Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
705 FXHW60 PHFO 221306 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 306 AM HST Tue Apr 22 2025 .SYNOPSIS... A deepening low to the far northwest of the state drifts southward and lingers west of the state over the next several days. Southerly winds will help to bring increased showers across the state. Unsettled weather conditions are expected today through Wednesday, with scattered showers across the state especially during the afternoon hours. Heavier showers and isolated thunderstorms will be possible around Kauai later today and into Wednesday. && .DISCUSSION... As expected, there are some modifications to the forecast this morning, but no significant changes. The surface low is about 800 miles to the northwest, and the associated front is about 500 miles northwest of Kauai early this morning. Thunderstorms have been observed ahead of the front overnight, including some making it into the offshore waters to the northwest of the coastal waters. The GFS and ECMWF continue to be in good agreement with the surface system and its associated strengthening upper level low sinking south over the next few days. While both models bring precipitable water (PW) values well above normal (1.5 to 1.7) over the western islands, the ECMWF is the wetter of the two models. The 2 am sounding from Lihue saw PW increase from 1.18 inches yesterday afternoon to 1.59 inches. Meanwhile, the Hilo sounding was 1.37 inches at 2 am. PW values are expected to continue to increase over the western end of the state. Both models bring 500 mb temperatures of -10 to -12C which is colder than normal. The 2 am sounding had -10C at 500 MB for Lihue and -08C for Hilo. This amount of moisture and instability continues to support the heavier showers and thunderstorms in the forecast over the western islands. Models continue to keep the system just west of Kauai, which will keep Kauai and Niihau as the main focus for the heavier showers, and thus the ones under the Flood Watch starting at noon today. Southerly flow over the remainder of the state will bring increased moisture which will result in produce humid conditions and scattered showers. With PW values being so far above normal, some brief downpours are possible especially during the afternoon and evening hours. Widespread rainfall is not expected for rest of the state, but some showery conditions will be possible at times. Thursday into Friday, the upper level low should lift further north and away from the state which should bring the return of stable conditions, although somewhat unsettle weather may continue near Kauai. Winds will continue to remain light to moderate and out of the southeast direction, which will produce warm and humid conditions through the rest of the week. Trade winds should return sometime during the first half of next week. && .AVIATION... Winds will veer southerly in advance of a developing low pressure system to the northwest of Kauai today. This should focus SHRA over south-facing coasts and slopes, along with some sea breeze development bringing low cigs and SHRA to north and east- facing coasts and slopes. The proximity of the low could bring +RA and isol TSRA to the western end of the state beginning this afternoon. MVFR/IFR conds will be possible in any SHRA/TSRA, otherwise VFR prevails. No AIRMETs are currently in effect. && .MARINE... A low roughly 700 NM northwest of Kauai will dig south today before stalling roughly 500 NM northwest of Kauai Wednesday. In response, gentle to moderate east southeast winds will continue to veer southerly around the western end of the state and moderate to locally fresh southeast will develop over the eastern end of the state. The western end of the state will have the highest chance of heavy showers and thunderstorms due to the proximity of the low. The low is expected to move north Friday into the weekend allowing winds to gradually ease and shift more easterly. A 2-3 foot, 12-14 second northwest swell will peak today, providing a little boost in surf along north and west facing shores. This swell will subside into Wednesday. Expect minimal surf late Wednesday through Friday. A slightly larger small, long period northwest swell is expected to fill in over the weekend. A small, long period south-southwest swell is filling in this morning with wave energy increasing in the 14 to 18 second band range as seen in the Barbers Point, Kaumalapau and Pear Harbor near shore CDIP buoys. This new swell is expected push surf along south facing shores above summer average as it peaks tonight and holds into Wednesday. The swell will slowly decline the rest of the week. Surf along east facing shores will remain near or below average due to the lack of strong trade winds over or upstream of the islands. East shore surf may see some side shore chop wrap in from the southeasterly wind swell over the next several days. && .HFO WATCHES/WARNINGS/ADVISORIES... Flood Watch from noon HST today through Wednesday afternoon for Niihau-Kauai- && $$ DISCUSSION...M Ballard AVIATION...Walsh MARINE...Almanza