Harvey’s Hazard Affecting Land
• This is a serious situation
• It is likely that 12 year hurricane drought will end with Harvey
• 6-10 feet of storm surge
• Up to 30” of rainfall in isolated areas
• At least category 3 but a chance at category 4 hurricane force winds up to 110 mph or greater
Synopsis of Harvey
Did you know that the longest time between hurricane landfalls for the state of Texas was eleven years? This was back in 1855-1865 but, of course, some parts of the Texas coastline were pretty sparsely populated during that time period. However, just as impressive is the current eight year drought of hurricane landfalls for Texas (2009-2016), and this is where the current landfall drought will stand as Harvey is now forecasted to be at least a category 1 hurricane when it makes landfall early Saturday morning along the Central Texas coastline.
As I mentioned in the BMS Tropical Update on Monday, I was very concerned that any systems that form in the Bay of Campeche often strengthen rapidly and continue strengthening until landfall. Given that the current sea surface temperatures are some of the warmest in the water (above 86 degrees) there is plenty of fuel for Harvey to strengthen, which it is currently doing. In fact, just this morning from 4 AM CDT to 7 AM CDT the central pressure of Harvey dropped 11 millibars which technically meets the NHC definition of a rapid deepening named storm, which is defined by a drop of 1.75 mbar/hour or 42 millibars in a day.
Track & Intensity
Harvey’s landfall track has been fairly stable over the last 48 hours with a landfall location estimated between Corpus Christi and Freeport, TX. The main uncertainty is what will happen after Harvey moves inland: how much will it weaken and will it stall or will it move more south or north or even back out over the warm Gulf of Mexico. All of these are options that are produced by the weather models at this point as overall the steering currents will collapse and Harvey will meander. The most logical scenario would be a move northeast back over the Gulf of Mexico early next week.
— Philippe Papin (@pppapin) August 24, 2017
As mentioned, there is nothing holding back Harvey from intensifying until landfall. The only constraint I see is time and with another full day over very warm waters there is no doubt Harvey will be a hurricane. The question is will it become a major hurricane before landfall? Some models are suggesting this, but often there is a lack of confidence in such forecasts because they often over estimate these intensity ramps. However, I think this time it’s different. The statistical SHIP model suggests an increase of 45 kts (50 mph) within the next 36 hours which would make Harvey a 100 kt (115 mph) storm or a major hurricane at landfall. However, at this time there is good possibility that Harvey could be a Category 3 at landfall, but some of the newest model are even suggesting a Category 4.
In meteorology they always teach that the trend is your friend when forecasting, but in this case the trend in intensity forecasts below over the last 24 hours is not our friend with a much stronger system expected now then event 24 hours ago.
Because Harvey is expected to be a major hurricane at landfall it will likely be the strongest hurricane to hit the area since Celia of 1970 which in today’s dollars would cause an estimated of $8B USD. Beulah 1967 ($6B), Allen 1980 ($2.5B), and Brett 1999 ($110M) can also be used as analog hurricanes at this early stage to get an idea of potential impact.
One factor that is difficult to estimate is the flood related losses from a landfalling storm and as mentioned in the last update the flooding rains that will result from Harvey look to be historic as Harvey is forecasted to stall out. Rainfall estimates keep increasing: the National Weather Service is forecasting up to 20 – 25” for rain for parts of coastal Texas with rainfall over 10” stretching from San Antonio, Austin , Collage Station, and into Western Louisiana. I expect wind driven rain to be a major factor over much of the Texas coastline.
To put this into prospective below is some work by Ryan Maue at Weatherbell who has use the PRISM data from 1981 – 2015 to show what the maximum rainfall is over a three day period. Keep in mind the rainfall totals forecasted for Harvey are over the next 7 days. Needless to say I expect some 3 day rainfall records to be broken from Harvey.
— Ryan Maue (@RyanMaue) August 18, 2016
Below is the estimated percent of homes that have flood insurance through the NFIP. As Bryan Wood who works for Assurant suggest the overall percentages if homes inland with flood insurance are uncomfortably and on or off flood plain homes are at risk from Harvey heavy rainfall forecast.
— Bryan Wood (@bryanwx) August 23, 2017