| Current Conditions | ||
| Temp |  | -0.4 C |
| RH | | 11 % |
| Wind | | SSW 13 mph |
| Road | | Closed |
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Hawaiian Islands Synoptic Discussion and Guidance 183 FXHW60 PHFO 201316 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 316 AM HST Tue Apr 20 2021 .SYNOPSIS... Scattered showers capable of producing locally heavy rain and possibly a thunderstorm are expected today. Light and variable winds will continue into mid-week followed by a return to moderate to breezy trades this weekend. && .DISCUSSION... Weakly convergent southwest flow is supporting scattered showers around Kauai while dry weather and increasing high clouds prevail over the smaller islands. Some increase in shower coverage as far south as Oahu will be possible through the morning, but otherwise conditions should remain fairly static for the next several hours. The morning sounding out of Lihue shows PWATs have increased to 1.38", which is in line with expectations. The inversion has also risen by a couple thousand feet, to around 9kft, compared to yesterday afternoon. And the deep mid-level dry layer has eroded considerably, but is still stout enough to keep a lid on convection this morning. As a result, showers have been small and have produced only modest rainfall, the highest reading as of 2am HST being 0.25" atop Mount Waialeale. Forecast soundings indicate this dry layer eroding by mid-morning which will ease capping and allow convection to grow in vertical extent. Overall, the official forecast reasoning remains unchanged. The advection of warm and moist air within a deep layer of confluent southwesterly flow will contribute to modest forcing for ascent through the day while a fairly dynamic 60kt jet will provide favorable upper-level support by this afternoon. This will support an increasing coverage of showers capable of producing locally heavy rainfall, particularly over Kauai and Oahu. With that said, convection is expected to remain unremarkable thanks to a combination of paltry mid-level lapse rates, weak forcing, and limited surface-based instability due to increasing high clouds. The vigor of the sea breeze circulation will likewise be reduced by the increase in high clouds, so prospects for thunder appear quite limited. The lone exception will be leeward Big Island where high clouds will hold off until later today yielding a greater opportunity for afternoon destablization. A return to stable conditions can be expected by late tonight as mid-level subsidence overspreads the area from northwest to southeast. A building mid-level ridge over the central Pacific will cause moderate to breezy trades to return by the end of the week. Model consensus is improving that a mid-level trough moving southward along the periphery of the building ridge will move over or just east of the area Friday and Saturday. This will bring a reduction in stability and a trend toward wetter trades. && .AVIATION... The cold front far northwest of the Hawaiian Islands continues its slow march eastward. Light variable winds will shift to the south today as the front edges closer to Kauai. Current satellite imagery reveals increasing layered clouds across the western third of the state spreading eastward. Radar indicates minimal shower activity, but showers are expected increase in both coverage and intensity across Kauai by morning. These prefrontal showers should then spread across Oahu by noon and the remainder of the smaller islands by evening. On the Big Island, afternoon sea breezes will encourage cumulus buildups over the island's interior. Precipitable water values are sufficient to produce locally heavy downpours that could linger through the evening hours. The air mass over the state is expected to become increasingly unstable. Slight chance of thunderstorms over Kauai, Oahu, and Interior Big Island today. No AIRMETs in effect at this time. However, mountain obscuration will likely become an issue for Kauai later this morning. && .MARINE... A front approaching from the northwest will keep moderate east- southeast winds in place over the eastern islands and light and variable winds over the western end of the state through Wednesday. A few heavy showers and even a thunderstorm will be possible through this evening over the Kauai and Oahu waters as an upper level disturbance moves through. Moderate trades are expected to return from east to west across the state Wednesday night and Thursday as high pressure builds north of the islands. The trades are forecast to strengthen into the fresh to strong range Friday through the weekend, and Small Craft Advisories will likely be needed for some marine zones at that time. Surf along north facing shores will remain small through Tuesday. A new short period north-northwest swell will then give north shore surf a boost during the middle of the week, with a slow decline expected Friday through the weekend. Head high to slightly over head sets can be expected along north facing shores as this swell peaks, but surf will remain well below advisory thresholds. East shore surf will remain small through the next 7 days, although we should see a slow trend upward Thursday into the weekend as the trades over and upstream of the islands strengthen. Surf along south facing shores will remain small during the next 7 days, with mainly background south and south-southwest swell energy moving through. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...JVC AVIATION...Bedal MARINE...M Ballard Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office |
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