textproduct: Honolulu

SYNOPSIS

Limited showers windward and mauka into next week as moderate trades become breezy tonight into Sunday night. More widespread rain develops over the western end of the state by Wednesday. Periods of heavy rain will be possible over and around Kauai and potentially Oahu during the second half of the week.

DISCUSSION

Today through Tuesday. Stable, moderate trades prevail as the state remains under prominent mid-level ridging and is no longer enveloped in the moist SE flow trailing Hurricane Nolo. The inversion hovers around 8kft at Hilo but is rather ill-defined at Lihue courtesy of a sheared out upper wave that is slowly migrating past the state. Breezy trades develop tonight through Sunday night in response to a weak tropical disturbance that is advancing westward well south of the state. Limited coverage of windward and mauka showers during this time. By Tuesday, the gradient may be sufficiently weak to support sea breeze development and attendant development of afternoon showers over island interiors.

Wednesday through Saturday. Excellent consensus exists among the guidance that a very deep and early-season mid-latitude trough will extend south into the subtropics and be on the doorstep of Kauai by Wednesday morning. Cyclogenesis will occur around 30N latitude to the NW of Kauai causing deep layer flow to veer to southerly. This southerly flow is modeled to entrain the aforementioned tropical disturbance (presently around 14N/150W) providing a boost to a plume of deep tropical moisture that will be drawn northward Wednesday night into Thursday. Characterized by PWATs of 2" or higher and dewpoints in the 70s, this airmass will easily support areas of heavy rainfall. Divergence within the right entrance of a strong upper jet is modeled to be outstanding over the western end of the state as this moisture advances northward while mid-level forcing for ascent remains rather broad and lacks focus. Nonetheless, deep layer ascent beneath the right entrance should be adequate to maintain a decent coverage of showers during this time. The GFS suggests that mid-level entrainment of dry air will maintain higher stability and limit potential for deep convection while the EC considerably more aggressive with its depiction of actual rainfall. Either way, limited availability of conditional instability and paltry mid- level lapse rates will keep updraft intensity in check and will limit potential for thunder.

Model consensus breaks down during Thursday owing to the 12z GFS's depiction of a suspiciously intense and compact surface low. This low then quickly occludes and wraps westward beneath the upper low allowing trades to aggressively return as early as Thursday night. The EC's depiction is considerably more reasonable and suggests a longer-lived threat for heavy rain and flash flooding potential still focused mainly on Wednesday night through Thursday for Kauai and Oahu but then potentially lingering over Kauai into the weekend. The 18z GFS has trended toward the EC's surface low depiction and consequently favors a longer lived event and increases confidence in this diagnosis. A Flood Watch may eventually be needed for this time period for Kauai and/or Oahu.

AVIATION

Trade wind pattern will continue into Sunday with mostly dry and prevailing VFR conditions. Isolated showers remain possible across windward and mountain areas tonight and Sunday.

No AIRMETs in effect.

MARINE

Moderate to locally fresh trade winds will build in this evening as the high pressure strengthens to the north. A Small Craft Advisory goes into effect this evening for the windier waters and channels around Maui County and the Big Island through Sunday afternoon and may need to be extended. Locally strong trades may persist into early next week before high pressure to the north weakens again and winds start to diminish. By the middle of next week, troughing developing northwest of the islands may cause winds to veer out of the southeast once again.

Surf along south facing shores will steadily decline through the weekend. A new small, long-period south swell, originating from a large fetch of gale force winds southeast of New Zealand earlier this week, is expected to slowly fill in Sunday evening and peak Monday through Wednesday. This will push surf to slightly above seasonal averages for the first half of next week.

A small bump in surforecast to arrive beginning Sundaf along north-facing shores will decrease today as a small, short period north swell fades. Another small bump in north shore surf is possible early next week as a small, medium-period northwest swell arrives late Sunday and peaks on Monday. This will be followed by a slightly larger long-period northwest swell peaking on Wednesday, giving a good boost to north shore surf for mid-week. Surf along east facing shores will increase and become more choppy through tonight as easterly trades build back into the region.

FIRE WEATHER

Moderate trades will become breezy tonight through Sunday night as drier air works into the area during the day tomorrow. RH values are forecast to hover just above the critical threshold for fire weather but may nonetheless approach it during the afternoon. Fire weather risk then diminishes through the remainder of the week as moisture increases, winds decrease, and showers increase in coverage especially over the western end of the state. The inversion will range from 6kft to 8kft for the next couple of days.

HFO WATCHES/WARNINGS/ADVISORIES

Small Craft Advisory from 6 PM this evening to 6 PM HST Sunday for Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.