Day 20: First Tornado Watch

Posted by on 20 Jan 2010 | Comments Off

As was discussed in yesterday’s post, the wave train of short-wave troughs (upper-level lows) has aided in the development of an active weather pattern.  Overnight into this morning, portions of Iowa received over an inch of ice accumulation as the result of freezing rain.  At last check, 20,000+ customers were without power throughout Iowa.

Day 20 (c)

To the south, severe thunderstorms and tornadoes were developing across the southern United States, in what is known as the warm sector (A region of warm surface air between a cold front and a warm front.).  The first tornado watch of the year was issued today by the Storm Prediction Center (SPC).  For those who have never seen the text of a tornado watch, I’ve included the text for this watch below.

day020a 
SEL1

   URGENT - IMMEDIATE BROADCAST REQUESTED
   TORNADO WATCH NUMBER 1
   NWS STORM PREDICTION CENTER NORMAN OK
   315 PM CST WED JAN 20 2010

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

          PARTS OF LOUISIANA
          SOUTHWEST MISSISSIPPI
          SOUTHEAST TEXAS

   EFFECTIVE THIS WEDNESDAY AFTERNOON AND EVENING FROM 315 PM UNTIL
   1000 PM CST.

   TORNADOES...HAIL TO 1.5 INCHES IN DIAMETER...THUNDERSTORM WIND
   GUSTS TO 70 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE
   AREAS.

   THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 55 STATUTE
   MILES NORTH AND SOUTH OF A LINE FROM 80 MILES WEST OF FORT POLK
   LOUISIANA TO 60 MILES EAST OF MC COMB MISSISSIPPI.  FOR A
   COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE
   UPDATE (WOUS64 KWNS WOU1).

   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...TSTMS SHOULD CONTINUE TO INCREASE IN COVERAGE AND
   INTENSITY THIS AFTERNOON INTO EVENING...DRIVEN BY BROAD LOW-LEVEL
   WAA REGIME.  AMBIENT AIR MASS HAS BECOME WARM...MOIST AND MODERATELY
   UNSTABLE WITH MLCAPE OF 1000-2000 J/KG.  WHEN COUPLED WITH 40-45 KT
   OF DEEP-LAYER LAYER AND STEADILY STRENGTHENING LOW-LEVEL SHEAR ALONG
   LLJ AXIS...ENVIRONMENT WILL FAVOR THE DEVELOPMENT OF SUPERCELLS
   CAPABLE OF A FEW TORNADOES...IN ADDITION TO HAIL AND LOCALLY
   DAMAGING WIND GUSTS.

   AVIATION...TORNADOES AND 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 24035.

   ...MEAD/WEISS

Shortly after issuing the first tornado watch, tornado watch number two was issued to the northwest of tornado watch number 1.  It is certainly shaping up to be an active evening and overnight across the southern United States.  As the severe thunderstorm even unfolds, I’ll attempt to provide reports that I’ve heard about via my Twitter account.

day020b