Tropical Storm Joaquin


27-30th 2015 Late September Rain Storm

Showers and rain continue in Bermuda as the stationary front continues to lift across Bermuda. Today, scattered showers and downpours are resulting from deep tropical moisture and low level convergence along a stationary front between easterly flow around high pressure to the north and southeasterly flow around newly formed Tropical Storm Joaquin, located to the southwest of the island. Storm total rainfall since the stationary front entered the picture on Saturday are 2.50″ to over 4.00″ in central and western parishes, accounting for more than half of this September’s total rainfall.

29 Sep 2015 1945UTC RGB Annotations
This evening’s RGB enhanced satellite imagery of the Western Atlantic showing Joaquin and its five-day forecast track from the National Hurricane Center – keep in mind that the 3 to 5 day period is currently an unusually low confidence forecast for Joaquin.

Joaquin has shown signs of organization today as strong northwesterly vertical wind shear abated, deep convection developed, and Joaquin continued to be embedded in a moist environment. Investigatory flights into the tropical cyclone have found that a relatively fast pace of strengthening has coincided with the improved organization. As of the 6pm advisory Joaquin had 65mph maximum 1-minute sustained winds and a minimum central pressure near 990mb.

This is a marked change from Monday when there was little forecast model support for Joaquin to strengthen. Vertical wind shear pushed the cyclone’s deep convection off to the near southeast of the surface center. This decoupled the cyclone, which means that instead of the surface circulation being vertically aligned with the mid and upper level circulations, it was tilted toward the southeast with altitude. Because low level steering flow was weak on Monday, and remains weak today, the deep convection was able to ‘pull’ the low level circulation southeastwards underneath the mid and upper level circulations while the cyclone as a whole drifted to the west. This convective re-alignment was poorly caught by the suite of forecast models yesterday.

Now that Joaquin is becoming more organized, forecast models will have a better handle on the cyclone’s short term track. A continued slow track to the west-southwest over the next 2-3 days will take Joaquin towards the northern Bahamas Islands. Joaquin’s environment will remain at least marginally favorable, if not increasingly favorable for additional development during this time. As a result, it is likely that Joaquin will become a hurricane as it approaches the Bahamas, and as of the 6pm update this evening, this scenario is now being reflected in the National Hurricane Center official forecast. While a landfall or a direct hit on any of the Bahamas Islands is not certain, tropical storm conditions are quite possible and Watches or Warnings might be required for the Northern and Central Bahamas within the next 12 hours.

The question then becomes: how far west-southwest does Joaquin go before being picked up by a quickly deepening mid-latitude system? As of the latest forecast model guidance, there appears to be several reasonable scenarios as both this mid-latitude system approaching from the northwest and the remnants of Ida approaching from the east make the steering pattern exceedingly complex – the location, amplification, and timing of both systems will have bearing on the track of Joaquin past day 3.

One scenario takes Joaquin north then northwestward into the United States East coast somewhere either in the Mid-Atlantic or Northeastern regions. Another scenario takes Joaquin northward between the US and Bermuda then out to sea. A further scenario takes Joaquin northward then northeastward toward Bermuda. Because there is so much spread in the potential track of this system, this is an unusually low confidence track forecast, particularly after day three. Hopefully, aircraft observations today will improve the accuracy of the model guidance and increase confidence in a single forecast track scenario.

Further, because Joaquin is not expected to be within 400 nm of Bermuda in the next 72 hours, the storm is not currently a threat to Bermuda at this time. However, because the potential exists for this storm to turn towards Bermuda after 72 hours it is imperative to follow updated official information from the Bermuda Weather Service and National Hurricane Center as the forecast evolves.

Japan Met Agency Radar of Typhoon Dujuan south of the Ryukyu islands over the weekend.
Japan Met Agency Radar of Typhoon Dujuan south of the Ryukyu islands over the weekend.

Recently, in the Western Pacific, Typhoon Dujuan made a direct hit on the southernmost Ryukyu islands of Japan, then a landfall in Taiwan both as a category four equivalent typhoon, followed by a second landfall in eastern China as a much weaker tropical storm. Peak wind gusts measured at 130-180mph occurred in those islands of Japan and parts of coastal northeastern Taiwan where several feet of rain also fell. Quite impressive that such measurements were possible in real time as instruments and/or communications typically fail well before winds reach that level of ferocity – a testament to the strength of local infrastructure that hopefully represents the general state of affairs in those impacted regions.

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