TROPICAL STORM ROSLYN DISCUSSION NUMBER 4
NWS NATIONAL HURRICANE CENTER MIAMI FL EP182016
200 AM PDT MON SEP 26 2016
The sheared cloud pattern in infrared satellite imagery has changed
little since the previous advisory. However, a 0304Z SSMI/S pass
indicated the low-level structure had improved markedly, with a
sharp hooked band wrapping more than half around the now
well-defined low-level circulation center. The initial intensity has
been increased to 35 kt based on a Dvorak classification of T2.5/35
kt from TAFB and the improved low-level structure indicated in
SSMI/S 37 GHZ data. This makes Roslyn the seventeenth named storm of
the 2016 eastern North Pacific hurricane season.
The initial motion estimate is 020/4 kt. The advisory position had
to be adjusted more than 30 nmi to the east of the previous advisory
track based on recent SSMI/S and AMSU microwave center fixes.
Otherwise, the previous track forecast rationale remains unchanged.
A large mid- to upper-level low currently located over the central
Baja California peninsula is expected to continue moving westward to
west-southwestward during the next day or two. The combined
southwesterly flow between that low and a deep-layer ridge located
to the east and southeast of Roslyn should keep the tropical cyclone
moving slowly north-northeastward to northeastward for the next 48
hours. After that, Roslyn is expected to weaken into a shallow
remnant low, turning northward on day 3 and then northwestward on
day 4. The new NHC track forecast is to the right of the previous
advisory track, mainly due to the more eastward initial position,
and lies close to the various consensus models.
Little if any strengthening is expected due to gradually increasing
southwesterly vertical wind shear and a very dry mid-level moisture
regime characterized by humidity values less than 40 percent. By 36
to 48 hours, the vertical shear is forecast to increase to more than
30 kt, which is expected to erode the deep convection and induce
steady weakening. As a result, Roslyn is forecast to become a
remnant low by 48 hours, but this could occur sooner. The GFS and
ECMWF models show the remnant low dissipating by the end of the
forecast period when Roslyn will be moving over sub-24C SSTs. The
new intensity forecast is identical to the previous advisory and
closely follows the IVCN intensity consensus model.
TROPICAL DEPRESSION EIGHTEEN-E SPECIAL DISCUSSION NUMBER 1
NWS NATIONAL HURRICANE CENTER MIAMI FL EP182016
1000 AM PDT SUN SEP 25 2016
Deep convection associated with the low pressure area located well
southwest of the Baja California has become better organized
overnight, and early morning visible satellite imagery indicate
that the circulation has become better defined. Based on these
observations, and Dvorak classifications of T2.0 and T1.5 from
TAFB and SAB respectively, advisories are being initiated on a
30-kt tropical depression at this time. The depression is currently
located over warm water and within an area of low vertical wind
shear, which is favorable for some gradual strengthening today.
However, in about 24 hours, the tropical cyclone is forecast to move
into an area of higher southwesterly shear caused by large mid- to
upper-level low and associated trough over northwestern Mexico. The
global models suggest that the shear will become quite strong in 36
to 48 hours as the cyclone begins moving over cooler waters. This
should result in a rapid spin down, and the system is likely to
become a post-tropical remnant low within 3 days. The official
intensity foreast is slightly above the SHIPS guidance but is in
good agreement with the intensity consensus.
Since the depression is still in its formative stage, the initial
motion is a somewhat uncertain 360/4 kt. The cyclone is expected to
be steered slowly northward or north-northeastward during the next
couple of days around the western portion of a weak mid-level ridge
that extends southwestward from southern Mexico. By 72 hours, the
low should turn northwestward in the low-level steering flow after
it weakens and becomes a vertically shallow system. Although the
ECMWF and GFS models are on opposite sides of the guidance envelope,
the track models are in general agreement on the overall scenario
and the NHC forecast lies near the multi-model consensus.
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