Tropical Storm Pena could reach 60 mph by Nov. 12 while passing well south of Hawaii

Tropical Storm Pena was spinning about 530 miles south-southeast of South Point, Hawaii, with maximum sustained winds of 50 mph at 11 a.m. HST on Nov. 7. The NOAA hurricane specialists tracking it expect the storm to keep its distance from the islands, and no coastal watches or warnings were in effect.

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A forecast written only hours later changed the intensity outlook. By 11 p.m. HST on Nov. 7, the center had trimmed Pena’s initial winds to 40 knots, or 45 mph, and the forecast peak that earlier guidance had placed at hurricane strength was pulled back to about 60 mph.

Advisory 5 and the position south of South Point

The public advisory issued at 11 a.m. HST on Nov. 7 placed the center at 11.5 degrees north and 153.9 degrees west, about 1,105 miles east-southeast of Johnston Island. Pena was moving west-northwest at 9 mph with a minimum central pressure of 998 millibars. Forecasters Pierce and Cangialosi wrote that the system remained steady while passing well south of the Hawaiian Islands.

That distance is the reason no watch or warning has been needed. Pena sits at roughly 11.5 degrees north, in the latitude band where Central Pacific storms form and drift westward under the ridge to their north, far below the main islands. Any rain reaching the Big Island, which some models had suggested before the storm formed, would come from the outer edge of its circulation rather than the center.

The Honolulu Star-Advertiser reported on Nov. 3 that the National Weather Service office in Honolulu had said any cyclone from the disturbance had a high chance of passing well south of the islands. It also noted that forecasters held low confidence in a stretch of stronger trade winds over that weekend.

A tilted vortex and a weaker forecast peak

The forecast discussion issued at 11 p.m. HST on Nov. 7 explains the downgrade. Pena’s structure was deteriorating: increased vertical tilt left the surface circulation running ahead of the mid-level vortex, and an ASCAT-B scatterometer pass found a peak wind of only 31 knots, even though satellite Dvorak estimates ran from 45 to 55 knots.

Two things work against a small storm in that setup. Monsoonal outflow from the south is disrupting the circulation, and Pena’s compact size makes it more sensitive to less-than-ideal surroundings. The forecaster, Papin, kept sea-surface temperatures near 29 degrees Celsius and deep-layer shear mostly under 10 knots on the favorable side of the ledger.

The intensity numbers that follow are modest. The forecast calls for 35 knots at 12 and 24 hours, 40 knots at 36 hours, 45 knots (50 mph) at 48 to 60 hours, and 50 knots (60 mph) from 72 to 120 hours. The forecast and advisory product carries the same path through the outlook points, 50 knots by Nov. 12 and again on Nov. 13, and notes that intensity errors have averaged near 20 knots per day.

Put side by side, the two products show how fast a small storm can change character. At 11 a.m. HST the maximum sustained wind stood at 50 mph with higher gusts, and the next complete advisory was set for 5 p.m. By the 11 p.m. discussion, the estimate had dropped to 45 mph, and the specialists wrote that the structure was weakening rather than organizing.

Pressure moved the same direction. The estimated minimum central pressure was 998 millibars in the daytime advisory and 1002 millibars in the overnight product, with gusts of 50 knots, a drift upward that fits a storm losing organization instead of consolidating around a tighter core.

Hurricane threshold of 64 knots and the model spread

An earlier Central Pacific forecast had called for 60 knots by Nov. 9 to 10 and 65 knots by Nov. 11 to 12, which would have crossed the 64-knot line that separates a tropical storm from a hurricane. The latest numbers sit well under it, and the discussion describes confidence in the intensity forecast as low.

The spread among guidance is wide. Hurricane regional models show Pena remaining a weak tropical storm or degenerating into a trough, and the 00 UTC runs of the HAFS model suggest it could dissipate. Gradual intensification is expected only after about 36 hours, and only if the storm survives its current disruption.

Track guidance moved too. Pena is heading west at about 6 knots, steered by the ridge, and may lose some latitude between 24 and 60 hours before gaining it late in the period. The center nudged the track south for the 36 to 72 hour window, which if anything widens the margin from Hawaii. Residents can follow each update from the NWS Honolulu forecast office, which relays the Central Pacific Hurricane Center products.

The unresolved question is how long Pena lasts. If the tilt persists and the dissipating HAFS runs are right, the 60 mph peak never arrives. If the circulation realigns, Pena could strengthen gradually as forecast while still staying far south of the islands.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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