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Hawaii Island Weather Outlook for March 7th, 2021

A Flash Flood Watch has been issued for the Island of Hawaii through late Tuesday evening (March 9th). The possibility of heavy rains with thunderstorms over the next few could bring the flash flooding conditions around the island.

Also, the possibility of small accumulations of snowfall on top of the summits on Hawaii Island is expected over the next few days.

Published by: HIRSC - Weather Team



The Flash Flood Watch continues for

* The Big Island.

* Through late Tuesday night

* Deep moisture will spread over the Big Island from the east and
  southeast tonight and Monday, then linger into at least Tuesday,
  bringing the potential for persistent heavy rain that could lead
  to flash flooding.

* Flood prone roads and other low lying areas may be closed due to
  elevated runoff and overflowing streams. Landslides may also occur
  in areas with steep terrain.


Be prepared to take action if a Flash Flood Warning is issued. If
you experience heavy rain or rising waters, head to higher ground

A Flash Flood Watch means that conditions are favorable for flash
flooding. Flash Flooding is LIFE THREATENING. Do not cross fast
flowing water in your vehicle or on foot.


Cloudy and showery weather is expected for much of the upcoming week as an upper level disturbance interacts with moisture moving in from the east and southeast. Breezy trade winds will give way to lighter winds by mid-week as a surface trough moves over the islands.


Breezy to potentially strong easterly trade winds will prevail for the next couple of days, weakening during the second half of the upcoming week as a surface trough moves over the islands. An increase in deep-layer moisture is expected starting tonight over the Big Island, spreading to Maui County on Monday, bringing the potential for heavy showers that may overspread all islands Tuesday and Wednesday. However, there still remains a significant amount of uncertainty in the forecast, as the strength, location, and movement of a developing surface trough will strongly dictate winds and weather (and waves) seen in the islands. A 1038 MB high centered well N of the islands is supporting breezy to locally strong trade winds, while a persistent trough aloft W of the islands continues to support a moderately unstable island atmosphere. Incoming low-level moisture diminished today, but it is on the increase once again, especially near and upwind of the Big Island and Maui. Additionally, a subtle reorganization of the flow aloft over the islands is expected tonight and Monday, leading to increasing mid-and upper-level moisture over Maui and the Big Island. This moisture will spread to the other islands by Tuesday as the trough becomes N-S trough oriented and sharpens W of Kauai. This destabilizing trough aloft is expected to induce the formation of a low-level trough near or E of the islands initially, with the trough then moving W or NW through at least mid-week. Model guidance continues to offer subtle differences in the strength and location of the trough, but these are significant enough to introduce a large amount of uncertainty in most aspects of the forecast. We are certain that the forecast will need to be fine-tuned as things evolve, with potentially dramatic changes at times. If the trough sharpens just E of the islands on Tuesday (per latest GFS), the pressure gradient could tighten to the point that sustained 30- 35 mph E/SE winds, and widespread heavy rain, would develop. If the trough is just a little farther to the NE (per latest ECMWF), much lighter winds, and a bit less precipitation, would be expected. A somewhat broad brush was used to "paint" the forecast grids over the next couple of days, and includes a slight chance of thunderstorms as well as winter weather over the Big Island Summits. Temperatures aloft are not expected to be sufficiently cold to support winter weather over Haleakala. Despite the model differences for Tuesday and beyond, there is growing consensus that deep-layer moisture will increase over the Big Island tonight and Maui County on Monday, spreading to the other islands on Tuesday. A Flash Flood Watch (FFA) has been issued for Maui County and the Big Island (with slight differences in their start times), and a Winter Weather Advisory Advisory is now in effect for the Big Island Summits. A Winter Storm Warning may be required later Monday if anticipated snowfall develops due to even deeper moisture moving in. Generally speaking, the low-level trough will linger near the islands for most of the upcoming week, as will a trough aloft, so a significant drying trend is not anticipated anytime soon, and the FFA will likely include all islands at one point or another.

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