Breezy trades weaken slightly during midweek then restrengthen by late in the week. Potential tropical development threatens to bring heavy rainfall to eastern portions of the state during the latter half of the week, though uncertainty remains higher than usual at this time.
This afternoon, skies were partly to mostly cloudy windward/mauka and mostly sunny to sunny leeward. At 3:30 PM HST, radar detected showers in most windward and mauka areas, with a few making it over to leeward areas on the breezy trades. Trades will continue at moderate to breezy speeds today through tomorrow.
The chance of formation of the remnants of TD 15-E into Nolo (the next name on the list) is up to 60% in the next 48 hours and 90% over the next 7 days according to NHC. Satellite is showing increasing rotation this afternoon, and convection is increasing. Model spread remains large on the track of this system, and this is not likely to improve until Tuesday or Tuesday night, at the earliest. The latest ensembles continue to display a large spread in timing and location of the track. The general consensus continues to show the storm remaining south of the islands. However, there is a large spread concerning how far south, from just barely (brushing right up against the Big Island) to several hundred miles. The latest Google DeepMind ensemble shows this spread well.
Ensembles in general show the developing system moving WNW to a position roughly several hundred miles south of the Big Island sometime Wednesday night, then turning north Thursday. After that, there are too many possible solutions to reasonably describe with words. We are looking forward to when NHC begins to issue forecasts, and that should be sometime Tuesday or Wednesday.
This continues to be a rapidly evolving situation. Our best guess at this time is that impacts could start with increased rainfall on Big Island (and perhaps Maui County) as early as Wednesday or as late as Thursday. Winds will increase across the eastern end of the state regardless of which track the system takes, most likely Thursday into Friday. How strong the system gets, combined with how far away it is, will determine how high the wind speeds get. Rainfall totals covering the period from 6 AM HST Wednesday to 6 AM HST Saturday could exceed 10 inches on windward Big Island and 3 inches leeward, with over 4 inches on windward Maui. We currently expect 1 to 3 inches elsewhere (leeward Maui and all other islands). Note that this forecast may change *considerably*.
Isolated thunderstorms are possible, mainly over the eastern end of the state, from Tuesday night through Saturday night. Moist and deep tropical air will be advected northward over Big Island starting Wednesday (PW values over 2 inches). This airmass will likely move over Maui County as well, perhaps Wednesday night into Thursday. It is presently not clear if it will make it to Oahu and Kauai, as it depends on the exact track of the storm. This airmass will obviously make it easy to get heavy rain rates and increase the risk of flash flooding. A flood watch will likely be issued before Wednesday, and the timing of issuance depends on how confidence in the track evolves.
Moderate to locally breezy trade winds will persist as a disturbance in the deep tropics moves to within a few hundred miles south of the Big Island on Tuesday. Moisture along the northern periphery of the disturbance will gradually spreads over the island chain, leading to an increase in mainly windward rainfall. MVFR ceilings will become more widespread over windward slopes. AIRMET Sierra for mountain obscuration remains in effect for windward Big Island and will likely be expanded to additional islands overnight and Tuesday. Aside from scattered afternoon SHRA over the Kona slopes of Big Island, VFR will dominate over leeward areas.
With trades remaining at moderate to breezy strength, AIRMET Tango for moderate low level turbulence downwind of island terrain will
The gradient between high pressure to the northeast and an approaching trough that may develop into a tropical cyclone to the southeast will maintain moderate to locally strong trade winds through Wednesday. A Small Craft Advisory is in effect for the typical windy waters of Maui County and the Big Island through early Wednesday morning. Wind speeds will strengthen during the latter half of the week as the trough or potential tropical disturbance moves near the islands. Shower coverage will also increase, along with the potential for isolated thunderstorms, particularly across the eastern waters.
Surf along south facing shores will remain above average through this afternoon, then will gradually drop heading into midweek as the current south-southwest swell fades. A slightly smaller, long-period south-southwest swell will fill in late this week into next weekend generated by a series of lows near New Zealand. West facing shores will experience a bit of the southerly wrap through most of the week.
Breezy trade winds will maintain moderate surf along east facing shores for the next several days. Surf along north facing shores will remain flat to tiny through tonight, but will see a bump heading into Tuesday and Wednesday as a small, medium-period north-northwest swell fills in from an Aleutian low. Then a small, long- period west-northwest swell generated by Typhoon Dujuan as it curves northeast near Japan will build locally Friday into next weekend, increasing surf along north and west facing shores.
Small Craft Advisory until 6 AM HST Wednesday for Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters- Maalaea Bay-Pailolo Channel.
