textproduct: Honolulu

SYNOPSIS

Moderate trades focus showers windward and mauka through the weekend. A strengthening low then brings potential for heavy rain to the islands beginning Tuesday or Wednesday. Given recent flood events and soils that remain only partially recovered, the potential for runoff and flash flooding impacts may increase rapidly.

DISCUSSION

Today through Monday. Moderate trades focus showers windward and mauka. A zonally oriented subtropical jet has sagged south allowing high clouds to clear. The STJ will get a slight bump north again tomorrow allowing high clouds to return. Benign stable trade wind weather through Monday.

Tuesday. Potent shortwave of interest is seen over Japan on this morning's water vapor imagery. The existing longwave pattern will serve as a waveguide allowing this fast-moving wave to deepen the existing trough northwest of the islands early next week. The first sign of its arrival will be upstream surface pressure falls that will cause winds to weaken and veer to southeasterly Monday night into Tuesday. Combined with blocked flow in the shadow of the Big Islands, Tuesday will be favorable for sea breeze development. Steadily rising PWAT values during this time as tropical moisture is drawn north combined with ample mid-level forcing during midday will set the stage for potentially heavy afternoon showers over island interiors. There will be more clarity on this potential as this period enters the high res modeling window during during the next couple of days. This first wave will be on its way out by late Tuesday, its primary role serving to prime the local atmosphere by drawing PWATS approaching 2" northward. Any rain with this initial push of moisture is most likely to be stratiform in nature and not particularly heavy.

Wednesday through Thursday night. Similar evolution of a trailing wave is expected, this time achieving better phasing with mid- latitude energy allowing pressure falls to evolve into organized low pressure centered NW of the islands. With deep moisture in place and continuing to increase, mid-level forcing steadily increases Wednesday into late Thursday. Superpositioned with outstanding jet support by Wednesday night, the Wed-Thurs time period still appears to be the the target period for greatest flooding potential. At this time, guidance generally favors the western end of the state, but the entire state is within the margin of error at this lead time and flooding may still impact any/multiple islands. Of note, recent solutions have converged on a consensus for a stronger low developing in closer proximity to the islands. This will result in elevated potential for downsloping winds most likely on Thursday. While the ambient wind field is not overly strong, southerly 925mb winds in excess of 30 kts may be sufficient to generate Wind Advisory level gusts in favored locales located N/NE of steep terrain.

Friday onward. The north central Pacific will remain a favored area for deep troughing through mid-April. At the same time, the moisture band associated with next week's low will likely remain in place over the islands for some time. This raises the potential for additional waves of heavy rain to develop next weekend or beyond. The duration/ending time of this particular event is therefore much less certain than usual.

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 next week may quickly lead to enhanced runoff, renewed rises in streams and reservoirs, and localized flash flooding concerns. Also worth noting that while not not a kona low, the difference is largely academic and does not change the expectation for flooding impacts.

A Hydrologic Outlook summarizing current information has been issued.

AVIATION

Moderate trade winds will slowly weaken over the next 24 hours. Isolated light showers will continue to be carried in to the windward slopes, with brief MVFR conditions are possible. Otherwise, VFR conditions are expected to prevail.

No AIRMETs are in effect and none are expected overnight.

MARINE

Moderate to locally fresh ENE trades ease through Sunday as high pressure north of the area advances east. By Monday a front will approach the region from the NW and a trough will move up from the S. As a result, winds will veer to ESE. Tuesday through the remainder of the week, SE to S winds will prevail and could become strong toward the later half of the week. These features will be driven by an upper level trough which may generate isolated thunderstorms over the coastal waters by the middle to latter half of next week.

Surf along N shores remains small into this evening as small medium period WNW (310 degree) energy fills in. Tonight into Sunday, small, medium- period NW (330-340 degree) energy will also build. These swells will bump surf slightly upward for N and W shores. Monday night through Wednesday, a moderate medium- period WNW (310 degree) swell will fill in and increase surf to just below the advisory threshold. This swell will linger and gradually decline through the latter half of the week.

Along S shores, expect minor pulses from the South Pacific to provide periods of small surf through the weekend until a larger long-period S swell fills in Monday and lasts through the week. Strengthening southerly winds will lead to choppy conditions by midweek.

E shores will subside as trades weaken and will remain small as winds veer to SE. By the middle of next week, a small medium period swell will provide a bump to surf.

HFO WATCHES/WARNINGS/ADVISORIES

None.