Tropical Cyclone Discussion
741 WTPA44 PHFO 080845 TCDCP4 Tropical Storm Pena Discussion Number 7 NWS Central Pacific Hurricane Center Honolulu HI CP042026 Issued by NWS National Hurricane Center Miami FL 1100 PM HST Sat Nov 07 2026 This evening, Pena's vertical coherence appears to be unraveling. On GOES-18 satellite, its structure is looking increasingly fragile, with the earlier deep convection weakening near the center, with a more recent burst primarily east of the center. A 04 UTC WSFM microwave pass suggested that Pena has become more vertically tilted, with the low-level cloud features on the 37-GHz channel indicating the surface circulation running out ahead of the mid-level vortex seen on the 89-GHz channel. This was recently confirmed by a 0644 UTC ASCAT-B pass, which showed an elongated low-level circulation centered east of the deepest convection. The peak wind retrieval from the instrument was only 31 kt, though an ASCAT-C pass just received had some higher winds. The 06 UTC subjective Dvorak estimates from PHFO and PGTW still supported 45-55 kt, but objective estimates from UW-CIMSS have decreased to 36-43 kt. Given the rather anemic scatterometer data, the initial intensity is lowered to 40 kt this advisory. The tropical storm's westward motion has slowed a bit over the past 6 h, with an estimated motion of 270 degrees at 6 kt. A zonally oriented low- to mid-level ridge stretches across the Central Pacific to the north of Pena. This feature should result in the tropical storm maintaining a general westward motion for the next 2-4 days. However, there is also a prominent low-level westerly-wind burst occurring south of Pena resulting in an enhanced monsoonal circulation. This feature, combined with a strengthening low-level ridge could allow Pena to lose some latitude in 24-60 h. Towards the end of the forecast period, the ridge begins to shift eastward, which could provide an opportunity for Pena to gain more latitude by the end of the forecast, assuming the tropical cyclone is still around. The track guidance this cycle is quite similar to the prior one for the first 24-36 h, though the better performing track aids (HCCA, GDMI, WN3I) now show a more southward track in 36-72 h. Thus, the updated NHC track forecast was nudged further south during this period once again, blending these more reliable track aids with the previous forecast. Pena's structural degradation this evening makes its intensity forecast rather tricky. Environmental conditions appear favorable at first glance, with warm 29 C sea-surface temperatures expected along the storm's track, and deep-layer shear remaining under 10 kt for the bulk of the SHIPS guidance forecast. However, the increasing vertical tilt of Pena's vortex appears to be preventing it from taking advantage of these favorable environmental conditions, and this evening's scatterometer data indicates Pena has weakened from earlier today. Another factor worth considering is the monsoonal circulation south of Pena, which is producing a lot of convective outflow in the path of the tropical storm. Pena is also a rather small tropical cyclone, and is likely more sensitive than usual to less than pristine environmental conditions. The intensity guidance this evening is lower than earlier, with all of the hurricane regional models now showing Pena remaining either a weak tropical storm or degenerating into a trough of low pressure. The updated NHC intensity forecast now shows a little short-term weakening, followed by gradual intensification beginning in 36 h. This assumes Pena survives, and is able to eventually respond to the more favorable environmental conditions. This forecast now leans more on a blend of the hurricane-regional model guidance and AI-based guidance, with less weight on the statistical-dynamical aids. It is also possible Pena dissipates as a tropical cyclone sometime during this forecast period, as suggested by the 00 UTC HAFS runs. Needless to say, this is a low confidence intensity forecast. FORECAST POSITIONS AND MAX WINDS INIT 08/0900Z 11.5N 155.0W 40 KT 45 MPH 12H 08/1800Z 11.5N 156.3W 35 KT 40 MPH 24H 09/0600Z 11.5N 158.1W 35 KT 40 MPH 36H 09/1800Z 11.4N 160.3W 40 KT 45 MPH 48H 10/0600Z 11.3N 162.5W 45 KT 50 MPH 60H 10/1800Z 11.1N 164.4W 45 KT 50 MPH 72H 11/0600Z 10.9N 166.4W 50 KT 60 MPH 96H 12/0600Z 11.0N 172.0W 50 KT 60 MPH 120H 13/0600Z 12.5N 177.4W 50 KT 60 MPH