December 1999 4-23
INFLIGHT AVIATION WEATHER ADVISORIES Inflight Aviation Weather Advisories are forecasts to advise en route aircraft of development of potentially hazardous weather. All inflight aviation weather advisories in the conterminous U.S. are issued by the Aviation Weather Center (AWC) in Kansas City, Missouri. The WFO in Honolulu issues advisories for the Hawaiian islands. In Alaska, the Alaska Aviation Weather Unit (AAWU) issues inflight aviation weather advisories. All heights are referenced MSL, except in the case of ceilings CIG, which indicate AGL.
There are three types of inflight aviation weather advisories - the Significant Meteorological Information (SIGMET), the Airman’s Meteorological Information (AIRMET), and Convective SIGMET. All of these advisories use the same location identifiers (either VORs, airports, or well-known geographic areas) to describe the hazardous weather areas (see Figures 4-11 and 4-12 on pages 4-45 and 4-46).
SIGMET (WS)/AIRMET (WA) SIGMETs/AIRMETs are issued corresponding to the FA areas (see Figures 4-5 and 4-6). The maximum forecast period is 4 hours for SIGMETs and 6 hours for AIRMETs. Both advisories are considered “widespread” because they must be either affecting or be forecasted to affect an area of at least 3,000 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 in actuality only a small portion of this total area would be affected at any one time.
SIGMET (WS) A SIGMET advises of nonconvective weather that is potentially hazardous to all aircraft. SIGMETs are unscheduled products that are valid for 4 hours. However, conditions that are associated with hurricanes are valid for 6 hours. Unscheduled updates and corrections are issued as necessary. In the conterminous U.S., SIGMETs are issued when the following phenomena occur or are expected to occur:
1. Severe icing not associated with thunderstorms
2. Severe or extreme turbulence or clear air turbulence (CAT) not associated with thunderstorms
3. Dust storms or sandstorms lowering surface or inflight visibilities to below 3 miles
4. Volcanic ash In Alaska and Hawaii, SIGMETs are also issued for:
1. Tornadoes
2. Lines of thunderstorms
3. Embedded thunderstorms
4. Hail greater than or equal to ¾ inch in diameter SIGMETs are identified by an alphabetic designator from November through Yankee excluding Sierra and Tango. (Sierra, Tango, and Zulu are reserved for AIRMETs.) The first issuance of a SIGMET will be labeled as UWS (Urgent Weather SIGMET). Subsequent issuances are at the forecaster’s discretion.
Issuance for the same phenomenon will be sequentially numbered, using the original designator until the phenomenon ends. For example, the first issuance in the Chicago (CHI) FA area for phenomenon moving from the Salt Lake City (SLC) FA area will be SIGMET Papa 3, if the previous two issuances, Papa 1 and Papa 2, had been in the SLC FA area. Note that no two different phenomena across the country can have the same alphabetic designator at the same time.
December 1999 4-24 Example of a SIGMET:
BOSR WS 050600 SIGMET ROMEO 2 VALID UNTIL 051000
ME NH VT
FROM CAR TO YSJ TO CON TO MPV TO CAR MOD TO OCNL SEV TURB BLW 080 EXP DUE TO STG NWLY FLOW. CONDS CONTG BYD 1000Z.
International SIGMET Some NWS offices have been designated by the ICAO as Meteorological Watch Offices (MWOs).
These offices are responsible for issuing International SIGMETs for designated areas that include Alaska, Hawaii, portions of the Atlantic and Pacific Oceans, and the Gulf of Mexico. The offices which issue International SIGMETs are the Alaskan Aviation Weather Unit in Anchorage, Alaska (AK);the Tropical Prediction Center in Miami, Florida (FL); the WFO in Honolulu, Hawaii (HI); the Aviation Weather Center in Kansas City, MO; and the WFO on Guam Island in the Pacific Ocean. These SIGMETs are considered “widespread” because they must be either affecting or be forecasted to affect an area of at least 3,000 square miles at any one time. The International SIGMET is issued for 12 hours for volcanic ash events, 6 hours for hurricanes and tropical storms, and 4 hours for all other events. Like the domestic SIGMETs, International SIGMETs are also identified by an alphabetic designator from Alpha through Mike and are numbered sequentially until that weather phenomenon ends. The criteria for an International SIGMET are:
1. Thunderstorms occurring in lines, embedded in clouds, or in large areas producing tornadoes or large hail
2. Tropical cyclones
3. Severe icing
4. Severe or extreme turbulence
5. Dust storms and sandstorms lowering visibilities to less than 3 miles
6. Volcanic ash Example of an International SIGMET:
ZCZC MIASIGA1L TTAA00 KNHC 121600 KZNY SIGMET LIMA 5 VALID 121600/122000 UTC KNHC- ACT TS OBS BY SATELLITE WI AREA BOUNDED BY 30N69W 31N64.6W 26.4N66.4W
27.5N69.4W 30N69W. CB TOPS TO FL480. MOV ENE 15 KT. INTSF.
AIRMET (WA) AIRMETs (WAs) are advisories of significant weather phenomena but describe conditions at intensities lower than those which require the issuance of SIGMETs. AIRMETs are intended for dissemination to all pilots in the preflight and en route phase of flight to enhance safety. AIRMET Bulletins are issued on a scheduled basis every 6 hours beginning at 0145 UTC during Central Daylight Time and at 0245 UTC during Central Standard Time. Unscheduled updates and corrections are issued as necessary. Each AIRMET Bulletin contains any current AIRMETs in effect and an outlook for conditions expected after the AIRMET valid period. AIRMETs contain details about IFR, extensive mountain obscuration, turbulence, strong surface winds, icing, and freezing levels.
December 1999 4-25 There are three AIRMETs - Sierra, Tango, and Zulu. AIRMET Sierra describes IFR conditions and/or extensive mountain obscurations. AIRMET Tango describes moderate turbulence, sustained surface winds of 30 knots or greater, and/or nonconvective low-level wind shear. AIRMET Zulu describes moderate icing and provides freezing level heights. After the first issuance each day, scheduled or unscheduled bulletins are numbered sequentially for easier identification.
Example of AIRMET Sierra issued for the Chicago FA area:
CHIS WA 121345 AIRMET SIERRA UPDT 3 FOR IFR AND MTN OBSCN VALID UNTIL 122000 .
AIRMET IFR...SD NE MN IA MO WI LM MI IL IN KY FROM 70NW RAP TO 50W RWF TO 50W MSN TO GRB TO MBS TO FWA TO CVG TO HNN TO TRI TO ARG TO 40SSW BRL TO OMA TO BFF TO 70NW RAP OCNL CIG BLW 010/VIS BLW 3SM FG/BR. CONDS ENDG 15Z-17Z.
.
AIRMET MTN OBSCN...KY TN
FROM HNN TO TRI TO CHA TO LOZ TO HNN MTNS OCNL OBSC CLDS/PCPN/BR. CONDS ENDG TN PTN AREA 18Z- 20Z..CONTG KY BYD 20Z..ENDG 02Z.
....
Example of AIRMET Tango issued for the Salt Lake City FA area:
SLCT WA 121345 AIRMET TANGO UPDT 2 FOR TURB VALID UNTIL 122000 .
AIRMET TURB...NV UT CO AZ NM FROM LKV TO CHE TO ELP TO 60S TUS TO YUM TO EED TO RNO TO LKV OCNL MOD TURB BLW FL180 DUE TO MOD SWLY/WLY WNDS. CONDS CONTG BYD 20Z THRU 02Z.
.
AIRMET TURB...NV WA OR CA CSTL WTRS FROM BLI TO REO TO BTY TO DAG TO SBA TO 120W FOT TO 120W TOU TO BLI OCNL MOD TURB BTWN FL180 AND FL400 DUE TO WNDSHR ASSOCD WITH JTSTR. CONDS CONTG BYD 20Z THRU 02Z.
....
Example of AIRMET Zulu issued for the San Francisco FA area:
SFOZ WA 121345 AIRMET ZULU UPDT 2 FOR ICE AND FRZLVL VALID UNTIL 122000 .
AIRMET ICE...WA OR ID MT NV UT
FROM YQL TO SLC TO WMC TO LKV TO PDT TO YDC TO YQL LGT OCNL MOD RIME/MXD ICGICIP BTWN FRZLVL AND FL220. FRZLVL 080-120. CONDS CONTG BYD 20Z THRU 02Z.
.
AIRMET ICE...WA OR FROM YDC TO PDT TO LKV TO 80W MFR TO ONP TO TOU TO YDC LGT OCNL MOD RIME/MXD ICGICIP BTWN FRZLVL AND FL180. FRZLVL 060-080. CONDS CONTG BYD 20Z THRU 02Z.
.
FRZLVL...WA...060 CSTLN SLPG 100 XTRM E.
OR...060-070 CASCDS WWD. 070-095 RMNDR.
NRN CA...060-100 N OF A 30N FOT-40N RNO LN SLPG 100-110 RMNDR.
December 1999 4-26
CONVECTIVE SIGMET (WST) Convective SIGMETs are issued in the conterminous U.S. for any of the following:
1. Severe thunderstorm due to:
a. surface winds greater than or equal to 50 knots
b. hail at the surface greater than or equal to ¾ inches in diameter
c. tornadoes
2. Embedded thunderstorms
3. A line of thunderstorms
4. Thunderstorms producing precipitation greater than or equal to heavy precipitation affecting 40% or more of an area at least 3,000 square miles Any convective SIGMET implies severe or greater turbulence, severe icing, and low-level wind shear. A convective SIGMET may be issued for any convective situation that the forecaster feels is hazardous to all categories of aircraft.
Convective SIGMET bulletins are issued for the western (W), central (C), and eastern (E) United States.
(Convective SIGMETs are not issued for Alaska or Hawaii.) The areas are separated at 87 and 107 degrees west longitude with sufficient overlap to cover most cases when the phenomenon crosses the boundaries. Bulletins are issued hourly at H+55. Special bulletins are issued at any time as required and updated at H+55. If no criteria meeting convective SIGMET requirements are observed or forecasted, the message “CONVECTIVE SIGMET…NONE” will be issued for each area at H+55. Individual convective SIGMETs for each area (W, C, E) are numbered sequentially from number one each day, beginning at 00Z. A convective SIGMET for a continuing phenomenon will be reissued every hour at H+55 with a new number. 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.
Example of a convective SIGMET:
MKCC WST 251655 CONVECTIVE SIGMET 54C VALID UNTIL 1855Z WI IL FROM 30E MSN-40ESE DBQ DMSHG LINE TS 15 NM WIDE MOV FROM 30025KT. TOPS TO FL450. WIND GUSTS TO 50 KT POSS.
CONVECTIVE SIGMET 55C VALID UNTIL 1855Z WI IA FROM 30NNW MSN-30SSE MCW DVLPG LINE TS 10 NM WIDE MOV FROM 30015KT. TOPS TO FL300.
CONVECTIVE SIGMET 56C VALID UNTIL 1855Z
MT ND SD MN IA MI LINE TS 15 NM WIDE MOV FROM 27020KT. TOPS TO FL380.
OUTLOOK VALID 151855-252255 FROM 60NW ISN-INL-TVC-SBN-BRL-FSD-BIL-60NW ISN
December 1999 4-27
IR STLT IMGRY SHOWS CNVTV CLD TOP TEMPS OVER SRN WI HAVE BEEN WARMING STEADILY INDCG A WKNG TREND. THIS ALSO REFLECTED BY LTST RADAR AND LTNG DATA. WKNG TREND OF PRESENT LN MAY CONT…HWVR NEW DVLPMT IS PSBL ALG OUTFLOW BDRY AND/OR OVR NE IA/SW WI BHD CURRENT ACT.
A SCND TS IS CONTG TO MOV EWD THRU ERN MT WITH NEW DVLPMT OCRG OVR CNTRL ND. MT ACT IS MOVG TWD MORE FVRBL AMS OVR THE WRN DAKS WHERE DWPTS ARE IN THE UPR 60S WITH LIFTED INDEX VALUES TO MS 6. TS EXPD TO INCR IN COVERAGE AND INTSTY DURG AFTN HRS.
WST ISSUANCES EXPD TO BE RQRD THRUT AFTN HRS WITH INCRG PTNTL FOR STGR CELLS TO CONTAIN LRG HAIL AND PSBLY DMGG SFC WNDS.
ALASKA, GULF OF MEXICO, AND INTERNATIONAL AREA FORECASTS (FAs)
ALASKA AREA FORECAST (FA) The Alaska Aviation Weather Unit in Anchorage, Alaska, produces the FA for the entire state of Alaska.
The Alaska FA combines the FA, SIGMETs, and AIRMETs into one product. Each FA contains a regional synopsis, 12-hour geographic specific forecasts, and an 18-hour outlook for each geographic area. Forecast weather elements are sky condition, cloud height, mountain obscuration, visibility, weather and/or obstructions to visibility, strong surface winds (direction and speed), icing, freezing level, and mountain pass conditions. Hazards and flight precautions, including AIRMETs and SIGMETs, may be found in their respective geographic area. AIRMETs and SIGMETs are also issued as separate products.
Partial example of Alaska FA:
JNUH FA 191445 .
EASTERN GULF COAST AND SE AK… .
AIRMET VALID UNTIL 230300 TS IMPLY POSSIBLE SEV OR GREATER TURB SEV ICE LLWS AND IFR CONDS.
NON MSL HEIGHTS NOTED BY AGL OR CIG .
SYNOPSIS…VALID UNTIL 231500 990 MB LOW VCY PACV DRFTG E AND WKN. CDFNT S FM LOW BCMG STNR AND WK ICY BAY SWD BY 15Z. E PACIFIC LOW S 50N MOV N TO 975 MB CNTR 50 SM W PASI AT 15Z WI OCFNT SWD.
.
LYNN CANAL AND GLACIER BAY JB…VALID UNTIL 230900 …CLOUDS/WX… …AIRMET MT OBSC…TEMPO MT OBSC INCLDS. NC… SCT030 SCT-BKN050 BKN100 TOP 160. TEMPO HI LYRS TOP FL250. TEMPO BKN030 ISOL -RA. SFC WND S 15 KT G25 KT LYNN CANAL.
OTLK VALID 230900-240300…VFR RA. 18Z MVFR CIG RA.
PASSES…WHITE AND CHILKOOT…MVFR CIG RASN.
…TURB… LYNN CANAL…ISOL MOD TURB BLW 060. ELSW ..NIL SIG.
…ICE AND FZLVL… TEMPO LGT RIME ICEIC 050-120. FZLVL 030.
December 1999 4-28 Figure 4-7. Alaska Area Forecast Sectors.
December 1999 4-29
GULF OF MEXICO AREA FORECAST A specialized FA for the Gulf of Mexico is issued by the Tropical Prediction Center in Miami, Florida.
The product combines the FA, inflight aviation weather advisories, and marine precautions. This product is intended to support both offshore heliport and general aviation operations. The Gulf of Mexico FA focuses on an area which includes the coastal plains and coastal waters from Apalachicola, Florida, to Brownsville, Texas, and the offshore waters of the Gulf of Mexico, in an area west of 85W longitude and north of 27N latitude. Each section of the FA describes the weather conditions expecting to impact the area and will include the descriptor none if no significant weather is forecast to occur. Amendments to this FA are issued the same as amendments to the domestic FAs.
Partial example of Gulf of Mexico FA:
FAGX01 KNHC 151030 151100Z-152300Z OTLK…152300Z-161100Z AMDT NOT AVBL 0200Z-1100Z
TROPICAL ANALYSIS AND FORECAST BRANCH
TROPICAL PREDICTION CENTER MIAMI FLORIDA GLFMEX N OF 27N W OF 85W…CSTL PLAINS CSTL WTRS AQQ-BRO. HGTS MSL UNLESS NOTED.
TS IMPLY POSS SEV OR GTR TURB…SEV ICE…LOW LVL WS AND STG SFC WND…HIGH WAVES…CIG BLW 010…AND VIS BLW 3SM.
01 SYNS… WK SFC TROUGH FM 31N84W TO 26N88W AT 11Z DRIFTING E THROUGH 23Z. WK HIGH PRES ACRS RMNDER GLFMEX THRU FCST AND OTLK PD.
… 02 FLT PRCTNS… NONE.
… 03 MARINE PRCTNS… NONE.
… 04 SGFNT CLD/WX… CSTL PLAINS CSTL WTRS BRO-LCH AND OFSHR WTRS W OF 94W… FEW040. OTLK…VFR.
… CSTL PLAINS LCH-AQQ… FEW015. OCNL VIS 3-5SM BR. AFT 14Z SCT100. AFT 19Z SCT/BKN020-030 BKN/SCT070-090.
WIDELY SCT TSRA/ISOL +TSRA.
… 05 ICE AND FZ LEVEL BLW 120… NONE. FZ LEVEL ABV 120.
… 06 TURB BLW 120… NONE.
…
December 1999 4-30 07 WND BLW 120… CSTL PLAINS CSTL WTRS LCH-GPT AND OFSHR WTRS 94W-89W…SFC- 120 NE-E 10-15 KT.
OTLK…NOSIG.
… 08 WAVES… CSTL WTRS BRO-AQQ…1-2 FT. OTLK…NOSIG.
NNNN
INTERNATIONAL AREA FORECASTS FAs from the surface to 25,000 feet are also prepared in international format for areas in the Atlantic Ocean, Caribbean Sea, and the Gulf of Mexico. Moreover, significant weather forecasts for 25,000 feet to 60,000 feet are prepared in chart form and in international text format for the Northern and Western hemispheres.
Example of an International FA from the surface to FL250:
FANT2 KWBC 091600 091800Z TO 100600Z ATLANTIC OCEAN WEST OF A LINE FROM 40N 67W TO 32N 63W. SFC TO FL250.
SYNOPSIS.
RIDGE OVER AREA MOVING TO EAST. FRONTAL SYSTEM MOVING OFF COAST BY 06Z.
SIGNIFICANT CLDS/WX.
N OF 34N AND W OF 71W…PATCHES OVC005/015 TOP 030/040 OTHERWISE BKN/OVC015/025 BKN/OVC200/240. BY 06Z INCREASING IMC IN SHRA/TS SPREADING ACROSS AREA FROM WEST. TS TOPS ABOVE 240.
S OF 34N AND W OF 75W…SCT/BKN 015/250. BY 06Z INCREASING IMC IN SHRA/TS SPREADING ACROSS AREA FROM WEST. TS TOPS ABOVE 240.
ELSEWHERE…CLR OCNL SCT015/025. BY 06Z INCREASING BKN080/100.
ICE.
FZ LVL 080/090 N SLOPING TO 120/130 S. MOD IN SHRA. SEV IN TS.
TURB.
MOD IN SHRA. SEV IN TS.
OUTLOOK.
100600Z TO 101800Z FRONT CONTINUING SLOWLY EWD. INCREASING IMC IN SHRA/TS SPREADNG E OVER AREA. SHRA/TS ENDING SW PORTION AFTER FRONTAL PASSAGE.
December 1999 4-31 Example of international significant weather forecast for FL250 to FL600:
FAPA1 KWBC 141610 SIG WX PROG FL250-FL600 VALID 150600Z ISOL EMBD CB TOPS 400 NE OF 11N173W 14N166W 11N164W 01N174W ISOL EMBD CB TOPS 400 07N158W 08N137W 11N137W 12N158W 07N158W ISOL EMBD CB TOPS 400 19N157W 32N143W 22N162W 15N162W
MDT OR GRTR TURB AND ICG VCNTY ALL CBS MDT TURB 310-410 19N145W 25N144W 19N163W 15N162W 19N145W The groups of numbers and letters are the boundary points of the areas in latitude and longitude. For example, “11N173W” is latitude 11 degrees north and longitude 173 degrees west.
TRANSCRIBED WEATHER BROADCAST (TWEB) TEXT PRODUCTS NWS offices prepare transcribed weather broadcast (TWEB) text products for the contiguous U.S., including synopsis and forecast for more than 200 routes and local vicinities. (See Figure 4-8.) (Not all NWS forecast offices issue all three of these products.) These products may be used in the Telephone Information Briefing Service (TIBS), Pilot’s Automatic Telephone Weather Answering Service (PATWAS), Low/Medium Frequency (L/MF) and VHF omni-directional radio range (VOR) facilities as described in Section 1. TWEB products are valid for 12 hours and are issued 4 times a day at 0200Z, 0800Z, 1400Z, and 2000Z. A TWEB route forecast or vicinity forecast will not be issued if the TAF for that airport has not been issued. A NIL TWEB will be issued instead.
A TWEB route forecast is for a 50-nautical-mile wide corridor along a line connecting the end points of the route. A TWEB local vicinity forecast covers an area with a radius of 50 nautical miles. The route and vicinity forecasts describe specific information on sustained surface winds (25 knots or greater), visibility, weather and obscuration to vision, sky conditions (coverage and ceiling/cloud heights), mountain obscurement, and nonconvective low-level wind shear. If visibility of 6SM or less is forecast, obstructions to vision and/or weather will be included. Thunderstorms and volcanic ash will always be included regardless of the visibility. Cloud bases can be described either in MSL or AGL (CIGS or BASES). It depends on which statement is used: “ALL HGTS MSL XCP CIGS.” or “ALL HGTS AGL XCP TOPS.” Use of “AGL,” “CIGS,” and “BASES” should be limited to cloud bases within 4,000 feet of the ground. Cloud tops, referenced to MSL, should also be forecasted following the sky cover when expected to be below 15,000 MSL using the sky cover contractions FEW, SCT, or BKN. Nonconvective low-level wind shear will be included when the TAF for the airport involved has issued a nonconvective low-level wind shear forecast. Expected areas of icing and turbulence will not be included.
Example of TWEB route forecast:
249 TWEB 251402 KISN-KMOT-KGFK. ALL HGTS AGL XCP TOPS. KISN-50NM E KISN TIL 00Z P6SM SKC…AFT 00Z P6SM SCT050 LCL P6SM -TSRA BKN050. 50NM E KISN-KDVL TIL 20Z P6SM SCT070…AFT 20Z P6SM SCT070 LCL SFC WNDS VRB35G45KT 3-5SM TSRA CIGS OVC030-040. KDVL-KGFK TIL 16Z P6SM SCT-BKN020 AREAS 3-5SM BR…AFT 16Z P6SM SCT040.
December 1999 4-32 Explanation of route forecast:
249 - route number TWEB - TWEB route forecast 25 - twenty-fifth day of the month 1402 - valid 14Z on the twenty-fifth to 02Z on the twenty-sixth (12 hours) KISN-KMOT-KGFK - route: Williston, North Dakota (ND), to Minot, ND, to Grand Forks, ND Remainder of the message explained: All heights AGL except cloud tops. KISN-50NM E KISN until 00Z, visibility greater than 6SM with clear skies. After 00Z, visibility greater than 6SM with scattered clouds at 5,000 feet AGL. Local areas of visibility greater than 6SM, thunderstorm with light rain showers, and broken clouds at 5,000 feet AGL. 50 NM E KISN-KDVL (Devil’s Lake, ND) until 20Z, visibility greater than 6SM, scattered clouds at 7,000 feet AGL. After 20Z, visibility greater than 6SM, scattered clouds at 7,000 feet AGL, local surface winds variable at 35 gusting to 45 knots, visibility 3- 5SM, thunderstorm with moderate rain showers, overcast ceilings 3,000-4,000 feet AGL. KDVL-KGFK until 16Z, visibility greater than 6SM, scattered to broken clouds at 2,000 feet AGL, areas of visibility 3- 5SM with mist. After 16Z, visibility greater than 6SM, scattered clouds at 4,000 feet AGL.
An example of TWEB vicinity forecast:
431 TWEB 021402 LAX BASIN. ALL HGTS MSL XCP CIGS. TIL 18Z P6SM XCP 3SM BR VLYS BKN020…18Z-22Z P6SM SCT020 SCT-BKN100…AFT 22Z P6SM SKC.
Explanation of vicinity forecast:
431 - TWEB vicinity number TWEB - TWEB forecast 02 - second day of the month 1402 - valid 14Z on the second to 02Z on the third (12 hours) LAX BASIN - The weather conditions in the Los Angeles basin until 18Z, visibility greater than 6SM except 3SM due to mist in the valleys and broken clouds at 2,000 feet MSL. Between 18Z and 22Z, visibility greater than 6SM and scattered clouds at 2,000 feet AGL; also scattered to broken clouds at 10,000 feet MSL. After 22Z, visibility greater than 6SM and sky clear.
A TWEB synopsis forecast is a brief description of the weather systems affecting the route during the forecast valid period. The synopsis describes movement of pressure systems, movement of fronts, upper air disturbances, or air flow.
An example of a TWEB synopsis:
BIS SYNS 250820. LO PRES TROF MVG ACRS ND TDA AND TNGT. HI PRES MVG SEWD FM CANADA INTO NWRN ND BY TNGT AND OVR MST OF ND BY WED MRNG.
Explanation of synopsis:
BIS - Bismarck, ND, WFO issuing the synopsis and route forecast SYNS - Synopsis for the area covered by the route forecast 25 - twenty-fifth day of the month 0820 - Valid from 08Z on the twenty-fifth to 20Z on the twenty-fifth (12 hours) The remainder of message explained: Low pressure trough moving across North Dakota today and tonight. High pressure moving southeastward from Canada into northwestern North Dakota by tonight and over most of North Dakota by Wednesday morning.
December 1999 4-33 An example of another TWEB synopsis:
CYS SYNS 101402 STG UPSLP WNDS OVR WY TIL 01Z WITH WDSPRD IFR CONDS IN LGT SN AND BLOWING SN. CONDS WL IPV FM N TO S ACRS WY AFT 01Z WITH DCRG CLDS.
Explanation of synopsis:
CYS - Cheyenne, WY, WFO issuing the synopsis and route forecast SYNS - Synopsis for the area covered by the route forecast 10 - tenth day of the month 1402 - Valid from 14Z on the tenth to 02Z on the eleventh (12 hours) The remainder of the message explained: Strong upslope winds over Wyoming until 01Z with widespread IFR conditions in light snow and blowing snow. Conditions will improve from north to south across Wyoming after 01Z with decreasing clouds.
December 1999 MI MO KFAr Figure 4-8. TWEB Route Map.
249 ND NE
SD KPIR KCYS KMLS NM 195 382 AZ TWEB ROUTE NETWORK LEGEND • Local Vicinity Forecast • Local Area Forecast 235 Route Forecast (Number) 400 CA 4-34