Hawaii Weather Synopsis: Bouts of unsettled weather will continue this week. An upper level low just south of the Big Island will maintain a threat for heavy showers over the eastern end of the state today with thunderstorms possible. Strengthening trade winds will focus rainfall along windward terrain, though leeward areas will experience showers at times. Another round of unsettled weather is expected state-wide Friday through the weekend.
A Winter Weather Advisory is in effect until 4 pm HST this afternoon for the Big Island summits.
A Small Craft Advisory is in effect until 4 pm HST this afternoon for waters around Kauai.
Click on the following link for the latest National Weather Service advisories and warnings: Wind/Surf/Flood related advisories or warnings
Broad Overview of Hawaii’s Weather
Based on the NWS Area Forecast Discussion for Hawaii, issued 2 Nov 2011 at 9:30am HST
Deep layered high pressure north of the state is keeping a mean positively tilted weakness situated over Hawaii. The orientation of the main upper low will support the threat for heavy rain and thunderstorms over the eastern portion of the state today. The remaining islands will continue to see persistent windward showers which will reach most leeward areas throughout the day with breaks in the clouds and showers allowing for periods of sun. Some convection may still produce light snow showers through this afternoon. Although conditions are looking better right now, NWS will leave the Winter Weather Advisory posted through 4 pm HST to catch any stray convection capable of producing light frozen precipitation above the freezing level. A cancellation before 4 pm HST is possible.
GOES water vapor imagery depicts another short wave impulse approaching the state passing 30°N. This will reinforce the low and cold pool aloft locally once it reaches Hawaii around sunrise tomorrow. This will again increase chances for heavy rain and thunderstorms across the state Thursday and Friday. Falling temperatures will drop to freezing levels so more frozen precipitation is possible across the Big Island summits.
The deeper layered high to the north (see latest weather map) will keep upper-level low pressure trapped over the area in a “rex block” (NWS definition: a blocking pattern where there is an upper level high located directly north of a closed low) for a prolonged period of time along with breezy trades at the surface. The low aloft will weaken and fill gradually over the weekend as it drifts west. This should provide some relief and drying late over the weekend, however confidence remains low at this point.
Trade winds will prevail next week. GFS and ECMWF models maintain a mean weakness aloft, however some stabilization should occur. Boundary layer moisture fields remain uncertain beyond the weekend but are still suggestive of a wet trade pattern.
Previous discussion issued at 400 am HST Wed morning, Nov 2: A wet, trade-wind shower pattern will continue today, with heavy showers possible over the eastern end of the state. An upper-level low centered just southeast of the Big Island is maintaining deep moisture over the state, as seen in the 1.6 to 1.8 inches of precipitable water registered on the overnight soundings. Instability associated with the upper low continues to trigger thunderstorms over waters just east of the Big Island. The upper low will remain just southeast of the Big Island through the day, maintaining a threat for heavy showers over Maui and the Big Island and creating a slight chance of thunderstorms on the Big Island.
High pressure far north of the islands will drive gradually increasing trade winds that will keep showers moving. This will limit the threat for flooding, but we will closely monitor windward areas of the Big Island where deep moisture and instability are greatest. On the smaller islands, showers will be focused across windward and mauka areas, while leeward sections will see passing showers.
A Winter Weather Advisory for the Big Island summits has been extended through the day. Blow-off from the thunderstorms to the east has been covering the summits overnight and likely producing freezing rain. Expect these conditions to persist this morning with snow showers possible in the afternoon.
Another round of unsettled weather is possible beginning tomorrow. The upper level low currently south of the Big Island will drift back over the state and will be reinvigorated as a shortwave aloft drops into the area from the north. Very cold temperatures aloft associated with the upper low will likely trigger locally heavy showers and thunderstorms. For now, we have broad brushed the entire state with a slight chance of thunderstorms Thursday and Friday. During this time strong high pressure far north of the state will drive moderate to locally breezy trade winds, likely setting the stage for a somewhat rare round of thunderstorms under a trade wind flow. The stronger trade winds should keep showers moving, keeping the risk for localized flooding across windward areas.
A possibly unsettled trade-wind rainfall pattern will continue into the weekend. This pattern falls into the category of a “rex block,” featuring high pressure aloft to the north and an upper level low to the south in the vicinity of the islands. There is fairly high certainty that this will translate into continued moderate to breezy trade winds. The rainfall picture is less clear. The GFS and ECMWF models move deep moisture away from the state while the upper low weakens a bit. Given this, we have opted to leave out heavy shower wording, but confidence is low.
In the marine environment, the band of stronger 25 kt winds around Kauai are expected to ease later today. The Small Craft Advisory for this area will likely be allowed to expire this afternoon. With overall increasing trades, the small craft focus will likely shift to the windier coastal zones and channels of Maui and Hawaii counties.
Numerous showers are expected across the coastal waters today. Thunderstorms may occur over the eastern zones. More unsettled weather will combine with increasing trades for generally poor conditions again Thursday and Friday.
Not much happening in the way of surf around the state today. The current northwest swell will drop further today. A small southern hemisphere swell will produce modest surf along south facing shores. Choppy trade swell along windward beaches and reefs will remain moderate to near high for the next several days.
The following numbers represent the strongest wind gusts (mph), along with directions Wednesday morning:
14 Princeville, Kauai – E
15 Bellows AFB, Oahu – ENE
10 Molokai – NNE
24 Kahoolawe – E
25 Lipoa, Maui – E
05 Lanai – ENE
29 South Point, Big Island – ENE
Here are the 24-hour precipitation totals (inches) for each of the islands as of Wednesday morning:
4.30 Mt. Waialeale, Kauai
2.01 Palisades, Oahu
0.73 Kihei #2, Maui
1.81 Hilo Airport, Big Island
Here is the latest NWS rainfall forecast
Here are the latest available satellite images:
SURF ZONE FORECAST – OAHU
NATIONAL WEATHER SERVICE HONOLULU HI
900 AM HST WED NOV 2, 2011
Surf along north facing shores will be 3 to 5 feet this morning and 2 to 4 this afternoon through Thursday.
Surf along south facing shores will be 3 to 5 feet today and 2 to 4 feet Thursday.
Surf along west facing shores will be 2 to 4 feet through Thursday.
Surf along east facing shores will be 3 to 5 feet through Thursday.
Outlook through Tuesday Nov 8: The current small south swell will subside Thursday night with another appearing Friday night through Saturday morning. A small to moderate northwest swell arriving Friday night will peak Saturday into Saturday night then gradually subside through Tuesday. A small northwest swell is expected Tuesday night.