Tropical weather analysis - May 19, 2012
Here's one I didn't anticipate. Tropical Storm Alberto formed off the coast of South Carolina today. As of the most recent NHC advisory, the following was posted:
Wind: 50 mph, higher gusts
Location: 31.8°N 78.5°W
Movement: SW at 6 mph
Pressure: 998 mb
Category: Tropical storm (Saffir-Simpson Hurricane Scale)
The center is difficult a bit difficult to locate, but based on a healthy slew of radar and satellite fixes, is estimated to be within the southern side of the convection. Assuming I'm correct, the cyclone is not as disheveled as it looks. However, disorganized systems like Alberto often contain mid-level circulations that are displaced from the surface circulations.
Figure 1. Latest infrared satellite image of Tropical Storm Alberto, courtesy of NOAA's Satellite Services Division (SSD).
Alberto's future track is highly uncertain, complicated by a well-defined upper low between Virginia and Bermuda. This system extends all the way down to 850 mb, so its influence on Alberto can be expected to be rather large. However, the global models unanimously move this low into eastern North Carolina and/or southern Virginia in about a day or so, and current water vapor images would seem to support this. Alberto is trapped in a col region between a large ridge over the Ohio Valley, and the aforementioned upper low. The combination of these features, and a shortwave trough over the central United States should cause a gradual bend of the storm to the north, and eventually northeast.
Large scale guidance is in decent agreement about this, though it really depends on exactly where Alberto's center is. If it is indeed within the recent burst of convection on the south side as I suspect it is, a landfall on the mainland is virtually inevitable. If the center is farther north, the storm might merely skirt the coast. I anticipate a landfall along the South Carolina/North Carolina border tomorrow afternoon. The timing and placement of landfall will also weigh heavily on the influence of the aforementioned upper low, which is forecast to fill as it moves inland, and also the progression of the relatively flat trough to the west.
After landfall, Alberto should move back out into the Atlantic. Passage over cold waters north of the North Carolina coast should preclude reintensification. Alberto is expected to become extratropical in about four days.
Given the complexities of the synoptic steering pattern, this is a rather low-confidence forecast.
5-day intensity forecast
Initial 05/19 0300Z 45 KT 50 MPH
12 hour 05/20 1200Z 45 KT 50 MPH
24 hour 05/21 0000Z 50 KT 60 MPH
36 hour 05/21 1200Z 50 KT 60 MPH
48 hour 05/22 0000Z 45 KT 50 MPH...inland
72 hour 05/23 0000Z 40 KT 45 MPH...over water
96 hour 05/24 0000Z 35 KT 40 MPH...extratropical
120 hour 05/25 0000Z...dissipated
Watches and warnings
WATCHES AND WARNINGS
CHANGES WITH THIS ADVISORY...
A TROPICAL STORM WATCH HAS BEEN ISSUED FOR THE COAST OF SOUTH
CAROLINA FROM THE SAVANNAH RIVER TO SOUTH SANTEE RIVER.
SUMMARY OF WATCHES AND WARNINGS IN EFFECT...
A TROPICAL STORM WATCH IS IN EFFECT FOR...
* SAVANNAH RIVER TO SOUTH SANTEE RIVER SOUTH CAROLINA
A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE
POSSIBLE WITHIN THE WATCH AREA...IN THIS CASE WITHIN 24 HOURS.
COASTAL INTERESTS ELSEWHERE FROM GEORGIA THROUGH THE OUTER BANKS OF
NORTH CAROLINA SHOULD MONITOR THE PROGRESS OF ALBERTO.
FOR STORM INFORMATION SPECIFIC TO YOUR AREA IN THE UNITED
STATES...PLEASE MONITOR PRODUCTS ISSUED BY YOUR LOCAL NATIONAL
WEATHER SERVICE FORECAST OFFICE.
The tropical disturbance we have been tracking in the eastern Pacific over the last several days is a little better organized today, but I use that term rather loosely. While convection is slowly regenerating, this activitty is poorly-organized, and the overall circulation remains broad and ill-defined.
Figure 2. Latest infrared satellite image of Invest 92E, courtesy of NOAA's Satellite Services Division (SSD).
Some modest easterly shear continues to afflict the system, but these winds are forecast to decrease over the next day or two. The largest impediment to development continues to be the very slow movement of the disturbance, which is putting it closer to the axis of a large ridge over the western Caribbean. It appears that this feature has been the primary cause of the shear the system has encountered over the last couple of days.
The large scale steering pattern for 92E remains in the hands of a developing storm system over the western Pacific. The upper low attendant to this feature that is expected to bring the trough into the US is located in the open Pacific between the Hawaiian Islands and the coast of Oregon. Based on the placement and speed of this feature, the trough appears to be pretty much on schedule, so a long-range threat to Mexico is still warranted.
The models, naturally, still disagree on the specifics -- namely the timing and amplitude of the trough. The CMC and NOGAPS show a slightly weaker trough and are thus farther west, taking about 12-24 hours more to recurve 92E into southwest Mexico. The GFS and ECMWF, typically more reliable, show a more clear-cut path into central Mexico, and landfall on Friday. It should be noted that the 12z run of the ECMWF actually sends the storm back offshore on Saturday beneath a developing mid-level ridge. This is likely a reflection of the lower tropospheric steering, as model height fields still show a definitive mid-level weakness in the central United States ridge during this time. And because the system is expected to be strong by then, it is unlikely to be steered by the lower- tropospheric flow.
Interests along the coast of Mexico from Puerto Vallarta to Acapulco should continue to monitor the progress of this disturbance.
Probability of genesis within the next 48 hours: 30%