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STORMcast Discussion 2017-09

Issued: 08Sept17

Time:  1330 EDT  

The STORMcasts shown here are intended for the students of UDHS's AGS and AES geoscience programs and are for educational/informational purposes only.  They frequently combine scientific concepts along with a sense of humor that often pokes fun at our behavior during weather events.  While intended to be informative, the STORMcasts should never be used to make life and death decisions during severe weather events.  Follow the appropriate bulletins posted by the National Weather Service and local media outlets for appropriate actions during these events.

 

Current National Weather Service advisory map as of STORMcast issuance...

HURRICANE WARNING - Until further notice.  Equivalent Cat 2 hurricane force winds at ground level.  Peak wind forecast: 90-110 mph with gusts to 140 mph. Tropical storm force winds Saturday afternoon           until early Monday morning.  Window for Hurricane force winds: early Sunday morning until Sunday evening.  "Devastating to Catastrophic" damage possible.

STORM SURGE WARNING - Until further notice for Miami Dade Co., north to the Everglades and south to include the Flordia  Keys.  Life-threatening storm surge possible.  Peak storm surge Inundation: The potential for 4-6 feet above ground somewhere within surge prone areas.  Window of concern: Begins early Saturday evening
 

HURRICANE WATCH - Until further notice for central Florida.  Equivalent Cat 1 hurricane force wind at ground level. Peak wind forecast: 65-85 mph with gusts to 130 mph.  Window for Tropical Storm force winds: early Sunday - Monday morning.  Window for hurricane force winds: Sunday afternoon until early Monday morning.
 


(Map courtesy of NWS)

Everyone,

I'm flattered by the number of emails, etc. that I have received over these informational STORMcasts.  However, I need to reinforce a few things...

1.  As indicated above the graphics, these STORMcasts should NEVER be used to decide to make life or death decisions.  If you are concerned about family or friends in the path of this hurricane, encourage them to listen to the course of action recommended by local emergency management officials.

2.  The STORMcasts are composed in my free time, during lunch, between classes, etc.  I simply cannot provide individualized forecasts for specific locations or people.  Please understand my situation.

3.  My main purpose is to report the scientific, meteorological elements of the event as they appear at the time of the STORMcast.  I cannot evaluate or presume to know all of the factors that go into the human-based decisions. 


IT'S JUST A MATTER OF HOW BAD...

Obviously, I've been real busy in the STORMcast office this week and with a magnitude 8.1 earthquake in Mexico on top of everything, it's been an interesting week to be an earth scientist.  Needless to say it's been a costly few weeks for North America and the Caribbean and we're really just getting started.  For days now, models have suggested that Irma would hit Florida and, although the tracks continue to oscillate back and forth, it seems extremely likely that some part of Florida (if not most of it) will be under the gun of this hurricane over the weekend.

Most Recent Jiggles

Just like our Nor'easters, a storm track deviation of only 20-40 miles could mean the difference between 4" and 16" of snow.  In this case, it could mean being on a completely different side of Irma's eye which literally turns the wind direction around by 180 degrees.  Unfortunately, there is not a computer model in the world that can isolate a hurricane's path down to the mile so very soon, there will come a time when it will be time to turn the computer off and just let nature take its course...which is about where I am right now. 

As of this afternoon, the model ensemble has pushed the expected path of Irma slightly west of yesterday's which means the storm could track through the entire Florida peninsula raking it from top to bottom.  Both the U.S. GFA and NAM models like a scenario (for the moment) that takes the center of Irma directly up through the Florida Peninsula.  If the storm tracks to the south of Miami over some part of the Keys, it will mean onshore winds on the east side of the state which will significantly enhance the storm surge and flood significant portions of Miami-Dade Co.  The Keys the eyewall actually goes over could face wholesale destruction and overwashing (we saw this during Sandy along the NJ barrier islands).  Peak winds, however, might miss the Miami proper.  The threat of a landfall in GA or SC will be greatly diminished.

If the storm takes a more easterly track (like projected yesterday), Miami could take the brunt of the eyewall's fury and see catastrophic damage even with enhanced building codes in place.  The worst case scenario would be if the eye straddles the coast and continues to feed in warm Atlantic water as it rakes the coastline before again striking land in GA or SC.  IF the storm takes such a track and maintains its intensity, severe wind damage could be spread out over a vast area instead of just near Miami.  The storm surge will also track up the entire length of Florida ahead of the storm as the CCW winds push onshore.  However, the western side of the state would likely be spared from significant damage.

Don't Let the Classification Fool You

Many in the business (I'm one of them) feel like the classification system use for so long should be abandoned since people often make irrational decisions when they hear a remar like "Irma has been downgraded to a Cat. 4."  In other words, the wind speeds might have dropped from 157 mph to 155 mph.  "Oh, well in that case, let's ride it out..."  The truth of the matter is that it is difficult not to be impressed with the power and structure of this storm which will likely go down as one of the strongest (and costliest) storms in history by the time it's all over.  Over the next 48 hours and prior to landfall, storms of this size and magnitude often fluctuate in intensity and even undergo "eyewall replacement cycles" when the storm might briefly form two eyewalls before one consumes the other.  This is normal and should not be interpreted as a weakening in intensity.  At this point, the only thing that will greatly weaken the storm is contact with land, whether highly populated or not.  This is not a storm to be trifled with.

Is there a "Hail Mary" Scenario Where We Might Have a Chance?

While it is possible the storm could overshoot Florida entirely before heading for the U.S. coast, I haven't seen a model run yet that suggested this scenario.  Plus, a hurricane like this in the Gulf would just mean a landfall further west into the Florida panhandle or somewhere else along the Gulf Coast.  The only freebie we have left is if the storm completely veers to the north ahead of the coast and misses the U.S. mainland altogether and I just don't see that happening.


Intensity Estimates and Timing (as of 0700 Fri. model run)

Landfall -
Somewhere in the middle Keys and then somewhere along the Everglades.  From there,Irma will lose strength but take approx. 36 hours to traverse the Florida Peninsula into Georgia.

Wind - There is nothing currently ahead of Irma that would lead to a significant drop in intensity so it should strike Florida near the Keys as a strong Cat. 4 with Cat. 5 wind gusts especially near the eyewall.  Virtually anything short of reinforced masonry will likely be heavily damaged or completel destroyed.  As the storm tracks up the peninsula, interaction with land with quickly decrease the wind threat but there will still be widespread damage especially of lesser-constructed buildings.

Rainfall - Since there are guiding elements to this storm, it will NOT become a Harvey so "typical" amounts of rain for tropical systems like this are to be expected.  Anywhere from 8"-15" with localized heavier amounts are expected.  No surprises here.

Storm Surge - While the Everglades could absorb a storm surge, highly urbanized areas cannot and most of Miami-Dade Co. is barely above sea level.  At a minimum, 4'-6' storm surges are expected but it would not surprise me if they are more like 10' or even higher very close to the eyewall.  In this regard, precise track is everything.  Overwashing is likely on any of the Keys that are situated near the eyewall.

Timing - Current models suggest sometime during the overnight hours Sat. into Sun. for a landfall over the Keys and into the Everglades.  1 AM-3 AM seems reasonable.  By lunchtime Monday, Irma should be crossing into GA as a minimal hurricane or tropical storm.

I will be moving into "data gathering" mode from here in order to collect as much as possible before weather instruments in the region are destroyed or disabled.  Therefore I will not likely be posting another STORMcast for this event.  Best wishes to anyone who may be impacted by this event.

rs

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