Tropical weather analysis - May 22, 2012
Apparently, the Northern Hemisphere tropics -- at least the Atlantic and Pacific, didn't bother taking a look at the calender, as there yet another area of interest in the Atlantic, "94L". The center of this broad disturbance has been quite difficult to locate, in part because westerly shear is so fast it has been masking the low cloud lines. Of course, the time of day doesn't help either. Based on a slew of observations, my best guess is somewhere between Yucatan and Grand Cayman. If true, the convection is well east of the center. CIMSS analysis diagnoses about 30 kt of westerly shear over the estimated low-level center.
Figure 1. Latest infrared of Invest 94L, courtesy of RAMMB imagery Colorado State University (CSU).
Model guidance suggests that upper-level winds could weaken somewhat over the next day or so, but would probably still be sufficient to keep the system from strengthening very much. Nonetheless, this has been a very active pattern for May, which if nothing else, is particularly interesting to me.
Water vapor imagery shows a large cutoff low moving across central Georgia. Current trends suggest that the low and its associated trough will bypass 94L, with no immediate recurvature anticipated. In about two days, another, more vigorous trough, is forecast to dive into the central US. Despite the distance of the trough relative to 94L, there should be enough of a weakness in the subtropical ridge to turn the system northeast or north-northeast toward south Florida. Model guidance is in good agreement with this. Thereafter, there are indications that the system could become trapped underneath a building ridge while moving across the western Atlantic. Such a pattern would favor a westward retrograde toward the southeast United States coast. However, there are simply too many uncertainties regarding both the synoptic pattern and the upper-level winds in the path of the system at that time, to state with any confidence what will happen in the future.
Regardless of development, locally heavy rainfall can be expected over the Cayman Islands, western Cuba, south Florida, and eventually the Bahamas.
Probability of genesis within the next 48 hours: 10%
Tropical Storm Bud continues churning in the Pacific far to the south of Mexico. Surprisingly, Bud has been a rather bewildering cyclone, in that it has not strengthened in what was deemed as a generally favorable environment. As of the most recent NHC advisory, the following was posted on the storm:
Wind: 40 mph, with higher gusts
Location: 12.9°N 105.9°W
Movement: NW at 14 mph
Pressure: 1004 mb
Category: Tropical storm (Saffir-Simpson Hurricane Scale)
Recent satellite photos suggest that Bud might finally be starting to organize, as the center is no longer exposed. A large convective burst with very cold cloud tops, up to -80C, has developed near and over the low-level center during the past couple of hours.
Figure 2. Latest infrared satellite image of Tropical Storm Bud, courtesy of NOAA's Satellite Services Division (SSD).
Upper-tropospheric outflow is well defined in all directions except the east, where it remains restricted, a symptom of easterly shear. However, high cloud motions suggest that the shear may finally be starting to diminish. Also, earlier microwave data, corroborated by recent satellite imagery, suggests Bud has developed a large and well-defined inflow band on the east side of the circulation. This southerly influx is helping to inject instability into the system, assisting intensification. Finally, this large band may, in the meantime (I expect it to dissipate as the inner wind core becomes better defined), help to shield Bud from any residual shear.
Assuming Bud is finally developing a well-defined inner core, at least steady intensification seems likely. However, the current outflow pattern is beginning to look anticyclonic, and this typically heralds strengthening. It is possible that Bud could undergo a brief period of rapid intensification if the ongoing convection persists and organizes. After 48 hours, Bud is forecast to encounter gradually increasing shear, cooler waters, and a very dry airmass near Baja. These factors should weaken the storm, and it is possible Bud will not be a tropical cyclone in five days.
The ridge to the north of Bud continues to deamplify, but water vapor imagery suggests that this process is occurring rather slowly, and the cyclone still has another 24 hours before recurving. This is a little slower than the NHC official forecast, as the ridge still looks strong. Models are in good agreement up to this point. Afterward, uncertainty arises as the express disagreement. However, the general trend over the last day or so has been for Bud to near the coast, stall, and execute a clockwise loop, presumably due to a weak low- to mid-level ridge over central Mexico.
Although a landfall is looking less likely, it wouldn't hurt to remind that, based on the running 5-year mean, there is still a 20 to 30% chance of tropical storm force winds affecting portions of the coast from Puerto Vallarta to Manzanillo. Besides, the quasi-stationary movement expected as Bud approaches the coast could produce a several day period of heavy rainfall, possibly leading to flash flooding and mudslides in areas of mountainous terrain.
5-day intensity forecast
Initial 05/24 0000Z 35 KT 40 MPH
12 hour 05/24 1200Z 45 KT 50 MPH
24 hour 05/25 0000Z 50 KT 60 MPH
36 hour 05/25 1200Z 65 KT 75 MPH
48 hour 05/26 0000Z 75 KT 85 MPH
72 hour 05/27 0000Z 65 KT 75 MPH
96 hour 05/28 0000Z 45 KT 50 MPH
120 hour 05/29 0000Z 30 KT 35 MPH
5-day track forecast
Figure 3. My 5-day forecast track for Tropical Storm Bud.