Leslie a threat to Bermuda; Isaac is gone

Tropical Storm Leslie is struggling today under a high 20 knots of wind shear, due to strong upper-level winds out of the northwest. The high shear is keeping heavy thunderstorms confined to the southeast quadrant of the storm, as seen on satellite loops. According to the latest SHIPS model forecast, the shear is expected to stay high through Wednesday, then drop to the low category, 5 - 10 knots, on Thursday and Friday. At that time, Leslie will be over warm ocean waters of 29°C, and may intensify to a hurricane by Friday, as predicted by most of the intensity forecast models. Steering currents for Leslie are expected to collapse by mid-week, as Leslie gets stuck between two upper level lows. The storm will slowly meander over the open ocean for many days, and the latest guidance from the GFS and ECMWF models shows Leslie making a very close pass by Bermuda on Saturday, and that island can expect a 3-day period of rough weather Friday through Sunday. Leslie will stay stuck in a weak steering current environment until a strong trough of low pressure approaches the U.S. East Coast on Saturday. This trough should be strong enough to pull Leslie to the north by Sunday, September 9. At that time, Leslie may be close enough to the coast that the storm will make landfall in Canada or New England by Monday, September 10. However, making forecasts of what a tropical cyclone might do a week in advance is a sketchy proposition. Leslie could miss land entirely; this all depends upon the timing and strength of the September 8 trough of low pressure. If Leslie survives its encounter with high wind shear early this week and becomes a hurricane late in the week, the storm will bring an extended period of high waves to the U.S. coast late this week. These waves will be capable of creating dangerous rip currents and beach erosion.


Figure 1. Morning satellite image of Tropical Storm Leslie.

Invest 99L in the Central Atlantic
An extratropical low pressure system that got cut off from the jet stream and is now spinning away in the Central Atlantic near 28°N 38°W has begun to develop a small amount of heavy thunderstorm activity, and was designated Invest 99L by NHC on Sunday morning. This storm is not a threat to any land areas, and in their 8 am EDT Tropical Weather Outlook on Sunday, NHC gave 99L a 10% chance of developing into a tropical cyclone by Tuesday morning.

Isaac declared non-tropical
Hurricane Isaac's remnants are no longer considered an organized tropical system, but ex-Isaac will bring 1 - 3" of rain to Indiana, Kentucky, and Tennessee today. Isaac's remnants may bring a few tornadoes to Tennessee, Alabama, and Mississippi today, and NOAA's Storm Prediction Center has placed portions of these states in their "Slight Risk" area for severe weather. As of late Saturday night, Isaac's preliminary tornado total was 39, putting it in a tie for 12th place for most tornadoes spawned by a tropical system, according to Dr. Greg Forbes of the Weather Channel. The record holder is Hurricane Ivan of 2004, with 127 tornadoes. Isaac brought storm surges as high as 13.6' to the coast (in Lake Borgne, LA), and dumped 20" of rain at one station in New Orleans. Isaac is being blamed for at least seven deaths in the U.S., 24 in Haiti, and five in the Dominican Republic.

A few notable rainfall totals from Isaac, through Saturday:

20.08" New Orleans, LA
17.63 Wilmer, AL
17.04" Kiln, MS
16.60" Vero Beach, FL
16.29" Royal Palm Beach, FL
15.02" Marion, MS
13.99" Pascagoula, MS
13.94" Baptist, LA
13.45" Hattiesburg, MS
10.85" Gulfport, MS
10.40" Slidell, LA
10.17" Biloxi, MS
9.85" Mobile, AL
9.08" Mount Pleasant, SC
8.64" West Palm Beach, FL
8.39" Pine Bluff, AR
7.36" Charleston, SC (2.8 mi NE)
5.95" Baton Rouge, LA
5.63" Greenville, IL


Figure 2. Rainfall total for Hurricane Isaac, ending at 8 am EDT Sunday September 2, 2012. Over ten inches of rain fell in portions of Louisiana, Mississippi, Alabama, Florida, and Arkansas. Image credit: NOAA/AHPS.

I'll have a short post on Monday.

Jeff Masters