of fair-weather waterspouts. This is intended to heighten public awareness on days when waterspout formation is likely and prevent a waterspout from "sneaking up" on an unsuspecting mariner or beach-goer.
Generally speaking fair weather waterspouts form in environments characterized by little to no vertical wind shear below 6000 feet in altitude, deep moisture, good instability (as measured by CAPE) and moderate to steep low-level lapse rates. The forecasting scheme weights each of these factors and arrives at a numerical and categorical forecast characterizing the risk of waterspout occurrence for the day.
Research at other offices and qualitative observations locally seem to indicate that waterspouts form preferentially in the vicinity of morning land-breeze boundaries. This seems logical given the boundary will naturally be a focus for converging air at the lowest levels. Helicity on the sub-mesoscale will also be enhanced in these areas. Additional research may attempt to factor in the probability of land-breeze formation and its effect on waterspout probability.
Shown below is an image of the Waterspout Calculator forecasters use to input meteorological data to derive the waterspout risk.
First, CAPE is observed over the entire sounding. Zero CAPE gives no correlation...up to 1200 J/kg gives moderate correlation and 1200 to 2000 J/kg gives a high correlation.
The distribution of instability is alos important. Waterspouts require instability to be present in the lower portion of the troposphere where convective vertical velocities can be wrapped into a waterspout's circulation. This is measured by comparing temperatures at 900mb and 600mb. The greater the difference in these temperatures indicates a larger amount of potential instability. This is true to a point...however very steep lapse rates nearing dry adibatic in the layer can occur when the atmosphere is too dry to support convection or waterspouts.
Up to 17.5 degrees C difference...no correlation
17.5 to 18 degrees C or greater than 20 degrees C...moderate correlation
18 to 20 degrees C...high correlation
Wind Shear Parameter
Analyze the winds in the sounding at the 1000...2000...4000 and 6000 foot agl layers. Shear between each of these layers can be computed by doing a vector subtraction of the u and v components of the winds at each level. The resultant wind...i.e. the shear...is an indication of how much distortion a rising updraft and growing cumulus cloud will enconter as it ascends. Waterspouts...despite their occasionally extreme wind speeds...are very fragile and can be disrupted by high values of wind shear.
The following chart shows values of wind shear which have been found to be correlated with waterspout formation:
No Correlation Moderatre Correlation High Correlation
1k-2k ft Vector Shear... >6kt 4 to 6kt <4kt
2k-4k ft Vector Shear... >6kt 4 to 6kt <4kt
4k-6k fr Vector Shear... >5kt 3 to 5kt <3kt
Since waterspouts are tied so closely to moist convective processes, the presence of sufficient moisture in the atmosphere is obviously important. Analysis of three years of soundings on days with waterspouts has revealed moist air(characterized by dewpoint depressions < 5 C) should extend from the surface up to 600mb.
First dry layer(<5C)layer
Below 750mb...no correlation
750mb to 600mb...moderate correlation
Not present or present above 600mb...high correlation
Rip currents are powerful, channeled currents of water flowing away from shore. They typically extend from the shoreline, through the surf zone, and past the line of breaking waves. Rip currents can occur at any beach with breaking waves, including the Great Lakes.
A daily rip current outlook is included in the Surf Zone Forecast
which is issued by many National Weather Service offices...including Wilmington North Carolina. A three-tiered structure of low, moderate, high is used to describe the rip current risk. This outlook is communicated to lifeguards, emergency management, media and the general public.
Just like the Waterspout Risk Calculator...there are several components that go into derivine the rip current risk. Click on the links to the right of the image below to read a description of the component.
Wind direction is very important to the development of strong rip currents. With the orientation of our beaches in this area...a subtle change in wind direction can have significant impacts on the development and intensity of rip currents.
For the Southeast facing beaches...like the ones in Pender...Horry...and Georgetown counties...a wind direction of 80 degrees to 170 degrees is important. For the beaches in New Hanover county...which face more to the east...a wind direction of 70 degrees to 160 degrees is important. For the Brunswick County beaches...which face more to the south...a wind direction of 150 degrees to 220 degrees is important.
The swell component in the wave spectrum also plays a significant role in the development and intensity of rip currents. The longer period swells...like the ones that eminate from a distant hurricane... provide more power by pushing more water on to the beach. In general...the swell direction must be normal or perpindicular to the beach or county in question.
Tides play a significant role in the development of rip currents. The gravitational influence of the earth's moon is stronger three days either side of a full or new moon. More weighing is given to the scheme during these periods.
Also studies have shown the frequency of rip currents is higher during low tide so more weighing is alos given if an extremely low tide occurs durng typical beach going hours.
Onshore Flow Parameter
The synoptic flow plays a role in the development of significant rip currents. Once again the direction is important for the respective beaches...with the southeast facing beaches favoring the direction range of 80 and 170 degrees. The south facing beaches favoring the direction range of 150 amd 220 degrees with the east facing beaches favoring a direction range of 70 and 160 degrees.
The longevity of the flow is important as well. Not only does the direction have to be in the favored range...the flow must have occurred 80% of the time during the previous 48 hours.
There are several good pages to view tropical weather forecasts and the various parameters that go into them. In the following sections...a brief description will be given of the field...followed by a link to the page. This section is designed to be a collection of the pages as well as a brief introduction and explanation of the parameter. A more rigorous explanation is generally available on each page.
After the National Hurricane Center designates an area of interest in the tropics...computer models...including track and intensity guidance...are initiated on the system. A great page for looking at the various model solutions is located at Tropical Cyclone Guidance Project
. More traditonal model guidance can be found at The Penn State Page
Saharan Air Layer Analysis
African dust is generally thought to be an inhibiting factor for the development of tropcial systems in the Atlantic. It is associated with very dry air at the mid levels of the atmosphere. Tropical systems develop and intensify in areas where the moisture content is high. The Saharan Air can be analyzed at The Cooperative Institute for Meteorological Satellite Studies...CIMMS.
Sea Surface Temperatures/Tropical Cyclone Heat Potantial
Warm water is a necessary component to tropical storm development. Water temperatures of 80 degrees Fahrenheit or about 26.5 degrees Celsius are the thresholds. A good site to view water temperatures can be found at the Tropical Cyclone Heat Potential Page.
The depth of the warm water...referred to as the heat potential...is also critical and can be examined at the same page.
Basically...wind shear refers to any change in wind speed or direction along a straight line. In the case of hurricanes, wind shear is important primarily in the vertical direction--from the surface to the top of the troposphere. Wind shear of less than 10 knots is very condusive to tropical cyclone formation. Increasing values of wind shear are less condusive for rapid development and values exceeding 20 knots are usually destructive. Once again...there are a couple of excellent sites to view current and forecast wind shear. The first is CIMMS.
The Penn State
site is anothe good reference. You need to select the 850-200MB parameter on the GFS model and step through the six hour panels.
Jeff Masters of The Weather Underground has an excellent wind shear tutorial.