Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 05/03/19

Area Forecast Discussion
National Weather Service Honolulu HI
351 PM HST Thu May 2 2019 .SYNOPSIS... Trade winds will weaken and shift southerly tonight as an advancing upper level trough gradually destabilizes the atmosphere. Clouds and showers will favor windward slopes this evening, with most showers moving offshore late. An unseasonably strong front will approach the state on Friday, leading to an unstable southerly flow that will produce spotty showers over most islands. The front will move down the island chain Saturday into early Sunday, likely bringing a period of heavy showers, gusty winds, and possibly thunderstorms. Cooler northwesterly winds are expected behind the front into the middle of next week. && .DISCUSSION... Trade winds are weakening, and the atmosphere is destabilizing this afternoon. Trades are on the decline as a cold front centered 600 miles northwest of Kauai weakens the subtropical ridge north of the state. The upper level trough driving the cold front is eroding the inversion, which has lifted up to as much as 10,000 ft according to afternoon soundings and recent aircraft data. This has led to isolated showers across leeward terrain on most islands, while a narrow band of low level moisture continues to fuel showers over windward slopes. The trades will continue to weaken and shift out of the southeast tonight as the atmosphere slowly destabilizes. The afternoon leeward showers will diminish around sunset, while windward showers persist, especially from Maui to Oahu. In fact, we could even see a brief flare up on windward Oahu this evening under the veering wind flow. Windward showers should push offshore late tonight as the shifting background flow causes land breezes to develop. However, Oahu and Kauai could see a few showers moving up from the south late. A somewhat unstable southerly flow will develop on Friday as the unseasonably strong front approaches the state. The upper trough supporting the front will continue to produce cooling aloft that will wipe out the inversion. Models show a developing prefrontal trough focusing moisture just west of Kauai, leaving the state under an unstable land and sea breeze regime with near normal precipitable water. Though there could be an isolated downpour at any time, showers over land will favor the afternoon hours. The front will reach Kauai early Saturday, then bring unsettled weather down the island chain through Saturday night. We expect the front will be near Kauai in the predawn hours on Saturday, reach Oahu midday, move to Maui County Saturday afternoon and evening, then possibly advance to the Big Island by predawn hours on Sunday. During this time, a potent upper low will drop southward into the upper trough and approach the state from the north. A jet stream along the southern flank of the upper low should move over the islands and produce forcing that will generate heavy showers along the front. The eventual position and strength of the compact upper low will be critical, but at this time, the greatest amount of forcing and coldest air aloft look to remain just northeast of the state. However, a line of heavy showers or thunderstorms along the front could be capable of producing strong, or even damaging, winds. Another item of note is the potential for gusty downsloping southwest to west winds along the front. It does not appear that a wind advisory will be need, though localized wind gusts of around 40 mph are a strong possibility on Saturday, especially on Oahu and parts of Maui County. At higher elevations, the high summits on the Big Island will almost certainly experience warning level winds, while advisory level conditions are expected on summit of Haleakala. On Sunday, the front will likely weaken near the Big Island as the compact upper level low passes north of the state. Latest runs of the GFS and ECMWF are hinting at a surface low developing below the upper level feature, potentially driving another band of showers and gusty northwest winds down the island chain. Stable and rather cool conditions are expected to fill in on the northwesterly flow later Sunday and Monday. The latest guidance drops dew points into the upper 50s, which will provide rather cool conditions for early May. As the upper level low weakens and lifts off to the northeast, stable and rather dry northerly winds will likely lead to dry conditions through the middle of next week. && .AVIATION... Expect scattered showers through tonight and into Friday. The showers through the day so far have included lots of areas for mountain obscuration along windward slopes. Therefore, AIRMET Sierra for mountain obscuration is in effect for all islands from Kauai to Maui and will probably stay in place through this evening. We are seeing some interior island showers this afternoon. Overall, the winds have been gradually weakening through today and should continue to do so into this evening and for Friday with a cold front beginning to close in from the northwest, loosening the pressure gradient. The winds may be light enough by tonight that we have a nighttime land breeze scenario. && .MARINE... Southeast flow is already evident across the coastal waters ahead of a front that is expected to reach the islands this weekend. Winds are expected to weaken some tonight with the high pressure north of the islands moving to the east. The high will be replaced by a developing low that will bring a front to the islands this weekend. The front will bring the potential for heavy showers, possibly a few thunderstorms, and gusty winds. The greatest focus will be to the northeast of the islands. Behind the front, moderate to locally strong northerly winds are expected which could prompt a Small Craft Advisory (SCA) for Saturday into Sunday night for some coastal waters. A small northwest swell at that time could also contribute to an uptick in seas, bringing seas near SCA levels mainly near Kauai. The latest model run from the NWPS show seas remaining below the 10 foot level, but will continue to monitor. The current small northwest swell is expected to persist through Saturday. The developing low referenced above will send a moderate sized short-period northwest swell toward the islands Saturday night through Monday, with surf expected to remain below advisory levels as the swell peaks late Sunday and Sunday night. A new small but longer period northwest swell will build Tuesday night, and persist through Thursday. Storm systems in the South Pacific will send small south and southwest swells our way through the middle of next week, with an uptick in swell energy expected over the weekend, and maybe again early next week. See the latest Oahu Surf Discussion (SRDHFO) for additional details on surf and swell. && .HFO WATCHES/WARNINGS/ADVISORIES... High Wind Watch from Friday evening through late Sunday night for Big Island Summits. && $$ DISCUSSION...Wroe AVIATION...Chevalier MARINE...M Ballard
Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
812 PM PDT Thu May 2 2019 .SYNOPSIS...02/739 PM. Low clouds are expected to develop along the coasts and valleys tonight into Friday morning with strong onshore flow. High pressure will remain in control on Friday and temperatures may warm slightly. An upper level low will approach the region on Saturday bringing cooler temperatures through the weekend and a chance of light showers Sunday and Monday. && .SHORT TERM (THU-SUN)...02/811 PM. A weak upper level ridge of high pressure will remain over the area through Friday. Near the surface, onshore flow was a bit stronger this afternoon compared to yesterday, and is expected to further strengthen on Friday afternoon, when LAX-Daggett gradient is expected to exceed +6 mb onshore. Also, a weak eddy circulation is expected to spin up the marine layer coverage for areas south of Point Conception overnight into Friday morning. Current satellite imagery showing low clouds across portions of the LA county coast. Current ACARS data showing the marine layer depth around 1300 feet across the LA basin this evening. Slight deepening of the marine layer is expected by Friday morning due to the eddy circulation. Low clouds are going to be most widespread overnight into Friday morning across LA county with coverage expected to reach the San Fernando and San Gabriel valleys. However due to the eddy circulation, low clouds and fog should spread northwest up the coast of Ventura and SBA counties. Little change in temperatures expected across the coast on Friday, with slight warming across inland areas due to increasing boundary layer temperatures. *** From previous discussion *** A cutoff upper level low pressure system, currently 1300 miles west of Los Angeles, will move over central California on Sunday. As it approaches the coast on Saturday, it will deepen the marine layer while strengthening the onshore pressure gradients. This should add up to a healthy inland push of low clouds into the coastal mountain slopes and maybe slow clearing at the coast, with even more clouds on Sunday. Cannot discount a little drizzle in some spots over the weekend, with the best chances on Sunday. By Sunday, some projections show a week and small frontal boundary moving through the area. While there is little moisture to work with, it could interact with the deep marine layer and result in some benefit showers, with the best chances over SBA and SLO Counties. If any showers result, amounts will be light. Although the low will be right over the area on Sunday, since the system has been spinning off the coast for awhile, the airmass is not terrible cold aloft. The thunderstorm threat is fairly minimal as a result, and snow levels should stay above 7000 feet. Temperatures will drop by a few degrees on Saturday, and even more on Sunday...especially over the interior. The stronger pressure gradients will result in gusty southwest winds over the interior sections, but looks to stay just under advisory strength at this time. .LONG TERM (MON-THU)...02/214 PM. The weekend low mentioned in the short term will still be in play on Monday. With the cutoff nature of this system, the latest computer projections have to be taken with a grain of salt, as these type of systems tend to outwit computers and forecasters alike. With that said, the latest guidance has the low wobbling over southern California Sunday night and Monday, before exiting the region by Tuesday. If this track holds, ample low level clouds and light showers will remain possible, with the better chances shifting to Ventura and LA Counties. Conditions will remain cool like Sunday. The latest projections call for some clearing and warming on Tuesday and Wednesday, but not a whole lot as onshore pressure gradients strengthen. Some moist northerly flow aloft could keep some clouds around, especially in the mountains where a stray shower to two is possible. Computer solutions diverge for the later half of next week, so went closer to climatology. && .AVIATION...03/0019Z. At 0000Z, the marine layer was around 1200 ft at KLAX. A weak inversion was in place. The top of the inversion was around 2000 ft at 16 degrees C. Moderate confidence overall in the 00Z TAFs. Low clouds and fog are expected to redevelop across most coastal and valley locations overnight into Friday morning, with cigs expected to move into KLAX by around 06z, spreading northwestward to KSBA by around 14Z Fri. The timing of the onset of the low clouds and changes in flight cats may be off +/- an hour or two. There is a 20%-30% chance of IFR cigs affecting KSBP at times late tonight into Fri morning. KLAX...High confidence overall in the 00Z TAF in regards to cigs returning overnight. Low clouds with MVFR cigs are expected to move into the airfield around 06Z then persist until around 19Z before scattering to VFR thru Fri afternoon. The timing of the onset of the low clouds may be off +/- an hour or two. KBUR...Moderate confidence overall in the 00Z TAF. Low clouds with MVFR cigs are expected to move into the airfield around 11Z late tonight then persist thru 17Z Fri. There is a 20 percent chance of IFR cigs late tonight into early Friday morning. && .MARINE...02/752 PM. For the Outer Waters...Moderate to high confidence in the current forecast. A low end Small Craft Advisory (SCA) will continue for gusty northwest winds through late tonight. Otherwise, winds and seas should remain under SCA thresholds through Tuesday. For the Inner Waters N of Point Conception...Moderate to high confidence that winds and seas should remain under SCA thresholds through Tuesday. For the Inner Waters S of Point Conception...Generally high confidence in the current forecast. Winds and seas are expected to remain under SCA thresholds through Tuesday. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...NONE. PZ...Small Craft Advisory in effect until 3 AM PDT Friday for zones 670-673-676. (See LAXMWWLOX). && .HAZARD POTENTIAL OUTLOOK (SUN-THU). No significant hazards expected. && $$ PUBLIC...Gomberg/Kittell AVIATION...Gomberg MARINE...Gomberg/Sirard SYNOPSIS...Stewart