827
WTNT42 KNHC 130855
TCDAT2
Tropical Storm Lorenzo Discussion Number 1
NWS National Hurricane Center Miami FL AL122025
500 AM AST Mon Oct 13 2025
The area of low pressure we have been monitoring over the last day
or two (AL97) has become better organized this morning. After
spending most of yesterday as an exposed low-level swirl, tonight a
large burst of deep convection with cloud top temperatures below
-80C has formed near and just east of the center. An earlier ASCAT-C
pass clipped the eastern side of the system, showing that it was
already producing tropical-storm-force winds, with a peak derived
wind of 36 kt. A pair of AMSR2 and GMI microwave passes at 04-06 UTC
near the system also showed the improved structural organization
under the cirrus. The subjective 06 UTC Dvorak fix from TAFB was
T2.5/35 kt, and the satellite presentation has only grown more
impressive since that time, with the convective cloud tops taking a
distinct comma shape pattern. Thus, advisories are being initiated
on Tropical Storm Lorenzo this morning, with an initial intensity
of 40 kt, assuming a little undersampling from the earlier
scatterometer wind data.
The initial motion is a somewhat uncertain 305/15 kt, a little
slower than the center being tracked earlier, potentially due to the
large convective burst tugging at the center. This general
northwestward motion with some additional slowdown is anticipated
today as Lorenzo approaches a weakness in the subtropical ridging
produced by an upper-level trough in the path of the tropical
cyclone. This trough is expected to fracture and shift to the
southwest of the system, and Lorenzo should turn northward between
this feature and the eroded mid-level ridge centered over the Cabo
Verde Islands. Ultimately, this ridge should result in the cyclone
recurving to the northeast around it, as it comes under influence of broad-scale mid-latitude westerly flow. The initial NHC track
forecast is in pretty good agreement with a fairly tightly clustered
track guidance envelope, and is roughly a blend of the HCCA and
Google DeepMind guidance this cycle, a little east of the overall
track envelope.
While Lorenzo's structure on satellite imagery has improved, it is
still currently experiencing about 25-30 kt of southwesterly
vertical wind shear. However, both the GFS and ECMWF-based SHIPS
guidance suggests this shear will soon lessen to less than 10 kts in
24-48 hours. Ordinarily this would lead to strengthening given the
sufficently warm 27-28 C ocean temperatures, but at the same time, environmental mid-level relatively humidity is also expected to dry substantially as the storm interacts with the upper-level trough
ahead of it. This trough could also still produce some residual
mid-level shear undercutting the outflow layer. The overall guidance
is not that enthusiastic about Lorenzo's intensity prospects, but it
is worth noting there is quite a bit of spread in both the ECMWF and
Google DeepMind ensemble members, ranging from a very weak tropical
cyclone to a category 2 hurricane. The initial NHC intensity
forecast will try to split the difference, showing little change in
strength in the short-term while the shear remains high, and then
just gradual intensification afterwards. This is higher than the
majority of the hurricane-regional model guidance this cycle, but
under the Google DeepMind ensemble mean and HWRF model.
FORECAST POSITIONS AND MAX WINDS
INIT 13/0900Z 14.2N 40.3W 40 KT 45 MPH
12H 13/1800Z 15.0N 41.7W 40 KT 45 MPH
24H 14/0600Z 16.2N 43.4W 40 KT 45 MPH
36H 14/1800Z 18.0N 44.4W 45 KT 50 MPH
48H 15/0600Z 20.1N 44.7W 50 KT 60 MPH
60H 15/1800Z 22.4N 44.2W 55 KT 65 MPH
72H 16/0600Z 25.1N 42.6W 60 KT 70 MPH
96H 17/0600Z 30.9N 36.9W 55 KT 65 MPH
120H 18/0600Z 33.2N 29.6W 45 KT 50 MPH
$$
Forecaster Papin
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