Tropical Depression Two and why one shouldn’t name disturbances
August 12, 2009
Late last night, after a few hours of watching the satellite loop of Tropical Depression Two, I created this unfortunate post on FriendFeed:
Barring a dramatic reversal in the next couple of hours, we will officially have Tropical Storm Ana in the Atlantic come 5 AM tomorrow morning. Also, it is starting to appear likely that there will be a Tropical Storm Bill by the weekend. (Both forming in the eastern Atlantic, with the latter appearing to be more troubling).
In it, I comitted the cardinal sin of naming disturbances before they actually earn the name***. It is a pet peeve of mine, but my discipline broke down as I was 90% convinced that we would see the National Hurricane Center upgrade the tropical depression to tropical storm status this morning. This, despite considerable uncertainty in where the center of the storm was at (the position of the center of circulation is an essential element of information in estimating intensity from conventional satellite imagery). A fair bit of my skepticism was washed away by this commentary in the evening discussion:
A RECENTLY- RECEIVED ASCAT PASS FROM 0000 UTC SUPPORTS INCREASING THE INTENSITY TO 30 KT. EVEN MORE RECENTLY...LATEST SATELLITE IMAGERY SUGGESTS A FURTHER INCREASE IN ORGANIZATION AND THE DEPRESSION MAY NOW BE CLOSE TO TROPICAL STORM STRENGTH. Tropical Depression Seven Discussion Number Four
In other words, the wind speed in the 11 PM advisory was based off 3 hour old information and the satellite imagery showed the storm becoming better organized since then. As it turned out, the next round of intensity estimates did not warrant an upgrade to tropical storm status at 5 AM. However, the subsequent round did, but microwave satellite imagery did not, so the cyclone remained a tropical depression at 11 AM. When the next estimates came in, they once again indicated a tropical storm and the NHC was right about to upgrade accordingly, but
...SINCE ABOUT 18Z THE CONVECTION HAS DECREASED SIGNIFICANTLY AND HAS MOVED FARTHER AWAY FROM THE EXPOSED LOW-LEVEL CENTER. BASED ON THIS...THE CYCLONE WILL REMAIN A 30 KT DEPRESSION FOR THIS ADVISORY. Tropical Depression Two Discussion Number Seven
The discussion goes on to cover the difficulties the cyclone is encountering, one of them being the disturbance to its east. The disturbance does not have any nomenclature attached to it yet, but was described thusly this afternoon in the Tropical Weather Discussion and Tropical Weather Outlook, respectively:
TROPICAL WAVE IS INTRODUCED ALONG 18W S OF 17N MOVING W AT 15 KT. SATELLITE IMAGERY AND SSMI TOTAL PRECIPITABLE WATER IMAGERY SHOWED A WELL DEFINED WAVE OFF THE COAST OF WEST AFRICA. UPPER AIR SOUNDINGS FROM DAKAR SENEGAL ALSO DEPICTED WAVE PASSAGE. WIDELY SCATTERED MODERATE CONVECTION IS FROM 6N-16N BETWEEN 15W-25W.A LARGE AREA OF SHOWERS AND THUNDERSTORMS BETWEEN THE CAPE VERDE ISLANDS AND AFRICA IS ASSOCIATED WITH A VIGOROUS TROPICAL WAVE. SLOW DEVELOPMENT OF THIS SYSTEM IS POSSIBLE OVER THE NEXT FEW DAYS AS IT MOVES TO THE WEST AT 10 TO 15 MPH. THERE IS A MEDIUM CHANCE...30 TO 50 PERCENT...OF THIS SYSTEM BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HOURS.
Forecast models have been very agressive in developing this storm; far more so than they had been for now Tropical Depression Two. They are advertising a tropical storm by Saturday with significant development to follow. It may be overly pessimistic on TD Two’s chances to think that the disturbance to its east becomes a tropical storm (especially since the NHC continues to forecast TD 2 becoming a tropical storm in the next 12 hours), but it is more plausible than was the case yesterday.
*** 2005 had at least two good examples of the problems in doing this. The tropical depression that became Cindy existed a couple of days before the one that became Dennis, but it was a close run thing that the tropical storm upgrades actually occurred in order of formation. An actual “bust” happened later in the season when Tropical Depression Ten was presumed to become Jose. People looking for Jose in the Atlantic, east of the Antilles were a bit surprised to find him in the Bay of Campeche instead.