Particularly Dangerous Situation

A Particularly Dangerous Situation (PDS) is a type of enhanced wording first used by the Storm Prediction Center (a national guidance center of the National Weather Service, the meteorological agency of the United States) 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.[1]

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] 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.

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] 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 (such as 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 Weather Prediction Center's Excessive Rainfall Outlooks.

On April 24, 2011, the National Weather Service Weather Forecast Office in Memphis, Tennessee 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 tornado watches are issued when there is a higher than normal risk of multiple strong to violent tornadoes – especially those that are predicted to be long-track in nature, with path lengths of more than 20 miles – in the watch area (usually amounting to damage consistent with EF4 or EF5 tornadoes), in addition to including significant wind and hail damage. 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. These types of tornado watches represent about 90% of PDS watches issued by the Storm Prediction Center.[2]

The PDS Tornado Watch shown below, was issued covering Alabama during the early stages of the extreme tornado outbreak that killed over 300 people (which clearly verified) as explained above.

NOTE: The color of all text in the original alert issued was black, except for the text "THIS IS A PARTICULARLY DANGEROUS SITUATION", which was rendered in red (not bolded) in the actual alert.[4]

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.

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.

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 (accounting 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).[2]

The PDS Severe Thunderstorm Watch shown below was from May 30, 2011:

NOTE: 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 (not bolded) in the actual alert.[5]

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.

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 local NWS Weather Forecast Offices, not the Storm Prediction Center.

...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 SOUTHERNMISSOURI 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.

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.

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.

PDS tornado warnings are currently issued on an experimental basis by the 38 National Weather Service Weather Forecast Offices within the Central Region.[7] 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). While the intention of this experimental warning may be to replace the loosely defined tornado emergency, PDS tornado warnings are structured as the second highest level of tornado warning within the Impact Based Warning system (an experiment – which also includes tags within warning products illustrating radar indications or physical observations of tornadoes, and damage potential – participated by the 33 Weather Forecast Offices within the Central Region, and will expand to eight additional offices within the Western, Eastern and Southern regions in the spring of 2014[8]); a tornado emergency, the highest warning level, is used within the United States for destructive tornadoes approaching more densely populated areas. These are the first warnings issued with PDS wording, and like PDS flash flood watches, are issued by local forecast offices.[9][10]

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 color of all text in the original alert issued was black, including the text "THIS IS A PARTICULARLY DANGEROUS SITUATION", which was neither rendered in red nor bolded in the actual alert. The wording "SIGNIFICANT" on the tornado damage threat was changed to "CONSIDERABLE" starting with the 2013 tornado season.[11]

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.

PDS wind chill warnings are issued when there is an enhanced risk of frost bite, hypothermia, and eventually death due to extremely low wind chills. These warnings are issued by the National Weather Service Weather Forecast Offices rather than the Storm Prediction Center.

The PDS Wind Chill Warning shown below was issued by the National Weather Service in the Twin Cities on January 5, 2014.

NOTE: The color of all text in the original alert issued was black, except for the text "THIS IS A PARTICULARLY DANGEROUS SITUATION", which was neither rendered in red nor bolded in the actual alert.[12]

...HISTORIC AND LIFE-THREATENING COLD AIR HAS ARRIVED......THIS IS A PARTICULARLY DANGEROUS SITUATION...

.THE COLDEST AIRMASS SINCE 1996 CONTINUES TO MOVE SOUTHEAST OUT
OF CANADA AND INTO MINNESOTA AND WISCONSIN. WIND CHILL VALUES WILL
RANGE BETWEEN 30 AND 45 BELOW TODAY AND CONTINUE TO FALL THROUGH
MONDAY MORNING. AIR TEMPERATURES TONIGHT WILL DROP INTO THE 20S
AND 30S BELOW ZERO. GUSTY WEST OR NORTHWEST WINDS COMBINED WITH
THESE EXTREMELY COLD TEMPERATURES WILL PRODUCE WIND CHILLS OF 50
TO 65 BELOW ZERO LATE TONIGHT AND EARLY MONDAY.

WIND CHILLS COLDER THAN 50 BELOW CAN CAUSE EXPOSED FLESH TO
FREEZE IN ONLY 5 MINUTES. A WIND CHILL WARNING IS IN EFFECT FOR
CENTRAL AND SOUTHERN MINNESOTA AND WEST CENTRAL WISCONSIN THROUGH
NOON TUESDAY.

THE GUSTY WINDS WILL ALSO BRING AREAS OF BLOWING SNOW TO MUCH OF
THE AREA TODAY AND TONIGHT. WHERE GUSTS REACH 35 TO 45 MPH OVER
WESTERN AND SOUTHERN MINNESOTA...VISIBILITIES MAY OCCASIONALLY BE
REDUCED TO LESS THAN A HALF MILE IN NEAR BLIZZARD CONDITIONS. THIS
WILL BRING AN ADDITIONAL LEVEL OF DANGER TO ANYONE STRANDED.

CHILLS OF 35 BELOW...AND IN 5 MINUTES WITH WIND CHILLS OF 50 BELOW OR COLDER.

OTHER IMPACTS...WINDS GUSTING BETWEEN 35 AND 45 MPH THIS

AFTERNOON AND TONIGHT WILL LEAD TO BLOWING SNOW WITH
VISIBILITIES OCCASIONALLY DROPPING TO 1/2 MILE OR LESS IN NEAR
BLIZZARD CONDITIONS. SHOULD YOUR VEHICLE BECOME STRANDED...YOUR
LIFE WILL BE AT RISK. CONSIDER POSTPONING ALL TRAVEL.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

A WIND CHILL WARNING MEANS THE COMBINATION OF VERY COLD AIR AND
STRONG WINDS WILL CREATE DANGEROUSLY LOW WIND CHILL VALUES. THIS
WILL RESULT IN FROST BITE AND LEAD TO HYPOTHERMIA OR DEATH IF
PRECAUTIONS ARE NOT TAKEN.

CHILLS OF 35 BELOW...AND IN 5 MINUTES WITH WIND CHILLS OF 50
BELOW OR COLDER.

OTHER IMPACTS...AREAS OF BLOWING SNOW ARE POSSIBLE AS WINDS GUST

AS HIGH AS 35 MPH THIS AFTERNOON AND TONIGHT.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

A WIND CHILL WARNING MEANS THE COMBINATION OF VERY COLD AIR AND
STRONG WINDS WILL CREATE DANGEROUSLY LOW WIND CHILL VALUES. THIS
WILL RESULT IN FROST BITE AND LEAD TO HYPOTHERMIA OR DEATH IF
PRECAUTIONS ARE NOT TAKEN.

While the use of PDS wording for other types of watches and warnings has not been used, PDS verbiage could theoretically be applied to any kind of watch or warning to alert the public to weather events where there exists an increased risk of loss of life or widespread damage to property. Some such situations could include PDS watches or warnings for blizzards, ice storms, high winds, extreme heat, or fire danger.