December 1999 11-5 intersection of the surface freezing level line and freezing level contours encloses an area with surfacebased multiple freezing levels.
Turbulence Areas of moderate or greater turbulence are enclosed by bold, long dashed lines. Turbulence intensities are identified by symbols. The vertical extent of turbulence layers is specified by top and base heights in hundreds of feet. Height values are relative to MSL with the top and base heights separated by a line. A top height of "240" indicates turbulence at or above 24,000 feet. (The upper limit of the prog is 24,000 feet.) The base height is omitted where turbulence reaches the surface. For example, "080/ " identifies a turbulence layer from the surface to 8,000 feet MSL. Thunderstorms always imply a variety of hazardous conditions to aviation including moderate or greater turbulence. Generally, turbulence conditions implied with thunderstorms is not depicted on the chart. However, for added emphasis, moderate to severe turbulence surface to above 24,000 feet is depicted for areas that have thunderstorms with more than half coverage on the surface prog. Intensity symbols and layer altitudes appear within or adjacent to the forecast area.
USING THE CHART The low-level significant weather prog chart provides an overview of selected flying weather conditions up to 24,000 feet for day 1. Much insight can be gained by evaluating the individual fields of pressure patterns, fronts, precipitation, weather flying categories, freezing levels, and turbulence displayed on the chart. In addition, certain inferences can be made from the chart. Surface winds can be inferred from surface pressure patterns. Structural icing can be inferred in areas which have clouds and precipitation, above freezing levels, and in areas of freezing precipitation. The low-level prog chart can also be used to obtain an overview of the progression of weather during day 1. The progression of weather is the change in position, size, and intensity of weather with time. Progression analysis is accomplished by comparing charts of observed conditions with the 12- and 24-hour prog panels. Progression analysis adds insight to the time-continuity of the weather from before flight time to after flight time. The low-level prog chart makes the comprehension of weather details easier and more meaningful. A comprehensive overview of weather conditions does not provide sufficient information for flight planning. Additional weather details are required. Essential weather details are provided by observed reports, forecast products, and weather advisories. Weather details are often numerous. An effective overview of observed and prog charts allows the many essential details to fit into place and have continuity.
36- AND 48-HOUR SURFACE PROG The 36- and 48-hour surface prog chart (Figure 11-2) is a day 2 forecast of general weather for the conterminous United States. The chart is an extension of the day 1 U.S. low-level significant weather prog chart issued from the same observed data base time. These two prog charts make up a forecast package. The chart is issued twice daily. The observation data base times for each issuance are 00Z and 12Z. For example, a chart issued based on 00Z Tuesday observations has a 36-hour valid time of 12Z Wednesday and a 48-hour valid time of 00Z Thursday. The chart is composed of two panels and a forecast discussion. The two panels contain the 36- and 48-hour surface progs.
SURFACE PROG PANELS The surface prog panels display forecast positions and characteristics of pressure patterns, fronts, and precipitation.
December 1999 11-6 Surface Pressure Systems Surface pressure systems are depicted by pressure centers, troughs, and isobars. Formats used for each feature are the same as used for the surface prog panels of the U.S. low-level significant weather prog chart.
Fronts Surface fronts are depicted by using the standard symbols and three-digit characterization code used on the surface analysis chart. (See Section 5.)
Precipitation Precipitation areas are outlined on each panel. Formats used to locate and characterize precipitation are the same as used for the surface prog panels of the U.S. low-level prog chart.
FORECAST DISCUSSION The forecast discussion is a discussion of the day 1 and day 2 forecast package. The discussion will include identification and characterization of weather systems and associated weather conditions portrayed on the prog charts.
USING THE CHART The 36- and 48-hour surface prog chart provides an outlook of general weather conditions for day 2. The 36- and 48-hour prog can also be used to assess the progression of weather through day 2.
HIGH-LEVEL SIGNIFICANT WEATHER PROG The high-level significant weather prog (Figures 11-3 and 11-4) is a day 1 forecast of significant weather.
Weather information provided pertains to the layer from above 24,000 to 60,000 feet (FL250-FL600).
The prog covers a large portion of the Northern Hemisphere and a limited portion of the Southern Hemisphere. Coverage ranges from the eastern Asiatic coast eastward across the Pacific, North America, and the Atlantic into Europe and northwestern Africa. The prog extends southward into northern South America. The area covered by the prog is divided into sections. Each section covers a part of the forecast area. Some sections overlap. The various sections are formatted on polar or Mercator projection background maps and issued as charts. Each prog chart is issued four times a day. The valid times are 00Z, 06Z, 12Z, and 18Z. Conditions routinely appearing on the chart are jet streams, cumulonimbus clouds, turbulence, and tropopause heights. Surface fronts are also included to add perspective. Other conditions will appear on the chart as pertinent. They are tropical cyclones, squall lines, volcanic eruption sites, and sandstorms and dust storms.
December 1999 11-7 FL390
ISOL EMBD CB 350 xxx Jet Streams Jet streams with a maximum speed of more than 80 knots are identified by bold lines. Jet stream lines lie along the core of maximum winds. Arrowheads on the lines indicate the orientation of each jet stream.
Double hatched lines positioned along the jet core identify changes of wind speed. These speed indicators are drawn at 20-knot intervals and begin with 100 knots. Wind speed maximums along the jet core are characterized by wind symbols and altitudes. A standard wind symbol (shaft, pennants, and barbs) is placed at each pertinent position to identify velocity. The altitude in hundreds of feet prefaced with "FL" is placed adjacent to each wind symbol.
Example:
Cumulonimbus Clouds Cumulonimbus clouds (CBs) are thunderstorm clouds. Areas of CBs meeting select criteria are enclosed by scalloped lines. The criteria are widespread CBs within an area or along a line with little or no space between individual clouds, and CBs are embedded in cloud layers or concealed by haze or dust. The prog does not display isolated or scattered CBs (one-half or less coverage) which are not embedded in clouds, haze, or dust. Cumulonimbus areas are identified with "CB" and characterized by coverage and tops. Coverages are identified as isolated (ISOL), occasional (OCNL), and frequent (FRQ). Isolated and occasional CBs are further characterized as embedded (EMBD.) Coverage values for the identifiers are:
isolated - less than 1/8; occasional - 1/8 to 4/8; and frequent - more than 4/8. Tops are identified in hundreds of feet using the standard top and base format. Bases extend below 24,000 feet (below the prog's forecast layer) and are encoded "XXX." The identification and characterization of each cumulonimbus area will appear within or adjacent to the outlined area. Thunderstorms always imply a variety of aviation hazards including moderate or greater turbulence and hail.
Examples:
Turbulence Areas of moderate or greater turbulence are enclosed by bold dashed lines. Turbulence conditions identified are those associated with wind shear zones and mountain waves. Wind shear zones include speed shears associated with jet streams and areas with sharply curved flow. Turbulence associated with thunderstorms is not identified. (Thunderstorms imply turbulence.) Turbulence intensities are identified by symbols. The vertical extent of turbulence layers is specified by top and base heights in hundreds of feet. Turbulence bases which extends below the layer of the chart are identified with "XXX." Top and
OCNL EMBD CB 450 xxx
December 1999 11-8 220 L 35 15 base heights are separated by a line. Height values are pressure altitudes. For example, "310/XXX" identifies a layer of turbulence from below FL240 to FL310.
Example:
Tropopause Heights Tropopause heights are plotted in hundreds of feet at selected locations. Heights are enclosed by rectangles. Centers of high and low heights are identified with "H" and "L" respectively along with their heights and enclosed by polygons.
Examples:
Surface Fronts Surface fronts are depicted on the prog to provide added perspective. Symbols used are the standard symbols used on the surface analysis chart. Movements of fronts are identified at selected positions. A vector with a number plotted adjacent to the vector identifies the direction and speed of movement. (See Section 5.)
Example:
310 xxx 300
December 1999 11-9 Tropical Cyclones The positions of hurricanes, typhoons, and tropical storms are depicted by symbols. The only difference between the hurricane/typhoon symbol and tropical storm symbol is the circle of the hurricane/typhoon symbol is shaded in. When pertinent, the name of each storm is positioned adjacent to the symbol.
Cumulonimbus cloud activity meeting chart criteria is identified and characterized relative to each storm.
Example:
Squall Lines Severe squall lines are lines of CBs with 5/8 coverage or greater. Squall lines are identified by long dashed lines, and each dash is separated by a "v." Cumulonimbus cloud activity meeting chart criteria is identified and characterized with each squall line.
Example:
V V Volcanic Eruption Sites Volcanic eruption sites are identified by a trapezoidal symbol. The dot on the base of the trapezoid locates the latitude and longitude of the volcano. The name of the volcano, its latitude, and its longitude are noted adjacent to the symbol. Pertinent SIGMETs containing information regarding volcanic ash will be in effect.
Example:
TUNGURAHUA
1.5S 78.4W Tropical Storm
December 1999 11-10 Sandstorms and Dust Storms Areas of widespread sandstorms and dust storms are labeled by symbol. The symbol with the arrow depicts areas of widespread sandstorm or dust storm, while the symbol without the arrow depicts severe sandstorm or dust haze.
Example:
USING THE CHART The high-level sig weather prog is used to get an overview of selected flying weather conditions above 24,000 feet. Much insight can be gained by evaluating jet streams, cumulonimbus clouds, turbulence, associated surface fronts, significant tropical storm complexes including tropical cyclones, squall lines, sandstorms, and dust storms.
December 1999 . 120 40N 30N 05 8 1998 Figure 11-1. U.S. Low-Level Significant Weather Prog.
АЛЛЛЛЛАЛАЛЛі LOW LVL 120W 18Z VT THU JAN 8 1998 THU SIG WY PROG 70W 30N 80t 90W 80/ 40 08 100W 200/100 80 40 JAN 8 1998 12HR SI6 X 906 D001 551 80 120W 06Z THU 120W VT 06Z 11-11
December 1999 105' 38 1028 VT: 12Z SAT 09 JAN 1999 TO SPREAD NEND TONIGHT THRU THE CNTRL APPALACHIANS AND INTO THE MID ATL AND NERN U.S. ON FRI IN ASSOC WITH A LEAD AREA OF HARM ADVECTION PUSHING NEND AHEAD OF THE AMPLIFYING UPSTREAM TROF. CHANGE OVER TO A MIX BAG OF PCPN IS LIKELY ACRS NEARLY ALL OF THE FRI INTO SAT AS WARMER AIR UL STREAM NEND AHEAD OF THE ACRS THE LOHER MS VALLEY/CNTRL GULF UPSTREAM AMPLIFYING TROF. COAST...LOCALLY HVY RAINS HL LIKELY CONT INTO AT LEAST TONIGHT ALONG THEN SPREAD NEED INTO AS SLEY FRI INTO SAT COLONION SOLEVEL FRONTAL CONVECTION SHOULD DYNAMICS STRENGTHEN ALONG THE AFOREMENTIONED N-S STRENGTHENING ACRS THE MID SECTION OF THE NATION...A REINFORCING SHOT OF ARCTIC AIR HL CONT TO PUSH QUICKLY SHD INTO THE NRN PLAINS TONIGHT AND THEN SHO ACRS THE REMAINDER OF THE PLAINS FRI INTO SAT. POST FRONTAL LIGHT SNOWS LIKELY IN THE ARCTIC AIR ACRS THE NRN PLAINS ADDITIONAL LIGHT SNOHS MAY STREAK FROM TONIGHT INTO EARLY SAT. SEHD INTO THE NRN PLAINS ON SAT AHEAD OF A DEVELOPING ALBERTA TRANQUIL UX FOR THE CLIPPER PUSHING SEND ALONG THE ARCTIC FRONT. GIBASIN AND W THIS PERIOD AS SFC HIGH PRESSURE BUILDS IN RESPONSE TO THIS GIBASIN SFC RIDGE HL STRENGTHEN THE UPSTREAM AMPLIFYING RIDGE. THE SANTA ANA'S ACRS SRN CA BY SAT HITH HIGH HINDS POSSIBLE. ORAVEC/FORECAST OPERATIONS BRANCH IMPLEASE NOTEM: THE HPC HEB ADDRESS HAS CHANGED: GRAPHICS ARE NOW Figure 11-2. U.S. Low-Level 36- and 48-hour Significant Weather Prog.
NERN U.S. FRONTAL BNORY.
t050 1467 DISCUSSION FROM 1200Z THU JAN 07 1999 HYDROMETEOROLOGICAL PREDICTION CENTER...NHS...CAMP SPRINGS HO REF 12 TO 48 HR MANUAL GRAPHICS THE LATEST MODEL RUNS ARE IN GOOD AGRANT WITH THE OVERALL TREND OF AMPLIFICATION TO THE BROAD CYCLONIC FLOW ACRS THE U.S. AS THE UPR RIDGE BUILDS OVR THE NERN PAC AND A DOHNSTREAM TROF AMPLIFIES OVR THERE ARE...HOHEVER... SOME FAIRLY SOFNT DIFFS HITH THE DETAILS OF THIS AMPLIFICATION AND THE ASSOC SFC FEATURES. AT TH... HE PREFER THE NGH/AVN SOLN WHICH IS FCSTG A SLIGHTLY MORE AMPLIFIED TROF DIGGING SEND BY EARLY SAT INTO THE MID MS VALLEY GIVEN THE AMPLITUDE OF THE UPSIRE THEY PRIMPRIRILAR ON REGIMES IN NOT IN ADDITION... THE ETA THE SENSIBLE UX FCSTG ENOUGH AMPLIFICATION TO DIGGING VORTS. EFFECTS FROM THIS MORE AMPLIFIED SOLN HOULD BE A SLOWER END PUSH TO THE MAIN N-S BAROCLINIC BNDRY STRENGTHENING OVR THE OH VALLEY THIS MORE AMPLIFIED UPR TROF FRI INTO SAT THAN THE ETA IS SUGGESTING. SHOULD ALSO SUPPORT INCREASED POST FRONTAL PCPN DEVELOPING OVR THE MID MS VALLEY LATE FRI AND SPREADING NEHD INTO THE OH VALLEY BY THIS POST FRONTAL PCPN HL LIKELY BE CHANGING FROM RAIN TO SNOW AS THE TROF AMPLIFIES AND PULLS IN COLDER AIR WITH POSSIBLE ACCUMULATING SNOHS ALSO LIKELY ACCUMULATIONS IN THE 3 TO 6- RANGE.
1999 FCSTR: DRAVEC 36HR SFC PROG VTs OOZ SAT 09 JAN NCEP I THE MS VALLEY. EARLY FRI.
11-12
December 1999 340 45 C [530] Figure 11-3. U.S. High-Level Significant Weather Prog.
FL340 RAFC WASHINGTON 250-600 SIGWX FL 10 APR 1998 SYMBOLS & AND CB IMPLY MOD OR SEVERE TURBULENCE AND HAIL CHECK SIGMETS FOR VOLCANIC ASH 530 11-13
December 1999 WASHINGTON 250-600 FL 00Z FRI 10 APR 1998 SYMBOLS & AND CB IMPLY MOD OR SEVERE TURBULENCE AND HAIL SIGMETS FOR VOLCANIC ASH SIGWX KPOB Figure 11-4. International High-Level Significant Weather Prog 11-14
December 1999 12-1 Section 12
CONVECTIVE OUTLOOK CHART The convective outlook chart (Figure 12-1) delineates areas forecast to have thunderstorms. This chart is presented in two panels. The left-hand panel is the Day 1 Convective Outlook, and the right-hand panel is the Day 2 Convective Outlook. These guidance products are produced at the Storm Prediction Center (SPC) in Norman, OK.
DAY 1 CONVECTIVE OUTLOOK The Day 1 Convective Outlook (Figure 12-1) outlines areas in the continental United States where thunderstorms are forecasted during the Day 1 period. It is issued five times daily. The first issuance is 06Z and is the initial Day 1 Convective Outlook that is valid 12Z that day until 12Z the following day.
The other issuances are 1300Z, 1630Z, 2000Z, and 0100Z, and all issuances are valid until 12Z the following day.
The outlook issued qualifies the level of risk (i.e., SLGT, MDT, HIGH) as well as the areas of general thunderstorms.
DAY 2 CONVECTIVE OUTLOOK The Day 2 Convective Outlook contains the same information as the Day 1 Convective Outlook. It is issued twice a day. It is initially issued at 0830Z during standard time and 0730Z during daylight time. It is updated at 1730Z. The timeframe covered is from 12Z the following day to 12Z the next day. For example, if today is Monday, the Day 2 Convective Outlook will cover the period 12Z Tuesday to 12Z Wednesday.
The outlook issued qualifies the level of risk (i.e., SLGT, MDT, HIGH) as well as the areas of general thunderstorms.
LEVELS OF RISK Risk areas come in three varieties and are based on the expected number of severe thunderstorm reports per geographical unit and forecaster confidence. Table 12-1 indicates the labels that appear on both the Day 1 and Day 2 Convective Outlook charts.
December 1999 12-2 Table 12-1 Notations of Risk
NOTATION EXPLANATION SEE TEXT Used for those situations where a SLGT risk was considered but at the time of the forecast, was not warranted.
SLGT (Slight risk) A high probability of 5 to 29 reports of 1 inch or larger hail, and/or 3-5 tornadoes, and/or 5 to 29 wind events,…or…a low/moderate probability of moderate to high risk being issued later if some conditions come together MDT (Moderate risk) A high probability of at least 30 reports of hail 1 inch or larger; or 6-19 tornadoes; or numerous wind events (30).
HIGH (High risk) A high probability of at least 20 tornadoes with at least two of them rated F3 (or higher), or an extreme derecho causing widespread (50 or more) wind events with numerous higherend wind (80 mph or higher) and structural damage reports SEE TEXT is used for those situations where a slight risk was considered, but at the time of the forecast, was not warranted. Although there is no severe outlook for the labeled area, users should read the text of the convective outlook (AC) forecast message to learn more about the potential for a threat to develop if some particular conditions do come together.
Slight (SLGT) risk implies well-organized severe thunderstorms are expected but in small numbers and/or low coverage.
Moderate (MDT) risks imply a greater concentration of severe thunderstorms, and in most situations, greater magnitude of severe weather.
High (HIGH) risk almost always means a major severe weather outbreak is expected, with great coverage of severe weather and enhanced likelihood of extreme severe events (i.e., violent tornadoes or unusually intense damaging wind events). SPC issues a public information statement (PWO) describing a "particularly dangerous situation" when HIGH risk areas are in effect, and it sometimes issues a PWO for MDT risk situations. Some National Weather Service (NWS) offices will include in their public forecasts the phrase "some thunderstorms may be severe" when a MDT or HIGH risk is issued.
In addition to the severe risk areas, general thunderstorms (non-severe) are outlined, but with no label on the graphic map.
USING THE CHART The Day 1 and Day 2 Convective Outlooks Charts are flight planning tools used to determine forecast areas of thunderstorms.