Mostly dry and pleasant trade wind weather will continue into Monday, with brief passing showers favoring windward and mauka areas. A significant pattern change is then expected by Tuesday as an upper disturbance and surface low pressure evolve northwest of the islands. This system is expected to draw deep tropical moisture northward over the state, leading to an extended period of southerly winds, increasing chances for widespread rainfall, and renewed flooding concerns from Tuesday through next weekend.
A relatively benign trade wind pattern remains in place across the Hawaiian Islands this morning and is expected to persist into Monday. Moderate easterly trades will continue to focus mainly light showers along windward and mauka slopes, while leeward areas remain dry.
Global guidance remains in good agreement and depicts a notable amplification of the large-scale pattern across the central Pacific by Tuesday. A series of shortwave troughs digging southeastward toward the state will translate to large-scale upper height falls a cross the region. The surface ridge to the north will gradually erode as an attendant broad surface trough evolves to the west. This will disrupt the typical trade wind flow by Monday night and allow low-level winds to gradually veer out of the southeast to south Tuesday through Wednesday.
This transition to southerly flow will draw deep tropical moisture northward into the islands beginning Tuesday and especially by Wednesday. Precipitable water values are forecast to climb toward 2 inches or higher, signaling a marked increase in moisture. This increasing moisture combined with daytime heating, sea breezes, and a potent shortwave trough moving through may even trigger some heavy showers over interior areas on Tuesday.
Confidence is increasing that the second half of the week may present the highest potential for widespread rainfall and flooding impacts across the state. During this time, upper-level forcing is expected to strengthen as a strong shortwave rounds the base of the trough, potentially enhancing lift over the region while low pressure organizes west of the islands. The combination of deep moisture, increasing large-scale forcing for ascent, and gusty southerly winds favors a transition toward a more widespread and organized rainfall event. In addition to the rainfall concerns during this period, some strong to severe thunderstorms along with gusty downsloping winds for north and windward sides of the islands can't be ruled out.
Although forecast confidence begins to lower by next weekend due to model differences, the general large-scale pattern could linger. This scenario suggests the potential for rain may persist through next weekend and beyond for at least parts of the state or where the deep moisture axis anchors.
Regardless of the precise evolution, antecedent conditions remain a significant concern. The islands have experienced significant flash flood events in recent weeks, and despite several days of drier trade wind weather, soils remain vulnerable in many areas. As a result, any period of heavy rainfall may quickly lead to enhanced runoff, renewed rises in streams and reservoirs, and localized flash flooding concerns.
This system will continue to be monitored closely over the coming days, as additional adjustments to timing, duration, and impact messaging are likely as this change in the pattern evolves.
Moderate trade winds will continue today and tonight. Isolated light showers will focus mainly on windward slopes, with brief MVFR conditions are possible. Otherwise, VFR conditions are expected to prevail.
No AIRMETs are in effect and none are expected today.
Moderate to locally fresh trades will persist during the next couple days as high pressure north of the islands moves slowly eastward. Winds will ease and shift southeasterly Monday night through Wednesday as a front stalls out and merges with a trough just west of the islands. The winds will turn more southerly and increase to moderate and strong levels on Thursday as a storm system develops along the trough west of the state.
A series of overlapping small northwest swells will keep some small surf in place along north facing shores through Monday. A new long-period northwest swell will fill in Monday night and Tuesday, giving a more noticeable boost to north shore surf late Tuesday through Wednesday, followed by a gradual decline Thursday into next weekend.
Overlapping southerly swells will keep some small surf in place along south facing shores through Monday. A new long duration, and slightly larger long-period south swell, will fill in Monday night. This swell will give a more noticeable boost to south shore surf Tuesday into next weekend. Strengthening southerly winds could lead to choppy conditions by late next week.
East shore surf will remain small and below climatological levels during the next 7 days due to a lack of strong trade winds over and upstream of the islands.
None.
