Harvey has arrived in Houston.
Numerous flash flood warnings are posted right now across the Greater Houston area, as an eastern flank feeder band of the storm flows ashore.

These heavy rains are likely to continue throughout the day today. There may be lulls at times, but the dominant weather we see in Houston today is heavy rainfall, leading to street and possibly bayou flooding.
Rain totals have begun to add up. Generally 1-3″ north and east and 3-6″+ south and west since yesterday.

As the day progresses, rain totals will balloon further. Some models show really impressive rain totals in the Houston area today. I would expect an average of 4-8″ today across the region, with higher amounts possible, especially on the south and west sides and perhaps a few spots east of the city with lesser amounts.
Bottom line in all this, you need to be prepared for significant travel inconvenience and disruption today. And if you can stay put, please do so.