Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
210 AM EST Sat Mar 7 2026

Valid 00Z Sun 08 Mar 2026 - 00Z Sun 15 Mar 2026

...Confidence Increasing for a Prolonged, Potentially Significant 
Flash Flood Threat Next Week...

A significant regime shift is expected across the Hawaiian Islands
during the forecast period. The current dry and stable pattern,
dominated by moderate to breezy east-southeast flow, will persist
through the weekend. However, a transition to a highly-unstelled
and potentially hazardous weather pattern is forecast to begin
Monday night, as the leading edge of a 130-140kt 250mb jet streak
will migrate down the western periphery of an upper trough, 
causing the trough to become more negatively tilted as it draws
closer to the western islands. A multi-day flash flood risk is 
becoming increasingly likely over Hawaii the rest of next week,
beginning over the western islands late Monday, as the pattern
transitions to a southerly "Kona" wind regime. Guidance consensus
is good, especially with the blocky upper pattern maintaining the
unstable, moist environment through the end of the period. 

Given the blocked upper flow pattern, deep-layer forcing east of 
the upper low, bolstered significantly within the broad left-exit
region of the 130-150kt Pacific Jet, will persist through the 
forecast period. This will draw tropical moisture northward across
the islands, with PWATs expected to reach or exceed 2.00" at 
times next week per the latest guidance. Deepening warm cloud 
layers will enhance rainfall efficiency by early next week, which 
along with the cooling aloft will result in a rapid increase in 
coverage and intensity of the rain Monday night into Tuesday, then
continuing through the rest of the forecast period. 

Deep moisture next week will be highly anomalous for March across
the islands, not just in terms of volume, but persistence as well.

* Per the CW3E,the latest ECMWF shows integrated water vapor
  transport (IVT) values as high as 700+ kg/m/s by midweek.

* 850-700 mb moisture flux anomalies per the GEFS are expected to
  reach 5+ standard deviations above normal during the middle-
  latter part of next week and persist through next weekend. This 
  at the 500 mb low northwest of the islands peaks between 4 and 5
  standard deviations below normal. 

* PWATs are also expected to be near 4 standard deviations above 
  normal per the GEFS, while the latest ECMWF Extreme Forecast 
  Index (EFI) exceeds 0.85, indicative of very unusual influx of 
  tropical moisture for March. 

The ECMWF EFI also indicates positive shift-of-tails (SOT) values
for the western and central islands, suggesting that at least 10%
of the ensemble members are forecasting an "extreme" event. The
SOT highlights the potential for localized, multi-day rainfall 
totals to significantly exceed historical March averages.

Hurley