AIRMET MTN OBSCN…ME NH VT MA NY PA  FROM CAR TO MLT TO CON TO SLT TO SYR TO CAR  MTNS OCNLY OBSCD BY CLDS/PCPN/BR. CONDS  CONT BYD 02Z THRU 08Z  Explanation:  AIRMET SIERRA was issued for the Boston area at 1945Z  on the 21st day of the month. SIERRA contains information  on IFR and/or mountain obscurations. This is the third  updated issuance of this Boston AIRMET series as indicated  by “SIERRA UPDT 3” and is valid until 0200Z on the  22nd. The affected states within the BOS area are: Maine,  New Hampshire, Vermont, Massachusetts, New York, and  Pennsylvania. Within an area bounded by: Caribou, ME;  to Saint Johns, New Brunswick; to 150 nautical miles east  of Nantucket, MA; to Newark, NJ; to Ottawa, Ontario;  to Caribou, ME. The effected states within Caribou, ME  to Millinocket, ME to Concord, NH to Slate Run, PA to  Syracuse, NY to Caribou, ME will experience ceilings  below 1,000 feet/visibility below 3 SM, precipitation/mist.  Conditions will continue beyond 0200Z through 0800Z.  SIGMETs (WSs) are inflight advisories concerning non- convective weather that is potentially hazardous to all  aircraft. They report weather forecasts that include severe  icing not associated with thunderstorms, severe or extreme  turbulence or clear air turbulence (CAT) not associated with  thunderstorms, dust storms or sandstorms that lower surface  or inflight visibilities to below three miles, and volcanic ash.  SIGMETs are unscheduled forecasts that are valid for 4 hours  unless the SIGMET relates to a hurricane, in which case it  is valid for 6 hours.  A SIGMET is issued under an alphabetic identifier, from  November through Yankee. The first issuance of a SIGMET  is designated as an Urgent Weather SIGMET (UWS).  Reissued SIGMETs for the same weather phenomenon are  sequentially numbered until the weather phenomenon ends.  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 of 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.  Convective SIGMETs are issued for each area of the  contiguous 48 states but not Alaska or Hawaii. Convective  SIGMETs are issued for the eastern (E), western (W), and  central (C) United States. Each report is issued at 55 minutes  past the hour, but special Convective SIGMETs can be  issued during the interim for any reason. Each forecast is  valid for 2 hours. They are numbered sequentially each day  from 1–99, beginning at 00Z time. If no hazardous weather  exists, the convective SIGMET is still issued; however, it  states “CONVECTIVE SIGMET…NONE.”  A Convective SIGMET (WST) is an inflight weather  advisory issued for hazardous convective weather that affects  the safety of every flight. Convective SIGMETs are issued  for severe thunderstorms with surface winds greater than 50  knots, hail at the surface greater than or equal to ¾ inch in  diameter, or tornadoes. They are also issued to advise pilots  of embedded thunderstorms, lines of thunderstorms, or  thunderstorms with heavy or greater precipitation that affect  40 percent or more of a 3,000 square mile or greater region.  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 location of  the jet stream. These conditions will begin after 0200Z and  continue beyond the forecast scope of this SIGMET of 0530Z.  Convective Significant Meteorological Information  (WST)  SIGMET  Example:  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:  Example:  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  13-12 