902 FXHW60 PHFO 290634 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 834 PM HST Wed Jun 28 2017 .SYNOPSIS... Breezy trade winds associated with high pressure north of the state will continue through Friday, before trending down slightly over the weekend. Dry and stable conditions will limit shower activity over the state through Thursday. Showery weather will become more likely beginning Thursday night and continuing through the weekend. Windward and mountain areas will remain the focus for most of the shower activity, with showers reaching leeward areas more regularly Thursday night through the weekend. The trade winds will strengthen on Monday and reach breezy levels over the 4th of July. A drier airmass will begin to move into the islands from the east on Monday, decreasing the coverage and intensity of trade wind showers through the middle of next week. && .DISCUSSION... Currently at the surface, an east to west oriented ridge of high pressure is located around 550 miles north of Kauai, and is driving breezy trade winds across the island chain this evening. Infrared satellite imagery shows partly cloudy skies in place across the State, with some more showery clouds developing upstream (to the northeast) of the islands. Radar imagery shows scattered showers moving into windward sections of Maui and the Big Island, with isolated showers over windward sections of Kauai, Oahu and Molokai, as well as leeward sections of the Big Island. Elsewhere, rain free conditions prevail. Main short term concern revolves around rain chance. Tonight and Thursday, The ridge of high pressure will remain in place to the north of the State, keeping a breezy and gusty trade wind flow in place across the island chain. The airmass is initially quite dry, with precipitable water (PW) values from the afternoon soundings indicating 0.94 and 1.00 inches at PHLI and PHTO respectively. Additionally, the low level inversions from the soundings were between 5 and 7 kft, indicating quite stable conditions remain in place. Model solutions show moistening trend over the next 24 hours, with PW values climbing into the 1.2 to 1.4 inch range by Thursday evening. As a result, we expected to see an uptick in trade wind shower activity beginning after midnight tonight and continuing through the morning hours on Thursday. The shower coverage should decrease Thursday afternoon as the sun erodes some of the incoming showery clouds moving in with the trades. Rainfall amounts are expected to remain light through the period. Thursday night through Sunday night, Breezy and gusty trade winds are expected to continue through the Friday night as the ridge of high pressure remains in place to the north of the State. The trades are then expected to trend down slightly over the weekend as an inverted trough pushes westward to the north of the islands, acting to disrupt the trade wind flow a bit. Both the GFS and ECMWF are in good agreement showing an increase in moisture over the State beginning Thursday night and continuing through the weekend. PW values climb to between 1.4 and 1.7 inches, and inversion heights will rise in response to lowering upper level heights as an upper low tracks by to the north of the island chain. As a result, we expect a showery and wetter than normal trade wind pattern beginning Thursday night and continuing through the weekend. Showers will continue to favor windward and mauka areas, but will reach leeward areas more regularly due to the elevated inversion heights and deeper moisture over the region. Monday through next Wednesday, The GFS and ECMWF exhibit some minor timing differences early next week, but overall they both agree on a return of a drier trade wind weather and strengthening trades. For now will utilize a blended approach for forecast details. As a result, we expected the trade winds to strengthen on Monday, becoming breezy and gusty for the 4th of July and into the middle of next week. Showery trades will likely continue into Monday morning, with a drier airmass overspreading the state Monday afternoon and Monday night and continuing through mid week. && .AVIATION... Expect a VFR evening as a relatively dry and stable trade wind airmass remains in place across the main Hawaiian Islands. The latest infrared satellite imagery shows a patch of scattered mainly light showers affecting Hana area of Maui, stretching southeast to the waters off the NE facing coast of the Big Island. Radar is also picking up an isolated shower over the Koolau Mtns of Oahu. Otherwise, relative dry condition prevails elsewhere. Satellite is showing a bit more low clouds with embedded showers approaching the islands from the NE. These trade showers will be favoring the windward and mountain areas of the larger islands. Brief MVFR cigs can be expected with these showers. They will be mainly light in intensity with restriction to vis limited to no less than 5 miles. A late night popup shower may occur along the Kona coast as well. Strength of the trades remains strong enough to keep AIRMET Tango in place through at least 1600 UTC. No other AIRMETs are expected. && .MARINE... A Small Craft Advisory (SCA) is in effect and includes the Kaiwi Channel, Maui County windward waters, Oahu leeward waters, and the typically windy zones around Maui and the Big Island, due to strong trade winds. The SCA is in effect through Thursday, and may continue for some zones into Friday, followed by slightly weaker trade winds this weekend. Trades will then strengthen again early next week. The locally strong trades will produce choppy surf along east facing shores. Long-period swells out of the southwest and southeast will generate moderate surf along south facing shores through the weekend. Surf will taper down around Sunday night or Monday. A small, short-period northwest swell is forecast from Friday into the weekend. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 PM HST Thursday for Oahu Leeward Waters-Kaiwi Channel-Maui County Windward Waters-Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. && $$ DISCUSSION...Jelsema AVIATION...Lau MARINE...Kinel |