Tropical Storm Humberto Forecast Discussion

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WTNT44 KNHC 152051
TCDAT4

Tropical Storm Humberto Discussion Number  13
NWS National Hurricane Center Miami FL       AL092019
500 PM EDT Sun Sep 15 2019

Humberto's convective structure continues to improve in satellite
imagery and reflectivity data from the Melbourne NOAA Doppler
weather radar, including intermittent appearances of a closed eye.
An Air Force Reserve reconnaissance aircraft investigating Humberto
this afternoon found SFMR winds of 58-59 kt in the northeastern
quadrant outside of the heaviest rain areas, and that the pressure
had fallen 4 mb during the past 2 hours, now down to 989 mb.  Based
on these data, the intensity has been increased to 60 kt.

The initial motion estimate is now 360/05 kt. There are no
significant changes to the previous forecast track or reasoning.
Humberto is likely located on or near an east-west oriented ridge
axis, and a sharp turn toward the northeast is likely during the
next 6-12 hours. The latest NHC model guidance remains highly
convergent on the previous forecast track and continues to show the
cyclone gradually being accelerated toward the northeast and
east-northeast during the 24-120 hour period. The large deep-layer
trough and associated cold front that will be digging southward out
of eastern Canada and the northeastern United States is forecast by
the ECMWF model to develop a cutoff low over the northwestern
Atlantic that results in some binary interaction with Humberto on
day 5. However, this is a new development and is considered to be an
outlier scenario at this time. The new forecast track is similar to
the previous advisory and lies close to the TVCN and HCCA consensus
track models.

Likewise, there is no significant change to previous intensity
forecast. All of the available model guidance continues to support
steady strengthening for the 3 days. In 48-72 hours, Humberto is
forecast to be moving into the right-rear quadrant of a strong,
anticyclonically curved 300-200 mb jet maximum. The associated
strong baroclinic/dynamical forcing is expected to produce strong
pressure falls and strengthening despite the hostile vertical wind
shear conditions of at least 30 kt. On days 4 and 5, the combination
of slightly cooler waters, drier and more stable air, and much
stronger vertical shear of 40-50 kt should cause steady weakening
despite the favorable jetstream dynamics. The previous NHC intensity
forecast remains unchanged, and continues to show Humberto reaching
its peak intensity in 72 hours, coincident with the warmest
sea-surface temperatures and strongest jetstream dynamical forcing.

Large swells from Humberto will affect portions of the northwestern
Bahamas and southeastern United States coast during the next few
days.  These swells are expected to produce dangerous surf
conditions and life-threatening rip currents.  See products issued
by your local weather office for additional information.

FORECAST POSITIONS AND MAX WINDS

INIT  15/2100Z 29.3N  78.0W   60 KT  70 MPH
 12H  16/0600Z 29.8N  77.6W   65 KT  75 MPH
 24H  16/1800Z 30.1N  76.7W   70 KT  80 MPH
 36H  17/0600Z 30.4N  75.4W   80 KT  90 MPH
 48H  17/1800Z 30.7N  73.6W   85 KT 100 MPH
 72H  18/1800Z 31.7N  68.9W   95 KT 110 MPH
 96H  19/1800Z 35.0N  62.0W   85 KT 100 MPH
120H  20/1800Z 39.6N  56.1W   60 KT  70 MPH

$$
Forecaster Stewart



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