NWS Infrared Satellite Image Loop

BIG ISLAND
333 PM HST MON OCT 5 2009

FLASH FLOOD WATCH IN EFFECT THROUGH TUESDAY AFTERNOON

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 PM HST MON OCT 5 2009

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 PM HST MON OCT 5 2009

SYNOPSIS
A short wave upper trough passing across the islands will keep the threat of locally heavy showers tonight and Tuesday…and well as a slight chance of thunderstorms overnight. A light and variable wind pattern is expected over the next several days…as a ridge of high pressure settles in near the state. Clouds and showers will tend to favor interior areas each afternoon and evening…with some isolated showers possible elsewhere at times. Generally muggy conditions will prevail until the expected return of the trade winds over the weekend.

DISCUSSION
The atmosphere over and around the main Hawaiian Islands is rather moist and unstable. The instability is due to upper level short wave troughs passing by just north of the state. a much broader and deeper trough is seen on water vapor imagery northwest of the state. this is forecast to pass through the islands tonight and Tuesday…likely resulting in continued instability. The flash flood watch has been extended through the day Tuesday due to the possibility of locally heavy rainfall. There is also a chance of thunderstorms overnight due to the instability associated with the upper trough.

Once the trough passes by…the atmosphere will become a bit more stable but precipitable water values are expected to remain high. showers will then tend to favor the afternoon and evening hours especially over interior areas. Also with all this moisture around…conditions will remain rather muggy.

At the surface…a frontal boundary is approaching the main Hawaiian Islands from the northwest…pushing the subtropical ridge southward ahead of it. We expect the front to stall northwest of Kauai and not affect the islands directly. However…the effects of the approaching ridge are already being felt in the form of light and variable winds. Models show that winds will remain light through the most of the week. the forecast continues to show the return of the trades over the weekend.