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Tropical Storm Darby Update: Tropical Storm Strength Through the Weekend

High clouds have been streaming toward the northeast which indicates the cyclone is beginning to feel an approaching upper level trough. One of the later passes through Darby by the 53rd Weather Reconnaissance Squadron’s WC-130J helped confirm the center position of the system toward the end of the morning mission though the aircraft had to fly lower to find it. Based on the morning recon mission and the maintenance of deep convection, the initial intensity has been held at 50 kt. Note that this is higher than the subjective Dvorak estimates of 45 kt from PHFO and SAB, and 30 kt from JTWC. The next aircraft mission into Darby is scheduled for this evening.

Click to enlarge

Click to enlarge

Darby is estimated to be moving at 280/11 kt to the south of a ridge. This ridge is forecast to weaken due to a low pressure system digging southward to the north of cyclone. This weakness is expected to decrease Darby’s forward motion over the next day, and increase the amount of vertical shear affecting the system this weekend.

The trusted dynamical models have remained largely consistent today, bringing Darby over the Big Island on Saturday. Thus, the current forecast has been nudged southward closer to the dynamical consensus with a landfall on the Big Island, followed by a path over Maui County and near Oahu. The latter part of this path assumes that Darby survives its impact on the Big Island which is not a certainty.

ProbabilityThe intensity forecast rationale remains the same since the last package. The main factors affecting the intensity forecast include marginal sea surface temperatures, the amount and timing of vertical wind shear, and the effects of any potential interactions with the Hawaiian Islands.

Sea surface temperatures will remain marginal near 26.5C over the next couple of days but vertical shear is expected to increase as the previously mentioned upper level trough digs farther south. This shear increase is not expected to become strong until later this weekend.

The forecast calls for only slow weakening with Darby maintaining tropical storm strength through the weekend. This is consistent with the previous package but slightly lower than the intensity consensus. Interactions with the Big Island may cause significant disruptions to Darby so the intensity forecast confidence is not high at this time.

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