textproduct: Honolulu

SYNOPSIS

Low pressure will draw deep tropical moisture over the state today. Waves of heavy rain capable of producing flash flooding are expected Wednesday through the end of the week. SE winds may become strong as well.

DISCUSSION

Today through Wednesday night. The initial mid-level shortwave is advancing ENE while remaining north of the islands today. Forcing associated with this feature has maintained a line of moderate to heavy showers that reached Oahu during midday. The showers are maintained by a corridor of weak low- level convergence and are encountering an increasingly moisture-rich environment as they advance east. This has allowed the band to hold together for much of the day, but diminishing forcing through this evening should allow this activity to dissipate with time.

A much more potent shortwave is seen on water vapor imagery anchoring the southern extent of a longwave trough that extends NE to Alaska. As longwave troughing amplifies immediately upstream of the islands, downstream ridge amplification will suppress deep convection tonight through much of Wednesday as rather strong mid- level subsidence takes shape through midday Wednesday. ESE-SSE winds gradually strengthen during this time in response to upstream pressure falls. Meanwhile, southerly winds aloft carry deep moisture characterized by PWAT values around 2" into the forecast area during the day Wednesday. Showers will become increasingly widespread in the afternoon, mainly over the western end of the state. The instability axis will lie over and immediately west of Kauai. This area will escape much of the developing subsidence as it remains under cyclonic flow aloft for much of the next 24 hours. Forcing is maximized within this corridor as well making it the favored location for the heaviest rainfall Wednesday afternoon through Wednesday night.

Thursday. Little change during the day Thursday as the upper low deepens but remains about the same distance west of Kauai. As a result, the heavy rain axis will remain largely stationary this time either over Kauai or just west of the island. Otherwise, waves of warm advection within the decreasingly stable environment will bring periods of locally heavy showers through Thursday.

Thursday night through Friday night. Intense upper-level forcing develops. The combination of right entrance dynamics and curved flow will maximize upper-level divergence over the western end of the state by Thursday evening. Meanwhile, vorticity advection is augmented by the descending/left entrance branch of the jet bringing a stratospheric potential vorticity intrusion as far down as 500mb, a testament to the intensity of emerging dynamic forcing. A mature jet-front system takes shape west of Kauai as this forcing rapidly organizes and ramps up and showers and thunderstorms along the frontal zone begin to intensify. At the same time, the strengthening surface low will cause prevailing ESE/SE winds at the surface to increase to 25-30kts sustained. 925mb southerlies will strengthen to around 40 kts by Thursday evening. This is not a traditional downsloping direction, making potential over typical downsloping areas like Windward Oahu a tough call in terms of wind potential. The southerly direction may instead favor the northern coast of Kauai. Convection, however, will be capable of mixing down these stronger winds making localized wind gusts around 50 mph a possibility Thursday evening into Friday afternoon. At this time, it appears that a Wind Advisory may be needed for select zones on Thursday. The front and attendant heavy rain then surge eastward Thursday night bringing the heaviest rainfall to Kauai late Thurs night into early Friday, Oahu during the first half of Friday, and Maui County and the Big Island during the second half of Friday.

High-end advisory winds are likely on the Big Island Summits and possibly Haleakala Summit on Maui Thursday night through Friday. Snow potential on the Big Island Summits will be limited by marginal temperatures.

Saturday onward. The resident band of tropical moisture lingers over the islands for the foreseeable future, even as trades potentially return during the middle of next week. Weakly cyclonic flow aloft, especially over the western end of the state, will favor the occasional upper-level disturbance tracking over the area and subsequent shower development. This may yield periods of localized heavy rain, but that remains to be seen.

AVIATION

Southeast winds have spread across the region, and a significant area of strong showers (and isolated thunderstorms) is moving over Oahu and heading towards Maui County. These are forecast to get to PHMK between 5 and 6 PM HST. Other strong showers were near PHTO. Additional showers are expected on all islands the rest of this afternoon and tonight. MVFR and isolated IFR conditions are likely in the stronger showers.

AIRMET Sierra is out for mountain obscuration on Kauai and Oahu. This will be expanded to Maui before the showers arrive.

MARINE

Southerly winds are expected to increase tomorrow and Thursday, as a low develops across a stalled front. Showers, some locally heavy, will be possible through Friday as a result of the southerly winds bringing tropical moisture to the north over the islands. A few isolated thunderstorms are moving down the island chain are thunderstorms are possible over the offshore waters through at least Friday.

A moderate long-period northwest swell is filling in boosting north shore surf late today through Wednesday. A small, short- period north-northeast swell will fill in today before peaking tomorrow and subsiding Thursday. This overlap will make for some rough choppy conditions along north facing shores.

A slightly larger, long- period south swell, will gradually increase today into tomorrow. Surf could reach advisory levels tomorrow before slowly easing Thursday into the weekend. Strengthening southerly winds will make for rough and choppy surf by late next week.

East shore surf will remain small and below the seasonal averages due to a lack of strong trade winds, Some wrap from the northerly swells is possible.

HFO WATCHES/WARNINGS/ADVISORIES

Flood Watch from Wednesday morning through Friday afternoon for all Hawaiian islands.

Winter Storm Watch from Wednesday morning through Friday afternoon for Big Island Summits.