Michael was a short-lived category two hurricane (on the Saffir-Simpson Hurricane Scale) that developed from subtropical origins off the northeast coast of Florida. Michael moved northeastward over the western and northern Atlantic as a tropical cyclone and eventually crossed Newfoundland as a strong extratropical low pressure system.
a. Synoptic History
The precursor low pressure system that eventually became Hurricane Michael developed as a result of an upper-level cold low that migrated southward from the mid-latitudes and interacted with a stationary front over the southeast Bahama Islands. A cold front pushed off the southeast U.S. coast on 7 October and moved slowly southeast for the next couple of days before it became stationary from near Bermuda to central Cuba on 10 October. High amplitude mid-latitude flow in the upper-troposphere allowed a cold low to drop southward just off the Florida east coast, which induced the formation of a surface low along the southern end of the front east of the central Bahamas on 12 October. The pressure gradient between the developing surface low and high pressure anchored over the eastern U.S. created an area of gale force winds several hundred miles northwest of the low center. The surface low remained nearly stationary for more than 24 hours before drifting slowly north-northeastward late on the 13th. On 14 October, the low deepened from 1010 mb to 1003 mb and moved north to a position about 800 nautical miles east of Cape Canaveral, FL. After remaining nearly stationary again for almost 24 h, the surface low turned westward and moved underneath the upper-level cold low on 15 October, and then stalled again about 650 nautical miles east of Jacksonville, FL, as a subtropical depression. Later that day, satellite classifications using the Hebert-Poteat technique indicated the pre-Michael low pressure system had strengthened into a subtropical storm. After remaining nearly stationary for an additional 48 h over very warm sea-surface temperatures (> 28° C), thunderstorms developed and persisted near the low-level center, which allowed the system to gradually acquire tropical characteristics.
Prior to tropical cyclone development, Dvorak satellite intensity estimates from the Tropical Analysis and Forecast Branch (TAFB), the Satellite Analysis Branch (SAB), and the Air Force Weather Agency (AFWA) indicated subtropical cyclone classifications as high as ST 2.5 or 40 mph (from the TAFB). However, by 0000 UTC, 17 October, satellite classifications suggested the low pressure system had acquired enough tropical characteristics to become Tropical Storm Michael. The transition from a subtropical to a tropical system was further supported by an earlier Advanced Microwave Sounder Unit (AMSU) satellite overpass, which indicated a weak warm core aloft and upper-level outflow had developed. Two QuikSCAT overpasses on the 16th also showed the radius of maximum winds had contracted from 150 nautical miles to less than 60 nautical miles between 1029 and 2252 UTC.
Later that day, U. S. Air Force Reserve (USAFR) reconnaissance aircraft indicated that Michael had strengthened based on 1500 ft flight-level wind reports of 69 mph and 990 mb surface pressure at 1717 UTC, and 83 mph and 988 mb pressure at 1906 UTC. Those values correspond to surface wind speed estimates of 59 mph and 70 mph, respectively, using a standard reduction of 0.85 for that altitude. Also, experienced hurricane hunter personnel estimated surface winds of 81 mph. Based on the reconnaissance data, it is estimated that Michael became a hurricane at 1800 UTC, 17 October. By 0615 UTC, 18 October, reconnaissance aircraft noted a 20 nautical miles circular closed eye at 850 mb, and observed a minimum surface pressure of 984 mb and flight-level winds of 84 mph. There were slight fluctuations in the flight-level winds and surface pressures for the next 36 hours until rapidly deepening occurred on the 19th. As Michael began to interact with the approaching strong mid-tropospheric trough, baroclinic effects may have played a role in Michael's 21 mb pressure drop from 986 mb at 1200 UTC to 965 mb at 1800 UTC. During the rapid deepening phase, the maximum flight-level wind and minimum pressure observed by reconnaissance aircraft was 109 mph (1500 ft) at 1829 UTC, from the NOAA Aircraft Operations Center (NAOC), and 979 mb from the USFAR, respectively, on 18 October.
Reconnaissance flights into Michael ceased after 1957 UTC on 19 October, and subsequent surface pressure and wind reports were obtained from ships, buoys, and land stations. While Michael was a tropical cyclone, the lowest pressure and maximum surface wind observed were 965.5 mb and 92 mph, respectively, which came from ship 3EHR6 (MSC Xingang) near the eastern eyewall at 1700 UTC, 19 October. By 2100 UTC, 19 October, a sharp shortwave trough embedded in strong southwesterly mid-level flow accelerated Michael northeastward toward a surface cold front. Surface data reports indicate merger with the front occurred about 100 nautical miles southwest of Harbour Breton, Newfoundland. The increasing vertical shear also likely played a role in the extratropical transition.
During landfall along the south coast of Newfoundland as an
extratropical system, maximum sustained winds of 79 mph with gusts to
107 mph and a minimum pressure of 967.7 mb were reported near Sagona Island.
A few hours prior to landfall, a Canadian reconnaissance aircraft observed
an unofficial wind speed of 156 mph at the top of the boundary layer.
However, the cool (4o to 10o C) and very stable
boundary layer likely prevented those strong winds from mixing down to
the surface. Radar imagery at 2145 UTC, 19 October, from Holyrood, Newfoundland,
indicated a well-defined eye at 1.5 km AGL. However, surface observations
and visible satellite imagery indicated that the position of the radar
eye was located about 75 nautical miles northeast of the surface position.
Decoupling of the mid- and upper-level circulation from the low-level
center further indicates that Michael was rapidly becoming extratropical.
b. Meteorological Statistics
Observations in Michael include satellite-based Dvorak technique intensity estimates from TAFB, SAB, and AFWA. In addition, flight-level observations are available from flights of the USAFR 53rd Weather Reconnaissance Squadron and the NAOC.
During and after landfall as an extratropical low pressure
system, the Environment Canada Weather Service reported surface winds
in excess of 58 mph (mainly along and east of the storm track) and pressures
below 975 mb over nearly all of Newfoundland between 2100 UTC, 19 October,
and 0600 UTC, 20 October. However, the Environment Canada Weather Service
did not considered it particularly unusual since "...those communities
are no strangers to such winds."
c. Casualty and Damage Statistic
There were no reports of injuries or deaths as Michael traversed Newfoundland as an extratropical low pressure system. Tree and structural damage on Newfoundland was reported as being light. Damage was confined mainly to home roofs and siding, and trees being downed.
Intensity For Hurricane Michael