438 
 WTPZ41 KNHC 152036
 TCDEP1
  
 TROPICAL STORM ALVIN DISCUSSION NUMBER   2
 NWS NATIONAL HURRICANE CENTER MIAMI FL       EP012013
 200 PM PDT WED MAY 15 2013
  
 ASCAT DATA FROM AROUND 1612 UTC SHOWED THAT THE DEPRESSION HAD WINDS
 VERY NEAR THE THRESHOLD FOR A TROPICAL STORM.  THE CLOUD PATTERN IN
 VISIBLE IMAGERY HAS BECOME MORE ORGANIZED SINCE THE TIME OF THAT
 PASS...AND IT IS LIKELY THAT THE SYSTEM HAS BECOME A TROPICAL
 STORM.  THE INTENSITY IS BEING SET AT 35 KT.
 
 THE ASCAT PASS ALSO HELPED TO BETTER LOCATE THE LOW-LEVEL
 CENTER...AND THE INITIAL MOTION ESTIMATE IS NOW 290 DEGREES AT 11
 KT.  ALVIN IS ON THE SOUTHWESTERN PERIPHERY OF A MID-LEVEL RIDGE
 THAT EXTENDS SOUTHWESTWARD FROM MAINLAND MEXICO OVER THE
 PACIFIC...AND THIS FEATURE SHOULD CONTINUE TO STEER THE STORM
 WEST-NORTHWESTWARD FOR THE NEXT FEW DAYS.  A MID-LEVEL TROUGH IS
 FORECAST TO AMPLIFY NEAR CALIFORNIA COAST IN 2 TO 3 DAYS...CREATING
 A WEAKNESS IN THE RIDGE.  THERE IS A SIGNIFICANT SPLIT IN THE MODEL
 GUIDANCE WITH THE STRONGER GFS AND GFDL SHOWING A TURN TOWARD THE
 NORTH AND THE WEAKER ECMWF AND UKMET MODELS SHOWING A CONTINUED
 WESTWARD MOTION BY DAY 5.  THE OFFICIAL FORECAST SOMEWHAT SPLITS
 THE DIFFERENCE BUT DOES FAVOR THE SCENARIO OF A STRONGER
 NORTHWESTWARD-TURNING STORM.
 
 MOST OF THE INTENSITY GUIDANCE SUPPORTS ALVIN REACHING HURRICANE
 STRENGTH IN A FEW DAYS.  THE DYNAMICAL GFDL AND HWRF MODELS ARE THE
 MOST AGGRESSIVE...BUT EVEN THE SHIPS MODEL SHOWS ALVIN GETTING
 STRONGER THAN WE INDICATED IN THE PREVIOUS OFFICIAL FORECAST. 
 ALVIN IS ABOUT TO MOVE OVER A WARM POOL OF WATER WITH SEA SURFACE
 TEMPERATURES AROUND 30.5C...AND VERTICAL SHEAR SHOULD BE MINIMAL
 FOR AT LEAST THE NEXT 3 DAYS.  THE NHC INTENSITY FORECAST CONTINUES
 TO SHOW A STEADY INTENSIFICATION AND HAS BEEN BUMPED UP SLIGHTLY
 FROM THE PREVIOUS FORECAST THROUGH DAY 3.  FASTER STRENGTHENING IS
 POSSIBLE...AND THE RAPID INTENSIFICATION INDEX NOW SHOWS A 1 IN 3
 CHANCE OF A 30-KT INCREASE IN WINDS OVER THE NEXT 24 HOURS. 
 WEAKENING SHOULD BE UNDERWAY IN 4 TO 5 DAYS MAINLY DUE TO STRONGER
 VERTICAL WIND SHEAR.
 
 FORECAST POSITIONS AND MAX WINDS
  
 INIT  15/2100Z  8.7N 105.1W   35 KT  40 MPH
  12H  16/0600Z  9.3N 106.5W   40 KT  45 MPH
  24H  16/1800Z 10.0N 108.2W   50 KT  60 MPH
  36H  17/0600Z 10.6N 109.8W   60 KT  70 MPH
  48H  17/1800Z 11.1N 111.4W   70 KT  80 MPH
  72H  18/1800Z 12.5N 114.0W   75 KT  85 MPH
  96H  19/1800Z 14.0N 116.0W   60 KT  70 MPH
 120H  20/1800Z 15.5N 117.5W   40 KT  45 MPH
  
 $$
 FORECASTER BERG