Posted by:futuremet, 12:00 PM GMT on August 29, 2009
Tropical Depression Danny
Tropical storm Danny has been downgraded into a tropical depression, as it moves northeastward at about 11 MPH. Despite the fact that it has weakened, satellite imagery indicates that intense convection has increased substantially with Danny. What is the cause of the increased convection?--An upper level low that is exiting the SE U.S. is interacting with Danny's convection; the baroclinic zone associated with the upper low is enhancing convective lift. Water vapor imagery also indicates that there is a significant moisture gradient associated with Danny, this will act as a frontal zone and reinforce convection. Because of all of these factors, Danny is an extremely asymmetrical tropical cyclone. When an asymmetrical tropical cyclone, it is more vulnerable to cold air intrusion to its western quadrant when it enters cooler waters--thus, it will convert to an extratropical cyclone faster. But, why does cold air intrusion hasten extratropical cyclogenesis?--The image below answers that question.
Figure 1: Naturally, cold air comes from the north. The counter clockwise spin of a tropical cyclone will cause colder air from the N to move from the NW toward the center. Now if the western quadrant has no convection associated with it, there is no barrier to prevent cold air from penetrating the center.
Figure 2: Eventually, it will covert to cold core.
Figure 3: Satellite imagery of Danny
To sum it up, the trough should keep Danny from entering deep into the U.S coast. Danny will bring windy and rainy conditions in the Northeast.
Invest 94L has lost most of its convection, but it still has an organized circulation. Some of the computer models like the ECMWF expect this to develop into a tropical depression by Tuesday. The road ahead of 94L is not easy one, because an elongated trough is causing quite a bit of wind shear to its northwest. There is a chance that it might survive, and if it does survive through next week, conditions will be really favorable for it.
Figure 4: Satellite imagery of 94L
Figure 5: favorable conditions by next week.
Long range outlook
Well, it looks the troughing pattern will end by the second week of September. The ensemble models continue to show a vigorous ridge setting over the east coast, and a deeply amplified trough over the west coast. This will help bring above average temperatures throughout much of the east, and cooler temperatures in the west. This pattern makes the east coast more susceptible to tropical cyclone landfalls, as the ridge blocks the storms from curving north. There is some uncertainty that this will occur, however. Within two days, the ambiguity of the model consensus should be low enough for us to know what will happen.