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Menu of Text Products for the Hawaiian Islands and the Tropical Pacific/Atlantic Oceans: Select a text product: Hawaiian Islands Synoptic Discussion and Guidance Hawaiian Islands Short Term Forecast Hawaiian Islands 7-day Zone Forecast Hydrologic Outlook Flash Flood Warning Flash Flood Watch Flash Flood Statement Tornado Warning Severe Weather Statement Severe Thunderstorm Warning Special Weather Statement Winter Storm Warnings/Watches/Advisories Urgent Weather Message Marine Weather Statement/High Surf Advisory/Warning Special Marine Warning Radar Message Special Messages Unknown Terminal Aerodome Foreasts (TAFS) Area Forecast for Hawaii and Adjacent Coastal Waters Upper-level Winds and Temperature Forecast for the 4 Major Hawaii AP AIRMETs for IFR Conditions, Turbulence, and Icing Thunderstorm SIGMETs Hawaiian Islands Satellite Interpetation Message Central/North Pacific Tropical Weather Outlook Central/North Pacific Tropical Weather Summary Central Pacific Tropical Cyclone Summary South Pacific Tropical Cyclone Summary Tropical Cyclone SIGMETs Buoy Winds and Wave Forecast Oahu Surf Observation and Forecast/Coastal Forecast for Hawaiian Waters Offshore Forecast for Hawaiian Waters North Pacific High Seas Forecast South Pacific High Seas Forecast Hawaiian Islands Automated Surface Observation System Hourly Weather Roundup Hawaiian Islands Wind Data Hawaiian Islands Rainfall Summary Hawaii Temperature and Precipitation Summary Daily Hawaii Cooperative Observations Honolulu AP Climate Summary Lihue AP Climate Summary Kahului AP Climate Summary Hilo AP Climate Summary Atlantic Tropical Cyclone Discussion Atlantic Tropical Weather Outlook East Pacific Tropical Cyclone Discussion East Pacific Tropical Weather Outlook Central Pacific Tropical Cyclone Discussion Central Pacific Tropical Weather Outlook West Pacific Tropical Cyclone Discussion West Pacific Tropical Weather Outlook

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Hawaiian Islands Synoptic Discussion and Guidance 919 FXHW60 PHFO 191400 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 400 AM HST Thu Apr 19 2018 .SYNOPSIS... High pressure north of the state will keep a moderate to breezy trade wind flow in place through the weekend, before strengthening to locally windy levels for the first half of next week. A band of an enhanced clouds and showers associated with an old front combined with an upper level trough moving through the state, will keep showery weather and abundant cloud cover in place through Friday, particularly over windward areas. Some of the showers may be heavy at times and an isolated thunderstorm can't be entirely ruled out. A more typical trade wind pattern will return Friday night through the middle of next week, with showers favoring windward and mauka areas. && .DISCUSSION... Currently at the surface, an east to west oriented ridge of high pressure centered around 600 miles north of Kauai, is driving moderate trade winds across the island chain early this morning. Aloft, an upper level trough is located around 250 miles west- northwest of Kauai, with an upper level ridge a few hundred miles to the east of the Big Island. Infrared satellite imagery shows mostly cloudy to overcast conditions across the state, with an extensive shield of mid and high cloud extending well to the southwest of the island chain. Meanwhile, radar imagery shows scattered to numerous showers moving into windward areas in association with an old frontal band, with the greatest coverage over Maui County and the Big Island. Main short term concern continues to revolve around the potential for heavy rainfall and flash flooding. Today through Friday, A ridge of high pressure will remain to the north of the state through the period, maintaining moderate to locally breezy trade winds across the island chain. The old frontal remnants will linger across the state over the next couple days, while the upper level trough shifts slowly southeastward, moving through the islands tonight and Friday. As a result, the airmass will destabilize, and deep tropical moisture will increase. Precipitable Water (PW) values will climb into the 1.6 to 1.9 inch range today and hold at these levels through tonight. The airmass will begin to stabilize and deep moisture will decrease from northwest to southeast down the island chain on Friday as the upper trough shifts southeastward, with PW values ranging from 1.2 inches near Kauai to around 1.6 inches over the Big Island by early Friday evening. As for sensible weather details, shower coverage and intensity is expected to increase statewide today as the old frontal remnants and upper level trough begin to interact, with showery weather then continuing tonight. As deep moisture decreases and the upper level trough shifts southeastward on Friday, shower coverage and intensity will decrease from northwest to southeast. The highest potential for heavy rainfall is expected from Oahu east to the Big Island, particularly for windward areas. Given the high levels of saturation from recent rainfall over Kauai however, any additional moderate to heavy rain that occurs could quickly result in flooding problems. As a result, the Flash Flood Watch remains in effect for the entire state from 6 AM this morning through 6 PM Friday. Thunderstorm probabilities don't appear all that great, but given the unstable airmass and deep tropical moisture in place, we can't rule out a rumble or two of thunder. As a result, we will keep a slight chance of a thunderstorm in the forecast, but we wouldn't be surprised if no thunder occurs. Additionally, with temperatures near and below freezing and deep moisture increasing over the Big Island summits, snow is expected to develop for elevations above 12000 feet today. Periods of snow, heavy at times, are then expected to continue through Friday. As a result, the Winter Storm Watch has been upgraded to a Winter Storm Warning for the highest elevations of Mauna Kea and Mauna Loa from 6 AM this morning through 6 PM Friday. Friday night through Wednesday, A ridge of high pressure north of the state will maintain moderate to locally breezy trade winds Friday night through the weekend. High pressure will then strengthen as it shifts to the northeast of the state Monday through the middle of next week, allowing the trade winds to strengthen to breezy and locally windy levels. Overall, a fairly typical trade wind pattern is expected through the period, with bands of enhanced clouds and showers drifting through the islands from time to time. Windward and mauka areas will remain the primary targets for showers, with an occasional shower spreading leeward due to the strength of the trades. Model time/height relative humidity cross sections show the mid and high cloud cover decreasing as the upper level trough exits eastward, so more sunny conditions are expected to return to the island chain. That said, some high cloud cover will likely linger through the weekend, particularly over the eastern islands, as a weakening upper level jet remains overhead. && .AVIATION... Moderate trade winds will continue across the Hawaiian Islands producing moderate low level mechanical turbulence over and downstream of mountains for the next 24 hours. Low cloud ceilings and showers will linger over mountain areas of all islands. A band of unstable clouds and showers will enhance shower activity across all islands with MVFR ceilings along windward slopes producing widespread mountain obscurations. AIRMET Sierra in effect for Tempo Mountain Obscurations over north through east facing slopes of all islands for elevations above 2000 feet. AIRMET Tango in effect for low level mechanical turbulence for all islands over and south through west of mountains below 7000 feet. Another AIRMET Tango remains in effect for upper level moderate turbulence due to the jet stream between FL280 and FL400. && .MARINE... Mariners should be aware that an upper-level trough moving over the area will likely produce unstable weather across the state during the next couple of days. This will bring the threat of locally heavy downpours and a chance of thunderstorms through Friday. A surface ridge far north of the area will maintain locally strong trade winds over the area today. Therefore, a Small Craft Advisory (SCA) remains in effect for most of the waters surrounding the islands of Maui County and the Big Island through this afternoon. The trade winds are expected to weaken slightly by this evening as the high moves farther east and a front approaches from the north-northwest. Starting this evening, the SCA has been scaled back to only include the typically windy waters adjacent to the islands of Maui County and the Big Island. This SCA is in effect through Friday afternoon for now. However, the latest forecast suggests that it will likely be extended from Friday night into early next week for these same waters. Surf will likely remain below the High Surf Advisory (HSA) thresholds along all shorelines through this weekend. Rough, moderate surf is expected to persist along most east facing shores through Sunday night. Surf heights will increase again along east facing shores early next week due to strengthening trade winds. Surf may reach the HSA threshold along east facing shores starting late Monday, and persist into the middle of next week. A moderate northwest swell arriving later today is expected to peak Friday, and then gradually subside this weekend. A slightly larger northwest swell is expected to spread down the island chain late Sunday and peak on Monday. This swell will gradually subside from Tuesday through Wednesday. A series of small swells from the southern hemisphere will maintain modest surf along most south facing shores across the island chain through this weekend. && .HFO WATCHES/WARNINGS/ADVISORIES... Flash Flood Watch from 6 AM HST this morning through Friday afternoon for the entire state of Hawaii. Winter Storm Warning from 6 AM this morning to 6 PM HST Friday for Big Island Summits. Small Craft Advisory until 6 PM HST Friday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. Small Craft Advisory until 6 PM HST this evening for Kaiwi Channel-Maui County Windward Waters-Big Island Windward Waters. && $$ DISCUSSION...Jelsema AVIATION...Bohlin MARINE...Houston Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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