Weather Glossary: S's

The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 categorization based on the hurricane's intensity at the indicated time. The scale provides examples of the type of damage and impacts in the United States associated with winds of the indicated intensity.

The scale does not address the potential for other hurricane-related impacts, such as storm surge, rainfall-induced floods, and tornadoes. It should also be noted that these wind-caused damage general descriptions are to some degree dependent upon the local building codes in effect and how well and how long they have been enforced.

The SAWRS program addresses the concerns of users who depend on weather observations for air operations. If the cooperator is collocated with a commissioned automated system, they ensure continuity during outage periods of the automated system.

The requirement for a SAWRS arises from the FAA validated need for observations to satisfy FAR 121 or 135 operations or for the safe conduct of other aircraft. The classification of a SAWRS operation is directly related to the automated observing platforms. There are three distinct SAWRS classifications:

1. SAWRS Indicates manual weather observations are the primary source of reporting the weather at the airport.
2. Backup SAWRS Indicates automated observations taken by a commissioned version 3 of an AWOS, better known as an AWOS III, is the primary source of reporting the airport's weather. AWOS III systems may be FAA owned units or privately non-federal units owned by airports or local governments.
3. SAWRS-II Indicates automated observations taken by a commissioned Automated Surface Observing System (ASOS), is the primary source of reporting the airport's weather. An ASOS unit is federally owned by the Federal Aviation Administration or the National Weather Service.

1. An official sky cover classification for aviation weather observations, descriptive of a sky cover of 3/8 to 4/8. This is applied only when obscuring phenomenon aloft are present--that is, not when obscuring phenomenon are surface-based, such as fog.
2. A National Weather Service convective precipitation descriptor for a 30%, 40%, and 50% chance of measurable precipitation (> Trace). See Probability of Precipitation (PoP).

A standing wave oscillation in any enclosed lake which continues after the forcing mechanism has ceased. In the Great Lakes, this forcing mechanism may be either strong winds along the axis of a lake, a pressure jump, or down draft winds associated with fast moving squall lines over the lake.

In either case, water is piled up at one end. The water sloshes from one end of the lake to the other causing fluctuations of perhaps several feet before damping out.

It consists of an extensive squall line which is oriented such that the angle between the mean wind direction and the squall line axis is small. A series of LEWPs and bow echoes move along the line. The downburst activity is associated with the LEWPs and bows.

A serial Derecho tends to be more frequent toward the north end of the line during the late winter and spring months. It occurs less frequently than its cousin the progressive derecho.

It is associated with a linear type mesoscale convective system that moves along and in advance of a cold front or dry line. These boundaries are often associated with a strong, migratory surface low-pressure system and strong shortwave trough at 500 mb (strong dynamic forcing).

Lifted Indices are typically -6 or lower and the advection of dry air in the mid-troposphere (9,800 to 23,000 ft / 3,000 to 7,000 km above ground) by relatively strong winds leads to high convective instability and increased downdraft potential.

The bow echoes move along the line in the direction of the mean flow, often southwest to northeast. These storms move at speeds exceeding 35 knots (64 km/h). Squall line movement is often less than 30 knots (56 km/h).

It is potentially the most dangerous of the convective storm types. Storms possessing this structure have been observed to generate the vast majority of long-lived strong and violent (EF2-EF5) tornadoes, as well as downburst damage and large hail. It is defined as a thunderstorm consisting of one quasi-steady to rotating updraft which may exist for several hours.

Supercells usually move to the right of the mean wind. These are called "right movers" and they are favored with veering winds. Occasionally, these thunderstorms will move to the left of the mean wind. These thunderstorms are called "left movers".

These supercells typically don't last as long as their "right mover" cousins and they usually only produce large hail (1" in diameter or greater) and severe wind gusts in the excess of 50 knots (58 mph / 93 km/h). Left Movers are favored when you have backing winds.

Radar will observe essentially one long-lived cell, but small perturbations to the cell structure may be evident. The stronger the updraft, the better the chance that the supercell will produce severe (hail greater than 1" in diameter, wind gusts greater than 58 mph (93 km/h), and possibly a tornado) weather.

Severe supercell development is most likely in an environment possessing great buoyancy (CAPE) and large vertical wind shear. A Bulk Richardson Number of between 15 and 35 favor supercell development.

Typically, the hodograph will look like a horse shoe. This is due to the wind speed increasing rapidly with height and the wind direction either veering or backing rapidly with height.

A thunderstorm that produces either of the following: winds of 50 kt (58 mph / 93 km/h) or greater (these speeds can result in structural or tree damage), hail 1" (25 mm) in diameter or larger, or a tornado. Lightning frequency is not a warning criterion for issuing a severe thunderstorm warning.

Severe thunderstorms can result in the loss of life and property. They can also produce a tornado with little or no advanced warning. A table of hail sizes can be found in this glossary, under the definition of hail. See approaching (severe).

This is issued when either a severe thunderstorm is indicated by the WSR-88D radar or a spotter reports a thunderstorm producing hail 1" (25 mm) or larger in diameter and/or winds equal or exceed 50 kt (58 mph / 93 km/h); therefore, people in the affected area should seek safe shelter immediately.

Severe thunderstorms can produce tornadoes with little or no advance warning. Lightning frequency is not a criterion for issuing a severe thunderstorm warning. They are usually issued for a duration of one hour. They can be issued without a Severe Thunderstorm Watch being already in effect.

Like a Tornado Warning, the Severe Thunderstorm Warning is issued by your National Weather Service Forecast Office (NWFO). Severe Thunderstorm Warnings will include where the storm was located, what towns will be affected by the severe thunderstorm, and the primary threat associated with the severe thunderstorm warning.

If the severe thunderstorm will affect the nearshore or coastal waters, it will be issued as the combined product--Severe Thunderstorm Warning and Special Marine Warning.

If the severe thunderstorm is also causing torrential rains, this warning may also be combined with a Flash Flood Warning. After it has been issued, the issuing weather forecast office will followed up periodically with Severe Weather Statements.

These statements will contain updated information on the severe thunderstorm and they will also let you know when warning is no longer in effect. Severe weather reports will either be reported in one the following 3 products:

1. Another severe weather warning (Tornado or Severe Thunderstorm),
2. In a Severe Weather Statement, or
3. In a Local Storm Report.

This is issued by the National Weather Service when conditions are favorable for the development of severe thunderstorms in and close to the watch area. A severe thunderstorm by definition is a thunderstorm that produces 1" (25 mm) hail or larger in diameter and/or winds equal or exceed 50 kts (58 mph / 93 km/h).

The size of the watch can vary depending on the weather situation. They are usually issued for a duration of four to eight hours. They are normally issued well in advance of the actual occurrence of severe weather.

During the watch, people should review severe thunderstorm safety rules and be prepared to move a place of safety if threatening weather approaches.

A Severe Thunderstorm Watch is issued by the Storm Prediction Center in Norman, Oklahoma. Prior to the issuance of a Severe Thunderstorm Watch, SPC will usually contact the affected local National Weather Service Forecast Office and they will discuss what their current thinking is on the weather situation.

Afterwards, SPC will issue a preliminary Severe Thunderstorm Watch and then the affected NWFO will then adjust the watch (adding or eliminating counties/parishes) and then issue it to the public by way of a Watch Redefining Statement.

During the watch, the local forecast office will keep the you informed on what is happening in the watch area and also let the you know when the watch has expired or been canceled.

A low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms).

Unlike the roll cloud, the shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn. It is accompanied by gusty, straight-line winds and is followed by precipitation.

It is a stability index used to determine thunderstorm potential. The SWI is calculated by lifting an air parcel adiabatically from 850 mb to 500 mb. The algebraic difference between the air parcel and the environmental temperature at 500 mb represents the SWI.

It is especially useful when you have a shallow cool airmass below 850 mb concealing greater convective potential aloft. However, the SWI will underestimate the convective potential for cool layers extending above 850 mb. It also does not take in account diurnal heating or moisture below 850 mb. As a result, one must be very careful when using this index.

This type of thunderstorm develops in weak vertical wind shear environments. On a hodograph, this would appear as a closely grouped set of random dots around the center of the graph. They are characterized by a single updraft core and a single downdraft that descends into the same area as the updraft.

The downdraft and its outflow boundary then cut off the thunderstorm inflow. This causes the updraft and the thunderstorm to dissipate. Single cell thunderstorms are short-lived. They only last about 1/2 hour to an hour. These thunderstorms will occasionally become severe, but only briefly. In this case, they are called Pulse Severe Thunderstorms.

This is issued by the National Weather Service to alert small boats to sustained (more than 2 hours) hazardous weather or sea conditions. These conditions may be either present or forecasted.

The threshold conditions for it are usually sustained winds of 18 kt (21 mph / 34 km/h) (less than 18 knots in some dangerous waters) to 33 kt (38 mph / 61 km/h) inclusive or hazardous wave conditions (such as 4 feet/1.2 meters or greater).

Along the coastal regions of the East Coast, Gulf of Mexico, and West Coast, this advisory relates to conditions out to as much as 100 nautical miles (185 kilometers) of shore (coastal waters). As a result, these will be only issued in the Coastal Marine Forecast

Mariners learning of this advisory are urged to determine immediately the reason by turning their radios to the latest marine broadcast. Decisions as to the degree of the hazard will be left to the boater, based on experience and size and type of boat. There is no legal definition for a "small craft".

A "see-saw" in surface pressure in the tropical Pacific characterized by simultaneously opposite sea level pressure anomalies at Tahiti, in the eastern tropical Pacific and Darwin, on the northwest coast of Australia.

The SO was discovered by Sir Gilbert Walker in the early 1920's. Walker was among the first meteorologists to use the statistical techniques to analyze and predict meteorological phenomena. Later, the three-dimensional east-west circulation related to the SO was discovered and named the "Walker Circulation".

The SO oscillates with a period of 2-5 years. During one phase, when the sea level pressure is low at Tahiti and High at Darwin, the El Niño occurs.

The cold phase of the SO, called La Niña, is characterized by high pressure in the eastern equatorial Pacific, low in the west, and by anomalously cold sea surface temperature (SST) in the central and eastern Pacific. This is called El Niño Southern Oscillation or ENSO.

This center provides real-time monitoring and forecasting of solar and geophysical events, conducts research in solar-terrestrial physics, and develops techniques for forecasting solar and geophysical disturbances.

SWPC's Space Weather Operations is jointly operated by NOAA and the U.S. Air Force and is the national and world warning center for disturbances that can affect people and equipment working in the space environment.

This is issued by the National Weather Service for hazardous weather conditions (thunderstorms over water, thunderstorms that will move over water, cold air funnels over water, or waterspouts) usually of short duration (2 hours or less) and producing sustained winds or frequent gusts of 34 knots or more that is not covered by existing marine warnings.

These are tone alerted on NOAA Weather Radio. Boaters will also be able to get this information by tuning into Coast Guard and commercial radio stations that transmit marine weather information.

This WSR-88D radar product depicts a full 360° sweep of spectrum width data indicating a measure of velocity dispersion within the radar sample volume. It is available for every elevation angle sampled, it provides a measure of the variability of the mean radial velocity estimates due to wind shear, turbulence, and/or the quality of the velocity samples.

It is used to estimate turbulence associated with boundaries, thunderstorms, and mesocyclones; check the reliability of the velocity estimates; and locate boundaries (cold front, outflow, lake breeze, etc.).

This WSR-88D radar product displays a vertical cross section of spectrum width on a grid with heights up to 70,000 feet (21 kilometers) on the vertical axis and distance up to 124 nm (230 km) on the horizontal axis.

Two end points to create cross section are radar operator selected along a radial or from one AZRAN to another AZRAN within 124 nm (230 km) of the radar that are less than 124 nm (230 km) apart.

It is used to:

1. Verify features on the Reflectivity Cross Section (RCS) and Velocity Cross Section (VCS) and to evaluate the quality of the velocity data
2. Estimate vertical extent of turbulence (aviation use).

• Auxiliary or Emergency Spillway - A secondary spillway designed to operate only during exceptionally large flood flows. Allows inflows from large storms to be released from the reservoir before the water level raises high enough to overtop the dam.
• Fuse Plug Spillway - An auxiliary or emergency spillway comprising a low embankment or a natural saddle designed to be overtopped and eroded away during flood flows.
• Primary (or Principal) Spillway - The spillway which would be used first during normal inflow and flood flows.
• Shaft or Morning Glory Spillway - A vertical or inclined shaft into which flood water spills and then is conducted through, under, or around a dam by means of a conduit or tunnel. If the upper part of the shaft is splayed out and terminates in a circular horizontal weir, it is termed a "bellmouth" or "morning glory" spillway.

This tabular product is issued by the National Weather Service once in the morning and evening. The morning product will contain the current weather conditions, yesterday's daytime high temperature, the 12-hour low temperature ending at a specified time, and 24-hour precipitation ending at a specified time from available reporting stations within the state or NWFO forecast area.

The evening product will contain the same information; however, the daytime high temperature will be today's high temperature instead of yesterday's high temperature. In the winter time, this product will contain the snow depth in inches if it is available.

These reports are broken up regionally. When the complementary satellite product is not available, reports from unaugmented ASOS stations will report "fair" in the sky/weather column when there are few or no clouds (i.e., scattered or less) below 12,000 feet (3,650 meters) with no significant weather and/or obstructions to visibility.

It forms as cold air moves over warm water. Water evaporates from the warm water surface and immediately condenses in the cold air above. Heat from the water warms the lower levels of the air creating a shallow layer of instability. It rises like smoke from the warm surface.

The low-level convection can become quite turbulent. Steam fog is most common in Arctic regions where it is called "Arctic Sea Smoke", but it can and does occur occasionally at all latitudes.

This WSR-88D radar product depicts a 27 nm by 27 nm (50 km by 50 km) region of storm relative mean radial velocity centered on a point which the operator can specify anywhere within a 124 nm (230 km) radius of the radar. The storm motion subtracted defaults to the motion of the storm closest to the product center, or can be input by the operator.

It is used to examine the 3-dimensional storm relative flow of a specific thunderstorm (radar operator centers product on a specific thunderstorm; aid in displaying shear and rotation in thunderstorms and storm top divergence that might otherwise be obscured by storm motion; and gain higher resolution velocity product.

A rise above the normal water level along a shore caused by strong onshore winds and/or reduced atmospheric pressure. The surge height is the difference of the observed water level minus the predicted tide. Most hurricane deaths are caused by the storm surge. It can be 50 or more miles (80 kilometers or more) wide and sweeps across the coastline around where the hurricane makes landfall.

The maximum rises in sea-level move from under the storm to the right of the storm's track, reaching a maximum amplitude of 10 to 30 feet (3 to 9 meters) at the coast. The storm surge may even double or more in height when the hurricane's track causes it to funnel water into a bay.

The storm surge increases substantially as it approaches the land because the normal water depth decreases rapidly as it approaches the beaches. The moving water contains the same amount of energy; thus, resulting in an increase of storm surge. Typically, the stronger the hurricane, the greater the storm surge.

This WSR-88D radar product displays the total precipitation (in inches) as a graphical image. It displays hourly precipitation total (in inches) as a graphical image. Currently, this product is done in a polar format with resolution 1.1 nm (2 km) by 1 degree. It will reset after one hour of no precipitation.

It is used to monitor total precipitation accumulation; observe short term trends of precipitation tracks with time lapse of this product; and estimate total basin runoff and ground saturation.

It has globular masses or rolls unlike the flat, sometimes definite, base of stratus. This cloud often forms from stratus as the stratus is breaking up or from spreading out of cumulus clouds. They usually consist of mainly water vapor and are located between the ground and 6,500 feet (1980 meters).

Stratocumulus often reveals the depth of the moist air at low levels, while the speed of the cloud elements can reveal the strength of the low-level jet.

This heavy rain signature is often difficult to detect on satellite. These warm top thunderstorms are often embedded in a synoptic-scale cyclonic circulation. Normally, they occur when the 500 mb cyclonic circulation is quasi-stationary or moves slowly to the east or northeast (about 2° per 12 hours). The average surface temperature is 68°F (15°C) with northeasterly winds.

The average precipitable water (P) value is equal to or greater than 1.34" (34 mm) and the winds veer with height, but they are relatively light. The heavy rain often occurs north and east of the vorticity maximum across the lower portion of the comma head about 2° to 3° north or northeast of the 850 mb low.

• Cold Low Type - This type has a circulation extending from the surface to the upper troposphere, with the maximum sustained low-level winds typically extending to a radius of 100 miles or more from the center.

• Mesoscale (Sub-Synoptic Scale) Cyclone Type - This type develops in or near a dying frontal zone with horizontal wind shear. This low is compact and develops a tight pressure gradient with the maximum sustained low-level winds, which can reach hurricane intensity, typically located less than 30 miles (48 kilometers) from the center.

  The whole storm circulation may initially be no more than 100 miles (161 kilometers) in diameter. These lows are typically short-lived and spend their lives usually over water. They may be cold core or warm core. This strange hybrid was once referred to as a "neutercane" after being discovered by satellite imagery.

A thunderstorm with a persistent rotating updraft. Supercells are rare, but are responsible for a remarkably high percentage of severe weather events - especially tornadoes, extremely large hail and damaging straight-line winds. They frequently travel to the right of the main environmental winds (i.e., they are right movers).

Radar characteristics often (but not always) include a hook or pendant, bounded weak echo region (BWER), V-notch, mesocyclone, and sometimes a TVS.

Visual characteristics often include a rain-free base (with or without a wall cloud), tail cloud, flanking line, overshooting top, and back-sheared anvil, all of which normally are observed in or near the right rear or southwest part of the storm.

Storms exhibiting these characteristics often are called classic supercells; however HP storms and LP storms also are supercell varieties.

A stability index developed by the Air Force which incorporates instability, wind shear, and wind speeds. The index combines the effects of low-level moisture (850 mb dew point), convective instability (Total Totals (TT) Index), jet maxima (850 mb and 500 mb wind speed), and warm air advection (veering directional shear between 850 mb and 500 mb).

It was designed to discriminate between ordinary and severe convection by incorporating thermodynamic information (850 mb dew point and Total Totals Index) and kinematic information (low- and mid-level flow characteristics showing strong wind fields and veering directional shear).

SWEAT Index - Severe Weather ThrEAT index as follows: SWEAT = (12 Td 850) + (20 [TT-49]) +(2 f 850) + f 500 + (125 [s+0.2]) where

• Td 850 is the dew point temperature at 850 mb,
• TT is the total-totals index,
• f 850 is the 850-mb wind speed (in knots),
• f 500 is the 500-mb wind speed (in knots), and
• s is the sine of the angle between the wind directions at 500 mb and 850 mb (thus representing the directional shear in this layer).

SWEAT values of about 250-300 or more indicate a greater potential for severe weather, but as with all stability indices, there are no magic numbers. The SWEAT index has the advantage (and disadvantage) of using only mandatory-level data (i.e., 500 mb and 850 mb), but has fallen into relative disuse with the advent of more detailed sounding analysis programs.