textproduct: Honolulu

SYNOPSIS

Unsettled weather will persist across the Hawaiian Islands through the week in the wake of the recent kona storm. Although a brief lull in widespread heavy rainfall is expected through midweek, deep tropical moisture will remain in place and support periods of showers, particularly over the eastern end of the state. Conditions are expected to deteriorate again during the second half of the week as another upper-level disturbance and surface low approach from the west, bringing increasing chances for widespread rainfall and renewed kona winds. With soils already saturated, additional rainfall could quickly lead to runoff and flooding impacts.

DISCUSSION

In the wake of the significant kona low that impacted the state over the past several days, a broad and persistent upper-level trough remains anchored over the region. While the kona low has weakened and lifted away, the larger-scale pattern remains largely unchanged, with deep tropical moisture continuing to stream northward over the state. Precipitable water values remain anomalously high, maintaining a moist air mass across the region.

Model guidance continues to indicate a relative decrease in widespread heavy rainfall through the next couple of days, particularly over the western end of the state. However, this should not be interpreted as a return to benign conditions. Periods of showers will persist within the moist/deep southwesterly flow, with locally heavy rainfall still possible, especially across Maui County and the Big Island where the axis of deepest moisture is expected to remain focused through midweek.

By the latter half of the week, the upper-level trough is forecast to deepen west of the state. This feature is expected to re-expand the axis of anomalous moisture back across the entire island chain while increasing large-scale ascent. At the surface, low pressure is forecast to develop and approach the islands, with potentially breezy kona winds and dewpoints climbing back toward 70 degrees.

This evolving pattern will support a renewed increase in shower coverage and intensity from late week into the weekend, with model consensus suggesting an additional several inches of rainfall possible over portions of the state during this period. While this next system does not currently appear to be as intense as the recent event, it will impact an already highly saturated environment.

Given the antecedent conditions, including elevated streams and saturated soils, even moderate rainfall rates may quickly lead to runoff issues. The threat for flash flooding will remain elevated statewide through the week, with the potential for more significant impacts returning late in the week into next weekend.

AVIATION

Showers continue to bring intermittent MVFR and IFR conditions to PHKO on the Big Island this evening. Bands of showers continue to move in from the west this evening as a surface trough moves into the state. This band of showers will move through Kauai, Oahu, and Maui before reinforcing the showers on the Big Island Tuesday. Occasional MVFR is likely with isolated periods of IFR through the forecast period.

AIRMET Sierra has been expanded for mountain obscuration for Kauai and maintaining it for Maui, Lanai, and the Big Island and will likely be extended through Tuesday morning.

MARINE

The gradient will remain rather weak through Wednesday, with light and variable winds prevailing over the waters. A brief period of light to moderate trades will be possible Thursday as a weak ridge builds north of the state, followed by light to moderate S winds on Friday as a new system approaches from the west.

Surf along S shores is trending down as S to SW winds ease. A new long period south swell will then arrive late tonight and Tuesday giving an extended boost to S shore surf through late this week.

A moderate short-period W swell associated with the recent kona storm is affecting N and W facing shores today, but surf remains below the advisory threshold. The short-period W swell will lower tonight and Tuesday while a new medium-period NNW swell fills in. This NNW swell could elevate surf to near the advisory threshold Tuesday night and Wednesday. The swell then shifts more N and lowers Thursday into the weekend.

Surf along E facing shores will remain well below normal through late week as trades over and upstream of the islands remain disrupted.

HFO WATCHES/WARNINGS/ADVISORIES

Wind Advisory until 6 PM HST Wednesday for Haleakala Summit-Big Island Summits.