Dorian Update, Saturday Morning (31 August) – Why you should use NHC forecasts

As described yesterday, the main feature guiding Dorian is a ridge of high pressure just to the north of the storm.  The forecast for how strong the western side of that feature will be in 3 days is the key to the track, and if the storm will hit Florida, Georgia, SC, even stay out to sea.  Note that for the northern Bahamas, looks like you are going to get at least part of this powerful storm, so continue to follow local guidance based on the warnings in place.

Here’s the latest “spaghetti map”; the various global models, as well as the specialized models like HMON and HWRF that kept the storm striking South Florida long after the global models had shifted, have now shifted well offshore.  In fact, the NHC track (the red lines) are now well to the left of the “envelope” …

So what’s going on, and, as some have criticized, why is NHC so “slow” to change their forecast? And what about the chorus of those singing “but my <insert favorite local TV weather person or blogger> got it right!”  It’s important to realize that where the storm is most likely to go, and where you should plan on the storm going, are often not the same thing.  And as always it is vital to separate actual empirical data from emotional impressions and anecdotal evidence.

The goal of the NHC forecast is the issuance of watches and warnings so that you have enough time to prepare safely for a storm, without disrupting your normal life.  It’s a tough balance.  Once plans are initiated, they are difficult to change.  In addition, too frequent warnings can cause complacency (“crying wolf”).  Also, as any follower of this blog knows, the guidance can shift back and forth from run to run in these models.  That can cause the “windshield wiper effect” where the track jumps back and forth.  NHC wants to avoid that, so they look for trends.  Sometimes that can mean they are not the first to jump on what later does turn out to be a real trend (such as may be happening here).  But it also means that over time they are more right, and more consistent.  Just how consistent?  Here is last year’s NHC forecast verification report.  They do one every year.  What about my modeling and research?  Try this.  Or this.  Does your favorite TV meteorologist or blogger have an r-square, threat score, or mean error for their five day forecast?  Maybe they got it right this time.  But how many times have they said “the storm is going to turn” and it didn’t?  Or said it wouldn’t and it did?  Got data?  That is not to be snarky, and not to say some broadcast meteorologists are not quite good.  They do care about being right: in fact, they often care too much about not only being right, but being the first to be right.  That leads to more errors in the long run. So for something highly specialized like hurricane forecasts, you’re literally betting your life and livelihood on this stuff.  You should probably make sure it has rigorous analysis to back it up, so for planning purposes, you won’t go wrong, and have a lot less stress, if you go with the NHC forecast.

I often hear the phrase “better safe than sorry.”  Well, to be blunt, that’s foolish.  Overreacting carries a cost – not just in economic terms, but in actual human lives.  Stress aside, evacuating too early kills people in traffic and other accidents, and is especially hard on the elderly and those with medical conditions.  Surgeries are delayed, and those recovering are placed at risk from being moved prematurely.  And in our society, economic losses often translate in to lives shortened or lost.

So, where do we stand, based on the NHC forecast track and potential damage?  Here’s the obligatory map …

As for actions, the basics are right here.  As noted the ridge and steering is forecast to weaken in about three days, causing the storm to stall, then turn north.  That still brings it very close to the Florida shoreline.  People in that area should be ready to act and wary of wobbles that could greatly increase the potential damage.  Even a brush with the coast, such as the NHC track is showing, is a $6 or $7 Billion dollar event.

What about Florida, Georgia and SC?  Again, NHC probably said it best:

Life-threatening storm surge and devastating hurricane-force
winds are still possible along portions of the Florida east coast
by the early to middle part of next week, but since Dorian is
forecast to slow down and turn northward near the coast, it is too
soon to determine when or where the highest surge and winds will
occur. Residents should have their hurricane plan in place, know
if they are in a hurricane evacuation zone, and listen to advice
given by local emergency officials.

 

The risk of strong winds and life-threatening storm surge is
increasing along the coasts of Georgia and South Carolina during
the middle of next week. Residents in those areas should continue
to monitor the progress of Dorian.

If you are in GA/SC and have a hurricane plan, there is plenty of time to wait another day or two to watch what happens.  Impacts would be Wednesday/Thursday for Savannah on this track, which is still rather uncertain.  It’s scary to see the line, and the “Severe/Catastrophic Damage” categories near your home.  But there is really no need to freak out just yet.  If the track is still on or near the GA Coast tomorrow, then it will be time to start taking some actions, and I’m sure the local emergency management officials will start to roll out their plans.

Also as noted earlier, the GA/SC coast is having higher than normal tides.  It doesn’t look like last night peaked as high as forecast, but it was right at flood stage.  Tonight’s high tide at 10pm will be in that same ball park, as will Sunday night’s high tide.  Then it should decrease a bit Monday, but after that, it depends on where the storm goes.

One thought on “Dorian Update, Saturday Morning (31 August) – Why you should use NHC forecasts

  1. Seems that “ridge of high pressure” north of storm has been a recurring factor in past hurricane seasons that increased landfall threat for southeast coast. Has such a ridge been more prevalent than usual and, if so, why? Thanks!

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