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Hawaii Island Weather Update for August 4th, 2019

Flash Flood Watch for Hawaii Island will go into effect Monday morning and High Surf Warning for the East Facing Shores of Hawaii Island remains in effect until 6 am Monday morning (August 5th), as effects from Flossie are expected to come on shore Monday afternoon into Tuesday.

The National Weather Service (NWS) has issued the following advisories for Hawaii Island:


FLASH FLOOD WATCH IN EFFECT FROM MONDAY MORNING THROUGH LATE MONDAY NIGHT The National Weather Service in Honolulu has issued a * Flash Flood Watch for Maui and the Big Island. * From Monday morning through late Monday night


Moisture associated with Tropical Cyclone Flossie moving into the islands from the east, along with an upper level trough just northwest of the state, will provide the ingredients for heavy rainfall and possible flooding issues for Maui and the Big Island Monday and Monday night. As the moisture moves westward, the Flash Flood Watch may need to be expanded to other islands as well PRECAUTIONARY/PREPAREDNESS ACTIONS... A Flash Flood Watch means that conditions are favorable for flash flooding. Flash flooding is LIFE THREATENING. Do not cross fast flowing water in your vehicle or on foot. Monitor later forecasts and be prepared to take action if a Flash Flood Warning is issued. If you experience heavy rain or rising water, head to higher ground immediately.



SURF ALONG EAST FACING SHORES FROM TROPICAL CYCLONE FLOSSIE Swells from Tropical Cyclone Flossie will produce hazardous surf through Monday HIGH SURF WARNING REMAINS IN EFFECT UNTIL 6 AM MONDAY * SURF...10 to 15 feet along east facing shores of the Big Island and Maui. * TIMING...Through tonight. * IMPACTS...High...Expect ocean water occasionally sweeping across portions of beaches, very strong breaking waves, and strong currents. PRECAUTIONARY/PREPAREDNESS ACTIONS... Large breaking surf and dangerous currents make entering the water very hazardous. Anyone entering the water could face death.


Road and Facility Closures as of August 4th at 4:45 pm:


- Hawaii Police Department (HPD) and Hawaii Department of Transportation (HDOT) reports all road are open at this time and any closure can come without notice. They ask all motorist to drive with caution on our roadways.


- No other closures were reported at this time.

Utility Outages as August 4th at 4:45 pm:


- Hawaii Electric Light Company (HELCO), Hawaiian Telcom, Spectrum, Department of Water Supply (DWS) all report no outages at this time.


Hawai'i Weather Outlook (as of 4:35 pm Sunday afternoon)


Synopsis

The weather pattern for the next few days will be highly dependent on the size, track and intensity of Tropical Cyclone Flossie. A wetter weather pattern is setting up as the moisture field associated with Flossie moves across the area from east to west. Instability caused by an upper level trough just northwest of the area will keep the airmass a bit unstable allowing for a slight chance of thunderstorms over some areas. A trade wind weather pattern is forecast to make a return later in the week and on into next weekend.

Discussion

A mid and upper level trough is located just to the northwest of Kauai. It's associated instability along with lingering moisture from Tropical Cyclone Erick has allowed for thunderstorms to form over and around Kauai. Thus a slight chance of thunderstorms are possible there through tonight. Elsewhere, moisture embedded in the low level flow is providing for some passing showers over some windward and mauka locations, while most leeward locations are generally dry. Tropical Cyclone Erick, currently located several hundred miles southwest of the state, continues to move westward away from the area. The lingering impacts from Erick are higher humidity levels along with a swath of high cirrus clouds which will be moving over the west end of the state overnight. Tropical Cyclone Flossie is currently centered several hundred miles east of the state. Our upcoming weather pattern will be highly dependent on the size, track and intensity of Flossie as it nears the vicinity of main Hawaiian Islands.

The 11 AM HST forecast for Flossie from the Central Pacific Hurricane Center has the center passing by just north of the state Monday and Tuesday. A mid and upper level trough currently centered just northwest of the state is forecast to linger over the next few days. This will keep the airmass a bit less stable than normal. The strongest winds associated with Flossie are located in the north semicircle of the system. Thus on its projected track, winds are not expected to play much of a factor in our forecast at this time. The biggest concern is the moisture field associated with the system interacting with the mid and upper level trough.

Shower activity is expected to ramp up later tonight and Monday over the east end of the chain, then spread up the island chain late Monday into Tuesday. Also instability caused by the mid and upper level trough will keep the airmass a bit on the unstable side. The increased moisture from Flossie along with the instability associated with the trough will allow for for the potential for heavy rainfall and possible thunderstorms over the east end of the state Monday and Monday night. Thus a Flash Flood Watch has been issued for Maui and the Big Island beginning Monday morning. This may later need to be expanded further up the chain as the moisture spreads westward.

By Wednesday, the east end of the state is expected to become drier as the moisture field from Flossie moves westward. However, the global models continue to keep higher precipitable water values over the central and western end of the state. With the trough lingering just to our northwest, a continued wet weather pattern may continue across some areas.

Later in the week and on into next weekend, we expect trade winds to make a return as high pressure builds in northeast of the area the the effects of Flossie pull away to the north. However the trough is forecast to linger just to our northwest. Shower activity will be highly dependent on incoming moisture availability mainly affecting windward and mauka areas. However confidence in the details that far out in the forecast is rather low at this time.



Tropical Storm Flossie Update for August 5th @ 5pm

Tropical Depression Flossie Discussion Number 31 NWS Central Pacific Hurricane Center Honolulu HI EP072019 500 PM HST Sun Aug 04 2019

Deep convection in the northeast quadrant of Flossie collapsed around 1800 UTC due to westerly vertical wind shear in excess of 20 kt. Since then, the low-level circulation center (LLCC) has been completely exposed, and isolated convection has been displaced over 175 nm to the east. Subjective Dvorak current intensity estimates ranged from a 1.5/25 kt from SAB to 2.0/30 kt from JTWC to 2.5/35 kt from HFO, while data T numbers were all lower. Based on these inputs and the absence of central convection for more than six hours, the initial intensity is lowered to 30 kt, and Flossie is now a tropical depression. The exposed LLCC of the depression is moving toward the west (270 degrees) at 11 kt. The continued westward motion, which will persist into Monday, gives increased confidence that the weakening system is becoming shallow and under a greater steering influence from the low- to mid-level ridge to the north. As the ridge weakens, Flossie is forecast to turn toward the west-northwest late Monday and remain on that motion near or over the main Hawaiian Islands until dissipation. The forecast track is nudged slightly south from the prior advisory and remains close to the ECMWF and HWRF near the middle of the guidance envelope. Flossie will gradually spin down over the next couple of days. An upper-level trough northwest of Hawaii will remain in place during this time, maintaining strong west-southwest winds aloft. As Flossie moves westward under these strong upper-level winds, vertical wind shear will increase to 30 to 40 kt tonight and Monday, causing the depression to slowly weaken as it moves near or over the Hawaiian Islands. The forecast is in line with the dynamical and statistical guidance but keeps Flossie slightly stronger than nearly all guidance in the next 12 to 24 hours. Interests in the main Hawaiian Islands should refer to products issued by the National Weather Service in Honolulu for information on flooding and surf impacts from Tropical Depression Flossie. FORECAST POSITIONS AND MAX WINDS INIT 05/0300Z 18.9N 149.8W 30 KT 35 MPH 12H 05/1200Z 19.1N 151.8W 30 KT 35 MPH 24H 06/0000Z 19.8N 154.4W 30 KT 35 MPH 36H 06/1200Z 20.8N 156.7W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 07/0000Z 21.8N 158.3W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 08/0000Z...DISSIPATED

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