Post-Tropical Cyclone Sam

Post-Tropical Cyclone Sam Summary

Sam's low-level circulation center has become exposed on the south
side of an east-to-west oriented cloud band containing cloud tops
that have warmed to around -50 deg C. The cyclone's overall cloud
pattern in satellite imagery resembles that of an occluded
extratropical low pressure system, and surface analyses from the
NOAA Ocean Prediction Center indicate that Sam's inner-core is now
interacting with a frontal system. Furthermore, the system is
located over sea-surface temperatures of around 14 deg C. Although
the system likely still has a warm-core thermal structure in the
mid- and upper-levels of the troposphere based on earlier 04/2321Z
AMSU-A/-B data, the low-level circulation now is comprised of an
extensive field of cold-air stratocumulus clouds. Based on these
data, Sam has made the transition to an extratropical cyclone over
the far north Atlantic. The initial wind speed has been lowered to
70 kt based on a standard decay rate.

The initial motion estimate is northward or 010/20 kt.
Extratropical-Sam has been moving north-northeastward at 35 kt
since the previous advisory, but the latest global and regional
models are all in very good agreement that the system will slow
down significantly during the next 6-12 hours as it interacts with
an approaching baroclinic mid- to upper-level trough/low. By late
Tuesday, the post-tropical cyclone should turn east-northeastward,
and then make a counter-clockwise loop southwest of Iceland on
Wednesday. By Thursday, the large cyclone is expected to move
east-northeastward or eastward and pass very near the southern coast
of Iceland on days 4 and 5 as a weakening extratropical low pressure
system. The new NHC track forecast is very similar to the previous
advisory track, and lies close to the tightly packed simple- and
corrected-consensus track models.

Extratropical-Sam is expected to maintain hurricane-force winds for
the next 12-18 hours, followed by a slow weakening trend as the
cyclone loses its baroclinic forcing. However, Post-Tropical-Sam
is still expected to pass close to Iceland in the 96-120-h period as
an expansive low pressure system producing gale-force winds over a
very large area.

This is the last advisory issued by the National Hurricane Center on
Sam. Additional information on this system can be found in High
Seas Forecasts issued by the National Weather Service, under AWIPS
header NFDHSFAT1, WMO header FZNT01 KWBC, and online at
ocean.weather.gov/shtml/NFDHSFAT1.php.

Key Messages:

1. Swells generated by Sam will impact the northern Leeward
Islands, the Greater Antilles, portions of the Bahamas, and
southeastern Newfoundland through midweek. These swells
could cause life-threatening surf and rip current conditions.
Please consult products from your local weather office.

Hurricane and Tropical Weather Links

National Hurricane Center
Weather Underground
NOAA GOES Satellite Imagery
South Florida Water Management District