Area Forecast Discussion for Hawaii

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Issued: Apr 28, 2017 4:46 AM HST

Synopsis
A cold front and a couple of upper level disturbances will bring an extended period of unsettled weather to the main Hawaiian Islands that will likely last through Monday of next week. There is the potential for heavy rain and and a slight chance for thunderstorms each day beginning today.

Discussion
A good bit of the evening showers left the land masses for the adjacent waters as an offshore land breeze formed. The exceptions were the Big Island where some heavy showers occurred along the leeward Kohala coast and near Waikoloa Village between midnight and 3 am, prompting a flood advisory. On the windward side of the Big Island, scattered showers were streaming into the Puna district and skirting the Kau coastline. The wind flow over the Big Island is a bit different from elsewhere with a steady easterly flow. The wind pattern there, however, will be changing to the SE this afternoon. The other island is Kauai where the showers would time to time ping its coastline. Otherwise, the evening has been quiet and dry.

cor...The forecast looks on track. Shower activity will be picking up especially this afternoon, not only because of the front reaching Kauai but also due to daytime heating and the light winds. On the latest satellite imagery, the front is not as distinctive as earlier in the evening as light northerly winds have snuck ahead of the front. As of 3 am HST, the front is roughly 90 miles W of Kauai. An ASCAT pass shows some 25 kt or small craft advisory (SCA) level winds behind the front. Because of this data, a SCA is in effect for the waters primarily around Kauai. SCA level winds are expected through tonight only. On water vapor imagery, the upper trough is noted 165 miles W of Kauai. It is on track in reaching Kauai shortly after day break this morning.

The front is expected to travel as far as northern Big Island by Saturday afternoon. A cool dry air mass with dew point to as low as the upper 50's will accompany the brisk northerly winds that reach only as far as Oahu. East of Oahu, the weather will be of scattered showers, a few locally heavy, and a slight chance of thunderstorms.

The nice weather over Kauai and Oahu will only be brief. The weather after Saturday evening, mainly after midnight, will start to turn sour across Kauai and Oahu as the next upper level disturbance, a low with a pool of very cold air of minus 19 degrees C at 500 mb, comes into play. The models are in sync with this feature settling over Kauai and Oahu by Sunday daybreak.

This is an messy picture synoptically speaking. The surface front remains stalled over Maui County on Sunday which may act as a enhancer to the development of the heavier showers. In short, the active weather favors Maui County and the Big Island over the weekend. A Flash Flood Watch for at least Maui is still not out of the question at this point. We will see how persistent the models are with this unsettled scenario.

The front weakens to a trough over the central islands Sunday night with a moist southerly wind flow east of the trough. The upper low stalls out some 200 miles SSW of Kauai, and lingers there through Monday. The ECMWF and GFS solutions differ thereafter in the placement of the upper low. The GFS has it just to the south of Oahu Monday night while the EC still has it 200 miles SSW of Kauai. Either scenarios points to unsettled weather extending into Monday night and perhaps Tuesday.

Aviation
A slow moving front will move through Kauai later today and through Oahu tonight. Light winds ahead of the front will allow for daytime sea breezes in most areas from Oahu eastward, while the winds will pick up out of the north on Kauai later this morning and Oahu tonight behind the slow moving front. Moisture ahead of the front along with an upper level disturbance moving overhead will keep conditions unsettled, with scattered showers and temporary MVFR cigs/vsbys possible across the entire island chain through the next 24 hours. The best chances for reduced cigs/vsbys will be this afternoon and this evening as showers develop as a result of daytime heating. Additionally, MVFR cigs/vsbys are expected to become more widespread across Kauai later today and Oahu tonight along and behind the slow moving front, with the most unsettled conditions expected over windward and mauka areas.

AIRMET Sierra remains in effect for mountain obscuration across northwest thru northeast sections of Kauai. These conditions will likely remain in place through much of the day. The AIRMET will likely need to be expanded to include the interior of some of the other islands this afternoon.

Marine
A complex weather pattern is expected for the next several days, introducing a slight chance for thunderstorms across most of the coastal waters today through Monday. A front and upper level trough just west of Kauai will continue to move southeast before stalling over the central islands Saturday. Small Craft Advisory winds and seas will fill in over the waters surrounding Kauai today as the front passes. A small break in thunderstorm chances should occur behind the front, before spreading across all waters late Saturday as an upper low and developing surface low slide over the island chain. The forecast confidence remains low on the exact placement of thunderstorm activity, but mariners should stay tuned to the updated forecast today and this weekend.

Surf along north facing shores will see a small bump tonight into Saturday from a short period choppy swell associated with the stronger winds behind the front. Surf will stay below advisory conditions. A series of small, longer period northwest swells are then expected for next week.

A series of long period south swells will grace the south facing shores into next week. The first set of swells will bring advisory level surf (8 ft) to exposed south facing shores today into early Saturday morning. A High Surf Advisory has been posted. Please refer to the Coastal Hazard Message, CFWHFO, for more information. Surf this weekend, along with a reinforcing south swell Monday and Tuesday remain borderline for advisory conditions.

Watches/Warnings/Advisories
High Surf Advisory until 6 AM HST Saturday for south facing shores of Niihau, Kauai, Oahu, Molokai. Lanai, Maui, and leeward Big island.

Small Craft Advisory until 6 AM HST Saturday for waters around Kauai and windward waters of Oahu.


Lau...Discussion

Eaton...Marine

Jelsema...Aviation