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Tropical Storm ERIKA


500 AM AST THU AUG 27 2015

Data from a NOAA Hurricane Hunter aircraft indicate that Erika has
strengthened overnight, although it is not clear as to how much. The
central pressure dropped to around 1001 mb around 0540Z, but was
up to 1003 mb on the next fix about an hour later. The highest
850-mb flight-level winds sampled by the aircraft were 48 kt, which
would correspond to an intensity of around 40 kt. However, the
aircraft found SFMR winds of 45 to 55 kt near the center. Based on a
blend of these data, the initial intensity has been conservatively
raised to 45 kt for this advisory. An Air Force Hurricane Hunter
aircraft will be investigating Erika again later this morning.

Aircraft data and satellite imagery show that Erika is still a
sheared tropical cyclone, with the deepest convection found east and
southeast of the center due to about 20 kt of westerly shear. The
models show the environment remaining unfavorable for significant
strengthening in the first 48 hours, with shear associated with an
upper-level trough west of Erika expected to increase to 25 to 30
kt. Given this, most of the intensity guidance shows little change
during the first couple of days and so does the NHC forecast. After
that time, the upper trough weakens and Erika should encounter a
more favorable upper-level pattern and warmer SSTs, which should
support intensification assuming that the cyclone survives the next
48 hours. The HWRF and GFDL are much stronger than the statistical
models this cycle, but the GFS and ECMWF now keep Erika weaker than
they did previously. The NHC forecast has been adjusted upward
slightly late in the period, but is well below the intensity
consensus given the large uncertainty and spread in the guidance.

Aircraft fixes and radar data from Guadeloupe were helpful in
finding the low-level center and determining an initial motion of
280/14. The steering flow from the subtropical ridge to the north
should result in a west-northwestward heading for the next 2 to 3
days. After that time, the spread in the guidance increases as the
cyclone moves between the southwestern edge of the ridge and a
mid/upper-level trough over the southeastern U.S. and Gulf of
Mexico. The models are in poor agreement on the eventual structure
and track of Erika late in the period. The ECMWF is weaker this
cycle and is on the left side of the dynamical model envelope. The
GFS, HWRF, and UKMET are all farther east but have trended west
this cycle. Given the large spread and the continued run-to-run
variability, little change was made to the NHC track forecast. The
new NHC forecast is closest to the latest GFS model prediction at
day 3 and beyond.

One should remember to not focus on the exact forecast track,
especially at the long range where the average NHC track errors
during the past 5 years are about 180 miles at day 4 and 240 miles
at day 5.


INIT  27/0900Z 16.8N  61.5W   45 KT  50 MPH
 12H  27/1800Z 17.6N  63.5W   50 KT  60 MPH
 24H  28/0600Z 19.0N  66.2W   50 KT  60 MPH
 36H  28/1800Z 20.3N  69.0W   50 KT  60 MPH
 48H  29/0600Z 21.5N  71.6W   50 KT  60 MPH
 72H  30/0600Z 24.0N  76.0W   55 KT  65 MPH
 96H  31/0600Z 26.5N  78.5W   65 KT  75 MPH
120H  01/0600Z 29.0N  79.5W   75 KT  85 MPH

Forecaster Brennan