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5.3.1 G-AIRMET Issuance.
G-AIRMETs are issued by the AWC every 6 hours and are updated/amended as necessary, coinciding with the text AIRMET products.
Table 5-7. G-AIRMET Issuance Schedule 1st Scheduled Issuance (UTC) 2nd Scheduled Issuance (UTC) 3rd Scheduled Issuance (UTC) 4th Scheduled Issuance (UTC) CONUS 0300 0900 1500 2100
5.3.2 G-AIRMET Content.
G-AIRMET snapshots are graphical forecasts of en route weather hazards valid at discrete times no more than 3 hours apart for a period of up to 12 hours into the future (00, 03, 06, 09, and 12 hours). 00 hour represents the initial conditions, and the subsequent 3-hourly graphics depict the area affected by the particular hazard at that valid time.
The AWC’s website provides several options to display the G-AIRMET. There is an interactive display as well as snapshots that may be viewed as static (single), combined, and looped images using the tools provided on the Web page. The following are examples of G-AIRMET snapshot static images.
G-AIRMET depicts the following en route aviation weather hazards:
• IFR Ceiling and Visibility (see Figure 5-12, G-AIRMET—Ceiling and Visibility (IFR) Snapshot Example).
o Areas of cloud ceilings with bases less than 1000 ft AGL and/or areas of surface visibilities below 3 sm, including the weather causing the visibility restriction. The cause of the visibility restriction includes only PCPN, FU, HZ, BR, FG, and BLSN.
Figure 5-12. G-AIRMET-Ceiling and Visibility (IFR) Snapshot Example Ceiling & Visiblity Turb High Turb Low Icing Mountain Obscuration Low Level Wind Shear Surface Winds Freezing Level lower Loop speed Faster ISSUED: 1445 UTC MON 26 APR 2010 Celling & I Visibility Turb High Turb Low Icing Obsch CL63/BR - - - LL Wind [LLWS € > Sfc Winds Frzg Lvl 0°: 120
• Mountain Obscuration (see Figure 5-13, G-AIRMET-Mountain Obscuration Snapshot Example).
• Areas of widespread mountain obscuration where Visual Meteorological Conditions (VMC) cannot be maintained, including the weather causing the obscuration. The weather causing the obscuration includes only CLDS, PCPN, FU, HZ, BR, and FG.
Figure 5-13. G-AIRMET-Mountain Obscuration Snapshot Example Ceiling & Visiblity Turb High Turb Low Icing Low Level Wind Shear Surface Winds Freezing Leve SueT Loop speed Faster · 26 APR 2H11 Turb High dộc Icing Mtn Sfc Winds → Frzg Lvl [0*: 120 5-25
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• Icing (see Figure 5-14, G-AIRMET—Icing Snapshot Example).
o Areas of moderate airframe icing, other than convectively induced, including the vertical extent (base and top). Altitude variations in the base of icing layers may be denoted (e.g., “080 / 060” indicates the altitude varies between 6,000 and 8,000 ft MSL).
Figure 5-14. G-AIRMET—Icing Snapshot Example
• Fre ezing Level (see Figure 5-15, G-AIRMET—Freezing Level Snapshot Example).
o Freezing level is defined as the lowest freezing level above the ground or at the SFC as appropriate. Freezing levels above the ground are delineated at 4,000 ft intervals MSL. Areas with multiple freezing levels above the ground are delineated including the vertical extent (base and top).
Figure 5-15. G-AIRMET-Freezing Level Snapshot Example Ceiling & Visiblity Turb High Turb Low Icing Mountain Obscuration Low Level Wind Shear Surface Winds Freezing Level Clear Slower Loop speed Faster Play 03Z APR ISSUED: 1445 UTC MON 26 APR 2018 Ceiling & Visibility Turb High Turb Low Icing Mtn SIc Winds ® Frzg Lvl 0": 120] tw Obscn
• Turbulence (see Figure 5-16, G-AIRMET-Turbulence-High Snapshot Example, and Figure 5-17, G-AIRMET-Turbulence-Low Snapshot Example).
o Areas of moderate turbulence, other than convectively induced, including the vertical extent (base and top).
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Figure 5-16. G-AIRMET-Turbulence-High Snapshot Example Ceiling & Visiblity Turb High Turb Low Icing Mountain Obscuration Low Level Wind Shear Surface Winds Freezing Level Slower Loop speed Faster 03Z
G-AIRME I UTC MON ISSUED: 1445 UTC MON 26 APR 201 Ceiling & Visibility Turb High Icing Mitn CLOS/ BR --- LL Wind [LLWS → Sfc Winds Frzg Lvl 0*: 120 Obscn Shear Figure 5-17. G-AIRMET-Turbulence-Low Snapshot Example Ceiling & Visiblity Turb High Turb Low Icing Mountain Obscuration Low Level Wind Shear Surface Winds Freezing Leve Slower Loop speed Faster Play 03Z G-AIRMET VALID: 1 UTC MON ISSUED: 1445 UTC MON 26 APR 2010 Ceiling & BR Visibility Turb High 280 Turb Low Icing ObSch CLbS/BR --- Mitn LL Wind [LLWS < Shea Stc Winds Frzg Lvl [0*: 120 5-28
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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.