Tropical Depression Fifteen Discussion Number 1
NWS National Hurricane Center Miami FL AL152019
500 PM AST Mon Oct 14 2019
The large low pressure system that moved off the coast of Africa
yesterday has gradually become better defined based on earlier ASCAT
scatterometer wind data and recent visible satellite imagery.
Several fragmented curved bands have been developing during the past
several hours, especially in the northern semicircle, and the
aforementioned ASCAT passes indicated that winds of 30-32 kt were
present northwest of the center. The initial intensity is set at 30
kt based on a blend of the scatterometer wind data and a satellite
intensity estimate of T1.5/25 kt from SAB, and the rather broad
nature of the depression.
A surge of mid-level dry air has recently penetrated into the
inner-core region, causing some erosion of the central deep
convection. However, this is expected to be a temporary condition
with convection returning later tonight and early Tuesday during the
normal nocturnal convective maximum period. However, the large size
of the cyclone -- more than 1000 nmi wide -- should prevent any
rapid or significant intensification. With very low vertical shear
conditions forecast by the GFS and ECMWF models to continue for the
next 24 to 36 hours, at least some modest strengthening seems likely
given the unusually warm SSTs of near 28.5 deg C that the cyclone
will be traversing during that time. By 48 hours, the shear is
forecast to increase from the southwest in excess of 25 kt,
resulting in weakening into a remnant low shortly thereafter. By 72
hours and beyond, the shear is expected to increase to more than 40
kt, which will cause rapid weakening and dissipation by the 120-h
period, if not sooner. The official intensity forecast is similar to
a blend of the NOAA-HCCA and IVCN consensus models, and the GFS-
and ECMWF-based Decay-SHIPS statistical intensity models.
The initial motion estimate is northwestward or 310/07 kt. The
latest NHC model guidance is in very good agreement on the tropical
cyclone moving generally northwestward around the southwestern
periphery of a deep-layer ridge for the next 48 hours. Thereafter,
the models diverge significantly based on how soon the cyclone
weakens and turns westward within the low-level easterly trade wind
flow. The ECMWF holds on to a vertically deeper system a little
longer than the GFS, UKMET, HWRF, and HMON models, resulting in a
longer northwestward track. However, since the cyclone will likely
have become a vertically shallow remnant low by 72 hours, the NHC
official forecast track is a little to the left of ECMWF solution,
closer to the TVCN and HCCA consensus model tracks at 72 and 96 h.
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