TS Dorian Making a Comeback?

After a long trek over the Atlantic Ocean from the coast of Africa, the remains of Tropical Storm Dorian (now called Invest 91L) have finally arrived at the shores of North America. Ex-Dorian is nearly stationary, and is situated over the Northwestern Bahama Islands, just off the coast of Southeast Florida. Satellite loops and Melbourne, Florida radar images show that ex-Dorian has only a limited amount of heavy thunderstorms, which are not well-organized. There does appear to be a surface circulation center trying to form just north of the storm’s heaviest thunderstorms, about 70 miles east of Vero Beach, Florida. However, dry air to the northwest, as seen on water vapor satellite loops, is inhibiting development. WInd shear is moderate, 10 – 20 knots, but is expected to rise to the high range, 20 – 30 knots, by Saturday morning. Ex-Dorian is expected to move slowly northwards and then north-northeastwards on Saturday. This motion will get ex-Dorian tangled up with a cold front that extends from Northern Florida northeastwards, just offshore from the Southeast U.S. coast. Before it merges with the front, ex-Dorian has some potential for regeneration into a tropical depression, and in their 8 am Friday Tropical Weather Outlook, NHC gave ex-Dorian a 30% chance of becoming a tropical cyclone by Sunday. Ex-Dorian will likely bring heavy rains to the Northwest Bahamas on Friday, and these heavy rains may also clip the coast of Southeast Florida. However, the bulk of ex-Dorian’s rains should stay offshore.

Figure 1. Morning radar image of ex-Dorian from the Miami radar.

from:    http://www.wunderground.com/blog/JeffMasters/show.html

TS Dorian, TS Flossie, European Heat Wave,

In the Central Pacific, Tropical Storm Flossie, a strong tropical storm with 65 mph winds, is headed west at 20 mph towards Hawaii. Satellite images show that Flossie is maintaining a modest area of heavy thunderstorms that are well-organized. The storm is over waters of 25°C, which is about 1°C below the water temperature typically needed to sustain a tropical storm. Flossie peaked in intensity Saturday morning, when the storm had 70 mph winds. As Flossie approaches the Big Island of Hawaii on Monday, these waters will warm to 26°C, but wind shear is expected to be in the moderate range, which should keep Flossie from strengthening. Dry air aloft will likely cause some weakening before landfall Monday morning, and Flossie will likely have top winds of 45 – 55 mph when is passes through the Hawaiian Islands. Flossie’s main threat will be heavy rains, with 6 – 10″ expected over The Big Island and Maui County, and 4 – 8″ in Oahu. Rains of this magnitude are capable of causing dangerous flash flooding and mudslides. Sunday’s 11 am EDT wind probability forecast from the Central Pacific Hurricane Center gave Hilo on the Big Island a 33% chance of experiencing sustained tropical storm force winds of 39 mph or greater from Flossie. These odds were 32% for Honolulu and 41% for Kahului.

Figure 1. MODIS satellite image of Tropical Storm Flossie taken at approximately 5 pm EDT Saturday July 27, 2013. At the time, Flossie had top winds near 50 mph. Image credit: NASA.

Tropical storms are uncommon in Hawaii
On average, between four and five tropical cyclones are observed in the Central Pacific every year. This number has ranged from zero, most recently as 1979, to as many as eleven in 1992 and 1994. August is the peak month, followed by July, then September. Tropical storms and hurricanes are uncommon in the Hawaiian Islands. Only eight named storms have impacted Hawaii in the 34 year period 1979–2012, an average of one storm every four years. Since 1949, the Hawaiian Islands received a direct hit from just two hurricanes–Dot in 1959, and Iniki in 1992. Both hit the island of Kauai. Only one tropical storm has hit the islands since 1949–an unnamed 1958 storm that hit the Big Island. A brief summary of the three most significant hurricanes to affect Hawaii in modern times:

September 1992: Hurricane Iniki was the strongest, deadliest, and most damaging hurricane to affect Hawaii since records began. It hit the island of Kauai as a Category 4 on September 11, killing six and causing $2 billion in damage.

November 1982: Hurricane Iwa was one of Hawaii’s most damaging hurricanes. Although it was only a Category 1 storm, it passed just miles west of Kauai, moving at a speed of nearly 50 miles per hour (80 km/h). Iwa killed one person and did $250 million in damage, making it the second most damaging hurricane to ever hit Hawaii. All the islands reported some surf damage along their southwest facing shores, and wind damage was widespread on Kauai.

August 1959: Hurricane Dot entered the Central Pacific as a Category 4 hurricane just south of Hawaii, but weakened to a Category 1 storm before making landfall on Kauai. Dot brought sustained winds of 81 mph with gusts to 103 mph to Kilauea Light. Damage was in excess of $6 million. No Dot-related deaths were recorded.

Figure 2. Tracks of all tropical storms and hurricanes to pass within 100 miles of the Hawaiian Islands, 1949 – 2012. During that time span, the Hawaiian Islands received a direct hit from just two hurricanes–Dot in 1959, and Iniki in 1992. Both hit the island of Kauai. One tropical storm also hit, and unnamed 1958 storm that hit the Big Island of Hawaii. Image credit: NOAA/CSC.

Remains of Dorian are worth watching
The remains of Tropical Storm Dorian will be passing just north of northern Lesser Antilles Islands today and just north of Puerto Rico tonight. Satellite images show no signs of a surface circulation, and just a moderate area of heavy thunderstorms associated with the storm. AIr Force hurricane hunter aircraft are on call to investigate Dorian’s remains on Sunday afternoon and Monday afternoon, if necessary. In their 8 am EDT Tropical Weather Outlook, NHC gave Dorian’s remains a 20% chance of regenerating by Tuesday.

Extreme heat wave in Europe
An extreme heat wave is baking Europe today, and at least five countries have a chance at setting a new all-time national heat record. The most likely candidate is Liechtenstein, where the forecast for Balzers Sunday calls for a high of 95°F. According to wunderground’s International Records database maintained by our weather historian, Christopher C. Burt, the current all-time heat record for Liechtenstein is 36°C (96.8°F) set at Vaduz on August 13, 2003.

from:    http://www.wunderground.com/blog/JeffMasters/show.html

Tropical Storm Dorian Forms

The season’s fourth named storm, Tropical Storm Dorian, is here. Born from a strong tropical wave that moved off the coast of Africa on Monday, Dorian formed unusually far east for so early in the season, at longitude 29.9°W. Only Hurricane Bertha of 2008, which became a tropical storm at 22.9°W longitude on July 3, formed farther to the east so early in the year. Satellite images show that Dorian is a small but well-organized system with a moderate amount of heavy thunderstorms. A large area of dry air lies to Dorian’s west, as seen on water vapor satellite images, but Dorian has moistened its environment enough that this dry air should not interfere with development for the next day. Dorian is under a low 5 – 10 knots of wind shear, which will tend to allow slow development. Ocean temperatures are barely adequate for maintaining strength of a tropical storm, about 26.5°C.

Figure 1. MODIS satellite image of Tropical Storm Dorian taken at approximately 8 am EDT July 24, 2013. At the time, Dorian had top winds near 50 mph. Image credit: NASA.

Forecast for Dorian
The SHIPS model predicts that wind shear will stay in the low range through Thursday, then rise to the moderate range Friday through Monday. Ocean temperatures will fall to 25 – 26°C Wednesday night through Thursday night, which may induce some weakening of Dorian. Thereafter, ocean temperatures will rise again, but wind shear will rise. This increase in wind shear will be capable of causing weakening, since there will still be a large area of dry air to Dorian’s west that the shear may be able to bring into Dorian’s core. Given its small size, Dorian is capable of relatively large changes in intensity in a short amount of time, and it would not surprise me if the storm dissipated by the end of the week–or became a Category 1 hurricane. However, the official NHC forecast of a tropical storm passing just north of the Lesser Antilles on Sunday is the most likely outcome; the 11 am wind probability forecast from NHC gave Dorian a 6% chance of being a hurricane at that time. Dorian should maintain a west-northwest track through the week, and spread heavy rains and gusty winds to the northern Lesser Antilles Islands beginning on Sunday. The usually reliable European model (ECMWF) has Dorian passing several hundred miles to the north of the Lesser Antilles Islands, while the other models show Dorian passing closer, within 100 miles. It currently appears that Dorian will be a potential threat to the Bahama Islands, Bermuda, and the U.S. East Coast next week. There will be a trough of low pressure capable of recurving Dorian out to sea before the storm reaches the Bahamas and U.S., but this trough is currently depicted as being fairly weak, reducing the chances of Dorian missing the Bahamas and U.S. East Coast.

Figure 2. Tracks of all Atlantic tropical depressions, tropical storms, and hurricanes (tropical cyclones) occurring in the months of June and July off the coast of Africa. Only Bertha of 2008 became a named storm farther east so early in the year, compared to Tropical Storm Dorian. Reliable satellite records of Eastern Atlantic tropical cyclones go back to 1966. Image credit: NOAA/CSC.

from:    http://www.wunderground.com/blog/JeffMasters/show.html