Particularly dangerous situation
A particularly dangerous situation (PDS) is a type of enhanced wording first used by the National Weather Service's Storm Prediction Center on certain severe weather watches. It is issued at the discretion of the forecaster composing the watch and implies that there is an enhanced risk of very severe and life-threatening weather, usually a major tornado outbreak or (much less often) a long-lived, extreme derecho event, but possibly another weather hazard.
On April 24, 2011, the National Weather Service office in Memphis issued the first PDS Flash Flood Watch to highlight the threat for widespread, significant, and potentially life-threatening flash flooding due to repeated rounds of severe thunderstorms.
PDS watches are quite uncommon; less than 3% of watches issued by the SPC from 1996 to 2005 were PDS watches, or an average of 24 each year. When a PDS watch is issued, there are often more PDS watches issued for the same weather system, even on the same day during major outbreaks, so the number of days per year that a PDS watch is issued is significantly lower.
PDS tornado watches are issued when there is a higher than normal risk of multiple strong, violent tornadoes in the watch area (usually amounting to damage consistent with EF4 or EF5 tornadoes), also including significant wind and hail damage, when in addition to tornadoes. While there are no set criteria for a PDS watch to be issued, they are usually issued when the potential exists for a major tornado outbreak. They represent about 90% of PDS watches issued.
PDS tornado warnings are currently issued on an experimental basis by five Forecast Offices in Kansas and Missouri. The criteria for a PDS warning is when a tornado on the ground has been spotted or confirmed, or a significant tornado is expected (due to radar signatures). The intention of this experimental warning may be to replace the loosely-defined tornado emergency. These are the first warnings issued with PDS wording, and like PDS flash flood watches, are issued by local forecast offices.
PDS severe thunderstorm watches are issued when there is a higher than normal risk of severe thunderstorm winds capable of major structural damage (in addition to large hail and perhaps a few isolated tornadoes), usually due to a strong and persistent derecho. These watches are very rare (an average of only two each year), as the risk for tornadoes must remain low enough to not warrant a tornado watch (a normal tornado watch would be issued if the tornado risk is significant alongside the extreme wind threat).
PDS flash flood watches are issued when there is a higher than normal risk of widespread, life-threatening flash flooding. These watches are issued by the NWS Local Forecast Offices, not the Storm Prediction Center.
The first PDS tornado watch was issued by Robert H. Johns for the April 2, 1982 tornado outbreak across the southern and central Great Plains. While historically applied only to Severe Thunderstorm, Tornado, and Flash Flood watches (i.e. Severe Local Storm "polygonal" events), PDS wording could theoretically be applied to other types of weather watches (i.e., Winter Storm, High Wind, Hurricane, or Fire Weather watches) when an enhanced threat for such conditions exists. These watches have generally (but not always) been issued during a high risk or an upper-end moderate risk either of severe storms from the SPC's convective outlooks or of Flash Flooding from the WPC's Excessive Rainfall Outlooks.
Sample PDS Tornado Watch
NOTE: The blue text in the alert below indicates links; the color of all text in the original alert issued was black, except for the text "THIS IS A PARTICULARLY DANGEROUS SITUATION", which was red in the actual alert.
URGENT - IMMEDIATE BROADCAST REQUESTED TORNADO WATCH NUMBER 235 NWS STORM PREDICTION CENTER NORMAN OK 145 PM CDT WED APR 27 2011 THE NWS STORM PREDICTION CENTER HAS ISSUED A TORNADO WATCH FOR PORTIONS OF MUCH OF ALABAMA NORTHWEST GEORGIA SOUTHEAST MISSISSIPPI SOUTHERN MIDDLE TENNESSEE EFFECTIVE THIS WEDNESDAY AFTERNOON AND EVENING FROM 145 PM UNTIL 1000 PM CDT. ...THIS IS A PARTICULARLY DANGEROUS SITUATION... DESTRUCTIVE TORNADOES...LARGE HAIL TO 4 INCHES IN DIAMETER... THUNDERSTORM WIND GUSTS TO 80 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS. THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 80 STATUTE MILES EAST AND WEST OF A LINE FROM 45 MILES NORTHEAST OF HUNTSVILLE ALABAMA TO 60 MILES WEST SOUTHWEST OF EVERGREEN ALABAMA. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU5). REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE FOR TORNADOES AND SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS AND POSSIBLE WARNINGS. OTHER WATCH INFORMATION...CONTINUE...WW 230...WW 231...WW 232...WW 233...WW 234... DISCUSSION...A CLASSIC TORNADO OUTBREAK SITUATION IS DEVELOPING ACROSS MUCH OF AL AS DISCRETE TORNADIC SUPERCELLS FORM OVER MS AND TRACK ACROSS THE WATCH AREA. STRONG LOW LEVEL AND DEEP LAYER VERTICAL SHEAR...COMBINED WITH A MOIST AND MODERATELY UNSTABLE AIR MASS...WILL POSE A DANGEROUS RISK OF STRONG/VIOLENT AND POTENTIALLY LONG-TRACK TORNADOES. AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT TO 4 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 70 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 500. MEAN STORM MOTION VECTOR 25040. ...HART
Sample PDS Severe Thunderstorm Watch
Although, as mentioned, PDS Severe Thunderstorm Watches are very rare, they are issued when the tornado threat is low and when the forecast wind gusts and hail sizes are very high. This PDS Severe Thunderstorm Watch shown here, was from May 30, 2011:
NOTE: Blue text indicates links, and the color of all text in the original alert issued was black, except for the text "THIS IS A PARTICULARLY DANGEROUS SITUATION", which was red in the actual alert.
URGENT - IMMEDIATE BROADCAST REQUESTED SEVERE THUNDERSTORM WATCH NUMBER 405 NWS STORM PREDICTION CENTER NORMAN OK 840 PM CDT MON MAY 30 2011 THE NWS STORM PREDICTION CENTER HAS ISSUED A SEVERE THUNDERSTORM WATCH FOR PORTIONS OF PARTS OF NORTHWEST IOWA PARTS OF CENTRAL KANSAS MUCH OF EASTERN NEBRASKA SMALL PART OF SOUTHEASTERN SOUTH DAKOTA EFFECTIVE THIS MONDAY NIGHT AND TUESDAY MORNING FROM 840 PM UNTIL 200 AM CDT. ...THIS IS A PARTICULARLY DANGEROUS SITUATION... WIDESPREAD DAMAGING WINDS WIND GUSTS TO 90 MPH LIKELY SCATTERED LARGE HAIL ISOLATED HAIL TO 1.5 INCHES IN DIAMETER POSSIBLE ISOLATED TORNADOES THE SEVERE THUNDERSTORM WATCH AREA IS APPROXIMATELY ALONG AND 75 STATUTE MILES EAST AND WEST OF A LINE FROM 35 MILES NORTHWEST OF SIOUX CITY IOWA TO 50 MILES SOUTH SOUTHEAST OF RUSSELL KANSAS. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU5). REMEMBER...A SEVERE THUNDERSTORM WATCH MEANS CONDITIONS ARE FAVORABLE FOR SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS AND POSSIBLE WARNINGS. SEVERE THUNDERSTORMS CAN AND OCCASIONALLY DO PRODUCE TORNADOES. OTHER WATCH INFORMATION...CONTINUE...WW 402...WW 403... DISCUSSION...INTENSE SQUALL LINE HAS DEVELOPED N/S ACROSS CENTRAL NEB AND CONTINUES TO DEVELOP SWD AS IT MOVES EWD. VERY STRONG PRES RISE/FALL COUPLET IS DRIVING THE LINE EWD WITH THE FORCING FROM LARGE SCALE TROUGH ROTATING ACROSS THE NRN/CENTRAL PLAINS PROVIDING BOTH SHEAR AND UPWARD MOTION TO RESULT IN POTENTIAL FOR VERY DAMAGING WINDS. IN ADDITION BRIEF TORNADOES ARE STILL POSSIBLE BUT DAMAGING WINDS NOW ARE THE DOMINANT THREAT. AVIATION...A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT TO 1.5 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 80 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 550. MEAN STORM MOTION VECTOR 23040. ...HALES
Sample PDS Flash Flood Watch
NOTE: The blue text in the alert below indicates links; the color of all text in the original alert issued was black, including the text "THIS IS A PARTICULARLY DANGEROUS SITUATION", which was not red in the actual alert.
URGENT - IMMEDIATE BROADCAST REQUESTED FLOOD WATCH NATIONAL WEATHER SERVICE MEMPHIS TN 239 PM CDT SUN APR 24 2011 ...VERY HEAVY RAINFALL THROUGH THE MIDDLE OF THIS WEEK WILL LIKELY LEAD TO SIGNIFICANT...WIDESPREAD FLASH FLOODING... ...THIS IS A PARTICULARLY DANGEROUS SITUATION... .A BOUNDARY WILL CONTINUE TO REMAIN STATIONARY ACROSS SOUTHERN MISSOURI INTO KENTUCKY THROUGH MONDAY. REPEATED ROUNDS OF THUNDERSTORMS WILL TRACK ALONG THE FRONT BRINGING HEAVY RAINFALL. THEN A LOW PRESSURE SYSTEM WILL TRACK ALONG IT INTO MISSOURI AND PUSH THE FRONT FURTHER SOUTH TO ALONG THE I-40 CORRIDOR MONDAY NIGHT THROUGH TUESDAY NIGHT. THIS WILL SHIFT THE HEAVY RAIN AXIS FURTHER SOUTH TO ALONG AND JUST NORTH OF THE I-40 CORRIDOR. A SECOND LOW PRESSURE SYSTEM WILL TRACK ALONG THE NEWLY STALLED BOUNDARY AND SET OFF ADDITIONAL TRAINING THUNDERSTORMS LATE TUESDAY NIGHT AND WEDNESDAY. THE FINAL COLD FRONT WILL PASS THROUGH LATE WEDNESDAY AFTERNOON...ENDING THE PERSISTENT HEAVY RAINFALL. ARZ026>028-035-036-048-049-058-MSZ001>014-TNZ003-004-019>021- 048>055-088>092-250400- /O.NEW.KMEG.FF.A.0007.110426T0000Z-110428T0000Z/ /00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/ CRAIGHEAD-POINSETT-MISSISSIPPI-CROSS-CRITTENDEN-ST. FRANCIS- LEE AR-PHILLIPS-DESOTO-MARSHALL-BENTON MS-TIPPAH-ALCORN- TISHOMINGO-TUNICA-TATE-PRENTISS-COAHOMA-QUITMAN-PANOLA-LAFAYETTE- UNION-WEAKLEY-HENRY-DYER-GIBSON-CARROLL-LAUDERDALE-TIPTON-HAYWOOD- CROCKETT-MADISON-CHESTER-HENDERSON-DECATUR-SHELBY-FAYETTE- HARDEMAN-MCNAIRY-HARDIN- INCLUDING THE CITIES OF...JONESBORO...HARRISBURG...BLYTHEVILLE... WYNNE...WEST MEMPHIS...FORREST CITY...HELENA...SOUTHAVEN... OLIVE BRANCH...CORINTH...IUKA...TUNICA...BOONEVILLE... CLARKSDALE...BATESVILLE...OXFORD...NEW ALBANY...MARTIN... DRESDEN...PARIS...DYERSBURG...HUMBOLDT...MILAN...HUNTINGDON... COVINGTON...JACKSON...LEXINGTON...BARTLETT...GERMANTOWN... COLLIERVILLE...MEMPHIS...MILLINGTON...SOMERVILLE...BOLIVAR... SAVANNAH 239 PM CDT SUN APR 24 2011 ...FLASH FLOOD WATCH IN EFFECT FROM MONDAY EVENING THROUGH WEDNESDAY EVENING... THE NATIONAL WEATHER SERVICE IN MEMPHIS HAS ISSUED A * FLASH FLOOD WATCH FOR PORTIONS OF EAST ARKANSAS...NORTH MISSISSIPPI AND WEST TENNESSEE...INCLUDING THE FOLLOWING AREAS...IN EAST ARKANSAS...CRAIGHEAD...CRITTENDEN...CROSS... LEE...MISSISSIPPI...PHILLIPS...POINSETT AND ST. FRANCIS. IN NORTH MISSISSIPPI...ALCORN...BENTON...COAHOMA...DESOTO... LAFAYETTE...MARSHALL...PANOLA...PRENTISS...QUITMAN...TATE... TIPPAH...TISHOMINGO...TUNICA AND UNION. IN WEST TENNESSEE... CARROLL...CHESTER...CROCKETT...DECATUR...DYER...FAYETTE... GIBSON...HARDEMAN...HARDIN...HAYWOOD...HENDERSON...HENRY... LAUDERDALE...MADISON...MCNAIRY...SHELBY...TIPTON AND WEAKLEY. * FROM MONDAY EVENING THROUGH WEDNESDAY EVENING. * THIS IS A PARTICULARLY DANGEROUS SITUATION. * TOTAL RAINFALL AMOUNTS OF 5 TO 8 INCHES ARE EXPECTED ALONG AND NORTH OF I-40 WITH 2 TO 5 INCHES EXPECTED SOUTH OF I-40. LOCALLY HIGHER AMOUNTS ARE LIKELY. * RAINFALL AMOUNTS SUCH AS THESE MAY LEAD TO WIDESPREAD... SIGNIFICANT...AND LIFE THREATENING FLASH FLOODING. THIS EVENT MAY BE AS SEVERE AS THE MAY 1–2, 2010 FLOODING IN PLACES. FLASH FLOODING OF CITIES...RURAL AREAS...RIVERS...AND SMALL STREAMS ARE POSSIBLE. PRECAUTIONARY/PREPAREDNESS ACTIONS... A FLASH FLOOD WATCH MEANS THAT CONDITIONS MAY DEVELOP THAT LEAD TO FLASH FLOODING. FLASH FLOODING IS A VERY DANGEROUS SITUATION. YOU SHOULD MONITOR LATER FORECASTS AND BE PREPARED TO TAKE ACTION SHOULD FLASH FLOOD WARNINGS BE ISSUED. && $$ ARZ008-009-017-018-MOZ113-115-TNZ001-002-250400- /O.EXT.KMEG.FF.A.0006.000000T0000Z-110428T0000Z/ /00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/ RANDOLPH-CLAY-LAWRENCE-GREENE-DUNKLIN-PEMISCOT-LAKE-OBION- INCLUDING THE CITIES OF...WALNUT RIDGE...PARAGOULD...KENNETT... CARUTHERSVILLE...UNION CITY 239 PM CDT SUN APR 24 2011 ...FLASH FLOOD WATCH NOW IN EFFECT THROUGH WEDNESDAY EVENING... THE FLASH FLOOD WATCH IS NOW IN EFFECT FOR * PORTIONS OF EAST ARKANSAS...SOUTHEAST MISSOURI AND WEST TENNESSEE...INCLUDING THE FOLLOWING AREAS...IN EAST ARKANSAS... CLAY...GREENE...LAWRENCE AND RANDOLPH. IN SOUTHEAST MISSOURI... DUNKLIN AND PEMISCOT. IN WEST TENNESSEE...LAKE AND OBION. * THROUGH WEDNESDAY EVENING. * THIS IS A PARTICULARLY DANGEROUS SITUATION. * ADDITIONAL RAINFALL AMOUNTS OF 6 TO 9 INCHES ARE EXPECTED. LOCALLY HIGHER AMOUNTS ARE LIKELY. THIS...IN COMBINATION OF THE 2 TO 4 INCHES THAT HAVE ALREADY FALLEN MAY LEAD TO TOTAL RAINFALL AMOUNTS IN EXCESS OF 12 INCHES IN MANY LOCATIONS. * RAINFALL AMOUNTS SUCH AS THESE WILL LIKELY LEAD TO WIDESPREAD... SIGNIFICANT...AND LIFE THREATENING FLASH FLOODING. THIS EVENT MAY BE AS SEVERE AS THE MAY 1-2 2010 FLOODING IN MANY PLACES. FLASH FLOODING OF CITIES...RURAL AREAS...RIVERS...AND SMALL STREAMS ARE POSSIBLE. PRECAUTIONARY/PREPAREDNESS ACTIONS... A FLASH FLOOD WATCH MEANS THAT CONDITIONS MAY DEVELOP THAT LEAD TO FLASH FLOODING. FLASH FLOODING IS A VERY DANGEROUS SITUATION. YOU SHOULD MONITOR LATER FORECASTS AND BE PREPARED TO TAKE ACTION SHOULD FLASH FLOOD WARNINGS BE ISSUED. && $$ BORGHOFF
Sample PDS Tornado Warning
PDS Tornado Warnings are issued by NWS offices serving Kansas and Missouri, usually when a tornado on the ground has been spotted or when a significant tornado is expected. This is currently experimental and is on a testing basis. One of the first PDS tornado warnings, if not the first, was issued on April 14, 2012 as shown below, by the National Weather Service in Wichita, Kansas.
NOTE: The blue text in the alert below indicates links; the color of all text in the original alert issued was black, including the text "THIS IS A PARTICULARLY DANGEROUS SITUATION", which was not red in the actual alert. The wording "SIGNIFICANT" on the tornado damage threat has been changed to "CONSIDERABLE" for the 2013 tornado season.
BULLETIN - EAS ACTIVATION REQUESTED TORNADO WARNING NATIONAL WEATHER SERVICE WICHITA KS 543 PM CDT SAT APR 14 2012 THE NATIONAL WEATHER SERVICE IN WICHITA HAS ISSUED A * TORNADO WARNING FOR... SOUTHEASTERN ELLSWORTH COUNTY IN CENTRAL KANSAS... NORTHWESTERN MCPHERSON COUNTY IN CENTRAL KANSAS... NORTHEASTERN RICE COUNTY IN CENTRAL KANSAS... SOUTHWESTERN SALINE COUNTY IN CENTRAL KANSAS... * UNTIL 630 PM CDT * AT 542 PM CDT...A CONFIRMED LARGE AND EXTREMELY DANGEROUS TORNADO WAS LOCATED 5 MILES SOUTHEAST OF GENESEO...AND MOVING NORTHEAST AT 35 MPH. THIS IS A PARTICULARLY DANGEROUS SITUATION. HAZARD...DAMAGING TORNADO. SOURCE...LAW ENFORCEMENT CONFIRMED TORNADO. IMPACT...MAJOR HOUSE AND BUILDING DAMAGE LIKELY AND COMPLETE DESTRUCTION POSSIBLE. NUMEROUS TREES SNAPPED. MAJOR POWER OUTAGES IN PATH OF TORNADO HIGHLY LIKELY. SOME ROADS POSSIBLY BLOCKED BY TORNADO DEBRIS. COMPLETE DESTRUCTION OF VEHICLES LIKELY. * LOCATIONS IMPACTED INCLUDE... MARQUETTE AND LINDSBORG. * PRECAUTIONARY/PREPAREDNESS ACTIONS... THIS TORNADO WARNING REPLACES THE SEVERE THUNDERSTORM WARNING THAT WAS IN EFFECT FOR THE SAME AREA. && LAT...LON 3878 9782 3853 9763 3836 9811 3849 9824 TIME...MOT...LOC 2243Z 236DEG 31KT 3848 9806 TORNADO...OBSERVED TORNADO DAMAGE THREAT...SIGNIFICANT HAIL...2.50IN $$ BILLINGS
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