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• Low-Level Wind Shear (LLWS) (see Figure 5-18, G-AIRMET—Low-Level Wind Shear (LLWS) Snapshot Example).
o LLWS is defined as windshear below 2,000 ft AGL, other than convectively induced, exceeding 10 kts per 100 ft (vector difference between two points in space).
Figure 5-18. G-AIRMET—Low-Level Wind Shear (LLWS) Snapshot Example
• Str ong Surface Winds (see Figure 5-19, G-AIRMET—Strong Surface Winds Snapshot Example).
o Areas of sustained surface winds greater than 30 kts. The direction and speed of winds are not depicted; only the area where sustained surface winds greater than 30 kts are occurring or forecast to occur.
5-30 Figure 5-19. G-AIRMET—Strong Surface Winds Snapshot Example
5.3.3 Use.
When using the G-AIRMET, users must keep in mind that if a 00-hour forecast shows no significant weather and a 03-hour forecast shows hazardous weather, they must assume a change is occurring during the period between the two forecasts. It should be taken into consideration that the hazardous weather starts immediately after the 00-hour forecast unless there is a defined initiation or ending time for the hazardous weather. The same would apply after the 03-hour forecast. The user should assume the hazardous weather condition is occurring between the snapshots unless informed otherwise.
5.4 Center Weather Advisory (CWA).
A CWA is an aviation weather warning for conditions meeting or approaching national in-flight advisory (AIRMET, SIGMET, or Convective SIGMET) criteria. The CWA is primarily used for aircrews to anticipate and avoid adverse weather conditions in the en route and terminal environments. CWAs are available on https://www.aviationweather.gov.
5.4.1 CWA Issuance.
CWAs are issued by the NWS Center Weather Service Units (CWSU). CWSU AORs are depicted on Figure 5-20, Center Weather Service Unit (CWSU) Areas of Responsibility.
5-31 Figure 5-20. Center Weather Service Unit (CWSU) Areas of Responsibility
C WAs are valid for up to 2 hours and may include forecasts of conditions expected to begin within 2 hours of issuance. If conditions are expected to persist after the advisory’s valid period, a statement to that effect is included in the last line of the text. Additional CWAs will subsequently be issued as appropriate. Notice of significant changes in the phenomenon described in a CWA is provided by a new CWA issuance for that phenomenon. If the forecaster deems it necessary, CWAs may be issued hourly for Convective activity.
5.4.2 CWA Criteria.
CWAs may be issued for the following situations:
• There is no existing AWC or AAWU advisory in effect.
• Any of the following conditions occur:
o Conditions meeting Convective SIGMET criteria.
o Icing—moderate or greater.
o Turbulence—moderate or greater.
o Heavy and extreme precipitation.
o Freezing precipitation.
o Conditions at or approaching low IFR (LIFR).
o Surface wind gust at or above 30 kts.
o LLWS (Surface—2,000 ft).
o Volcanic ash, duststorms, or sandstorms.
5-32 o When a hazard has grown significantly outside of the boundary defined by the AWC or AAWU advisory.
o To upgrade a thunderstorm advisory to include severe thunderstorms.
o To upgrade an AIRMET to include isolated severe turbulence or icing. If greater than isolated severe turbulence or icing is occurring, then a new (non-supplementary) CWA should be issued.
o To define a line of thunderstorms within a larger area covered by the AWC or AAWU advisory.
o To better define hazards expected at a major terminal already within an AWC or AAWU advisory.
o Anything that in the judgment of the CWSU forecaster will add value to an existing advisory.
• In the forecaster’s judgment the conditions listed above, or any others, may adversely impact the safe flow of air traffic.
5.4.3 CWA Format and Example.
Figure 5-21. Center Weather Advisory (CWA) Decoding Example
5-33 Table 5-8. Decoding a Center Weather Advisory (CWA) Line Content Description 1 ZDV ARTCC Identification 2 Phenomenon Number (single digit, 1-6) CWA Product Type (UCWA/CWA) 032140 Beginning and/or issuance UTC date/time 2 ZDV ARTCC Identification CWA Product Type 2 Phenomenon Number (single digit, 1-6) 02 Issuance Number (issued sequentially for each Phenomenon Number) VALID TIL 032340Z Ending valid UTC date/time 3 FROM FMN TO 10N FMN TO 20NE FMN TO 10E FMN TO FMN Phenomenon Location 4 ISOLD SEV TS NR FMN MOVG NEWD 10KTS.
TOP FL410. WND GSTS TO 55KTS. HAIL TO 1 INCH RPRTD AT FMN. SEV TS CONTG BYD 2340Z Phenomenon Description The CWA in Figure 5-21, Center Weather Advisory (CWA) Decoding Example, is decoded as follows:
(Line 1) Center Weather Advisory issued for the Denver ARTCC (ZDV) CWSU. The “2” after ZDV in the first line denotes this is the second meteorological event of the local calendar day. This CWA was issued/begins on the 3rd day of the month at 2140 UTC.
(Line 2) The Denver ARTCC (ZDV) is identified again. The “202” in the second line denotes the phenomena number again (2) and the issuance number (02) for this phenomenon. This CWA is the valid until the 3rd day of the at 2340 UTC.
(Line 3) From Farmington, New Mexico to 10 nautical miles north of Farmington, New Mexico to 20 nautical miles northeast of Farmington, NM to 10 nautical mile east of Farmington, New Mexico to Farmington, New Mexico.
(Line 4) Isolated severe thunderstorms near Farmington moving northeastward at 10 knots. Tops to Flight Level 410. Wind gusts to 55 knots. Hail to one inch reported at Farmington. Severe thunderstorms continuing beyond 2340 UTC.
5.5 Meteorological Impact Statement (MIS).
The MIS is a nontechnical plain language product intended primarily for FAA traffic managers and those involved in planning aircraft routing. MISs are issued by NWS Center Weather Service Units (CWSU).
MISs are available at https://www.aviationweather.gov, as well as CWSU websites.
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5.5.1 Valid Period.
The MIS valid times are determined according to local policy. The MIS is limited to not exceed a 48-hour valid period.
5.5.2 Content.
The MIS is a brief nontechnical discussion of meteorological events causing or expecting to cause the disruption of the safe flow of air traffic. This is followed by specifics such as what is causing the disruption, area, altitudes, and movement.
The MIS may refer to an online graphic, especially for complex situations, using a specific Web address and provide a brief description of the weather that is included in the text MIS. MIS products are numbered sequentially beginning at midnight local time each day. The MIS is disseminated and stored as a “replaceable product.” If the expiration time of the MIS is after the closing time of the CWSU, then a “No updates available after ddhhmmZ” message should be included at the end of the MIS text, where dd = date, hh = hour, mm = minutes.
5.5.3 Format.
The MIS format consists of a communication header line, the words “FOR ATC PLANNING PURPOSES ONLY,” and the text.
• Header Line: zzz MIS ii Valid ddtttt-ddtttt Zzz is the ARTCC identification (i.e. ZJX), MIS is the product type, ii is the 2-digit sequential issuance number, and ddtttt is the valid beginning and ending date/time UTC.
• The line immediately below the header line reads “FOR ATC PLANNING PURPOSES ONLY.”
• The m aximum length of the MIS is four lines. The MIS is nontechnical in nature to convey expected weather and impacts in the clearest and simplest manner possible to the user. References to a graphical product on the local CWSU website or https://www.aviationweather.gov may be included.
Example:
ZAB MIS 02 VALID 281300-290300 ...FOR ATC PLANNING PURPOSES ONLY...
AN UPPER-LVL DISTURBANCE OVER COLORADO COMBINED WITH A STRONG JET STREAM MOVING ACROSS THE SWRN U.S. IS FCST TO PRODUCE AREAS OF TURBULENCE ACROSS PORTIONS OF ZAB. THE TURBULENCE IS FCST TO SUBSIDE AFT 00Z AS THE DISTURBANCE AND JETSTREAM MOVE FURTHER EAST.
MIS issued by the Albuquerque, New Mexico, CWSU. The second MIS issuance of the day, valid from the 28th day of the month at 1300 UTC until the 29th at 0300 UTC. For air traffic control (ATC) planning purposes only. (Brief discussion of the meteorological events to affect the ARTCC.)
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5.6 Additional Products for Convection.
In addition to the SIGMETs, Convective SIGMETs, and CWAs already discussed, the NWS offers more products informing the aviation community about the potential for Convective weather.
5.6.1 Convective Outlooks (AC).
The NWS Storm Prediction Center (SPC) issues narrative and graphical Convective Outlooks to provide the CONUS NWS WFOs, the public, the media, and emergency managers with the potential for severe (tornado, wind gusts 50 kts or greater, or hail 1 in in diameter or greater) and non-severe (general) convection and specific severe weather threats during the following 8 days. The Convective Outlook defines areas of marginal risk (MRGL), slight risk (SLGT), enhanced risk (ENH), moderate risk (MDT), or high risk (HIGH) of severe weather based on a percentage probability, which varies for time periods from 1 day to 3 days, and then two probabilistic thresholds for days 4 through 8.
The day 1, day 2, and day 3 Convective Outlooks also depict areas of general thunderstorms (TSTMS). The outlooks in graphical (see Figure 5-22, Day 1 Categorical Convective Outlook Graphic Example) and text formats are available on the SPC web site.
5.6.1.1 Issuance.
Convective Outlooks are scheduled products issued at the following times:
Table 5-9. Convective Outlook Issuance Schedule Convective Outlook Issuance Time (UTC) Valid Period (UTC) Day 1 0600 1200 – 1200 1300 1300 – 1200 1630 1630 – 1200 2000 2000 – 1200 0100 0100 – 1200 Day 2 0600 (Daylight Saving Time) 0700 (Standard Time) Day 2/1200 – 1200 1730 Day 2/1200 – 1200 Day 3 0730 (Daylight Saving Time) 0830 (Standard Time) Day 3/1200 – 1200 Day 4-8 0900 (Daylight Saving Time) 1000 (Standard Time) Day 4/1200 – Day 8/1200 (1 day intervals provided in graphic format on SPC Web page) SPC corrects outlooks for format and grammatical errors and amends outlooks when the current forecast does not or will not reflect the ongoing or future Convective development.
5-36 Figure 5-22. Day 1 Categorical Convective Outlook Graphic Example
Fi gure 5-23. Categorical Outlook Legend for Days 1-3 Convective Outlook Graphic Example
For mat and Example of the Categorical Convective Outlook Narrative.
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK 1244 AM CDT WED MAR 25 2015 VALID 251200Z - 261200Z ...THERE IS AN ENH RISK OF SVR TSTMS LATE THIS AFTERNOON AND EVENING ACROSS PARTS OF NORTH CENTRAL AND NORTHEAST OKLAHOMA...ADJACENT SOUTHEAST KANSAS...SOUTHWEST MISSOURI AND NORTHWEST ARKANSAS...
...THERE IS A SLGT RISK OF SVR TSTMS SURROUNDING THE ENH RISK...ACROSS PARTS OF THE SOUTH CENTRAL PLAINS INTO THE OZARK PLATEAU...
...THERE IS A MRGL RISK OF SVR TSTMS ACROSS SURROUNDING AREAS OF THE
CENTRAL AND SOUTHERN PLAINS EAST NORTHEASTWARD INTO PARTS OF THE LOWER OHIO VALLEY...
5-37 ...SUMMARY...
SEVERE THUNDERSTORMS...MAINLY CAPABLE OF PRODUCING LARGE HAIL AND LOCALLY DAMAGING WIND GUSTS...ARE EXPECTED ACROSS PARTS OF THE SOUTH
CENTRAL PLAINS INTO THE OZARK PLATEAU LATE THIS AFTERNOON AND EVENING.
...SYNOPSIS...
SIGNIFICANT AMPLIFICATION IS ONGOING WITHIN A BRANCH OF MID-LATITUDE WESTERLIES ACROSS THE EASTERN PACIFIC INTO WESTERN NORTH AMERICA.
AS LARGE-SCALE MID/UPPER RIDGING CONTINUES TO BUILD NEAR THE PACIFIC COAST DURING THIS PERIOD...MODELS INDICATE THAT LARGE-SCALE TROUGHING WILL BEGIN TO EVOLVE DOWNSTREAM...EAST OF THE ROCKIES...THROUGH THE VICINITY OF THE APPALACHIANS.
WITHIN THIS LATTER REGIME...ONE NOTABLE SHORT WAVE IMPULSE IS FORECAST TO PIVOT RAPIDLY NORTHEASTWARD...FROM THE OHIO VALLEY THROUGH THE ST. LAWRENCE VALLEY BY LATE TONIGHT. AN ASSOCIATED
SURFACE LOW MAY CONSOLIDATE WITH ANOTHER LOW OVER THE GREAT LAKES REGION...BEFORE CONTINUING INTO QUEBEC LATE IN THE PERIOD...WITH A
TRAILING COLD FRONT ADVANCING SOUTHEASTWARD ACROSS THE CENTRAL UNITED STATES.
INITIALLY STALLED OVER PARTS OF THE LOWER OHIO VALLEY/OZARK PLATEAU AND CENTRAL PLAINS...MODELS SUGGEST THAT THE SOUTHWESTERN FLANK OF THIS FRONT WILL BEGIN A MORE RAPID SOUTHWARD SURGE...IN RESPONSE TO
ANOTHER SIGNIFICANT SHORT WAVE IMPULSE DIGGING SOUTHEAST OF THE CENTRAL ROCKIES THROUGH MUCH OF THE SOUTH CENTRAL PLAINS BY 12Z THURSDAY. THIS MAY BE ACCOMPANIED BY WIDESPREAD CONVECTIVE DEVELOPMENT...AND THE RISK FOR SEVERE STORMS.
...SOUTH CENTRAL PLAINS/OZARK PLATEAU INTO LOWER OHIO VALLEY...
SOME BOUNDARY LAYER MOISTENING CONTINUES ALONG AND SOUTH OF THE FRONT...BUT A SUBSTANTIVE RETURN FLOW OF MOISTURE OFF THE GULF OF MEXICO STILL APPEARS UNLIKELY DURING THIS PERIOD. MODEL SURFACE DEW
POINT FORECASTS FOR THIS AFTERNOON MAY STILL BE AT LEAST A BIT TOO HIGH. HOWEVER...NORTHEASTWARD ADVECTION OF WARM ELEVATED MIXED
LAYER AIR IS EXPECTED TO AGAIN CONTRIBUTE TO VERY STEEP MID-LEVEL LAPSE RATES BY THIS AFTERNOON. THIS PROBABLY WILL SUPPORT SIZABLE CAPE /PERHAPS AT OR ABOVE 2000 J PER KG/ WITHIN AT LEAST A NARROW CORRIDOR ALONG THE FRONTAL ZONE. IN THE PRESENCE OF STRONG SHEAR BENEATH 40-50 KT WESTERLY 500 MB FLOW...CONDITIONS SHOULD BECOME
CONDUCIVE TO THE DEVELOPMENT OF SEVERE STORMS FOR AT LEAST A PERIOD LATE THIS AFTERNOON INTO EARLY EVENING.
MODELS SUGGEST THAT FORCING FOR ASCENT ASSOCIATED WITH LOW-LEVEL
WARM ADVECTION MAY SUPPORT A RAPID INCREASE IN SEVERE STORM DEVELOPMENT BY 20-22Z NEAR THE CENTRAL AND EASTERN KANSAS/OKLAHOMA BORDER AREA. THIS MAY INITIALLY INCLUDE DISCRETE STORMS...BEFORE GROWING UPSCALE...WHICH SHOULD TEND TO TRACK EASTWARD ALONG THE FRONT INTO THE OZARK PLATEAU. ADDITIONAL SEVERE STORMS MAY FORM SHORTLY THEREAFTER ACROSS PARTS OF WEST CENTRAL OKLAHOMA. IT
APPEARS THAT THIS LATTER ACTIVITY WILL OCCUR JUST AHEAD OF THE SOUTHWARD SURGING COLD FRONT...AND STRONGER STORMS PROBABLY WILL BE CAPABLE OF PRODUCING LARGE HAIL AND LOCALLY STRONG WIND GUSTS.
AS LARGE-SCALE FORCING SUPPORTS INCREASINGLY WIDESPREAD THUNDERSTORM ACTIVITY THROUGH MUCH OF THE INSTABILITY AXIS...WHILE THE SURFACE
COLD FRONT BEGINS TO SURGE SOUTHWARD AND TENDS TO UNDERCUT
5-38 CONVECTION...IT REMAINS UNCLEAR AT THE PRESENT TIME HOW LONG THE SEVERE WEATHER POTENTIAL WILL CONTINUE INTO THE EVENING HOURS.
..KERR/MOSIER.. 03/25/2015
5.6.2 Watch Notification Messages.
The SPC issues Aviation Watch Notification Messages (SAW) to provide an area threat alert for the aviation meteorology community to forecast organized severe thunderstorms that may produce tornadoes, large hail, and/or Convective damaging winds as indicated in Public Watch Notification Messages within the CONUS.
SPC issues three types of Watch Notification Messages: SAW, Public Severe Thunderstorm Watch Notification Message, and Public Tornado Watch Notification Message. They are available on the SPC website.
The SAW was formerly known as the Alert Severe Weather Watch Bulletin (AWW). The NWS no longer uses that title or acronym for this product. The NWS uses the acronym SAW for the Aviation Watch Notification Message, but retains AWW in the product header for processing by weather data systems. The NWS uses the acronym AWW for their Aviation Weather Warning product, which is a completely different product from the SAW.
The Severe Thunderstorm and Tornado Watch Notification Messages were formerly known as the Severe Weather Watch Bulletins (WW). The NWS no longer uses that title or acronym for this product but retains WW in the product header for processing by weather data systems.
It is important to note the difference between a Severe Thunderstorm (or Tornado) Watch and a Severe Thunderstorm (or Tornado) Warning. A watch means severe weather is possible during the watch valid time, while a warning means that severe weather has been observed or is expected within the hour. Only the SPC issues Severe Thunderstorm and Tornado Watches, while only NWS WFOs issue Severe Thunderstorm and Tornado Warnings.
5.6.2.1 Aviation Watch Notification Message (SAW).
The SPC issues SAW to provide an area threat alert for the aviation meteorology community to forecast organized severe thunderstorms that may produce tornadoes, large hail, and/or Convective damaging winds as indicated in Public Watch Notification Messages.
The SA W product is an approximation of the area in a watch. For the official area covered by a watch, see the corresponding Public Watch product. To illustrate, Figure 5-24, Aviation Watch (polygon) Compared to Public Watch (shaded) Example, is an example of the Aviation Watch (polygon) compared to the P ublic Watch (shaded). Also, the SAW is easier to communicate verbally over the radio and telephone than reciting the entire Public Watch product.
5-39 Figure 5-24. Aviation Watch (polygon) Compared to Public Watch (shaded) Example
5.6.2.1.1 Issuance.
The SPC will issue the SAW after the proposed Convective watch area has been collaborated with the affected NWS WFOs defining the approximate areal outline of the watch.
Watch Notification Messages are nonscheduled, event-driven products valid from the time of issuance to expiration or cancellation time. Valid times are in UTC. SPC will correct watches for formatting and grammatical errors.
When tornadoes or severe thunderstorms have developed, the local NWS WFO will issue the warnings for the storms.
5.6.2.1.2 Format and Example of an Aviation Watch Notification Message (SAW).
SPC forecasters may define the area as a rectangle or parallelogram (X mi either side of the line from point A to point B) or (X miles north and south or east and west of the line from point A to point B). Distances of the axis coordinates should be in sm. The aviation coordinates referencing VOR locations and state distances will be in nautical miles (NM). Valid times will be in UTC. The watch half width will be in sm. The SAW will contain hail
5-40 size in inches or ½ in (forecaster discretion for tornado watches associated with hurricanes) surface and aloft, surface Convective wind gusts in knots, maximum cloud tops and the Mean Storm Motion Vector, and replacement information, if necessary.
The SAW will refer users to the accompanying public product, known as the Watch Outline Update (WOU) message, which provides the names of all counties or parishes in the watch area. Note the letter “n” refers to the issuance number, e.g., 2 means the second issuance of the WOU message.
WWUS30 KWNS 271559 SAW2 SPC AWW 271559 WW 568 TORNADO AR LA MS 271605Z - 280000Z AXIS..65 STATUTE MILES EAST AND WEST OF LINE..
45ESE HEZ/NATCHEZ MS/ - 50N TUP/TUPELO MS/ ..AVIATION COORDS.. 55NM E/W /18WNW MCB - 60E MEM/ HAIL SURFACE AND ALOFT..3 INCHES. WIND GUSTS..70 KNOTS. MAX TOPS TO
550. MEAN STORM MOTION VECTOR 26030.
LAT...LON 31369169 34998991 34998762 31368948 THIS IS AN APPROXIMATION TO THE WATCH AREA. FOR A COMPLETE DEPICTION OF THE WATCH SEE WOUS64 KWNS FOR WOU2.