Tropical Storm Rosa Discussion Number 25
NWS National Hurricane Center Miami FL EP202018
200 AM PDT Mon Oct 01 2018
Convection has been steadily decreasing due to the combination
of southwesterly shear of about 30 kt and cool SSTs of less than
24 deg C. As a result, only a small patch of thunderstorms
remains in the northeastern quadrant of Rosa's circulation. ASCAT
passes around 0414Z and 0516Z only indicated wind speeds of 41-42 kt
in the eastern semicircle. Assuming that some undersampling is
occurring, the intensity has only been decreased to 45 kt for this
advisory. Additional weakening is expected due to a further increase
in the vertical wind shear and decreasing ocean temperatures beneath
the cyclone. The official intensity forecast calls for Rosa to be
near 35 kt at the time of landfall later today, but it is possible
that more weakening could occur than currently indicated.
Regardless, strong tropical-storm-force wind gusts will be
widespread across the higher elevations of the northern Baja
California peninsula. Rosa is expected to become remnant low before
the cyclone reaches Arizona due to the rugged terrain of Baja and
northwestern Mexico, and the continued strong wind shear.
Rosa is moving northeastward now, or 035/10 kt. A northeastward to
north-northeastward motion is expected until and after landfall,
with some acceleration beginning later tonight. The latest NHC
model guidance remains tightly clustered, and the official forecast
track lies close to the previous track and an average of the TVCE,
HCCA, and FSSE multi-model consensus aids.
Key Messages:
1. The main hazard expected from Rosa or its remnants is very heavy
rainfall in Baja California, northwestern Sonora, and the U.S.
Desert Southwest. These rains are expected to produce
life-threatening flash flooding and debris flows in the deserts, and
landslides in mountainous terrain. For more information about
potential rainfall in that area, please see products from the
Weather Prediction Center and your local NWS forecast office.
2. Tropical storm conditions are expected over portions of the
central and northern Baja California peninsula later today,
especially over higher elevations. These conditions could spread
into the northern Gulf of California later tonight. Interests in
those locations should monitor the progress of Rosa.
Hurricane Rosa Discussion Number 13
NWS National Hurricane Center Miami FL EP202018
200 AM PDT Fri Sep 28 2018
Although enhanced BD-curve infrared imagery shows that Rosa's 20 n
mi wide eye has continued to warm (+16C) this morning, the southwest
quadrant of the eyewall appears to have weakened, or collapsed, and
the inner ring cloud tops have warmed considerably. This change in
the cloud pattern maybe the early stage of an eyewall replacement
cycle (ERC). It's certainly worth noting, however, that the
satellite presentation about 3 hours ago indicated that Rosa may
have reached a peak intensity of 130-135 kt which was also
indicated in the ADT adjusted raw T-numbers. For this advisory,
the initial intensity is set at 125 kt and is based on a compromise
of the subjective and objective satellite intensity estimates.
Rosa has a small window of opportunity to complete its ERC and
restrengthen before it encounters decreasing oceanic temperatures,
and increasing southwesterly shear. These inhibiting factors, along
with Rosa moving into a more stable air mass, should cause a
downward intensity trend by early next week, and for the cyclone to
weaken to a tropical storm by Tuesday, and a tropical depression as
it quickly moves northeastward over the southwestern U.S. The
intensity forecast is similar to my predecessor's and is close to
the IVCN consensus through 48 hours, then quite similar to the
NOAA-HCCA intensity model beyond that forecast period.
The initial motion is estimated to be westward, or 270/6 kt. A
mid-level ridge over the southwestern United States is forecast to
weaken over the next couple of days in response to an approaching
mid-level shortwave trough from the northwest. This growing
weakness in the ridge should influence Rosa to gradually turn
northward through mid-period, and afterward, turn northeastward
within the strong mid- to upper tropospheric southwesterly flow
produced by the aforementioned trough. The NHC track forecast was
adjusted ever so slightly to the left of the previous one to light
more closely to the TVCN and HCCA multi-model guidance.
Hurricane Rosa Discussion Number 10
NWS National Hurricane Center Miami FL EP202018
800 AM PDT Thu Sep 27 2018
Rosa is becoming better organized. Conventional satellite imagery
show a large area of cold cloud tops near the center, and there is
a hint of an eye in the first-light visible images. Microwave
imagery indicates that the eye structure underneath the overcast
has become better defined, with less evidence of dry air entrainment
than seen yesterday. The initial intensity has been increased to
90 kt in best agreement with the subjective Dvorak estimate from
TAFB and the CIMSS ADT technique. The hurricane currently has good
outflow in all directions.
Conditions appear generally favorable for continued strengthening
for the next 24-36 hr, and the new intensity forecast now makes Rosa
a major hurricane in 12 h. It should be noted that if the current
strengthening is the start of the previously anticipated rapid
intensification Rosa could get stronger than the current forecast,
which shows a peak intensity of 105 kt near the upper edge of the
intensity guidance. After 36 h, the forecast track takes the
cyclone over decreasing sea surface temperatures, and by the end of
the forecast period it is expected to encounter strong southwesterly
vertical shear. This combination should cause significant
weakening, and Rosa is expected to be a tropical storm as it
approaches the Baja California peninsula near the end of the
forecast period.
The initial motion is a somewhat uncertain 270/10. For the next
12-24 h, the hurricane should move generally westward on the south
side of the subtropical ridge over the Pacific west of northern
Mexico and a mid-latitude ridge over California. Subsequently, Rosa
should turn northwestward and northward through a break in the ridge
caused by a large mid-latitude trough moving eastward through the
northeastern Pacific. By 96-120 h, Rosa should recurve
northeastward into the westerlies on the eastern side of the trough
and move in the general direction of northwestern Mexico and the
northern Baja California peninsula. There remains some spread in
the guidance forward speed after recurvature, with the GFS being
faster than the ECMWF. The new intensity forecast is a blend of
these extremes in forward speed at the 96 and 120 h points.
Overall, the guidance envelope has shifted a little west since the
previous advisory, and the new track is also shifted a little
westward.
Hurricane Rosa Discussion Number 6
NWS National Hurricane Center Miami FL EP202018
900 AM MDT Wed Sep 26 2018
Rosa is undergoing rapid intensification. Rosa's inner core has
quickly consolidated, indicated by a 1134 UTC SSMI/S image that
showed a solid mid-level ring with a strong well-developed banding
feature in the south semicircle. The lower frequency image from
the pass indicated a small break in the north portion, probably due
to the drier air and northeasterly shear that had been inhibiting
the cyclone's cloud pattern last night. Based on the conventional
and microwave imagery presentation, the initial intensity is raised
to 65 kt.
Rosa should continue on this fast strengthening trend during the
next 12 hours or so. Afterward, further intensification, at a
slower pace, is expected during the next couple of days. Through
the remaining portion of the forecast, gradual weakening is expected
as a result of decreasing sea surface temperatures, increasing
southwesterly vertical shear, and an invading stable, more drier,
marine layer air mass.
The initial motion is estimated to be west-northwestward, or 290/9
kt, within the easterly mid-tropospheric flow produced by a
ridge to the north of the hurricane. Around mid-period, the
large-scale models show a weakness developing the in the
aforementioned ridge in response to a mid-latitude trough
approaching the southwest U.S. and Baja California peninsula. As a
result of this synoptic pattern change, Rose should move
northwestward Saturday and then north-northwestward Sunday. The
large along and cross-track spread recognized in the guidance 24
hours ago has decreased significantly, increasing the forecast
confidence some beyond 48 hours. The NHC forecast is fairly close
to the previous one, and is based primarily on the multi-model
consensus.
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