Today will likely be the last day of trade winds, as winds become southeast Friday. A relatively dry and stable air mass will continue to support only light showers focused windward and mauka through the middle of next week. A weak cold front and upper level trough will bring increased moisture and a slight increase in instability early next week, keeping the chance of showers going. These showers will still be mainly windward and mauka. Trades are expected to return in the middle of next week.
Skies were partly cloudy across the state this afternoon. Light showers were noted on radar, mainly mauka and windward. The exception was in the Kona region of the Big Island, where afternoon cumulus buildups have developed into isolated showers. The atmosphere is relatively dry at low levels, and stable as well. Trade winds continue to blow today, but should turn and come out of the southeast Friday. These southeast winds are forecast to last into Wednesday of next week.
A ridge of high pressure at the surfaces stretches from southwest to north-northeast of the state this afternoon. And this ridge extends vertically as well, with the upper level ridge axis running pretty much due north. Surrounding this upper level ridge is a pair of troughs. The one to our northeast is associated with the flooding and severe storms along the west coast of the mainland. The one to our northwest will weaken as it approaches, pushing a weak cold front toward Kauai County Saturday night and Sunday. This front will then stall near but most likely just west of the state, before slowly retreating westward Monday. Depending on exactly how far east the front makes it, Kauai County could see a noticeable increase in showers.
The upper trough associated with this front will weaken and remain nearly stationary through Wednesday. This will bring slightly cooler temperatures aloft, help to increase low level moisture due to persistent southeast flow, and therefore help to slightly destabilize the atmosphere. PW values increase slowly through Tuesday before dropping back down Wednesday into Thursday, but remain below normal throughout this process. Therefore, showers will continue to be favored mauka and in windward areas, with an additional increase on southeast-facing shores that may not be traditionally considered windward. Weak ridging aloft is expected to move in late Wednesday and Thursday, and this will help strengthen a surface ridge to the north and should bring back east to northeast trade winds.
Trade winds continue through today before gradually weakening and transitioning to ESE tomorrow. Shower activity remains limited through the evening, but overnight and early morning windward and mauka showers are possible. Expect brief MVFR conditions within those showers, and VFR conditions otherwise.
No AIRMETS are in effect at this time.
Changes with the afternoon forecast package include extending the Small Craft Advisory (SCA) through Friday night. Otherwise little change to the forecast.
Most of the models, high resolution and global models, show SCA level winds remaining in place over the typically windier waters near Maui County and the Big Island into Friday night. With those models in good agreement with today's ASCAT passes, have extended the SCA through Friday night. High pressure remains far north of the islands, and the associated ridge is expected to sink southward and weaken winds Friday night.
A low pressure system far to the northwest of the islands has a cold front associated with it that is expected to enter the northwest offshore waters Saturday. As this front gets closer to the offshore waters, we will see the winds over the coastal waters weaken and veer to a more southeasterly direction. The front itself is expected to bring some thunderstorms to the northwest offshore waters Saturday through Monday. At this time, it looks like the instability that will trigger the thunderstorms will stay north of the coastal waters.
The current moderate, medium period north-northeast (010-030 degrees) swell continues to boost surf heights along north facing shores. This swell is expected to be on the decline Friday. A Marine Weather Statement remains in place for moderate harbor surges at Kahului and Hilo through tonight. A combination of the declining north-northeast swell and a small long period west- northwest swell will keep smaller surf in the forecast this weekend. The next large long period northwest swell is expected over the first half of next week.
Rough and choppy surf along east facing shores will gradually decline as trade winds ease over the next few days. Some wrap around energy from the medium period north-northeast swell will create choppy rough surf conditions along east sides of all islands. Surf along south facing shores will remain very small through the week.
Small Craft Advisory until 6 AM HST Saturday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.
