Sigmets

| August 9, 2020 | By: Severe VFR
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SIGMETS, for SIGnificant METeorlogical information, are inflight advisories concerning non-convective weather that is potentially hazardous to all aircraft.

Sigmets are issued as needed and are not scheduled weather products. Sigmets are valid for 4 hours unless the weather causing a sigmet is due to a hurricane. If the issuance of a sigmet is due to a hurricane, then the sigmet is valid for 6 hours.

Reasons For A Sigmet To Be Issued

Sigmets are issued for the following weather types:

  • Severe Icing
  • Severe, Extreme, and Clear Air Turbulence (CAT) not associated with thunderstorms
  • Dust Storms and Sand Storms lowering visibilities to less than three miles
  • Volcanic Ash

In Hawaii and Alaska, SIGMETs are also issued for:

  • Tornadoes
  • Lines of thunderstorms
  • Embedded thunderstorms
  • Hail greater than or equal to 3/4 inch in diameter

When a sigmet is issued, it is given an alphabetic identifier starting at November, ranging through Yankee. The initial issuance of a sigmet is designated as an Urgent Weather SIGMET (UWS). If a sigmet becomes reissued for the same reason, it is assigned a number that increases by one for every re-issuance. 

Sample issued sigmet:

SFOR WS 100130 SIGMET ROME02 VALID UNTIL 100530 OR WA FROM SEA TO PDT TO EUG TO SEA OCNL SEV CAT BTN FL280 AND FL350 EXPCD DUE TO JTSTR. CONDS BGNG AFT 0200Z CONTG BYD 0530Z .

Explanation:

This is SIGMET Romeo 2, the second issuance for this weather phenomenon. It is valid until the 10th day of the month at 0530Z time. This SIGMET is for Oregon and Washington, for a defined area from Seattle to Portland to Eugene to Seattle. It calls for occasional severe clear air turbulence between FL280 and FL350 due to the jet stream's location. These conditions will begin after 0200Z and continue beyond the forecast scope of this SIGMET of 0530Z.

Convective SIGMETs

A Convective SIGMET is in inflight weather advisory issued for hazardous convective weather that affects all aircraft types and operations' safety.

Convective SIGMETs are issued 5 minutes before every hour. A convective SIGMET may be issued for any convective situation in which the forecaster feels hazardous to all aircraft categories. Convective SIGMETs imply severe turbulence, severe icing, and low-level wind shear. Once issued, a convective sigmet remains valid for 2 hours.

Sigmets are numbered from 1-99, beginning at 00z. If no hazardous weather exists when a sigmet is supposed to be issued, it is still issued. However, the SIGMET will state, "CONVECTIVE SIGMET...NONE."

Reasons For A Convective SIGMET To Be Issued

Convective SIGMETs are issued for the following weather types:

  • Surface wind gusts greater than or equal to 50 knots
  • Hail greater than or equal to 3/4 inch in diameter
  • Embedded or severe thunderstorms expected to occur for more than 30 minutes
  • A line of thunderstorms at least 50 miles long with thunderstorms affecting 40% of its length
  • Tornadoes
  • Thunderstorms with strong radar reflectivity, or significant satellite or lightning signature, affecting 40% or more of a 3000 square mile area.

 Example of an issued convective sigmet:

MKCC WST 221855 CONVECTIVE SIGMET 20C VALID UNTIL 2055Z ND SD FROM 90W MOT-GFK-ABR-90W MOT INTSFYG AREA SEV TS MOVG FROM 24045KT. TOPS ABV FL450. WIND GUSTS TO 60KTS RPRTD. TORNADOES…HAIL TO 2 IN… WIND GUSTS TO 65KTS POSS ND PTN

Explanation:

Convective SIGMET was issued for the central portion of the United States on the 22nd at 1855Z. This is the 20th Convective SIGMET issued on the 22nd for the central United States, as indicated by "20C" and is valid until 2055Z. The affected states are North and South Dakota, from 90 nautical miles west of Minot, ND; to Grand Forks, ND; to Aberdeen, SD; to 90 nautical miles west Minot, ND. An intensifying area of severe thunderstorms moving from 240 degrees at 45 knots (to the northeast). Thunderstorm tops will be above FL 450. Wind gusts up to 60 knots were reported. Also reported were tornadoes, hail to 2 inches in diameter, and wind gusts to 65 knots possible in the North Dakota portion.

Convective SIGMETs are issued for the following weather types:

  • Surface wind gusts greater than or equal to 50 knots
  • Hail greater than or equal to 3/4 inch in diameter
  • Embedded or severe thunderstorms expected to occur for more than 30 minutes
  • A line of thunderstorms at least 50 miles long with thunderstorms affecting 40% of its length
  • Tornadoes
  • Thunderstorms with strong radar reflectivity, or significant satellite or lightning signature, affecting 40% or more of a 3000 square mile area.

 

Example of an issued convective sigmet:

MKCC WST 221855 CONVECTIVE SIGMET 20C VALID UNTIL 2055Z ND SD FROM 90W MOT-GFK-ABR-90W MOT INTSFYG AREA SEV TS MOVG FROM 24045KT. TOPS ABV FL450. WIND GUSTS TO 60KTS RPRTD. TORNADOES…HAIL TO 2 IN… WIND GUSTS TO 65KTS POSS ND PTN

Explanation:

Convective SIGMET was issued for the central portion of the United States on the 22nd at 1855Z. This is the 20th Convective SIGMET issued on the 22nd for the central United States, as indicated by "20C" and is valid until 2055Z. The affected states are North and South Dakota, from 90 nautical miles west of Minot, ND; to Grand Forks, ND; to Aberdeen, SD; to 90 nautical miles west Minot, ND. An intensifying area of severe thunderstorms moving from 240 degrees at 45 knots (to the northeast). Thunderstorm tops will be above FL 450. Wind gusts up to 60 knots were reported. Also reported were tornadoes, hail to 2 inches in diameter, and wind gusts to 65 knots possible in the North Dakota portion.

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