Trade winds will continue to strengthen today as Tropical Storm Moke moves from east to west, south of the islands. This system will deliver periods of rainfall to all islands from late tonight through Monday, but heaviest along windward and southeast slopes of the Big Island. A Flood Watch remains in effect for the Big Island. Typical drier trade wind weather will return by mid next week.
Latest IR satellite imagery loop continues to show deep convection several hundred miles southeast of the Big Island, in association with Tropical Storm Moke. The center of Tropical Storm Moke was located roughly 500 miles southeast of Hilo and moving at around 12 mph to the west northwest. Closer to home, mainly clear skies prevail with few low clouds pushing against the island's windward slopes. The 06Z Hilo sounding shows a fairly shallow subsidence inversion height near 5000 feet with relatively dry air both above and below the inversion. Thus, it comes as no surprise that the radar has been quiet with only isolated light showers dotted over the region. Trade winds have begun to slowly ramp up statewide the last several hours into a more moderate range, generally 10 to 20 mph, with gusts to around 30 mph at wind prone sites.
Local pressure gradients will further tighten today as Hawaii becomes sandwiched between the low pressure of Tropical Storm Moke (as it moves west, south of the islands) and a 1026 mb surface high far to the northeast of the region. Winds across select zones of Maui County and the Big Island, mainly the typical tradewind prone zones in the lee of mountains or passes, should reach advisory level by this afternoon. A Wind Advisory has been issued for these areas and has a starting time of noon. Hi-resolution model guidance indicates nearing High Wind Warning levels in the lee of the Kohala mountains on Big Island later this evening into tonight. If this comes to fruition, and winds increase above guidance, the advisory for this zone would need to be upgraded. Leeward Ko'olau mountains and Wai'anae mountain zones on Oahu may be added to the advisory on a later shift as winds near criteria this evening into tonight. Trade winds remain strong through Sunday, then begin to dwindle late Sunday into Monday morning.
By this evening, a slug of deeper low level moisture from Tropical Storm Moke begins to approach the Big Island from the east. Showers will become numerous along windward and southeastern sections of the Big Island late tonight through Monday with periods of embedded heavy rain. Both the GFS and ECMWF share similar solutions and the NHC track forecast for Moke has changed very little during the past several advisories. For this reason, forecast rain amounts were held steady overnight with no major changes. A Flood Watch for the Big Island will go into effect tonight and continue through Monday afternoon. Rainfall totals of 5 to 10 inches, locally up to 15 inches are possible over the windward and southeast slopes of the Big Island. It will not take a significant amount of rain to cause flash flooding due to saturated grounds from recent heavy rain. For the remaining islands, showers will also become more numerous as deeper low level moisture increases from Tropical Storm Moke. However, totals should be much less with only up to a few inches of rainfall accumulation expected. Isolated amounts up to 4 inches are possible over the eastern portion of Maui as moist easterly flow moves up the slope of Haleakala.
Early next week, as Tropical Storm Moke pushes further west, away from the islands, both the GFS and ECMWF indicate some residual moisture and instability left behind in its wake. This could spell more weather weather for the area, especially over the higher terrain of Maui and the Big Island. Details will be worked out in the coming days. By Wednesday, a surface high northeast will once again dominate and bring a return of more typical trade wind weather.
Trade winds will fill back in over the islands as Tropical Storm Moke approaches and passes south of the Big Island this weekend, increasing the pressure gradient across the state. Trades will gradually strengthening from east to west, becoming strong by this afternoon and continuing to build on Sunday. AIRMET Tango will likely be needed for low level turbulence downwind of island terrain.
While occasional pockets of MVFR conditions will be possible within passing showers along windward and mauka locations today, VFR conditions are expected to prevail. Tonight into Sunday, increasing moisture over the eastern end of the state as Tropical Storm Moke passes by to our south will result in increasing chances for heavy rainfall, along with low visibilities and ceilings, across the Big Island. This will bring more widespread MVFR and IFR conditions.
Trade winds will begin to strengthen across the coastal waters in response to Tropical Storm Moke passing south of the islands this weekend. A Small Craft Advisory (SCA) has been issued through this afternoon for the eastern two thirds of the coastal waters. The SCA has been expanded to include all nearshore waters from tonight through Sunday afternoon. Stronger winds and Moke generated wind wave swell may push significant seas to near 10 feet within Big Island's eastern waters this weekend.
A Tropical Storm Warning is in effect for the offshore waters as the passage of Tropical Storm Moke introduces tropical storm force conditions to these waters from tonight through Sunday.
East-facing shore surf will gradually build this weekend in response to strengthening trades. Surf along east and south-facing shores will be impacted by a short period east southeast swell generated by Tropical Storm Moke as well as northeast wind wave swell as a result of the stronger trades. As Moke passes by to the south this weekend, a mounting southeast swell will veer and become south to southwest into Monday. This small, short period captured easterly fetch from TS Moke may push Big Island east surf to advisory levels tonight into Sunday. Moke's wind field is forecast to be relatively small and, with potential minor deviations in forecast track, confidence for Big Island east-facing shore advisory surf remains low.
Surf along most south-facing shores will be slightly below summer norms but still remain somewhat elevated due to a lingering small, long period south southwest swell through the weekend. Another reinforcing small south swell is expected over the weekend which will hold these near summertime heights. A series of south swells next week on top of this resident swell may potentially increase south surf to near advisory levels.
Small, medium to long period west to north swell originating from Hurricane Lala will arrive along north and west-facing coasts this weekend.
Wind Advisory from noon today to 6 PM HST Sunday for Lanai Mauka- Kahoolawe-Maui Leeward West-Kohala-Big Island Interior-Molokai Southeast-Molokai North-Molokai West-Molokai Leeward South-Lanai Windward-Lanai Leeward-Maui Central Valley North-Maui Central Valley South-Windward Haleakala-South Haleakala-Big Island South- Big Island Southeast-Big Island North.
Flood Watch from late tonight through Monday afternoon for Big Island-
Small Craft Advisory from 6 PM this evening to 6 PM HST Sunday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel.
Small Craft Advisory until 6 PM HST Sunday for Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County Windward Waters-Maui County Leeward Waters-Maalaea Bay-Pailolo Channel- Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward Waters-Big Island Southeast Waters.
