A wet trade wind weather pattern developing over the Hawaiian Islands will produce increasing rainfall, heavy at times, along with a few thunderstorms into Saturday. All island areas will see enhanced shower activity with the highest rainfall amounts favoring the windward mountain areas. Showers will decrease a bit this weekend as the upper low drifts westward away from the state. However, another weak upper low moves in near Kauai and Oahu by Monday triggering another round of wet trade wind weather potentially lasting into next week Wednesday.
This mornings satellite imagery, water vapor channel, clearly shows two upper lows approaching the islands from the north. Divergent winds are shown along the southern and eastern flank of this low pressure system with thunderstorms forming around the cold core center of the low. In the lower levels, using the infrared satellite channel, we see long bands of unstable clouds likely associated with remnants of an old East Pacific cold front riding into the islands on the trade winds. These cloud bands produce enhanced showers caused by the instability surrounding the upper low and the convergent lifting of the trade winds forcing these unstable clouds orographically up the windward mountain slopes.
For today we expect to see easterly trade winds into the moderate to locally breezy range with increasing clouds and showers as the upper low moves in from the north and low level cloud bands riding in from the east begin to converge over the island chain. This mornings subsidence temperature inversion heights, as measured by upper air balloon soundings at Lihue and Hilo at 2 AM HST (12Z), have risen to the 7,000 to 8,000 foot level. This means scattered to numerous showers are likely near island mountains.
An extended period of wet weather will continue to affect all islands this week with periods of numerous showers falling across the state into Saturday. Some of these showers will be locally heavy at times, and thunderstorms are likely over the western islands of Niihau, Kauai and Oahu. The Big Island may also see a few thunderstorms on Thursday. Storm total rainfall estimates for this entire two to three day event may exceed 2 inches in terrain favored windward locations. Our confidence in which islands will see the heaviest showers is currently not high enough to trigger a flood watch at this time. However, a flood watch may be needed once we see how this pattern evolves in order to narrow down potential flooding threats to specific islands in the chain.
Showers will linger over the islands on Saturday, with a brief decrease in shower activity on Sunday as the upper low drifts westward moving away from the island chain. However, another weak upper low moves right in from the north on Monday, setting up in an almost identical location near Kauai and Oahu. Cold air surrounding this low and divergence aloft will bring yet another round of enhanced shower activity to the islands next week, from Monday into next Wednesday. Stay Tuned.
Moderate to locally breezy easterly trade winds are expected with increasing clouds and showers over the next 12 to 24 hours. An upper level low will move in from the north and set up over the western half of the state as a bands of unstable low clouds drift into the region from the east. The convergence of the upper low with these unstable convergent low cloud bands will increase shower activity into Saturday.
Brief periods of MVFR conditions are possible in passing showers mainly over windward and mountain areas favoring the overnight and early morning hours. Isolated IFR conditions are possible in heavier showers.
No AIRMETs currently in effect. AIRMET Sierra will likely be issued later today for mountain obscurations. AIRMET Tango may be issued later today for upper level turbulence associated with stronger winds surrounding the approaching upper low.
Moderate to fresh trade winds are expected today as a weak surface troughing over the western end of the state advances west and dissipates. A Small Craft Advisory (SCA) for winds around Maui County and the Big Island remains in effect until 6 AM HST Thursday. An approaching upper low brings a chance for thunderstorms to area waters from later tonight into the weekend.
A moderate to long period NW (310-320) swell that peaked yesterday will slowly decline through the rest of the week into the weekend.
Small, medium period S swell continues through the week with minor longer period pulses arriving this morning and Saturday. E shores remain small through the near term, but will see increasing short period action as trades strengthen mid-week.
Small Craft Advisory until 6 AM HST Thursday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.
