Monday, September 10, 2018

Two Hurricanes Threatening U.S.

Although Hurricane Florence is getting a vast majority of the media attention, there are actually two current hurricane threats to the U.S. at present as Hurricane Olivia is expected to weaken but still i mpact Hawaii.  Each of these systems is indicated by arrows in the analysis below. 

GFS analysis and observed satellite imagery at 1200 UTC 10 September 2018
We will begin with Florence.  According to the National Hurricane Center public advisory issued at 11 AM AST Monday September 10, Florence is currently a category 3 hurricane with maximum sustained winds of 115 mph, with higher gusts.  It is expected to remain a major hurricane through Thursday when it approaches the eastern seaboard.  

It is a blessing and a curse that modern science can track and predict the movement of these hurricanes days in advance.  The challenge is that while we have a decent idea that Florence will be a significant threat to the eastern seaboard, we can't pinpoint the track or intensity, or the details of the weather she will produce so far out.  This often yields days of speculation and media discussion of what might happen.  The most important thing at this stage for residents of the region is to put together a hurricane plan (see and monitor forecasts and recommendations by the National Weather Service, National Hurricane Center, and local officials.  

The National Hurricane Center "cone of uncertainty" indicates the probable path of Florence (about 2/3 of the time, the track will fall in this cone.  From Thursday through Saturday, there is a wide range of possible locations for the hurricane center covering a significant portion of the mid-Atlantic states, as indicated by the stippled region.  

That cone also doesn't describe the impact area, which can extend from the low center.  Further, while winds are a concern, most of the damage and deaths produced by hurricanes are produced by water — especially flooding produced by storm surge or heavy rainfall — and so it is important not to fixate on hurricane category, which is based entirely on wind speed. 

Meanwhile, the Hawaiian Islands are facing another tropical storm threat from Hurricane Olivia.  Olivia is currently a category one hurricane that is expected to move westward across the islands late Tuesday and Wednesday, as either a weak hurricane or a tropical storm.  Again, the cone below indicates that the low center could move across any of the islands and the central Pacific Hurricane Center discussion issued at 5 AM HST Monday September 10 notes that "It is important to not focus on the exact forecast track and intensity when planning for Olivia. Persons on the main Hawaiian
Islands east of Kauai should continue preparing for the likelihood of direct impacts from this system today and early Tuesday."

Again, avoid fixation on category as there are concerns about heavy rainfall, flooding, surf, and surge.  Olivia is expected to produce 10-15 inches of rain on the windward side of some islands, with local accumulations of 20 inches possible. 

Friday's post discussed the importance of discerning reliable sources and information given the firehose of frequently bad information we receive daily (see Not So Deep Thoughts on Reliability and Science).  When it comes to severe and hazardous weather, your best source of weather information in the U.S. is the National Weather Service, its local forecast offices, and its forecast centers.  Heed the recommendations of local emergency management officials. 


  1. Hey! So entirely and completely off topic...

    I was reading an article on KSL and switched over to the comments. Which normally makes me want to go and cry silently (or sometimes scream) in a corner over the ignorance and ugliness of what I read.

    However, I read an interesting one that said with Great Salt Lake at half of "normal" levels may be a contributing factor in our drought. That lower lake levels somehow equate to less lake effect snow.

    I'm not sure I believe this, yet it got me thinking in a new way I hadn't before. I'm curious your thoughts on this.

    1. I have two thoughts on this topic:

      1. The current drought is a regional scale phenomenon that covers much of the southwest and even portions of Oregon and Washington. It is most intense in the four-corners region. Drought on this scale has nothing to do with the Great Salt Lake.

      2. We have done substantial research on this topic and during the winter, lake effect contributes less than 10% of the total precipitation in the Wasatch Range. Further, the amount of lake effect in any given year is related more to the large-scale circulation than the lake size.

      Thus, the lower lake levels are having at most a very small impact.