The most likely area for flooding is the area tinted in blue.  Here are the seven day rainfall amounts as forecast by the NWS. 

At bottom right are the Hawaiian Islands. The small island farthest west (left) is Niihau and the next one to the right is Kauai. They were struck hard by now rapidly weakening Lowell.  On Kauai, the reported peak gust was at a place called Puu Lua which was 92 mph (I am not familiar with that location and cannot find it on a map). Nawiliwili had a gust of 86 mph, Barking Sands 84 mph and Lihue (where you land when you fly to Kauai) had 84, which was a bit higher than expected. Because it is still dark, we don't know much about damage except that it is said to be "extensive." On Kauai, poweroutage.us says 90% of the customers (homes and businesses) are without power.  We have not heard from Niihau and I'm not sure we will in the near future. It is a colony of native Hawaiians who often resist communicating with the outside world. 

8:40pm   Below is my last update of the evening. Things are pretty well locked in as far as the forecast is concerned and, without a functioning radar on Kauai, I can't do the precision work I had in mind.  Good bless everyone in Hawaii and Aloha for tonight.  8:20pm Update Via Molokai radar (Kauai's is still down), the eye is about to move over Niihau. Peak winds over Niihau will be 100 mph with a few higher gusts. Kauai will see gusts to near 100 around Barking Sands and the Na Pali Coast along with in mountain areas. Gusts as far east as Lihue will be near 60 mph.  The tornado risk continues until 4am HST. Flash flood warning continues for all of Kauai and Niihau.  Tens of thousands of homes and businesses are without power in Hawaii.  The eye of the storm is indicated by an E. The direction of movement by arrows. W = area of highest winds and it is intentionally wider on the east side. See storm surge forecast below.  7:08pm Update Using the radar ...

Updated forecasts are here .  4:30pm Update Tornado watch in effect for the Island of Kauai.  4pm Update With one tornado having likely already occurred on Kauai, the radar just failed again. Hopefully, we'll get it back but it is not likely as the winds and rains have been increasing.  So, our already limited tornado warning capability is about gone unless we get the radar back.  3:15pm Update:  Winds are gusting from the east to 45 mph at Lihue, which is on the side of Kauai away from the approaching hurricane (see map below).  At least two roads are already closed due to flooding. Heavy rains (yellow and orange below) are occurring.  Noon Update: Flash flood warnings  (green polygons) are now in effect for about the northeast half of Kauai. The rain (multi-colors) has tapered off the light showers for the time being after heavier squalls earlier in the day. Updated surge forecast: Remember: there will be waves of up to 15' feet on top of the su...

11am Hawaii Time: As of this time, there is no indication Lowell is weakening. It also appears to be moving slightly east of the path shown in the map below. Preparations should be rushed to completion.  7am Hawaii Time: It looks to me that the center of the storm may move on a slightly more eastern path than shown. As of 7am, the maximum sustained winds (with higher gusts) are 120 mph. This storm is likely to cause significant damage. Tropical storm WARNING now for Oahu. The Kauai radar is now operating. Thank you, NWS!! -- map updated 8am Hawaii time -- Hurricane Warnings are now in effect for Kauai and Niihau.  Tropical Storm Watch for Oahu (includes Honolulu). Red = Hurricane Warning. Yellow = tropical storm watch. White is the path of the center  of the storm at the specified times (Hawaiian Standard). Brown is the radius of sustained 75 mph or stronger winds. Amber is the radius of tropical storm force winds (39 to 74 mph).  The red line to the west (upper left...

As I feared (see below), the National Hurricane Center has moved the path of Hurricane Lowell farther east, which requires a hurricane watch for Kauai and Niihau. The pink/blue = tropical storm warning  with a hurricane watch. Red is a hurricane warning for the western Hawaii Atoll, which are largely unpopulated islands and mini-islands. Take a look at the brown area which is the radius of hurricane force winds ( ≥ 75 mph) and the amber is tropical storm force winds (39-74 mph). Remember, the white is the forecast path of the center  of the storm.  From the National Hurricane Center  The surf will be more than rough, it will be life-threatening. Below are the storm surges as forecast by NHC. Of course, the high waves generated by Lowell will be on top of the storm surge. There will be flooding, especially on Kauai and landslides are possible in the mountains.  While this is not as strong as Iniki (whose center went right over Kauai), all hurricanes are dangerous...