Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
666 FXHW60 PHFO 080632 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 832 PM HST Tue May 7 2024 .SYNOPSIS... The trades will ease tonight, then hold at moderate levels over the eastern islands through Thursday, while lighter trades and leeward land and sea breezes prevail over the western islands. Showers will favor windward slopes and coasts at night and interior and leeward areas each afternoon. More unsettled weather with thunderstorms and localized downpours appear in store as a disturbance aloft moves southeastward across the islands Friday and Saturday. Conditions should improve Sunday through early next week, as the most unsettled weather pushes east of the state. && .DISCUSSION... Currently at the surface, a cold front is located around 900 miles northwest of Kauai, while a 1032 mb high is centered 2000 miles northeast of Honolulu. Moderate to locally breezy trade winds prevail across the state, strongest over the eastern islands. Infrared satellite imagery shows partly to mostly cloudy conditions, with cloud coverage the greatest over windward areas and leeward sections of the Big Island. Radar imagery shows scattered showers moving into windward slopes and coasts, with mostly dry conditions in leeward areas, except for leeward Big Island where scattered daytime heating showers persist. Main forecast concern continues to revolve around the potential for some active and unsettled weather Friday and Saturday. The cold front to the distant northwest will shift southeastward and closer to the islands during the next couple of days, eventually weakening into a broad trough over the islands on Friday. As a result, the trades will ease during the next few days. Moderate trade winds will hold over the eastern islands Wednesday and Thursday, while lighter trades allow for land and sea breeze development in leeward areas. The trades lower even more on Friday, allowing for sea breeze development statewide. The broad trough to appears to gradually dampen out over the weekend, which could allow some light to moderate trade winds to build back over the islands, but there are conflicting signals on this. By early next week, a new approaching cold front will begin to shift the low level boundary layer flow around to the southeast, maintaining moderate easterly trades over the eastern islands, while land and sea breezes become more dominant over the western end of the state. As for the remaining weather details, a hybrid trade wind pattern should prevail through Thursday. Showers will favor windward and mauka areas overnight and during the morning, then transition over to interior and leeward areas each afternoon into the early evening. As the boundary layer flow becomes light and variable Thursday night, most of the shower activity should remain offshore or close to the islands coasts. The weather pattern becomes much more interesting beginning Friday and continuing through Saturday as an upper level low swings southeastward across the island chain, with broad surface troughing holding over the state. Precipitable water values don`t appear all that high, generally holding at or below 1.5 inches over the smaller islands, while reaching as high as 1.7 inches over southeastern sections of the Big Island. As a result, a widespread heavy rain threat doesn`t appear all that high, although some downpours will be possible that could bring some minor flooding impacts. We will continue to monitor the latest model trends as the need for a Flood Watch is still not out of the question, particularly over the eastern end of the state. The more concerning threat revolves around the potential for severe thunderstorms. Mid-level 700-500 mb lapse rates reach 7 to 7.5 C/KM as the upper level low pivots southeastward through the islands, while surface based CAPE values increase to between 1000 and 1500 J/KG, and 0-1 KM MUCAPE values reach as high as 2000 J/KG. This in combination with 0-6 KM Bulk Shear values of 25 to 40 knots could lead to some stronger thunderstorms capable of producing hail and strong gusty winds. The one element that seems to be holding back the potential for well organized convection is the lack of a significant surface trigger aside from sea breezes. Nevertheless, we will need to keep a close eye on how this pattern evolves during the next couple days. The most unstable airmass slides east of the state Saturday night, which should bring a return of a hybrid trade wind pattern featuring windward and mauka showers at night and interior and leeward showers during the afternoon and early evening hours. This pattern appears to generally hold through early next week. && .AVIATION... Moderate trade winds will gradually ease to become light to moderate tomorrow. Bands of light showers will continue to favor windward and mauka areas, and be most active during the overnight hours. Brief MVFR conditions are to be expected in passing showers, otherwise VFR conditions will prevail statewide. No AIRMETs are in effect at this time, nor are any expected to be needed...unless low clouds and showers increase in coverage across windward areas more than expected tonight into tomorrow morning. && .MARINE... Moderate to fresh trade winds will weaken over the next several days as high pressure northeast of the state moves further away and a front approaches the islands from the northwest. The front will weaken into a trough over the islands Friday as wind speeds look to ease and become light to gentle Friday and Saturday with southeast flow over the eastern half and northeast to east flow over the western half of the state. Gentle to locally moderate east to southeast flow looks to trend into the early half of next week across the coastal waters. An upper level low will move across the island chain late Friday through Saturday which will increase the chance for heavy showers and potential thunderstorms. The current northwest swell will decrease through Wednesday followed by a reinforcing short-period north northwest swell Friday into the weekend. Surf along south facing shores will remain steady through tonight. Forerunners from a larger and longer lived south- southwest swell will arrive Wednesday, as surf is expected to be near, or at, High Surf Advisory criteria when the swell peaks Thursday into Friday before declining into the weekend. Surf along east facing shores should remain small as winds remain light through the rest of the week. && .HFO WATCHES/WARNINGS/ADVISORIES... High Wind Warning until 6 AM HST Wednesday for Big Island Summits. && $$ DISCUSSION...Jelsema AVIATION...Farris MARINE...Shigesato