Hurricane Lorenzo Forecast Discussion Number 30

By | September 29, 2019

Issued at 500 PM AST Sun Sep 29 2019

WTNT43 KNHC 292045

Hurricane Lorenzo Discussion Number  30
NWS National Hurricane Center Miami FL       AL132019
500 PM AST Sun Sep 29 2019

The cloud pattern has continued to degrade throughout the day with
cold cloud tops filling in the eye, with a gradual erosion of the
convection over the southwest quadrant. A recent AMSR2 pass confirms
that the eyewall is indeed open on the western side, and suggests
there may be another eyewall replacement cycle occurring. The NOAA
Hurricane Hunters are currently performing multiple experimental
flights inside and around Lorenzo, and are providing NHC with
valuable data. The wind data from the SFMR are indicating that the
hurricane-force winds extend outward farther than previously
thought, about 60-70 n mi from the center in all quadrants. The
aircraft also reported an adjusted minimum central pressure of 948
mb, a peak flight level wind of 99 kt, and an image capture from an
on-board radar that confirmed the break in the eyewall. The
available objective and subjective intensity estimates range from
102 kt to 114 kt. Based on a compromise of these data, the initial
intensity has been set at 100 kt, but this may be a little generous
given much lower dropsonde wind speeds measured by the aircraft.

Lorenzo continues north-northeastward at 9 kt through a weakness in
the subtropical ridge. A mid-latitude trough will approach Lorenzo
from the west in a couple of days. Increasing southwesterly flow
ahead of the trough will force the hurricane to turn northeastward
and accelerate beginning on Monday. The forecast track confidence
remains very high through 48 hours, and the new forecast is
essentially an update of the previous one. By 72 hours, the guidance
continues to diverge significantly, likely due to how they are
handling the interaction between Lorenzo and a larger extratropical
cyclone to its north late in the forecast period. The ECMWF/UKMET do
not have Lorenzo being absorbed into the larger low. Instead, they
track Lorenzo to the east-northeast toward Europe ahead of the
associated upper trough. The rest of the guidance has Lorenzo
absorbed into the low several hundred miles west of the British
Isles. The forecast track confidence remains low beyond 72 hours,
and was nudged just a little to the left from the previous advisory.

The aircraft near-surface dropsonde data suggests that Lorenzo is
moving over cooler waters of 24-25 degrees C, a few degrees cooler
than model guidance indicates, due to the upwelling of the waters
with lower oceanic heat content.  The environmental conditions
for the next 48 hours are characterized by moderate southwesterly
shear, decent moisture, and strong upper-level divergence. Due to
the presence of the shear and cooler waters, gradual weakening is
anticipated for the next couple of days. By 72 hours, much stronger
shear and the interaction with the approaching mid-latitude trough
should cause the cyclone to weaken at a faster rate and begin to
transition to an extratropical cyclone. By 96 hours or so,
extratropical transition is expected to be complete. The official
intensity forecast was adjusted lower than the previous one
through 48 hours, and then is similar thereafter. This solution is
near the various multimodel consensus aids.

Key Messages:

1.  Lorenzo is a large and powerful hurricane, and its hurricane-
and tropical-storm-force wind fields are expected to expand further
during the next several days.  Regardless of Lorenzo's exact track
near the Azores, strong winds are becoming increasingly likely on
those islands Tuesday night and Wednesday, and residents there
should monitor the progress of the hurricane. Watches could be
issued for those islands later today or tonight.

2.  Large swells generated by Lorenzo will continue to spread across
much of the north Atlantic basin during the next few days.  These
swells will produce life-threatening surf and rip currents.


INIT  29/2100Z 26.9N  44.2W  100 KT 115 MPH
 12H  30/0600Z 28.2N  43.6W   90 KT 105 MPH
 24H  30/1800Z 30.1N  42.6W   85 KT 100 MPH
 36H  01/0600Z 32.4N  40.6W   85 KT 100 MPH
 48H  01/1800Z 35.6N  37.0W   80 KT  90 MPH
 72H  02/1800Z 44.6N  26.3W   70 KT  80 MPH
 96H  03/1800Z 52.2N  17.2W   55 KT  65 MPH...POST-TROP/EXTRATROP
120H  04/1800Z 55.0N  15.9W   45 KT  50 MPH...POST-TROP/EXTRATROP

Forecaster Latto

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