The changes up to cat 4 and back down to cat 1 were based on measured wind speeds at the time of change. Harvey is apparently remarkable for the rapidity of these changes, at least according to the weatherpeople I listened to..

Hurricanes absorb energy from warm sea water, which strengthens them. As the body of the storm moves over land it is cut off from this source of energy, and hence weakens. According to reports I read Harvey was Cat 4 at landfall, however as it moved inland it rapidly lost energy. I believe it has currently been downgraded to a tropical storm rather than a hurricane.

Some models were predicting that Harvey might move back offshore, where it could again draw energy from the warm water, and strengthen again. Apparently that hasn't happened as of yet.

EDIT: A good source of up to date information is from the NOAA National Hurricane Center.

Thanks for the comments about Harvey (hurricane).

I realize that the Hurricane should gain strength as it travels over warmer waters. And indeed - that did happen. In fact it happened remarkably quickly. Harvey was listed as going from a Cat 1 to a Cat 4 storm.

What I don't understand is why towns like Corpus Christi were not absolutely devastated by high winds. Don't get me wrong. I am happy that the towns on the Texas coast were spared this type of destruction. But how can a Cat-4 storm go to a Cat-1 storm in a matter of two or three hours? Something is seriously puzzling about the whole sequence. People in Texas have said ... Harvey is a bad storm. But it's nothing like the power we saw from Hurricane Ike. I think that meteorologists really need to take a look at this whole phenomenon with Harvey (i.e. the reported wind speeds).

Also ... forecasts now say that some parts of Texas may see more than 50 inches of rain. More rain than has ever been seen before. That would make this flooding event a once-in-every-500-years event. Or maybe, once-in-every-thousand-years-event. The highest data point recorded for torrential rain in a one-week period. It's a little puzzling why the towns and cities were 'caught by surprise". Warnings have been given for 2-3 days. You only have to look at the satellite photos of Harvey to realize that something 'big and awful' is bearing down on Texas. Travelers are now trapped at Hobby airport in Houston, eating the restaurant food to survive. The Red Cross Center is now surrounded by flood waters and inaccessible.

There seems to be a big 'lag time" in how authorities responded to this crisis.

LINK ...National Weather Service calls Hurricane Harvey an "unprecedented event"/
http://www.msn.com/en-us/weather/topstories/%E2%80%9Cthis-event-is-%E2%80%A6-beyond-anything-experienced%E2%80%9D-the-national-weather-service%E2%80%98s-ominous-warning-on-harvey/ar-AAqNyDf?li=AA4ZnC&ocid=spartandhp

Pete

I believe the eye of the storm made landfall to the east of Corpus Christi. That means Corpus was not in the most "dangerous quadrant" which is to the NE of eye. That is where the strongest winds will be.

Also, I think it is a bit too early to know just how much damage there was from the winds, as reports are still coming in. See this photo spread in The Atlantic for examples of some damage that did occur. Apparently the area around Rockport, which is east of Corpus, was especially hard hit. Years ago I spent a little time in Rockport, and my recollection is that it is (was?) a delightful little town.

EDIT: The "dangerous quadrant" is due to the relation of the storms movement, relative to the wind circulation within the storm. See the Wikipedia article on Buys Ballot's law. Quoting from that article:

I'm holding at HurCon 3 with all HurCon 2 preps completed. We're ready to go, if the need arises.

Based on current forecasts, my only bug out route is now to the east. Our bug out distance needs to be 300-400 miles.

wildman

I think they should have made you the Governor of Texas. Or at least, the state coordinator for emergencies. You were the only one listening.

Texas has now mobilized 3000 National Guard troops. That is a very smart move. This crisis is by no means over. Houston and other cities have to get through a whole week of this stuff. this is only Day #1. People are gonna' start running out of clean water, food and sanitation pretty quickly. It will be a real mess in the communities with the high flood waters.

AKSAR - maybe you are right. We never heard from the towns that got hit with the hardest winds. That story has not been told yet.

And finally - here is an article that asks the same question that I am asking ... why wasn't Houston evacuated??

http://www.msn.com/en-us/news/us/harvey-is-causing-%E2%80%98epic-catastrophic-flooding%E2%80%99-in-houston-why-wasn%E2%80%99t-the-city-evacuated/ar-AAqNzHI?li=AA4ZnC&ocid=spartandhp

Pete

Yeah, they tried evacuating a couple of million people out of the Houston area a few years ago when Hurricane Rita came calling. Didn't go well.

FEMA has also poured in 5,000 personnel to Texas. I think the biggest thing that will help Texas is if first responders pour in from many surrounding states - or from across the whole country. That's the only way they will get on top of this.

Pete

I just ordered a replacement Aquatainer WaterBob to replace the 1 of 2 that I used. These are single event water tanks because I'll never get the internal surfaces dry to restore them. The 2 waterbob's boosted my potable water storage to 260+ gals and I have 130 gals of fresh rainwater in reserve. I have no affiliation with Aquatainer but I do like this product.

Latest weather report indicates you may be seeing Missouri in the near future. Forecast is for what left of Harvey to move back over water, regroup and then come ashore closer to Houston. This storm is not over.