TROPICAL WEATHER DISCUSSION
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
805 PM EDT TUE 24 MAY 2005

...WHILE TROPICAL CYCLONE FORMATION IS NOT EXPECTED...THE LOW AND TROUGH SHOULD CONTINUE TO FOCUS SHOWERS/THUNDERSTORMS ACROSS THIS AREA TONIGHT THROUGH WED.

THEREFORE...THE POTENTIAL EXIST FOR LOCALIZED HEAVY RAINFALL AND FLOODING ESPECIALLY IN MOUNTAINOUS AREAS THROUGH WED UNTIL THE TROUGH AND ASSOCIATED AXIS OF SHOWERS/TSTMS LIFTS SLOWLY NWD INTO THE ATLC.

ELSEWHERE...MOISTURE OVER THE SW CARIBBEAN HAS SHIFTED INTO THE E PAC WITH LESS SHOWER/TSTM ACTIVITY TODAY. HOWEVER...DIURNAL CONVECTION WILL STILL BE FAVORED OVER INTERIOR SECTION OF CENTRAL AMERICA.

SIMILARLY...ACTIVITY OVER THE YUCATAN PENINSULA IS LESS WIDESPREAD BUT EXPECT INCREASING ACTIVITY AS AN UPPER JET OVER THE W ATLC STRENGTHENS PRODUCING INCREASING UPPER LEVEL DIVERGENCE OVER THE NW CARIBBEAN. LOW LEVEL MOISTURE SHOULD ALSO BE ON THE INCREASE AS A WEAK TROPICAL WAVE...CURRENTLY ALONG 70W...GETS PULLED NWD AROUND THE W PERIPHERY OF THE SUBTROPICAL RIDGE.
Carriage returns added to the above for emphasis on each area of the discussion
http://www.srh.noaa.gov/data/NHC/TWDAT.0505242312

You have the right ideas checking the Tropical Weather Products from NHC/ TPC first. I'm sure some of the Mets on the board can give actual numbers, but the tropical wave to tropical depression and higher categories ratio is probably close to 1 in 5 or 1 in 8 waves grows into something. That's a 12.5 to 20% chance that one will become a depression or higher. This time of year, ( Pre Hurricane Season ), I would think that there would be less than a 5% chance of a Tropical Wave becoming anyhting but a thunderstorm complex. However, We have seen an Eastward moving EPAC Hurricane in the last week, so all bets are off!
I'll see if I can find some links for the 'Earliest Tropical Depression' in the Atlantic.