textproduct: Honolulu

SYNOPSIS

A kona low will bring several rounds of moderate to heavy rainfall to the islands through the weekend resulting a risk of flash flooding.

SHORT TERM UPDATE

Issued at 907 AM HST Thu Mar 19 2026

Large scale environment generally remains weakly subsident this morning beneath subtle mid-level height rises and downward motion within a healthy right exit region of the upper jet. The low and mid- levels are nevertheless largely saturated courtesy of moisture left behind by the previous system, and a PWAT of 1.75" on this morning's Lihue sounding is largely representative of moisture confined to the lowest 25kft. A relatively deep layer of southwest flow has developed ahead of the nascent kona low which resides a few hundred miles west of Kauai at press time. Weak isentropic ascent within this corridor of SW flow is supporting a wide coverage of stratiform over the state this morning, particularly over Kauai and SW portions of the Big Island. The batch over the Big Island will likely be transient in nature, advancing over the S/SE slopes through today while the area of rainfall over & upstream of Kauai will have more staying power courtesy of upstream westerlies in the 850-700mb layer and attendant convergence aloft. Finally, weak convergence within SE flow in the boundary layer is supporting an increasing coverage of shallow convection upstream of Kauai. Radar observations suggest rain rates in excess of 1.0"/hr within these showers nad may pose a low-end nuisance flood threat for Kauai today. These showers may be aided by a very subtle shortwave embedded in the flow about 300 miles W of Kauai. Showers may therefore tend to develop eastward through the day eventually training over Oahu and posing a similar nuisance flood type of threat prior to the start time of the Flood Watch.

A wet night is in store for Oahu and Maui County as forcing gradually spreads eastward allowing rainfall intensity to increase as it shifts from Kauai toward the central portion of the state. Uncertainty increases by late tonight as the global models develop a secondary area of low pressure north of Kauai that appears to be mostly tied to convective feedback. While not entirely unrealistic, preference lies with something closer to the UKMet which allows a stronger surface low to develop as surface pressure falls remain tied to the strongest large scale forcing. The degree to which the models deepen the convectively-enhanced secondary low is the difference between The EC's eastward solution, the UKMet which is further west, and the GFS which lies in between. Given the solution preference and the slight westward trend noted in the GFS and UK, Kauai will be added to the Flood Watch with this morning's update.

PREV DISCUSSION

Deep cloud cover continues to develop over the Hawaii region this morning in response to a low pressure system moving in from the west with upper level troughing and sub tropical jet stream support. Winds will veer more southerly tonight as shower activity slowly increases across the state, pulling up additional moisture from the deep tropics, ahead of the approaching kona low. Two weak surface lows, supported by upper level troughing, will drift through the islands from later tonight through Sunday. The large scale forcing will move the heaviest showers to different islands in pulses of activity throughout this event.

A Flood Watch remains in effect for most of the Hawaiian Islands for this event. This watch may be expanded statewide as run to run model guidance continues to show differences in where and when the heaviest showers may develop.

The first pulse of increasing moderate to heavy showers and thunderstorms will move in ahead of the first surface low passing just north of the island of Kauai from tonight into Friday. The latest weather model consensus shows the best forcing for increasing moderate to heavy shower activity develops along a wind convergence band favoring the islands in Maui County and Oahu. The next pulse shifts into gear from Friday night into Saturday as the next surface low moves northeastward passing just north of Kauai. The band over Maui County shifts northward to Kauai, Niihau and Oahu during this time period. Stronger upper level dynamics from a passing trough, bulk wind shear from 35 to 50 knots and instability suggests stronger rotating thunderstorms will likely develop near the western islands. Kauai County may see increasing chances for heavy showers and thunderstorms from Friday night into Saturday.

By Sunday, the surface low rapidly moves away from the Hawaiian Islands towards the northeast direction. Cool northerly to northeasterly winds begin to filter into the western islands of Niihau, Kauai and Oahu with drying trends by Sunday night. Western islands in Maui County and the Big Island will continue to see periods of moderate to heavy showers.

On Monday, the heavy shower and thunderstorm threat ends as we quickly transition back to a northeasterly trade to last into Thursday with wind pattern. Cooler temperatures are expected to last into Thursday with enhanced showers due to a weak upper level troughing pattern, showers will favor the typical windward and mountain areas into next weekend.

AVIATION

Conditions will gradually deteriorate through the day as an upper-level disturbance and surface low approach from the west. Expect increasing chances for widespread rainfall across portions of the state. The low-level flow will gradually veer from the southeast today to south-southwest by Friday, with speeds increasing into the moderate range at exposed terminals. Expect periods of MVFR/IFR CIGS/VSBYS, especially in showers.

AIRMET Sierra for mountain obscuration can also be expected for much of the area by tonight. AIRMET Tango is in effect for moderate turbulence for FL200-350. These conditions are expected to persist today. AIRMET Zulu for light icing in layer 150-FL250 is also anticipated.

MARINE

Issued at 907 AM HST Thu Mar 19 2026 Light to moderate southeast winds will strengthen tonight as surface low pressure west of the islands and an associated trough stretching north of the state deepen. Moderate to locally fresh south to southwest winds will prevail Friday and Saturday, and periods of heavy showers and thunderstorms are expected. The low will lift to the north of the state on Sunday, likely allowing moderate northerly winds and lower chances for rainfall to develop around Kauai. On Monday, high pressure building north of the state will push a surface trough eastward over the islands. Fresh to strong northeast winds will require a Small Craft Advisory (SCA) over much of the island chain, though variable winds could linger around the Big Island through the day.

A moderate, medium period north-northwest swell will continue to trend down today, then shift out of the north-northeast Friday. A moderate reinforcing north-northeast swell is expected Saturday and Sunday, followed by a potentially larger pulse of north- northeast swell early next week that could produce surf near the High Surf Advisory level. A small, overlapping west-northwest swell is expected to arrive late Saturday and hold through early next week.

South shores will see continued surf just below advisory criteria as a long period south swell holds on today. The swell will diminish tonight and Friday, followed by a small south-southwest swell this weekend. Aside from areas exposed to wrapping north swell, surf along east facing shores will remain well below average. As northeast trade winds develop early next week, rough surf will return to east facing shores.

HFO WATCHES/WARNINGS/ADVISORIES

Flood Watch from 6 PM HST this evening through Sunday afternoon for Hawaii.