A significant pattern change is then expected by Tuesday as an upper level trough and surface low pressure evolve northwest of the islands. This system is expected to draw deep tropical moisture northward over the state, leading to an extended period of strong southerly winds, increased chances for widespread rainfall and thunderstorms, and renewed flooding concerns from from the middle of the week through this weekend.
Water vapor imagery shows an approaching upper level trough over the Northwest Hawaiian Islands this afternoon. Occasional cirrus clouds along the eastern flank of the trough will continue over the next day or so. At the surface, a ridge is northeast of the state with the axis passing through 30N140W. To the northwest, a cold front around 575 miles northwest of Kauai tracking east at 10 mph. Gentle to moderate southeast winds prevail with lighter winds/land breezes setting up leeward due to the Big Island wind blockage. Most islands will see decreasing showers and clearing tonight except for Kauai where clouds and showers will begin to enhance due to the upper trough.
A broad surface trough is expected to develop ahead of the front, just northwest of the islands early on Tuesday, reinforcing and strengthening south winds across the islands. A moisture band associated with the trough will also set up around Kauai on Tuesday around the time the upper trough moves across this area bringing a chance for heavy showers. Deep tropical moisture will be drawn northward in the south flow. The combination of daytime heating, sea breezes, and a potent shortwave trough moving through, may even trigger some heavy showers or thunderstorms over interior areas on Tuesday. 0-6 km bulk shear values in excess of 60 knots may allow some of these thunderstorms to become organized and produce brief strong gusts on Tuesday, before dropping off Tuesday night. The upper trough exits Tuesday night while the deep tropical moisture builds northward, but the brief returning stability may be enough to provide a period of less-intense showers Tuesday night into Wednesday morning.
Upper-level forcing is forecast to strengthen again Wednesday as a strong second shortwave rounds the base of the trough, potentially enhancing lift over the region while surface low pressure center organizes west of the islands. The combination of deep tropical moisture engulfing the state, increasing large- scale forcing for ascent point towards another widespread and organized rainfall event for the state. Therefore, a Flood Watch has been issued for all islands from Wednesday morning through the day Friday.
South winds will ramp up likely to advisory levels Wednesday night through Friday morning. Strong winds will occur in and around heavy showers and thunderstorms, as well as around mountain ridges and downsloping gusts over northern and windward slopes of all islands. Severe wind gusts (58 mph+) will be possible, though most areas will see max in the 40-50 mph range, and likely more of a Wind Advisory setup. Stay tuned for more information on that. Finally, a Winter Storm Watch has been posted for a combination of freezing rain and snow on the Big Island summits above 12,500 feet also running Wednesday through Friday.
Differences remain in the global models for the weekend, but the general consensus is that the active pattern could linger in the area. The ECMWF clears out much of the state of the bulk of the rain, but the GFS continues the potential for rain across at least portions of the state through the weekend and beyond, where the deep moisture axis anchors.
As previous discussions have mentioned, one important consideration with this week's rainfall is antecedent conditions. The islands experienced significant flash flood events in recent weeks, and despite several days of drier trade wind weather, soils remain vulnerable in many areas. As a result, any time period of higher rainfall rates could quickly lead to enhanced runoff, renewed rises in streams and reservoirs, and localized flash flooding concerns.
Trades will continue to weaken heading into the evening as an upper-level disturbance moves in and initiates a wet week across the islands. VFR conditions are expected to prevail through at least sunrise tomorrow, however rain may start to move in for Kauai and even Hilo by late morning Tuesday. Rain is then expected to slowly spread across the islands afterward.
AIRMET Tango remains in effect for upper level turbulence between FL300 and FL350 over the islands and is expected to continue through the period. AIRMET Sierra was dropped for windward Oahu, but another may be issued for other islands as this aforementioned disturbance moves in with more moisture.
Moderate to locally fresh trades will gradually ease and shift southeasterly tonight through Wednesday as a front stalls just west of the islands and high pressure far north of the islands moves slowly eastward. The winds will turn more southerly and increase to moderate and strong levels on Thursday as a low pressure system develops along the stalled frontal boundary.
Small northwest swells will keep some small surf along north facing shores through this evening. A moderate long-period northwest swell will fill in tonight and Tuesday, giving more of a boost to north shore surf late Tuesday through Wednesday. A small, short- period north-northeast swell will fill in during the day Tuesday before peaking Wednesday and subsiding Thursday. This overlap will make for some rough choppy conditions along north facing shores.
A small southerly swell will keep some small surf in place along south facing shores through this evening. A slightly larger, long- period south swell, will fill in tonight, giving a more noticeable increase to south shore surf Tuesday. Surf could reach advisory levels by Wednesday before slowly easing Thursday into the weekend. Strengthening southerly winds will make for rough and choppy surf by late next week.
East shore surf will remain small and below the seasonal average due to a lack of strong trade winds over and upstream of the islands, except for some wrap from the northerly swells.
Flood Watch from Wednesday morning through Friday afternoon for all Hawaii islands-
Winter Storm Watch from Wednesday morning through Friday afternoon for Big Island Summits.
