Tornado outbreak - February 10-11, 8+ dead

[CrazyC83]

SEL2

URGENT - IMMEDIATE BROADCAST REQUESTED
TORNADO WATCH NUMBER 12
NWS STORM PREDICTION CENTER NORMAN OK
930 PM CST TUE FEB 10 2009

THE NWS STORM PREDICTION CENTER HAS ISSUED A
TORNADO WATCH FOR PORTIONS OF

A LARGE PART OF WESTERN AND NORTHERN ARKANSAS
PARTS OF NORTHWESTERN LOUISIANA
PARTS OF EASTERN OKLAHOMA
A LARGE PART OF EASTERN TEXAS

EFFECTIVE THIS TUESDAY NIGHT AND WEDNESDAY MORNING FROM 930 PM
UNTIL 500 AM CST.

TORNADOES...HAIL TO 2.5 INCHES IN DIAMETER...THUNDERSTORM WIND
GUSTS TO 90 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE
AREAS.

THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 90 STATUTE
MILES EAST AND WEST OF A LINE FROM 25 MILES NORTHEAST OF HARRISON
ARKANSAS TO COLLEGE STATION TEXAS. FOR A COMPLETE DEPICTION OF
THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS
WOU2).

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...THIS TORNADO WATCH REPLACES TORNADO
WATCH NUMBER 9...TORNADO WATCH NUMBER 10. WATCH NUMBER 9 10 WILL
NOT BE IN EFFECT AFTER 930 PM CST. CONTINUE...WW 11...

DISCUSSION...INTENSE S/WV TROUGH AND ASSOCIATED VERY STRONG MID/UPR
WIND MAX WILL MOVE RAPIDLY EWD ACROSS SRN PLAINS TONIGHT. LINE OF
SEVERE THUNDERSTORMS WITH EMBEDDED TORNADIC SUPERCELLS HAS DEVELOPED
ERN OK INTO CENTRAL TX. IN ADDITION TO WIDESPREAD DAMAGING WINDS
ASSOCIATED WITH THE LINE...TORNADOS ARE ALSO LIKELY. IN ADDITION
POTENTIAL FOR MORE DISCRETE SUPERCELLS DEVELOPING AHEAD OF THE
SQUALL LINE WHICH WOULD RESULT IN AN INCREASE POTENTIAL FOR STRONG
TORNADOES.

AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL
SURFACE AND ALOFT TO 2.5 INCHES. EXTREME TURBULENCE AND SURFACE
WIND GUSTS TO 80 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO
500. MEAN STORM MOTION VECTOR 24045.


...HALES

[Ed Mahmoud]

90 mph?

What is the trigger for a PDS Severe T-Storm Watch?

[CrazyC83]

Looks like a major derecho event possible? It's getting strong fast!

[CrazyC83]

90 mph?

What is the trigger for a PDS Severe T-Storm Watch?

This would easily be a PDS Severe (with >95/90 in the wind section - usually 70% significant wind would trigger such), except for the fact the tornado threat is way too high. That is one of the reasons a PDS Severe Thunderstorm Watch is so rare. But 30% significant tornado threat is just too low for a PDS Tornado Watch (it should be at least 50% before such is warranted).

[Ed Mahmoud]

Straightline wind, I'd guess. I didn't see any indication of a tornado on the free NWS radar I can see at home...

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE FORT WORTH TX
937 PM CST TUE FEB 10 2009

..TIME.. ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE.. ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..

0935 PM TSTM WND DMG WACO 31.57N 97.18W
02/10/2009 MCLENNAN TX EMERGENCY MNGR

ROOF BLOWN OFF OF A RESTAURANT NEAR BAYLOR UNIVERSITY.

$$

[Bunkertor]

They just launched a tornado warning at the AAC.

[Texas Snowman]

Per the CBS 11 Weather Twitter:

PER LEWISVILLE PD: TORNADO SIGHTED AT FM 3040 AND EDMONDS

[CrazyC83]

Report of 88 mph winds per FOX DFW.

[Texas Snowman]

KOCO: "We have only one confirmed dead right now, but we have also been told that emergency relief from at least Norman and Washington are headed to the area. I really even hesitate to say more than that, but I also don’t want to paint an overly optimistic picture. Right now, we just don’t know how many people might have been killed or hurt."

Local TV station KXII (in Sherman/Denison) is reporting "extensive damage" in the Lone Grove area.

[jasons2k]

Did anyone else notice how the southern end of the squall line just exploded as it reached the SAT area? My GR3 went from almost nothing to a solid line in two frames.

[CrazyC83]

The squall line really did explode to the south - the cells are really back-building.

[jasons2k]

90 mph?

What is the trigger for a PDS Severe T-Storm Watch?

This would easily be a PDS Severe (with >95/90 in the wind section - usually 70% significant wind would trigger such), except for the fact the tornado threat is way too high. That is one of the reasons a PDS Severe Thunderstorm Watch is so rare. But 30% significant tornado threat is just too low for a PDS Tornado Watch (it should be at least 50% before such is warranted).

I've never seen numerical criteria used to issue a PDS watch. Usually, it's when the SPC is expecting an outbreak of long-lasting, violent tornadoes (F3+). It is strictly subjective.

Here is what the "official" word is from the SPC:

http://www.spc.noaa.gov/faq/

2.7 I noticed the wording "THIS IS A PARTICULARLY DANGEROUS SITUATION" in some of your watches. What does this mean? What is the criteria for a PDS watch?
The "particularly dangerous situation" wording is used in rare situations when long-lived, strong and violent tornadoes are possible. This enhanced wording may also accompany severe thunderstorm watches for exceptionally intense and well organized convective wind storms. PDS watches are issued, when in the opinion of the forecaster, the likelihood of significant events is boosted by very volatile atmospheric conditions. Usually this decision is based on a number of atmospheric clues and parameters, so the decision to issue a PDS watch is subjective. There is no hard threshold or criteria. In high risk outlooks PDS watches are issued most often.

[Ed Mahmoud]

Not PDS Tornado, I was asking for the usually reserved for major derecho event PDS T-Storm...

[CrazyC83]

Reports of an 18-wheeler blown off the road by high winds near Bruceville, TX.

[CrazyC83]

MESOSCALE DISCUSSION 0092
NWS STORM PREDICTION CENTER NORMAN OK
0946 PM CST TUE FEB 10 2009

AREAS AFFECTED...S AND SERN TX

CONCERNING...SEVERE POTENTIAL...WATCH NEEDED SOON

VALID 110346Z - 110415Z

SQUALL LINE MOVING THROUGH S TX FROM AUSTIN TO SOUTH OF SAN ANTONIO
IS EXPECTED TO CONTINUE EWD TO THE COAST. DAMAGING WIND AND LARGE
HAIL WILL REMAIN LIKELY. A WW WILL BE ISSUED SOON.

..DIAL.. 02/11/2009


ATTN...WFO...LCH...HGX...CRP...EWX...

LAT...LON 29379479 28349660 28599823 29949747 30299643 30089434
29379479

[jasons2k]

Even more rare - this is what Wikipedia says:

http://en.wikipedia.org/wiki/Particular ... _situation

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 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 would be issued if the tornado risk is significant alongside the wind threat).[2]

[Ed Mahmoud]

MESOSCALE DISCUSSION 0092
NWS STORM PREDICTION CENTER NORMAN OK
0946 PM CST TUE FEB 10 2009

AREAS AFFECTED...S AND SERN TX

CONCERNING...SEVERE POTENTIAL...WATCH NEEDED SOON

VALID 110346Z - 110415Z

SQUALL LINE MOVING THROUGH S TX FROM AUSTIN TO SOUTH OF SAN ANTONIO
IS EXPECTED TO CONTINUE EWD TO THE COAST. DAMAGING WIND AND LARGE
HAIL WILL REMAIN LIKELY. A WW WILL BE ISSUED SOON.

..DIAL.. 02/11/2009


ATTN...WFO...LCH...HGX...CRP...EWX...

LAT...LON 29379479 28349660 28599823 29949747 30299643 30089434
29379479

Picture, thousand words...

[RL3AO]

Said watch.

Code: Select all

URGENT - IMMEDIATE BROADCAST REQUESTED
   SEVERE THUNDERSTORM WATCH NUMBER 13
   NWS STORM PREDICTION CENTER NORMAN OK
   1000 PM CST TUE FEB 10 2009
   
   THE NWS STORM PREDICTION CENTER HAS ISSUED A
   SEVERE THUNDERSTORM WATCH FOR PORTIONS OF
   
          PARTS OF SOUTH TEXAS
          COASTAL WATERS
   
   EFFECTIVE THIS TUESDAY NIGHT AND WEDNESDAY MORNING FROM 1000 PM
   UNTIL 300 AM CST.
   
   HAIL TO 1.5 INCHES IN DIAMETER...THUNDERSTORM WIND GUSTS TO 70
   MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS.
   
   THE SEVERE THUNDERSTORM WATCH AREA IS APPROXIMATELY ALONG AND 95
   STATUTE MILES EAST AND WEST OF A LINE FROM 85 MILES NORTHWEST OF
   HOUSTON TEXAS TO 35 MILES EAST SOUTHEAST OF BEEVILLE TEXAS.  FOR
   A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH
   OUTLINE UPDATE (WOUS64 KWNS WOU3).
   
   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 11...WW 12...
   
   DISCUSSION...SQUALL LINE HAS EXTENDED SWD ALONG STRONG COLD FRONT
   MOVING EWD ACROSS CENTRAL AND SRN TX. THE STRONG SURFACE CONVERGENCE
   WILL ALLOW STORMS TO CONTINUE TO DEVELOP THROUGH THE SIGNIFICANT
   INHIBITION. DAMAGING WINDS AND MARGINAL HAIL WILL BE ASSOCIATED WITH
   THE LINE AS IT MOVES E/SEWD TOWARD TX COAST.
   
   AVIATION...A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT
   TO 1.5 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 60
   KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 450. MEAN STORM
   MOTION VECTOR 27030.
   
   
   ...HALES

[CrazyC83]

That was fast...this squall line is fooling everyone.

[Dave]

Still holding with multiple thunderstorm warnings, no tornado warnings at this time.