BMS Tropical Update 10/05/2017 10 AM CDT

By: - October 5th, 2017

Tropical Depression 16 has been upgraded as of this morning to Tropical Storm Nate, but it remains poorly organized as it interacts with the Central American coastline. Nate’s center is now just off the Nicaraguan coast. A well-defined core has not yet emerged at this stage, and any banding features are weak and transient. Unfortunately, environmental conditions are favorable for that to change in the coming days.

Forecasted Intensity
Water temperatures remain extremely warm in the western Caribbean Sea, as well as in the southern Gulf of Mexico. Tropical systems need ocean temperatures above 80F to develop and thrive, and the water in Nate’s path is well over that mark in the range of 84F to 87F.

Current Sea Surface Temperature analysis suggests waters warm enough to support rapid intensification. However, warm water is just one ingredient that is needed.  Temperatures above are in degrees Celsius.

Wind shear over the next few days will be light. Nate is expected to be located over the western Caribbean Sea by tomorrow, between Honduras and the Yucatan Peninsula. Wind shear values in this area, excluding those generated by the storm’s circulation, are near zero. Nate will have little to no trouble with wind shear as it moves northwest, and this combined with the warm water in its path leads me to believe that strengthening will occur.

The amount of land interaction that Nate has with the Yucatan Peninsula as it moves north will be critical in determining how strong Nate will be when it enters the southern Gulf of Mexico. Another potential inhibiter to Nate’s future intensity could be a bit of dry air it encounters as it approaches the Yucatan Peninsula and moves into the Gulf of Mexico. Notice the dry air stretching from Guatemala to the NE Gulf.

Current water vapor showing the amount of water in the air. The brown in the image above shows dry air in the path of Nate in the central Gulf of Mexico.

 

This is the 700 mb water vapor from the American Model initialization. This shows dry air is even at lower levels of the atmosphere, which is also backed up by the water vapor image above.

This is the dry air inhibiting development of the tropical wave near Florida this morning, and it may put a lid on how strong Nate can become as it enters the Gulf of Mexico.

Therefore, the intensity forecast remains even more uncertain than the track forecast at this time. If the storm can put together an inner core and keep the dry air at bay, intensification into a mid-grade hurricane is possible. If the storm’s inner core doesn’t become organized, or if dry air is able to work into the core, the system will likely remain either a strong tropical storm or a weak hurricane until landfall. However, as discussed yesterday, weaker systems tend to strengthen right up until landfall in the Gulf of Mexico, which appears to be possible with Nate.  Some model guidance is still suggesting that a Category 2 hurricane is possible at landfall.

However, given the inhibiting factors of the Yucatan Peninsula, the dry air and some increase in wind shear that is forecasted right before landfall, today there is a much lower possibility of Nate becoming a major hurricane prior to landfall, and it is more likely that it will be a weak hurricane or even a tropical storm at landfall.

Forecasted Landfall

Nate’s forecasted path has shifted slightly west over the last 24 hours, although there remains some uncertainty as to exactly where the storm will make landfall.  At this point, anyone from the Big Bend region of Florida over to western Louisiana should be watching this system closely.  The current model consensus takes the center of the storm near New Orleans, which is a perfectly reasonable scenario.

 

Current morning run of the American GFS model ensemble plot. The black line is the NHC OFCL forecast, which is east of most of the model guidance from this model.

 

Current morning run of the European ECMWF model ensemble plot. The black line is the NHC OFCL forecast, which is east of most of the model guidance from this model.  Since the GFS and ECMWF are all west of the NHC official forecast, I think the NHC will keep adjusting the landfall location west.

Regardless of landfall location, the model guidance on the timing of Nate’s northward track is now in better agreement with it making landfall on the Gulf Coast on Sunday morning. It should then weaken as it races northeast across the Appalachian Mountains early next week as a much weaker tropical system, bringing some much needed rain to the area. The fast movement of the system at this time is not conducive to a large inland flood threat.

Current rainfall forecast suggesting that the upward eastern slopes of the Appalachian Mountains will receive the most rain into early next week.  The amounts are all less than 5″ at this time.

Insurance Industry Impacts
Until we know Nate’s strength as it moves into the Gulf of Mexico, it is too early to determine storm surge impacts and potential insured loss. However, even a weak tropical storm such as Tropical Storm Lee that made landfall near New Orleans in 2011 caused insured loss along the Gulf Coast and inland states.

 

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