Light and variable winds will allow land breezes to develop overnight, helping to clear low clouds and any showers out of leeward and interior areas. A cold front will bring a brief round of showers and perhaps isolated thunderstorms early Wednesday morning into Thursday. This will be followed by a reinforcing shot of cool weather. Trades return briefly Friday into Saturday before they are disrupted again by another front approaching from the northwest.
Current radar and satellite imagery show some lingering clouds and a few showers over leeward slopes and island interiors due to sea breeze activity earlier this afternoon. A land breeze has kicked in and any lingering low level clouds should clear out overnight. A cold frontal band located roughly 200 miles northwest of Kauai has some middle to high clouds extending ahead of this boundary and filtering in from the northwest around Kauai and Oahu. Latest hi- resolution model guidance shows modest prefrontal clouds and showers as we head into the evening hours due to relatively dry and stable environment around the state. The front is expected approach and move over Kauai shortly after midnight before quickly moving over Oahu during the Wednesday morning traffic hours. The front is expected to weaken has it moves over Maui County Wednesday afternoon before becoming diffused over the Big Island and possibly shifting just east of the state Wednesday night into Thursday. Showers along the frontal boundary may produce locally heavy rainfall with slight chance of thunderstorms. These showers are expected to clear out quickly as moderate to locally breezy northwest winds fill in behind the front, ushering drier air with dew points dropping into the lower to mid 50s. There still remains some uncertainty regarding how far south this drier airmass will penetrate with confidence decreasing the further south down the island chain you go, especially as the front weakens and the strong northwest winds ease. For this evening's forecast package, even though the probabilities of thunderstorms remains fairly low we will continue to carry isolated thunderstorms along the frontal boundary as it starts to move down the island chain tomorrow morning, then keep it mostly over the eastern waters as the front weakens and gradually progresses further south.
On Thursday, the western end of the state will be cool and dry while the front dissipates over the Big Island and its remnant moisture lingers over the eastern end of the state, especially near the Big Island. By this point, the upper level low will have deepened and brought height falls and lower temperatures aloft across the island chain. Model guidance shows 500 mb temperatures dropping to -12C to -13C over the Big Island by Thursday afternoon. This instability combined with lingering moisture and diurnal heating may be enough to bring an isolated thunderstorm or two to the Big Island Thursday afternoon and a chance for wintery precipitation at the summits. We will continue to look into this more for the Wednesday morning update.
On Friday, winds will veer and easterly trades will strengthen to moderate levels as high pressure builds just north of the state. At the same time, moisture that was being pulled north across the eastern North Pacific earlier in the week will shift across the island chain in the redeveloping trades. That plume of moisture along with the remnant moisture from the dissipated front will increase the chance of windward and mauka showers Friday into Saturday and help dew points to recover back into the 60s. The pressure gradient will weaken again Saturday into Sunday in response to another approaching cold front from the northwest, disrupting the trades once again and bringing lighter winds across the area.
Forecast confidence decreases Sunday night and beyond due to model discrepancies. The general pattern looks to be similar to what we will see Wednesday and Thursday as another trough digs down across the Central Pacific and spawns a surface low that will drag a front into our area. However, the strength of this upper- level trough and resulting surface low is a bit uncertain and model guidance is hinting at an area of prefrontal convergence and shortwave energy that could bring rain to the western end of the state ahead of the next front from lingering moisture.
Vfr conditions and land breezes can be expected this evening. A cold front will then track from Kauai to Maui after midnight through Wednesday. A band of low clouds and showers are expected to accompany the front, bringing some MVFR cigs/vsbys with it, along with increasing north to northwest winds in its wake. VFR conditions will prevail over the Big Island through Wednesday evening.
No AIRMETs are currently in effect. AIRMET Sierra will likely be needed for mountain obscuration later tonight and Wednesday as the cold front moves southeastward down the island chain.
AIRMET Tango may also be needed for low level turbulence over and downwind of the terrain behind the front. AIRMET Tango may also be needed for moderate upper level turbulence Wednesday afternoon and Wednesday night across much of the state.
Winds across the coastal waters have become light and variable as the approaching front is entering the northwest offshore waters this evening moving southeast around 15 to 20 knots. Gentle to moderate southwest to west winds are expected to develop tonight into Wednesday in advance of the front. The front is expected to enter Kauai northwest waters in the early morning hours Wednesday, over Oahu waters mid- morning Wednesday, and Maui County waters Wednesday afternoon and evening. The front will likely be weakening and slowing in forward speed after crossing Oahu, dissipating near the Big Island Thursday. The front will bring a relatively brief period of rainfall, with the potential for thunderstorms and heavy showers, mainly over Kauai and Oahu waters.
A short-lived period of strong northwest winds following the frontal passage (with a longer-lived increase in seas) has prompted the issuance of a Small Craft Advisory (SCA) for many zones from Wednesday morning into Thursday. Winds will diminish as the front dissipates, with a surface ridge moving over the islands from the west by Thursday night. A surface high will pass north of the islands Friday into Saturday, supporting a moderate east-northeast trade wind flow. Long range guidance indicates the potential for another frontal passage late in the weekend.
A new moderate long period northwest swell will slowly building through Wednesday. Offshore NDBC buoys 51001/51101 in the northwest offshore waters have started to see long period forerunners this evening associated with this new swell. Surf along north and west facing shores are expected to potentially reach High Surf Advisory (HSA) conditions across most north and west facing shores from Kauai to Maui as the swell peaks Wednesday. This remains somewhat uncertain, and will let future buoy observations overnight assist with that decision.
Surf is expected to be larger Thursday and Friday over north facing shores as a developing storm low continues to intensify due north of the islands near 32N. The mix of 8 to 14 second north- northwest to north swell will make for jumbled surf as it overlaps with the diminishing longer period northwest swell. This swell will be slow to decline into next weekend as the low will remain nearly stationary. Additionally, a weaker low north of the islands has generated a short-period (8-10 seconds) northeast swell that will build overnight, peak Wednesday, and diminish Thursday. A new NW swell is possible next Monday, otherwise no other significant swells are expected.
Small Craft Advisory from 6 AM Wednesday to 6 PM HST Thursday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu Windward Waters-Kaiwi Channel.
