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 in Norman, Oklahoma 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.[1] 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.[2] There often are multiple PDS watches issued for the same weather system and on the same day during major outbreaks, so the number of days per year that a PDS watch is issued is significantly lower. 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.[3]
PDS flash flood watches are issued when there is a greater risk of widespread, potentially life-threatening flash flooding. These watches are issued by the local forecast offices, not the Storm Prediction Center.
PDS severe thunderstorm watches are issued when there is a greater risk of severe wind damage capable of major structural damage (in addition to large hail and perhaps a few isolated tornadoes), usually due to a strong and persistent derecho. They are very rare (an average of only two each year) as the tornado risk has to remain low enough to only warrant a severe thunderstorm watch (a tornado watch - generally not PDS - would be issued if the tornado risk is significant alongside the extreme wind threat).[2]
PDS tornado watches are issued when there is a greater risk of multiple strong, violent or extremely violent tornadoes in the watch area (damage consisting of EF4 or EF5), in addition to significant wind and hail damage. While there are no hard 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.[2]
These watches are generally (but not always) issued after a high risk or an upper-end moderate risk of severe storms is issued by the SPC in the convective outlooks.
While historically applied only to Severe Thunderstorm, Tornado, and Flash Flood watches, PDS wording could theoretically be applied to other types of weather watches (i.e., Winter Storm, High Wind, or Fire Weather watches) when an enhanced threat for such conditions exists.
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[edit] Sample PDS Flash Flood Watch
This watch was the first PDS Flash Flood Watch and was issued by the National Weather Service forecast office in Memphis, Tennessee on April 24, 2011, as mentioned above.[4]
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
[edit] 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 was from May 30th, 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.</sb |url=http://www.spc.noaa.gov/products/watch/ww0405.html |title=Storm Prediction Center: Particularly Dangerous Situation (PDS) Severe Thunderstorm Watch 405 |date=2011-05-30
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
[edit] Sample PDS Tornado Watch
This PDS Tornado Watch was issued covering Alabama in the heart of the extreme tornado outbreak that killed over 300 people (which clearly verified) as explained above.
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.[5]
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
[edit] References
- ^ NOAA (2004-04-18). "Storm Prediction Center Frequently Asked Questions (FAQ)". http://www.spc.noaa.gov/faq. Retrieved 2008-05-23.
- ^ a b c Dean, Andrew R. (2006-11-07). "PDS watches: how dangerous are these "particularly dangerous situations?" (2006 - 23SLS_23sls)". American Meteorological Society. http://ams.confex.com/ams/23SLS/techprogram/paper_115252.htm. Retrieved 2008-05-23.
- ^ Lewis, John (2007-11-03). "A Forecaster's Story: Robert H. Johns". Electronic Journal of Severe Storms Meteorology 2 (7). http://www.ejssm.org/ojs/index.php/ejssm/article/view/29.
- ^ http://mesonet.agron.iastate.edu/vtec/#2011-O-NEW-KMEG-FF-A-0006
- ^ NOAA (2011-04-27). "Storm Prediction Center: Particularly Dangerous Situation (PDS) Tornado Watch 232". http://www.spc.noaa.gov/products/watch/ww0235.html. Retrieved 2011-04-27.
[edit] External links
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