textproduct: Honolulu

SYNOPSIS

Moisture associated with the remnants of Norbert will move from east to west across the state through Friday and bring an increase in shower activity, especially for windward and mauka locations. Some showers will be heavy at times with a slight chance of thunderstorms. A Flood Watch remains in effect for all the main Hawaiian islands through late tonight. Drier air will move back in this weekend behind the departing trough. Moisture from the remnants of Tropical Depression 15-E may bring another round of enhanced trade showers by mid next week.

DISCUSSION

Latest IR satellite imagery loop shows an area of enhanced moisture and cloud cover continuing to edge over the Big Island and Maui windward coasts from the east. This batch of moisture is associated the remnants of former tropical cyclone Norbert, which has been reduced to a surface trough extending from the Big Island northward. Over the next 24 hours, the trough will push westward across the remaining islands and bring with it a swath of relatively deep moisture with over 2" of precipitable water. In tandem, models prog a subtle upper level trough to drop over the islands. Marginal upper level instability, combined with abundant moisture in the lower levels, will be enough to aid in the development of heavier showers and isolated thunderstorms over mainly windward and mauka locations along with the Kona slopes on the Big Island. A Flood Watch remains in effect for all main Hawaiian Islands through late tonight. The trough will slowly depart to the west of the state on Friday, but some residual moisture may be enough for heavier showers to stick around near Kauai. Atmospheric instability, produced by diurnal heating, could lead to another round of enhanced showers and isolated thunderstorms Friday afternoon over leeward slopes of the Big Island.

Saturday through Monday, moderate trades will help filter drier air back into the lower levels and reduce the shower coverage to scattered during mainly the late night/early morning hours. The aforementioned upper level trough, or possibly weak upper level low, will sit stagnant near Kauai and help advect some mid and upper level cloudiness over the state from the southwest.

Tuesday through Thursday, there is yet another potential push of deeper moisture from the remnants of tropical depression 15-E. Both the GFS and ECMWF show a low level wave associated with this moisture passing near or over the islands. This event is still well into the long range forecast, so confidence is rather low. However, it will still need to monitored closely for the potential of more heavy rain.

AVIATION

An approaching easterly trough, the remnants of TC Norbert, will be passing over the islands through Friday. This trough is pulling in a more moist air mass. This will result in a greater frequency of showers with brief periods of moderate intensity rain. Decks will lower tempo into IFR/MVFR categories during times of more intense rainfall. Typically breezy trades will persist through the period with wind-sheltered area breezes remaining light and variable.

AIRMET SIERRA remains in effect for island-wide, windward exposed mountain obscuration near or above 2,000 ft.

MARINE

A broad trough, associated with the remnants of Norbert, will allow moderate to locally strong northeast trades to hold through today. A Small Craft Advisory (SCA) for the typical windy waters around Maui and the Big Island is in effect. This SCA may be expanded to include other areas in the coming days. After the trough moves through the state through early Friday, a surface high far northeast of the state could allow moderate to locally strong trades to return over the weekend through the forecast period. Above normal moisture and upper level support will allow isolated thunderstorms with localized heavy rain and gusty outflows to develop over the offshore waters through Saturday.

A series of long to medium period south-southwest swells will continue falling. Surf along south facing shores should begin to see more consistent sets today near averages. A reinforcing southerly swell will fill in Friday bumping surf near High Surf Advisory thresholds by this weekend as swells shift more out of a south direction.

Surf along east- facing shores increases today into Thursday as trade winds strengthen and a small, medium- period east swell from the remnants of Norbert arrives. Flat to tiny surf along north facing shores is expected through the forecast period.

FIRE WEATHER

Fire weather risk should remain low through Friday due to moderate to breezy winds and increased relative humidity and moisture from the remnants of Norbert. Showers will decrease by this weekend. Inversion heights will be elevated or very weak.

HFO WATCHES/WARNINGS/ADVISORIES

Flood Watch through late tonight for all Hawaii islands-

Small Craft Advisory until 6 AM HST Saturday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.

Small Craft Advisory until 6 AM HST Saturday for Big Island Windward Waters.