AIRMETS - An AIRMET (AIRmans METeorological Information) advises of weather that maybe hazardous, other than convective activity, to single engine, other light aircraft, and Visual Flight Rule (VFR) pilots. However, operators of large aircraft may also be concerned with these phenomena. The items covered are:

In the AIRMET Sierra bulletin:

- Ceilings less than 1000 feet and/or visibility less than 3 miles affecting over 50% of the area at one time.

- Extensive mountain obscuration.

In the AIRMET Tango bulletin:

- Moderate turbulence.

- Sustained surface winds of 30 knots or more at the surface.

In the AIRMET Zulu bulletin:

- Moderate icing.

- Freezing levels.

These AIRMET items are considered to be widespread because they must be affecting or be forecast to affect an area of at least 3000 square miles at any one time. However, if the total area to be affected during the forecast period is very large, it could be that only a small portion of this total area would be affected at any one time.

AIRMETs are routinely issued for 6 hour periods beginning at 0245 UTC during Central Daylight Time and at 0145 UTC during Central Standard Time. AIRMETS are also amended as necessary due to changing weather conditions or issuance/cancellation of a SIGMET.

SIGMETs - A SIGMET (SIGnificant METeorlogical Information) advises of weather potentially hazardous to all aircraft other than convective activity. In the conterminous U.S., items covered are:

- Severe icing

- Severe or extreme turbulence

- Duststorms and sandstorms lowering visibilities to less that three (3) miles.

- Volcanic Ash

In Alaska and Hawaii, SIGMETs are also issued for the following events:

- Tornadoes

- Lines of thunderstorms

- Embedded thunderstorms

- Hail greater than or equal to 3/4 inch in diameter

For the lower 48 states and adjacent coastal waters, Convective SIGMETs are issued hourly for Thunderstorm-related aviation hazards.

These SIGMET items are considered to be widespread because they must be affecting or be forecast to affect an area of at least 3000 square miles at any one time. However, if the total area to be affect during the forecast period is very large, it could be that only a small portion of this total area would be affected at any one time.

SIGMETs are issued for 6 hour periods for conditions associated with hurricanes and 4 hours for all other events. If conditions persist beyond the forecast period, the SIGMET is updated and reissued.

Convective SIGMETs -CONVECTIVE SIGMETs are issued in the conterminous U.S. for any of the following:

Severe thunderstorms accompanied by:

- surface winds greater than or equal to 50 knots.

- hail at the surface greater than or equal to 3/4 inches in diameter.

- tornadoes.

- Embedded thunderstorms.

- Line of thunderstorms.

- Thunderstorms greater than or equal to VIP level 4 affecting 40% or more of an area at least 3000 square miles.

Any Convective SIGMET implies severe or greater turbulence, severe icing, and low level wind shear. A Convective SIGMETs may be issued for any convective situation which the forecaster feels is hazardous to all categories of aircraft.

Convective SIGMET bulletins are issued for the Eastern (E), Central (C), and Western (W) United States. The areas separate at 87 and 107 degrees west longitude with sufficent overlap to cover most cases when the phenomenon crosses the boundaries. Bulletins are issued hourly at Hour+55. The text of the bulletin consists of either an observation and a forecast or just a forecast. The forecast is valid for up to 2 hours.

Surface Analysis and Surface Weather Prog -

Weather Depiction -

Low Level Sig Prog -

- Updated 4 times daily for the 12hour and 24hour forecast. Estimated update is around 6z,9z,17,23z

- Updated 2 times daily for the 36hour and 48hour forecast. Estimated update is around 6z, 17z

- Cloud forecast should be interpretted as satellite would

- Precip areas should be interpretted as radar would.

- MSLP ilnes are placed at an 8 millibar interval

Winds Aloft -

Severe Weather Outlook -

Green - General (non-severe) thunderstorms expected

Yellow - Slight risk of severe thunderstorms expected

Red - Moderate risk of severe thunderstorms expected

Purple - High Risk of severe thunderstorms expected

Thunderstorm Probability - NGM (Nested Grid Model) is run twice a day at 0000z and 1200z. Products are updated by 0400z and 1600z respectively.

- Solid white lines are thunderstorm probability contours in 10% intervals

- Color contour lines are probabilities of severe weather in 10% intervals

- NOTE: For a thunderstorm to be classified as severe, it must produce winds greater than or equal to 58mph (50kts) and/or hailr 3/4 inch or greater in diameter.

High Level Sig Wx -

- Updated 4 times daily for the 12hour forecast. Estimated update around 6z,9z,17z,23z.

- CB areas represent thunderstroms and cumulonimbus clouds with cb areas depicted as isolated (iso), occassional (ocnl), or frequent (freq).

Station Model Plot -

>

Upper Air Analysis -

Observed Winds -The observed winds aloft charts are updated twice daily at approximately 0200Z (00Z data) and 1400Z (12Z data). Levels shown are the 2nd standard level (approximately 2,000 ft. above the surface), 14,000 ft. (600 mb level), 24,000 ft. (400 mb level), and 34,000 ft. (250 mb level). Temperatures are given in degrees Celsius.

Freezing Level Analysis -

Relative Humidity -The Average relative humidity is derived by taking the average of the relative humidities from the surface layer up to 500 millibars based on observations taken from a profile of the atmosphere. These observations are taken twice a day around 12z and again at 00z.

Preciptable Water -The precipitable water (PW) graphic depicts the observed quantity of water vapor between the surface and 500mb (18,000 feet MSL). The chart shows the amount of water (inches) the air would contain if all the vapor were condensed.

Some Constants:

- Warmer air has the ability to store more water vapor, hence the amounts are generally higher in the summer months.

- Higher elevations have less vertical atmosphere between surface and 500mb, hence they generally have lower PW values.

Lifted/K Index -The Lifted/K Index chart is updated twice daily at approximately 0330Z (00Z data) and 1530Z (12Z Data). The Lifted/K Index chart combines two indices commonly used to measure atmospheric instability and the related potential for thunderstorm development. The first, the Lifted Index (upper number), is a measure of the chance of severe thunderstorms. It is the difference between the observed 500 mb temperature and the temperature that a parcel of air would have if lifted from the boundary layer to the 500 mb level. If the temperature at 500 mb is warmer than the parcel lifted to that level, the air is stable and the chance of thunderstorms is low; if the temperature at 500 mb is colder than the parcel lifted to that level, the air is unstable and thunderstorms are likely. The difference in temperature is directly related to the chance of severe thunderstorms.

The second index, the K Index (lower number), is a measure of the probability of airmass thunderstorms. It takes into account the temperature lapse rate between 850 mb and 500 mb, the amount of moisture at 850 mb, and the dryness of the air at 700mb.

Active Tropical Storms - These charts are created in the Meteorological Operations department of WSI, using information supplied by the Tropical Prediction Center in Miami, FL. The charts are created for tropical systems in the Atlantic and Pacific, and production begins when the Tropical Prediction Center declares a disturbance as a Tropical Depression. Once begun, the charts are updated with every update issued by the TPC, which is every 6 hours normally, and every 2-3 hours as the storm approaches land.

The tropical system tracking charts provide information about current storms, including the latest statistics such as location, strength, and movement, categorical wind probabilities, watches and warnings in effect at the time, and 12 hr through up to120 hour forecasted positions and strengths.

The status of the storm is presented with the icon used to pinpoint location. A Tropical Depression is represented by an "L". A Tropical Storm is represented by an icon with an open center, and a Hurricane by the same icon with a closed center. Near the icon, the latest information about the storm is listed in the following order: name of the storm, time and date of the advisory from which this information was taken, latitude and longitude of the center of the storm, highest sustained winds recorded in the storm, direction and speed of movement of the storm, and the central pressure of the storm. Wind speed is also provided for the forecasted positions of the storm.

Any watches or warnings for areas being affected are highlighted along the coast in the appropriate color. Refer to the color chart on the image for the type of watch or warning in effect. A watch indicates that conditions are likely for that particular storm to affect the region, a warning indicates that the conditions are imminent. The color filled wind probability contours indicate the overall likelihood of winds reaching the thresholds of 40mph (yellow), 60mph(orange) and 75mph(dark red).There are dark line contours within these filled areas which show the probability of that threshold being met. There are 3 probability levels contoured, 10%, 40% and 70%, with the numeric label matching the color of the threshold being described.

Volcanic Ash Charts -The Volcanic Ash Forecast graphic depicts the concentrations of ash following a volcanic eruption. It is a composite of volcanic ash particle transport and dispersion from the surface to FL550. The graphic displays the volcano name, longitude and latitude, eruption date and time, eruption duration, summit elevation of the volcano, initial cloud ash column height, the AVN run the ash forecast is based off of, and valid forecast dates and times.

The Volcanic Ash Forecast chart is based off the Volcanic Ash Forecast Transport and Dispersion (VAFTAD) Chart. The VAFTAD is a three-dimensional time-dependent dispersion model developed by the National Oceanic and Atmospheric Administration (NOAA) Air Resources Laboratory (ARL). It uses National Center for Environmental Prediction (NCEP) AVN model winds aloft data to determine the location of ash concentrations.

The VAFTAD model is triggered by the occurrence of volcanic eruption. PIREPs and satellite imagery play an important part in determining if the model will be run. *Minor eruptions could be happening, but they may not be significant enough to cause the VAFTAD model to be triggered, and the Volcanic Ash Forecast graphic to be drawn. In this case, the following statement will be posted on the graphic:

PRESENTLY THERE ARE NO SIGNIFICANT AREAS OF

VOLCANIC ASH EXPECTED

UPDATED 12:00 GMT (current date)

Watches -Weather Watches are issued by the Storm Prediction Center (SPC). They are issued in advance of expected severe thunderstorms or tornados. The "Current Watches" advisory graphic will plot these watch boxes in yellow for severe thunderstorm watches, and red for tornado watches. The watch number and expiration time will also be plotted near the box. Occasionally the SPC issues a watch which is not immediately valid, but scheduled to become valid within an hour. These "impending" Watch Boxes will be plotted with dashed lines from the time of issuance until the time they are scheduled to become valid. At "valid" time, the dashed lines change to solid lines.

The SPC will also issues status messages referring to valid watch boxes. These messages will often cancel portions of the original box, as the severe weather threat diminishes. The effect on the plot will be for the box to become "trimmed" away.

Hurricane Tracking/Forecast Charts

These charts are created in the Meteorological Operations department of WSI, using information supplied by the Tropical Prediction Center in Miami, FL. The charts are created for tropical systems in the Atlantic and Pacific, and production begins when the Tropical Prediction Center declares a disturbance as a Tropical Depression. Once begun, the charts are updated with every update issued by the TPC, which is every 6 hours normally, and every 2-3 hours as the storm approaches land.

The tropical system tracking charts provide information about current storms, including the latest statistics such as location, strength, and movement, categorical wind probabilities, watches and warnings in effect at the time, and 12 hr through up to120 hour forecasted positions and strengths.

The status of the storm is presented with the icon used to pinpoint location. A Tropical Depression is represented by an "L". A Tropical Storm is represented by an icon with an open center, and a Hurricane by the same icon with a closed center. Near the icon, the latest information about the storm is listed in the following order: name of the storm, time and date of the advisory from which this information was taken, latitude and longitude of the center of the storm, highest sustained winds recorded in the storm, direction and speed of movement of the storm, and the central pressure of the storm. Wind speed is also provided for the forecasted positions of the storm.

Any watches or warnings for areas being affected are highlighted along the coast in the appropriate color. Refer to the color chart on the image for the type of watch or warning in effect. A watch indicates that conditions are likely for that particular storm to affect the region, a warning indicates that the conditions are imminent.

The color filled wind probability contours indicate the overall likelihood of winds reaching the thresholds of 40mph (yellow), 60mph(orange) and 75mph(dark red).There are dark line contours within these filled areas which show the probability of that threshold being met. There are 3 probability levels contoured, 10%, 40% and 70%, with the numeric label matching the color of the threshold being described. See the Hurricane Tracking/Forecast Sample chart for more details.

WSI SIGMETs -

Issued for significant turbulence, convection, icing, volcanic ash, or dust storm advisories. WSI SIGMETs are valid for up to 6 hours and are issued and updated as necessary.

Icing - moderate of greater icing forecasted for the highlighted area

Convection - 35% thunderstorm coverage or more

Volcanic ash - Ash above FL180 and forecasted to impact flight operations

Dust - Significant dust forecasted to reduce surface visibilities.

WSI Flight Plan Guidance (FPG) -

Depicts potential flight hazards including turbulence, thunderstorms, icing, volcanic ash, and dust storms. The interactive map display also includes ozone and other significant events.

Issued in 3 hour increments through +12 hours and updated a minimum of every 3 hours.