Hurricane Lorenzo blows ashore; Karen continues weakening; TD 14 forms

Hurricane Lorenzo blew ashore at about midnight local time on the Mexican coast as a Category 1 hurricane with 75 mph winds. Satellite imagery shows that this very small storm is already starting to dissipate, and Lorenzo's effects will be confined to a small area near the coast where the storm made landfall. A pass from NASA's TRMM satellite (Figure 1) showed rainfall amounts of up to one inch per hour from Lorenzo. Lorenzo could dump 5-10 inches of rain over a narrow region extending 50 miles inland from the landfall location. Widespread major flooding is not expected, but some serious local flooding will probably occur.


Figure 1. Rainfall rates estimated by NASA's TRMM satellite as Lorenzo made landfall at 11:26 pm EDT last night. Rainfall amounts as high as one inch per hour (red colors) were estimated in the eyewall and in the spiral band to the west of Lorenzo.

It's not unusual for tropical storms to intensify suddenly where Lorenzo did, in the extreme southwestern Gulf of Mexico's Bay of Campeche. According to a paper presented by NASA's Scott Braun at a hurricane conference in 2006, this may be due to the mountains along shore diverting the low-level winds and helping intensify a tropical storm's vortex. In the case of a model simulation done of 2005's Tropical Storm Gert the authors write:
"The simulation shows an easterly wave and surface trough moving westward over the southern Gulf of Mexico with somewhat disorganized convection occurring in its vicinity. Low-level easterly flow ahead of the trough impinges on the eastern side of the Sierra Madre mountains and leads to flow blocking. Because of the particular shape of the topography, this blocked flow causes northwesterly to westerly flow to occur in the southern Gulf that eventually pinches off the trough to form a closed cyclonic circulation. This topographically forced flow also helps to organize some of the convection in a linear band south of the vortex center. Once the closed cyclonic circulation is formed, convection increases and gradually intensifies the cyclone to tropical storm strength by the time of landfall.

Tropical Storm Karen
Tropical Storm Karen has weakened to a minimal tropical storm, thanks to continued high levels of wind shear. Strong westerly winds aloft are creating about 20-25 knots of wind shear over Karen. This morning's QuikSCAT pass showed a very elongated center of circulation, with winds of minimal tropical storm force to the east of the center. Satellite loops show Karen's exposed low-level center of circulation, now visible as a swirl of low clouds. Most of the heavy thunderstorm activity that was was on the storm's northeast side has died out this morning, and Karen may get downgraded to a tropical depression today. A small area of heavy thunderstorm activity has developed near the center on the east side, but Karen is looking very unhealthy now.

Karen has taken an unexpected jog to the north this morning, which puts the storm closer to a region of higher wind shear. A motion to the west-northwest is expected to resume later today. With Karen's more northerly position, the odds of the storm being torn apart have increased. The GFS model kills Karen by Sunday, and the SHIPS model (which is based on the GFS) is forecasting that 35-40 knots of wind shear will affect the storm tonight through Saturday night. The other models do not kill Karen, but none of them forecasted the northward jog that will bring Karen into higher wind shear sooner. There is about a 50% chance Karen will be gone by Sunday, destroyed by wind shear.

If Karen survives, an ominous possibility arises--a ridge of high pressure is expected to build over the eastern U.S. and western Atlantic, forcing Karen westward towards the U.S. An anticyclone with low wind shear might build over the storm, creating an environment favorable for intensification of Karen into a hurricane. The path Karen might take late next week is highly uncertain. The high pressure ridge expected to build to the north of Karen will be quite strong, which might force Karen westward into Florida, or even west-south-westward through the Bahamas and into Cuba. There will be several small "short wave" troughs moving through the ridge that may be able to turn Karen more northwesterly towards New England, though. Still another uncertainty is the possibility of a new tropical, subtropical, or extratropical cyclone forming off the coast of North Carolina on Sunday or Monday, something now predicted by all of the computer models. Some of the models take this new storm north-eastward out to sea, which would create a weakness in the ridge that would pull Karen northward. However, most of the models predict that the new storm will move west or west-southwest over Georgia or Florida. A storm-storm interaction between Karen and the new storm might ensue, an event the models are poor at handling.

In short, this is a very complicated situation, and we'll just have to wait and see how it unfolds.

Tropical Depression 14
Tropical Depression 14 formed about 50 miles southwest of the Cape Verdes Islands this morning. The storm is under about 10-20 knots of wind shear. This shear should keep any development slow today. This morning's high-resolution QuikSCAT pass showed that the storm down not have a well-organized surface circulation, but rather a crescent-shaped line of converging winds arcing along a 400-mile long line. A few 50-knot wind vectors appeared in squalls on the storm's east side. This system will likely never affect land, since it is starting out too far north and will gain additional latitude in the coming days. The storm is headed northwest towards a region of high wind shear, and may not survive long.

I'll have an update Saturday morning by 10am.

Jeff Masters