Trades will increase to moderate to breezy levels by this afternoon and continue through the weekend. An area of deeper moisture will approach the Big Island from the east late Saturday, then move westward through Monday and bring an enhancement of shower activity to mainly windward portions of Maui and the Big Island. A typical stable trade wind pattern will return by Tuesday of next week.
Water vapor imagery loop shows cyclonic spin from an upper low located around 425 miles northwest of Kauai. This feature continues to draw upper level moisture (in the form of isolated areas of thin cirrus clouds) northward over the islands from the tropics. Closer to the surface, some weak radar returns have been noted drifting towards the windward coasts, caught up in the trade wind flow. Accumulation has been light with these showers, however, with only a few hundredths of an inch of reported at a handful of windward and mauka stations.
Trade winds will increase to moderate to locally breezy levels as high pressure north of the region becomes more well established this afternoon through the weekend. A fairly shallow subsidence inversion height of around 5000 feet will help cap vertical cloud development and lead to mainly isolated to scattered showers along windward and mauka locations. Expect some cloud and shower development along the South Kona slopes on the Big Island during the afternoons, as local sea breezes develop.
By late Saturday, models have been consistent in showing a deeper push of low to mid layer tropical moisture reaching the Big Island from the east. The latest GFS time-height cross section indicates a weakening subsidence inversion with moisture deepening up to roughly 15000 feet. With modest instability and sufficient moisture in place, pockets of moderate to heavy showers will be possible. Both the ECMWF and GFS show some of the moisture pushing westward into windward Maui and to a lesser extent the rest of the smaller islands. This should at least add some enhancement to showers statewide from late Saturday to Monday. Though it is still too early to fine tune the forecast storm total amounts, typically wetter windward locations on Maui and the Big Island could pick up a couple inches of rainfall accumulation. Sufficient instability may exist for a rumble of thunder Sunday afternoon over the south or southwest slopes of Mauna Loa on the Big Island, but confidence is too low at this time to add to the current forecast package.
Typical summertime stable trade wind weather will return Tuesday through the end of the week with scattered showers mainly windward and mauka areas during the nights and early mornings.
Trade winds are forecast to ramp up into the weekend as the driving high strengthens and pushes slightly closer to Hawaii. Stability will also be increasing today as an upper low to the west lifts away. Leeward turbulence will likely increase around tonight or Saturday, but no AIRMET will be needed for now.
A band of low clouds and showers will move onto windward Big Island this morning, then become more diffuse as it spreads westward. Mountain obscurations/AIRMET Sierra will be possible for this area as this moisture tracks in.
Moderate trades are forecast to increase to fresh to locally strong by this afternoon as surface troughing to the northeast weakens today and an approaching trough to the southeast enhances the local pressure gradient. A Small Craft Advisory has been issued beginning 2 pm this afternoon for locally strong trades through the typical windy areas around Maui County and the Big Island. Dissipation of the trough to the northeast combined with renewed trough development south of the islands and strengthening high pressure to the northeast will keep fresh to locally strong trades in place through at least the weekend. Some weakening of the trades is possible early next week associated with transient mid-latitude troughing.
Background medium and long period energy will produce small surf for south facing shores today. A series of larger, medium to long period south swells arrive beginning on Saturday, causing surf to trend up for south and west facing shores this weekend into early next week. A moderate, long period south swell will peak near the High Surf Advisory threshold by the middle of next week.
Along east facing shores, trade wind swell and moderate trades will support surf slightly below seasonal averages. Surf will build this weekend in response to strengthening trade winds. Surf along north shores remains flat to tiny through the period, with a slight bump possible on Sunday from a small, medium-period north swell.
Small Craft Advisory from 2 PM this afternoon to 6 PM HST Saturday for Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.
